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A Seminar Report On “Spintronics Technology”
Session 2010-2011
Submitted To:
Submitted By:
Dr. R.S. Meena Mr. Pankaj Shukla Dept. of Electronics Engg. UCE, RTU, Kota
Shailendra Kumar Singh C.R.No. 07/126 Final Year, ECE UCE, RTU, Kota
Department of Electronics and Communication Engg. University College of Engineering Rajasthan Technical University, Kota Page 1
CERTIFICATE This is certify that the Seminar report titled “Spintronics Technology” has been submitted in partial fulfilment of the requirement for the award of Bachelor of Technology in Electronics & Communication Engineering by following student of final year B.Tech.
Shailendra Kumar Singh C.R.No:- 07/126 B.TECH. FINAL YEAR UCE, RTU, KOTA
Seminar Coordinators:
Head of the Department:
Dr R S Meena & Mr Pankaj Shukla (Associate Professors) Dept. Of Electronics Engg. UCE, RTU, Kota
Dr Rajeev Gupta Professor Dept. Of Electronics Engg. UCE, RTU, Kota
Page 2
ACKNOWLEDGEMENT It gives me great pleasure to present my seminar report on “Spintronics Technology”. No work , however big or small, has ever been done without the contributions of others. It would be a great pleasure to write a few words, which would although not suffice as the acknowledgement of this long cherished effort, but in the absence of which this report would necessarily be incomplete. So these words of acknowledgement come as a small gesture of gratitude towards all those people, without whom the successful completion of this project would not have been possible. I would like to express deep gratitude towards Dr. R S Meena (Associate Professor of Electronics Engineering Dept., UCE, Kota) & Mr. Pankaj Shukla (Associate Professor of Electronics Engineering Dept., UCE, Kota) who gave me their valuable suggestions, motivation and the direction to proceed at every stage.They are like a beam of light for us. Their kind guidance showed us the path of life and is unforgettable. They extended towards their valuable guidance, indispensable help and inspiration at times in appreciation I offer them my sincere gratitude. Last but not least we would like to thank the Department of Electronics Engineering, UCE, Kota for providing me with the facilities to lab, and all staff of communication lab, it would have been impossible for me to complete my project without their valuable guidance & prompt cooperation. I have tried my level best to make this seminar report error free ,but I regret for errors , if any.
SHAILENDRA KUMAR SINGH C.R.NO. - 07/126 B. TECH. FINAL YEAR, ECE UCE, RTU, KOTA
Page 3
CONTENTS S. No
Chapters
Page No
1.
Introduction
07
2.
Basic Principle
08
3.
Gaint Magnetoresistance
10
4.
Construction of GMR
12
5.
Memory Chips
14
6.
GMR Sensors
15
7.
Spin Valve GMR
16
8.
Spintronic Devices
17
9.
MRAM
18
10.
Spin Transistors
19
11.
Spintronic Scanner
22
12.
Conclusion
26
13.
Reference
27
Page 4
List of Figures S No.
Figure Name
Page No.
1.
Electron spinning
08
2.
Magnetic Orientation of electrons.
09
3.
A GMR read head
10
4.
A GMR Device
13
5.
A General Magnetic Field Sensor
14
6.
Spintronic Sensor
15
7.
Standard Geometry for GMR based Spin Valves
16
8.
GMR based Spin Valves for read head In hard drives
16
9.
256 K MRAM
18
10.
Spin Transistor
19
11.
Spin Polarised Field Effect Transistor
20
Page 5
ABSTRACT Spintronics is an emergent technology that exploits the quantum propensity of the electrons to spin as well as making use of their charge state. The spin itself is manifested as a detectable weak magnetic energy state characterised as ―spin up‖ or ―spin down‖. Conventional electronic devices rely on the transport of electrical charge carriers – electrons – in a semiconductor such as silicon. Now, however, device engineers and physicists are inevitably faced the looming presence of quantum mechanics and are trying to exploit the spin of the electron rather than its charge. Devices that rely on the electron‘s spin to perform their functions form the foundations of spintronics (short for spin-based electronics), also known as magnetoelectronics. Spintronics devices are smaller than 100 nanometre in size, more versatile and more robust than those making up silicon chips and circuit elements. The potential market is worth hundreds of billions of dollar a year. Spintronics burst on the scene in 1988 when French and German physicists discovered a very powerful effect called Giant Magnetoresistance (GMR). It results from subtle electron-spin effects in ultra thin multilayers of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. This resulted in the first spintronic device in the form of the spin valve. The incorporation of GMR materials into read heads allowed the storage capacity of a hard disk to increase from one to 20 gigabits. In 1997, IBM launched GMR read heads, into a market worth around a billion dollars a year. The field of spintronics is relatively young and it is difficult to predict how it will evolve. New physics is still being discovered and new materials being developed, such as magnetic semiconductors and exotic oxides that manifest an even more extreme effect called Colossal Magnetoresistance.
Page 6
Chapter 1 INTRODUCTION Conventional electronic devices rely on the transport of electrical charge carriers –electrons in a semiconductor such as silicon. Now, however, physicists are trying to exploit the ‗spin‘ of the electron rather than its charge to create a remarkable new generation of ‗spintronic‘ devices which will be smaller, more versatile and more robust than those currently making up silicon chips and circuit elements. Imagine a data storage device of the size of an atom working at a speed of light. Imagine a computer memory thousands of times denser and faster than today‘s memories and also imagine a scanner technique which can detect cancer cells even though they are less in number. The above-mentioned things can be made possible with the help of an exploding science – ―Spintronics‖.
Spintronics is a technology which deals with spin dependent properties of an electron instead of or in addition to its charge dependent properties. Conventional electronics devices rely on the transport of electric charge carries-electrons. But there is other dimensions of an electron other than its charge and mass i.e. spin. This dimension can be exploited to create a remarkable generation of spintronic devices. It is believed that in the near future spintronics could be more revolutionary than any other technology.
As there is rapid progress in the miniaturization of semiconductor electronic devices leads to a chip features smaller than 100 nanometers in size, device engineers and physicists are inevitable faced with a looming presence of a quantum property of an electron known as spin, which is closely related to magnetism. Devices that rely on an electron spin to perform their functions form the foundations of spintronics.
Information-processing technology has thus far relied on purely charge based devices ranging from the now quantum, vacuum tube today‘s million transistor microchips. Those conventional electronic devices move electronic charges around, ignoring the spin that tags along that side on each electron. Page 7
Chapter 2 BASIC PRINCIPLE The basic principle involved is the usage of spin of the electron in addition to mass and charge of electron. Electrons like all fundamental particles have a property called spin which can be orientated in one direction or the other – called ‗spin-up‘ or ‗spin-down‘ –like a top spinning anticlockwise or clockwise. Spin is the root cause of magnetism and is a kind of intrinsic angular momentum that a particle cannot gain or lose. The two possible spin states naturally represent ‗0‘and ‗1‘in logical operations. Spin is the characteristics that makes the electron a tiny magnet complete with north and south poles .The orientation of the tiny magnet ‗s north-south poles depends on the particle‘s axis of spin.
Fundamentals of spin: In addition to their mass, electrons have an intrinsic quantity of angular momentum
1.
called spin, almost of if they were tiny spinning balls. 2.
Associated with the spin is magnetic field like that of a tiny bar magnet lined up with the spin axis.
. Fig.1. Electron spinning 2.
Scientists represent the spin with a vector. For a sphere spinning ―west to east‖, the vector points ―north‖ or ―up‖. It points ―south‖ or ―down‖ for the spin from ―east to west‖.
4.
In a magnetic field, electrons with ―spin up‖ and ―spin down‖ have different energies.
5.
In an ordinary electronic circuit the spins are oriented at random and have no effect on current flow. Page 8
6.
Spintronic devices create spin-polarized currents and use the spin to control current flow.
Imagine a small electronically charged sphere spinning rapidly. The circulating charges in the sphere amount to tiny loops of electric current which creates a magnetic field. A spinning sphere in an external magnetic field changes its total energy according to how its spin vector is aligned with the spin. In some ways, an electron is just like a spinning sphere of charge, an electron has a quantity of angular momentum (spin) an associated magnetism. In an ambient magnetic field and the spin changing this magnetic field can change orientation. Its energy is dependent on how its spin vector is oriented. The bottom line is that the spin along with mass and charge is defining characteristics of an electron. In an ordinary electric current, the spin points at random and plays no role in determining the resistance of a wire or the amplification of a transistor circuit. Spintronic devices in contrast rely on the differences in the transport of spin-up and spin-down electrons.
Fig 2. Magnetic Orientation of electrons
Page 9
Chapter 3 Giant Magnetoresistance Electrons like all fundamental particles have a property called spin which can be orientated in one direction or the other – called „spin-up‟ or „spin-down‟ – like a top spinning anticlockwise or clockwise. When electron spins are aligned (i.e. all spin-up or all spin-down) they create a large-scale net magnetic moment as seen in magnetic materials like iron and cobalt. Magnetism is an intrinsic physical property associated with the spins of electrons in a material. Magnetism is already exploited in recording devices such as computer hard disks Data are recorded and stored as tiny areas of magnetised iron or chromium oxide. To access the information, a read head detects the minute changes in magnetic field as the disk spins underneath it. This induces corresponding changes in the head‟s electrical resistance – an effect called magnetoresistance. Spintronics burst on the scene in 1988 when French and German physicists discovered a much more powerful effect called „giant magnetoresistance‟ (GMR). It results from subtle electron-spin effects in ultra-thin „multilayers‟ of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. GMR is 200 times stronger than ordinary magnetoresistance. IBM soon realised that read heads incorporating GMR materials would be able to sense much smaller magnetic fields, allowing the storage capacity of a hard disk to increase from 1 to 20 gigabits. In 1997 IBM launched GMR read heads, into a market worth about a billion dollars a year. The basic GMR device consistsmof a three-layer sandwich of a magnetic metal such as cobalt with a nonmagnetic metal filling such as silver (see diagram). A current es through the layers consisting of spin-up and spin-down electrons. Those oriented in the same direction as the electron spins in a magnetic layer through quite easily while those oriented in the opposite direction are scattered. If the orientation of one of the magnetic layers can easily be changed by the presence of a magnetic field then the device will act as a filter, or „spin valve‟, letting through more electrons when the spin orientations in the two layers are the same and fewer when orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically.
Fig 3. A GMR read head Page 10
Magnetism is the integral part of the present day‘s data storage techniques. Right from the Gramophone disks to the hard disks of the super computer magnetism plays an important role. Data is recorded and stored as tiny areas of magnetized iron or chromium oxide. To access the information, a read head detects the minute changes in magnetic field as the disk spins underneath it. In this way the read heads detect the data and send it to the various succeeding circuits.
The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or anantiparallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment.
The magneto resistant devices can sense the changes in the magnetic field only to a small extent, which is appropriate to the existing memory devices. When we reduce the size and increase data storage density, we reduce the bits, so our sensor also has to be small and maintain very, very high sensitivity. The thought gave rise to the powerful effect called ―Giant Magnetoresistance‖ (GMR). GMR is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and non magnetic layers. The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR. Giant magnetoresistance (GMR) came into picture in 1988, which lead the rise of spintronics. It results from subtle electron-spin effects in ultra-thin ‗multilayer‘ of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. GMR is 200 times stronger than ordinary magnetoresistance. It was soon realized that read heads incorporating GMR materials would be able to sense much smaller magnetic fields, allowing the storage capacity of a hard disk to increase from 1 to 20 gigabits.
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Chapter 4 Construction of GMR The basic GMR device consists of a three-layer sandwich of a magnetic metal such as cobalt with a nonmagnetic metal filling such as silver. Current es through the layers consisting of spin-up and spin-down electrons. Those oriented in the same direction as the electron spins in a magnetic layer through quite easily while those oriented in the opposite direction are scattered. If the orientation of one of the magnetic layers can easily be changed by the presence of a magnetic field then the device will act as a filter, or ‗spin valve‘, letting through more electrons when the spin orientations in the two layers are the same and fewer when orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically. In an ordinary electric current, the spin points at random and plays no role in determining the resistance of a wire or the amplification of a transistor circuit. Spintronic devices in contrast, rely on differences in the transport of ―spin up‖ and ―spin down‖ electrons. When a current es through the Ferro magnet, electrons of one spin direction tend to be obstructed.
A ferromagnet can even affect the flow of a current in a nearby nonmagnetic metal. For example, in the present-day read heads in computer hard drives, wherein a layer of a nonmagnetic metal is sandwiched between two ferromagnetic metallic layers, the magnetization of the first layer is fixed, or pinned, but the second ferromagnetic layer is not. As the read head travels along a track of data on a computer disk, the small magnetic fields of the recorded 1‘s and 0`s change the second layer‘s magnetization back and forth parallel or antiparallel to the magnetization of the pinned layer. In the parallel case, only electrons that are oriented in the favored direction flow through the conductor easily. In the antiparallel case, all electrons are impeded. The resulting changes in the current allow GMR read heads to detect weaker fields than their predecessors; so that data can be stored using more tightly packaged magnetized spots on a disk. GMR has triggered the rise of a new field of electronics called spintronics which has been used extensively in the read heads of modern hard drives and magnetic sensors. A hard disk storing binary information can use the difference in resistance between parallel and antiparallel layer alignments as a method of storing 1s and 0s. A high GMR is preferred for optimal data storage density. Current perpendicular-to-plane (P) Spin valve GMR currently yields the highest GMR. Research continues with older current-in-plane configuration and in the tunnelling magnetoresistance (TMR) spin valves which enable disk drive densities exceeding 1 Terabyte per square inch. Page 12
Hard disk drive manufacturers have investigated magnetic sensors based on the colossal magnetoresistance effect (CMR) and the giant planar Hall effect. In the lab, such sensors have demonstrated sensitivity which is orders of magnitude stronger than GMR. In principle, this could lead to orders of magnitude improvement in hard drive data density. As of 2003, only GMR has been exploited in commercial disk read-and-write heads because researchers have not demonstrated the CMR or giant planar hall effects at temperatures above 150K. Magnetocoupler is a device that uses giant magnetoresistance (GMR) to couple two electrical circuits galvanicly isolated and works from AC down to DC. Vibration measurement in MEMS systems. Detecting DNA or protein binding to capture molecules in a surface layer by measuring the stray field from superparamagnetic label particles.
Fig 4. A GMR Device
Page 13
Chapter 5 Memory Chips Physicists have been quick to see the further possibilities of spin valves. Not only are they highly sensitive magnetic sensors (see Box), they can also be made to act as switches by flipping the magnetisation in one of the layers. This allows information to be stored as 0s and 1s (magnetisations of the layers parallel or antiparallel) as in a conventional transistor memory device. An obvious application is a magnetic version of a random access memory (RAM) device of the kind used in your computer. The advantage of magnetic random access memory (MRAM) is that it is „non-volatile‟ – information isn‟t lost when the system is switched off. MRAM devices would be smaller, faster, cheaper, use less power and would be much more robust in extreme conditions such as high temperature, or highlevel radiation or interference. The US electronics company Honeywell has already shown that arrays of linked MRAMS could be made to work. The potential market for MRAMS is worth 100 billion dollars annually. Over the past three years or so, researchers around the world have been working hard on a whole range of MRAM devices. A particularly promising device is the magnetic tunnel junction, which has two magnetic layers separated by an insulating metaloxide layer. Electrons can „tunnel‟ through from one layer to the other only when magnetisations of the layers point in the same direction, otherwise the resistance is high – in fact, 1000 times higher than in the standard spin valve. Even more interesting are devices that combine the magnetic layers with semiconductors like silicon. The advantage is that silicon is still the favourite material of the electronics industry and likely to remain so. Such hybrid devices could be made to behave more like conventional transistors. They could be used as non-volatile logic elements which could be reprogrammed using software during actual processing to create an entirely new type of very fast computing. The field of spintronics is extremely young and it‟s difficult to predict how it will evolve. New physics is still being discovered and new materials being developed, such as magnetic semiconductors, and exotic oxides that manifest an even more extreme effect called colossal magnetoresistance. What is certain is that the timespan from a breakthrough in fundamental physics to first commercial exploitation has been less than 10 years. The business opportunities for spintronics are still wide open. European research collaborations, some involving the UK, have a strong lead in developing the underlying physics and technology for this lucrative fledgling industry.
Fig 5. A general magnetic field sensor made of GMR multilayers ( iron-nickel with silver )
Page 14
Chapter 6 GMR SENSORS GMR sensors are already being developed in UK universities. They have a wide range of applications and the market is worth 8 billion dollars a year. Applications include: • Fast accurate position and motion sensing of mechanical components in precision and in robotics
engineering
• All kinds of automotive sensors for fuel handling systems, electronic engine control, antiskid systems, speed control and navigation • Missile guidance • Position and motion sensing in computer video games • Key-hole surgery and post-operative care
Fig 6. Spintronic sensor technology being tested on a Mercedes V8 engine at Oxford
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Chapter 7 Spin Valve GMR If the orientation of one of the magnetic layers can easily be changed by the presence of a magnetic field then the device will act as a filter, or ‗spin valve‘, letting through more electrons when the spin orientations in the two layers are the same and fewer when orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically.
Fig 7. Standard geometry for GMR based Spin Valves An electron ing through the spin-valve will be scattered more if the spin of the electron is opposite to the direction of the magnetisation in the FM layer.
Fig 8. GMR based Spin Valves for read head in hard drives Page 16
Chapter 8 Spintronic Devices
Spintronic devices are those devices which use the Spintronic technology. Spintronic-devices combine the advantages of magnetic materials and semiconductors. They are expected to be nonvolatile, versatile, fast and capable of simultaneous data storage and processing, while at the same time consuming less energy. Spintronic-devices are playing an increasingly significant role in high-density data storage, microelectronics, sensors, quantum computing and bio-medical applications, etc. Electronic Devices
Spintronic devices
1. Based on properties of charge of the electron
1. Based on intrinsic property spin of electron.
2. Classical property
2. Quantum property
4. Materials: conductors and semiconductors
4. Materials: ferromagnetic materials
5. Based on the number of charges and their energy
5. Two basic spin states; spin-up and spindown.
6. Speed is limited and power dissipation is high
6. Based on direction of spin and spin and spin coupling, high speed.
Some of the Spintronic devices are: Magnetoresistive Random Access Memory(MRAM) Spin Transistor Quantum Computer Spintronic Scanner
Page 17
Chapter 9 MRAM (Magnetoresistive Random Access Memory) An important spintronic device, which is supposed to be one of the first spintronic devices that have been invented, is MRAM. Unlike conventional random-access, MRAMs do not lose stored information once the power is turned off...A MRAM computer uses power, the four page e mail will be right there for you. Today pc use SRAM and DRAM both known as volatile memory. They can store information only if we have power. DRAM is a series of capacitors, a charged capacitor represents 1 where as an uncharged capacitor represents 0. To retain 1 you must constantly feed the capacitor with power because the charge you put into the capacitor is constantly leaking out. MRAM is based on integration of magnetic tunnel junction (MJT). Magnetic tunnel junction is a three-layered device having a thin insulating layer between two metallic ferromagnets. Current flows through the device by the process of quantum tunneling; a small number of electrons manage to jump through the barrier even though they are forbidden to be in the insulator. The tunneling current is obstructed when the two ferromagnetic layers have opposite orientations and is allowed when their orientations are the same. MRAM stores bits as magnetic polarities rather than electric charges. When a big polarity points in one direction it holds1, when its polarity points in other direction it holds 0. These bits need electricity to change the direction but not to maintain them. MRAM is non volatile so, when you turn your computer off all the bits retain their 1`s and 0`s.
Fig 9. 256 K MRAM Page 18
Chapter 10 SPIN TRANSISTORS Traditional transistors use on-and-off charge currents to create bits- the binary zeroes and ones of computer information. “Quantum spin field effect” transistor will use up-and-down spin states to generate the same binary data. One can think of electron spin as an arrow; it can point upward or downward; “spinup and spin-down can be thought of as a digital system, representing the binary 0 and 1. The quantum transistor employs also called “spin-flip” mechanism to flip an up-spin to a downspin, or change the binary state from 0 to 1. One proposed design of a spin FET (spintronic field-effect transistor) has a source and a drain, separated by a narrow semi conducting channel, the same as in a conventional FET. In the spin FET, both the source and the drain are ferromagnetic. The source sends spin-polarized electrons in to the channel, and this spin current flow easily if it reaches the drain unaltered (top). A voltage applied to the gate electrode produces an electric field in the channel, which causes the spins of fastmoving electrons to process, or rotate (bottom). The drain impedes the spin current according to how far the spins have been rotated. Flipping spins in this way takes much less energy and is much faster than the conventional FET process of pushing charges out of the channel with a larger electric filed.
Fig 10. In these devices a non magnetic layer which is used for transmitting and controlling the spin polarized electrons from source to drain plays a crucial role. For functioning of this device first the spins have to be injected from source into this non-magnetic layer and then transmitted to the collector. These non-magnetic layers are also called as semimetals, because they have very large spin diffusion lengths. The injected spins which are transmitted through this layer start precessing as illustrated in Figure before they reach the collector due to the spin-orbit coupling effect. Page 19
Vgg Collector
gate
Source
InAlAs
InGaAs
Fig.11 Spin polarized field effect transistor. Vg is the gate voltage. When Vg is zero the injected spins which are transmitted through the 2DEG layer starts processing before they reach the collector, thereby reducing the net spin polarization. Vg is the gate voltage. When Vg >> 0 the precession of the electrons is controlled with electric filed thereby allowing the spins to reach at the collector with the same polarization.
Hence the net spin polarization is reduced. In order to solve this problem an electric field is applied perpendicularly to the plane of the film by depositing a gate electrode on the top to reduce the spin-orbit coupling effect as illustrated in Figure 4. By controlling the gate voltage and polarity can the current in the collector can be modulated there by mimicking the MOSFET of the conventional electronics. Here again the problem of conductivity mismatch between the source and the transmitting layer is an important issue. The interesting thing would be if a Heusler alloy is used as the spin source and a semimetallic Heusler alloy as the transmitting layer, the problem of conductivity mismatch may be solved. For example from the Slater-Pauling curve Mt = Zt - 24, Heusler alloys with Mt >>0 can act as spin sources and alloys with Mt ~ 0 can act as semimetals. Since both the constituents are of same structure the possibility of conductivity
mismatch
may
be
less.
Traditional transistors use on-and-off charge currents to create bits—the binary zeroes and ones of computer information. ―Quantum spin field effect‖ transistor will use up-and-down spin states to generate the same binary data. One can think of electron spin as an arrow; it can point upward or downward; ―spin-up and spin-down can be thought of as a digital system, representing the Page 20
binary 0 and 1. The quantum transistor employs also called ―spin-flip‖ mechanism to flip an upspin to a downspin, or change the binary state from 0 to 1. One proposed design of a spin FET (spintronic field-effect transistor) has a source and a drain, separated by a narrow semi conducting channel, the same as in a conventional FET. In the spin FET, both the source and the drain are ferromagnetic. The source sends spinpolarized electrons in to the channel, and this spin current flow easily if it reaches the drain unaltered (top). A voltage applied to the gate electrode produces an electric field in the channel, which causes the spins of fast-moving electrons to process, or rotate (bottom). The drain impedes the spin current according to how far the spins have been rotated. Flipping spins in this way takes much less energy and is much faster than the conventional FET process of pushing charges out of the channel with a larger electric filed. One advantage over regular transistors is that these spin states can be detected and altered without necessarily requiring the application of an electric current. This allows for detection hardware that are much smaller but even more sensitive than today's devices, which rely on noisy amplifiers to detect the minute charges used on today's data storage devices. The potential end result is devices that can store more data in less space and consume less power, using less costly materials. The increased sensitivity of spin transistors is also being researched in creating more sensitive automotive sensors, a move being encouraged by a push for more environmentally-friendly vehicles A second advantage of a spin transistor is that the spin of an electron is semi-permanent and can be used as means of creating cost-effective non volatile solid state storage that does not require the constant application of current to sustain. It is one of the technologies being explored for Magnetic Random Access Memory (MRAM) Spin transistors are often used in computers for data processing. They can also be used to produce a computer's random access memory and are being tested for use in magnetic RAM. This memory is superfast and information stored on it is held in place after the computer is powered off, much like a hard disk.
Page 21
Chapter 11 Spintronic Scanner
Cancer cells are the somatic cells which are grown into abnormal size. The Cancer cells have different electromagnetic sample when compared to normal cells. For many types of Cancer, it is easier to treat and cure the Cancer if it is found early. There are many different types of Cancer, but most Cancers begin with abnormal cells growing out of control, forming a lump that's called a tumor. The tumor can continue to grow until the Cancer begins to spread to other parts of the body. If the tumor is found when it is still very small, curing the Cancer can be easy. However, the longer the tumor goes unnoticed, the greater the chance that the Cancer has spread. This makes treatment more difficult. Tumor developed in human body, is removed by performing a surgery. Even if a single cell is present after the surgery, it would again develop into a tumor. In order to prevent this, an efficient route for detecting the Cancer cells is required. Here, in this paper, we introduce a new route for detecting the Cancer cells after a surgery. This accurate detection of the existence of Cancer cells at the beginning stage itself entertains the prevention of further development of the tumor. This spintronic scanning technique is an efficient technique to detect cancer cells even when they are less in number. An innovative approach to detect the cancer cells with the help of Spintronics: The following setup is used for the detection of cancer cells in a human body: (a) Polarized electron source (b) Spin detector (c) Magnetic Field
Polarized electron source: A beam of electrons is said to be polarized if their spins point, on average, in a specific direction. There are several ways to employ spin on electrons and to control them. The requirement for this paper is an electron beam with all its electrons polarized in a specific direction. The following are the ways to meet the above said requirement: Photoemission from negative electron affinity GaAs Chemi-ionization of optically pumped meta stable Helium An optically pumped electron spin filter A Wein style injector in the electron source A spin filter is more efficient electron polarizer which uses an ordinary electron source along with a gaseous layer of
Page 22
Rb. Free electrons diffuse under the action of an electric field through Rb vapour that has been spin polarized in optical pumping. Through spin exchange collisions with the Rb, the free electrons become polarized and are extracted to form a beam. To reduce the emission of depolarizing radiation, N2 is used to quench the excited Rb atoms during the optical pumping cycle.
Spin detectors: There are many ways by which the spin of the electrons can be detected efficiently. The spin polarization of the electron beam can be analyzed by using: (a)Mott polarimeter (b)Compton polarimeter (c)Moller type polarimeter Typical Mott polarimeters require electron energies of ~100 kV. But Mini Mott polarimeter uses energies of ~25 keV, requiring a smaller overall design. The Mini Mott polarimeter has three major sections: the electron transport system, the target chamber, and the detectors. The first section the electrons enter is the transport system. An Einsel lens configuration was used here. Two sets of four deflectors were used as the first and last lens. The electrons next enter the target chamber. The chamber consists of a cylindrical target within a polished stainless steel hemisphere. A common material used for the high-Z nuclei target is gold. Low-Z nuclei help minimize unwanted scattering, so aluminum was chosen. Scattered electrons then exit the target chamber and are collected in the detectors. Thus there are many methods for detecting the spin polarization of electrons.
External Magnetic Field: An external magnetic field is required during this experiment. The magnetic field is applied after the surgery has undergone. First, it is applied to an unaffected part of the body and then to the surgery undergone part of the body. It is already mentioned that the magnetic field could easily alter the polarization of electrons.
Page 23
This technique using spintronics is suggested by us to identify tumor cells after surgery. The procedure for doing this experiment is as follows: Optical Spin Filter: After surgery and the removal of the tumor, the patient is exposed to a strong magnetic field. Now the polarized electron beam is applied over the unaffected part and spin orientation of electrons are determined using polarimeter. Then the same polarized beam is targeted over the affected part of the body and from the reflected beam, change in spin is determined. Based on these two values of spin orientation, the presence of tumor cells can be detected even if they are very few in number. Hence, we suggest this method for the detection purpose. A detailed view of this
innovative
approach
is
given
as
follows.
Spin Orientation of the unaffected part of the body: Applying Magnetic Field: When the magnetic field is applied to the unaffected part of the human body, the normal somatic cells absorbs the magnetic energy and retains it. Determinig the Spin orientation: When the electrons get incident on the cells the magnetic energy absorbed by the cells alters the spin orientation of the electrons. These electrons get reflected and it is detected by the Mott polarimeter. Then the change in spin orientation of the electrons is measured as Sx.
Spin Orientation of the surgery undergone part of the body: Applying Magnetic Filter: In the surgery undergone part of the body an external magnetic field is applied. The cancer cells which are present, if any, will absorb more magnetic energy than the normal cells since they differ in their electromagnetic pattern. Determinig the spin Orientation: Now an electron beam which is polarized is incident on the surgery undergone part of the body. The magnetic energy absorbed by the cancer cell alter the spin orientation of the electron beam. Since cancer cells absorb more magnetic energy, the change in orientation caused by them is also more. If no cancer cells are present the amount of change is equal to the previous case. The change in spin is measured by the polarimeter as Sy. Page 24
Inference: If the change in the spin in the unaffected part of the body is same as that of the surgery undergone part, i.e. If Sx=Sy Then, There are no cancer cells in the surgery undergone part of the body and all the cells have been removed by the surgery. If the change in spin in the unaffected part is not equal to the change caused by the surgery undergone part of the body, i.e. If Sx not equals Sy Then, There are some cancer cells in the surgery undergone part of the body and the cancer cells are not completely removed by the surgery. The steps involved are: 1) The patient is exposed to a strong magnetic field so that his body cell gets magnetized. 2) A beam of electrons with polarized spin is introduced on the unaffected part of the body and the change in spin is detected by a polarimeter. Let it be X 3) A beam of electrons with polarized spin is introduced on the part which had undergone surgery. And the corresponding change in spin be Y 4) If X - Y = 0, it indicates that cancer cells have been removed from the body, if not it indicates the presence of traces of cancer cells and it has to be treated again for ensuring complete safety to the patient.
Thus this technique efficiently identifies the presence of cancer cells in that part of the body that has undergone surgery to prevent any further development.
Page 25
CONCLUSION
Spintronics is one of the most exciting and challenging areas in nanotechnology, important to both fundamental scientific research and industrial applications. These spintronic-devices, combining the advantages of magnetic materials and semiconductors, are expected to be non-volatile, versatile, fast and capable of simultaneous data storage and processing, while at the same time consuming less energy. They are playing an increasingly significant role in highdensity data storage, microelectronics, sensors, quantum computing and bio-medical applications, etc. It is expected that the impact of spintronics to the microelectronics industry might be comparable to the development of the transistor 50 years ago. Today everyone already has a spintronic device on their desktop, as all modern computers use the spin valve in order to read and write data on their hard drive. It was followed immediately by the discovery of Tunneling Magnetoresistance (TMR) leading to the magnetic tunnel junction that has been utilized for the next generation computer memory known as Magnetic Random Access Memory (MRAM), another spintronic device for computers. Therefore, the initial driving force for spintronics has been the improvement of computer technology. At present the research has been concentrating on the fabrication of spin transistors and spin logics devices integrating magnetic and semiconductors, with the aim of improving the existing capabilities of electronic transistors and logics devices so that the future computation and thus the future computer could become faster and consume less energy. There are four main areas in spintronics: . 1) Understanding the fundamental physics, such as spin-dependant transports across the magnetic/ semiconductor interfaces and spin coherence length in semiconductors. 2) Synthesising suitable spintronic materials with Curie temperatures above room temperature, large spin polarisation at the Fermi level and matching conductivity between the magnetic and semiconductor materials. 3) Fabricating devices with nanometre feature sizes and developing new techniques for mass production. 4) Integrating spin-devices with current microelectronics and computing.
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REFERENCES 1. IEEE Digital Explore Library 2. School of Physics & Astronomy, University of Nottingham 3. Department of Physics and MARTECH , Florida State University 4. Department of Physics and Center for Advanced Photonic and Electronic Materials University at Buffalo ,The State University of New York 5. Research Councils UK www.rcuk.ac.uk 6. Engineering and Physical Sciences Research Council (EPSRC) www.epsrc.ac.uk 7. Particle Physics and Astronomy Research Council (PPARC) www.pparc.ac.uk 8. Council for the Central Laboratory of the Research Councils www.cclrc.ac.uk
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Session 2010-2011
Submitted To:
Submitted By:
Dr. R.S. Meena Mr. Pankaj Shukla Dept. of Electronics Engg. UCE, RTU, Kota
Shailendra Kumar Singh C.R.No. 07/126 Final Year, ECE UCE, RTU, Kota
Department of Electronics and Communication Engg. University College of Engineering Rajasthan Technical University, Kota Page 1
CERTIFICATE This is certify that the Seminar report titled “Spintronics Technology” has been submitted in partial fulfilment of the requirement for the award of Bachelor of Technology in Electronics & Communication Engineering by following student of final year B.Tech.
Shailendra Kumar Singh C.R.No:- 07/126 B.TECH. FINAL YEAR UCE, RTU, KOTA
Seminar Coordinators:
Head of the Department:
Dr R S Meena & Mr Pankaj Shukla (Associate Professors) Dept. Of Electronics Engg. UCE, RTU, Kota
Dr Rajeev Gupta Professor Dept. Of Electronics Engg. UCE, RTU, Kota
Page 2
ACKNOWLEDGEMENT It gives me great pleasure to present my seminar report on “Spintronics Technology”. No work , however big or small, has ever been done without the contributions of others. It would be a great pleasure to write a few words, which would although not suffice as the acknowledgement of this long cherished effort, but in the absence of which this report would necessarily be incomplete. So these words of acknowledgement come as a small gesture of gratitude towards all those people, without whom the successful completion of this project would not have been possible. I would like to express deep gratitude towards Dr. R S Meena (Associate Professor of Electronics Engineering Dept., UCE, Kota) & Mr. Pankaj Shukla (Associate Professor of Electronics Engineering Dept., UCE, Kota) who gave me their valuable suggestions, motivation and the direction to proceed at every stage.They are like a beam of light for us. Their kind guidance showed us the path of life and is unforgettable. They extended towards their valuable guidance, indispensable help and inspiration at times in appreciation I offer them my sincere gratitude. Last but not least we would like to thank the Department of Electronics Engineering, UCE, Kota for providing me with the facilities to lab, and all staff of communication lab, it would have been impossible for me to complete my project without their valuable guidance & prompt cooperation. I have tried my level best to make this seminar report error free ,but I regret for errors , if any.
SHAILENDRA KUMAR SINGH C.R.NO. - 07/126 B. TECH. FINAL YEAR, ECE UCE, RTU, KOTA
Page 3
CONTENTS S. No
Chapters
Page No
1.
Introduction
07
2.
Basic Principle
08
3.
Gaint Magnetoresistance
10
4.
Construction of GMR
12
5.
Memory Chips
14
6.
GMR Sensors
15
7.
Spin Valve GMR
16
8.
Spintronic Devices
17
9.
MRAM
18
10.
Spin Transistors
19
11.
Spintronic Scanner
22
12.
Conclusion
26
13.
Reference
27
Page 4
List of Figures S No.
Figure Name
Page No.
1.
Electron spinning
08
2.
Magnetic Orientation of electrons.
09
3.
A GMR read head
10
4.
A GMR Device
13
5.
A General Magnetic Field Sensor
14
6.
Spintronic Sensor
15
7.
Standard Geometry for GMR based Spin Valves
16
8.
GMR based Spin Valves for read head In hard drives
16
9.
256 K MRAM
18
10.
Spin Transistor
19
11.
Spin Polarised Field Effect Transistor
20
Page 5
ABSTRACT Spintronics is an emergent technology that exploits the quantum propensity of the electrons to spin as well as making use of their charge state. The spin itself is manifested as a detectable weak magnetic energy state characterised as ―spin up‖ or ―spin down‖. Conventional electronic devices rely on the transport of electrical charge carriers – electrons – in a semiconductor such as silicon. Now, however, device engineers and physicists are inevitably faced the looming presence of quantum mechanics and are trying to exploit the spin of the electron rather than its charge. Devices that rely on the electron‘s spin to perform their functions form the foundations of spintronics (short for spin-based electronics), also known as magnetoelectronics. Spintronics devices are smaller than 100 nanometre in size, more versatile and more robust than those making up silicon chips and circuit elements. The potential market is worth hundreds of billions of dollar a year. Spintronics burst on the scene in 1988 when French and German physicists discovered a very powerful effect called Giant Magnetoresistance (GMR). It results from subtle electron-spin effects in ultra thin multilayers of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. This resulted in the first spintronic device in the form of the spin valve. The incorporation of GMR materials into read heads allowed the storage capacity of a hard disk to increase from one to 20 gigabits. In 1997, IBM launched GMR read heads, into a market worth around a billion dollars a year. The field of spintronics is relatively young and it is difficult to predict how it will evolve. New physics is still being discovered and new materials being developed, such as magnetic semiconductors and exotic oxides that manifest an even more extreme effect called Colossal Magnetoresistance.
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Chapter 1 INTRODUCTION Conventional electronic devices rely on the transport of electrical charge carriers –electrons in a semiconductor such as silicon. Now, however, physicists are trying to exploit the ‗spin‘ of the electron rather than its charge to create a remarkable new generation of ‗spintronic‘ devices which will be smaller, more versatile and more robust than those currently making up silicon chips and circuit elements. Imagine a data storage device of the size of an atom working at a speed of light. Imagine a computer memory thousands of times denser and faster than today‘s memories and also imagine a scanner technique which can detect cancer cells even though they are less in number. The above-mentioned things can be made possible with the help of an exploding science – ―Spintronics‖.
Spintronics is a technology which deals with spin dependent properties of an electron instead of or in addition to its charge dependent properties. Conventional electronics devices rely on the transport of electric charge carries-electrons. But there is other dimensions of an electron other than its charge and mass i.e. spin. This dimension can be exploited to create a remarkable generation of spintronic devices. It is believed that in the near future spintronics could be more revolutionary than any other technology.
As there is rapid progress in the miniaturization of semiconductor electronic devices leads to a chip features smaller than 100 nanometers in size, device engineers and physicists are inevitable faced with a looming presence of a quantum property of an electron known as spin, which is closely related to magnetism. Devices that rely on an electron spin to perform their functions form the foundations of spintronics.
Information-processing technology has thus far relied on purely charge based devices ranging from the now quantum, vacuum tube today‘s million transistor microchips. Those conventional electronic devices move electronic charges around, ignoring the spin that tags along that side on each electron. Page 7
Chapter 2 BASIC PRINCIPLE The basic principle involved is the usage of spin of the electron in addition to mass and charge of electron. Electrons like all fundamental particles have a property called spin which can be orientated in one direction or the other – called ‗spin-up‘ or ‗spin-down‘ –like a top spinning anticlockwise or clockwise. Spin is the root cause of magnetism and is a kind of intrinsic angular momentum that a particle cannot gain or lose. The two possible spin states naturally represent ‗0‘and ‗1‘in logical operations. Spin is the characteristics that makes the electron a tiny magnet complete with north and south poles .The orientation of the tiny magnet ‗s north-south poles depends on the particle‘s axis of spin.
Fundamentals of spin: In addition to their mass, electrons have an intrinsic quantity of angular momentum
1.
called spin, almost of if they were tiny spinning balls. 2.
Associated with the spin is magnetic field like that of a tiny bar magnet lined up with the spin axis.
. Fig.1. Electron spinning 2.
Scientists represent the spin with a vector. For a sphere spinning ―west to east‖, the vector points ―north‖ or ―up‖. It points ―south‖ or ―down‖ for the spin from ―east to west‖.
4.
In a magnetic field, electrons with ―spin up‖ and ―spin down‖ have different energies.
5.
In an ordinary electronic circuit the spins are oriented at random and have no effect on current flow. Page 8
6.
Spintronic devices create spin-polarized currents and use the spin to control current flow.
Imagine a small electronically charged sphere spinning rapidly. The circulating charges in the sphere amount to tiny loops of electric current which creates a magnetic field. A spinning sphere in an external magnetic field changes its total energy according to how its spin vector is aligned with the spin. In some ways, an electron is just like a spinning sphere of charge, an electron has a quantity of angular momentum (spin) an associated magnetism. In an ambient magnetic field and the spin changing this magnetic field can change orientation. Its energy is dependent on how its spin vector is oriented. The bottom line is that the spin along with mass and charge is defining characteristics of an electron. In an ordinary electric current, the spin points at random and plays no role in determining the resistance of a wire or the amplification of a transistor circuit. Spintronic devices in contrast rely on the differences in the transport of spin-up and spin-down electrons.
Fig 2. Magnetic Orientation of electrons
Page 9
Chapter 3 Giant Magnetoresistance Electrons like all fundamental particles have a property called spin which can be orientated in one direction or the other – called „spin-up‟ or „spin-down‟ – like a top spinning anticlockwise or clockwise. When electron spins are aligned (i.e. all spin-up or all spin-down) they create a large-scale net magnetic moment as seen in magnetic materials like iron and cobalt. Magnetism is an intrinsic physical property associated with the spins of electrons in a material. Magnetism is already exploited in recording devices such as computer hard disks Data are recorded and stored as tiny areas of magnetised iron or chromium oxide. To access the information, a read head detects the minute changes in magnetic field as the disk spins underneath it. This induces corresponding changes in the head‟s electrical resistance – an effect called magnetoresistance. Spintronics burst on the scene in 1988 when French and German physicists discovered a much more powerful effect called „giant magnetoresistance‟ (GMR). It results from subtle electron-spin effects in ultra-thin „multilayers‟ of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. GMR is 200 times stronger than ordinary magnetoresistance. IBM soon realised that read heads incorporating GMR materials would be able to sense much smaller magnetic fields, allowing the storage capacity of a hard disk to increase from 1 to 20 gigabits. In 1997 IBM launched GMR read heads, into a market worth about a billion dollars a year. The basic GMR device consistsmof a three-layer sandwich of a magnetic metal such as cobalt with a nonmagnetic metal filling such as silver (see diagram). A current es through the layers consisting of spin-up and spin-down electrons. Those oriented in the same direction as the electron spins in a magnetic layer through quite easily while those oriented in the opposite direction are scattered. If the orientation of one of the magnetic layers can easily be changed by the presence of a magnetic field then the device will act as a filter, or „spin valve‟, letting through more electrons when the spin orientations in the two layers are the same and fewer when orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically.
Fig 3. A GMR read head Page 10
Magnetism is the integral part of the present day‘s data storage techniques. Right from the Gramophone disks to the hard disks of the super computer magnetism plays an important role. Data is recorded and stored as tiny areas of magnetized iron or chromium oxide. To access the information, a read head detects the minute changes in magnetic field as the disk spins underneath it. In this way the read heads detect the data and send it to the various succeeding circuits.
The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or anantiparallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment.
The magneto resistant devices can sense the changes in the magnetic field only to a small extent, which is appropriate to the existing memory devices. When we reduce the size and increase data storage density, we reduce the bits, so our sensor also has to be small and maintain very, very high sensitivity. The thought gave rise to the powerful effect called ―Giant Magnetoresistance‖ (GMR). GMR is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and non magnetic layers. The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMR. Giant magnetoresistance (GMR) came into picture in 1988, which lead the rise of spintronics. It results from subtle electron-spin effects in ultra-thin ‗multilayer‘ of magnetic materials, which cause huge changes in their electrical resistance when a magnetic field is applied. GMR is 200 times stronger than ordinary magnetoresistance. It was soon realized that read heads incorporating GMR materials would be able to sense much smaller magnetic fields, allowing the storage capacity of a hard disk to increase from 1 to 20 gigabits.
Page 11
Chapter 4 Construction of GMR The basic GMR device consists of a three-layer sandwich of a magnetic metal such as cobalt with a nonmagnetic metal filling such as silver. Current es through the layers consisting of spin-up and spin-down electrons. Those oriented in the same direction as the electron spins in a magnetic layer through quite easily while those oriented in the opposite direction are scattered. If the orientation of one of the magnetic layers can easily be changed by the presence of a magnetic field then the device will act as a filter, or ‗spin valve‘, letting through more electrons when the spin orientations in the two layers are the same and fewer when orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically. In an ordinary electric current, the spin points at random and plays no role in determining the resistance of a wire or the amplification of a transistor circuit. Spintronic devices in contrast, rely on differences in the transport of ―spin up‖ and ―spin down‖ electrons. When a current es through the Ferro magnet, electrons of one spin direction tend to be obstructed.
A ferromagnet can even affect the flow of a current in a nearby nonmagnetic metal. For example, in the present-day read heads in computer hard drives, wherein a layer of a nonmagnetic metal is sandwiched between two ferromagnetic metallic layers, the magnetization of the first layer is fixed, or pinned, but the second ferromagnetic layer is not. As the read head travels along a track of data on a computer disk, the small magnetic fields of the recorded 1‘s and 0`s change the second layer‘s magnetization back and forth parallel or antiparallel to the magnetization of the pinned layer. In the parallel case, only electrons that are oriented in the favored direction flow through the conductor easily. In the antiparallel case, all electrons are impeded. The resulting changes in the current allow GMR read heads to detect weaker fields than their predecessors; so that data can be stored using more tightly packaged magnetized spots on a disk. GMR has triggered the rise of a new field of electronics called spintronics which has been used extensively in the read heads of modern hard drives and magnetic sensors. A hard disk storing binary information can use the difference in resistance between parallel and antiparallel layer alignments as a method of storing 1s and 0s. A high GMR is preferred for optimal data storage density. Current perpendicular-to-plane (P) Spin valve GMR currently yields the highest GMR. Research continues with older current-in-plane configuration and in the tunnelling magnetoresistance (TMR) spin valves which enable disk drive densities exceeding 1 Terabyte per square inch. Page 12
Hard disk drive manufacturers have investigated magnetic sensors based on the colossal magnetoresistance effect (CMR) and the giant planar Hall effect. In the lab, such sensors have demonstrated sensitivity which is orders of magnitude stronger than GMR. In principle, this could lead to orders of magnitude improvement in hard drive data density. As of 2003, only GMR has been exploited in commercial disk read-and-write heads because researchers have not demonstrated the CMR or giant planar hall effects at temperatures above 150K. Magnetocoupler is a device that uses giant magnetoresistance (GMR) to couple two electrical circuits galvanicly isolated and works from AC down to DC. Vibration measurement in MEMS systems. Detecting DNA or protein binding to capture molecules in a surface layer by measuring the stray field from superparamagnetic label particles.
Fig 4. A GMR Device
Page 13
Chapter 5 Memory Chips Physicists have been quick to see the further possibilities of spin valves. Not only are they highly sensitive magnetic sensors (see Box), they can also be made to act as switches by flipping the magnetisation in one of the layers. This allows information to be stored as 0s and 1s (magnetisations of the layers parallel or antiparallel) as in a conventional transistor memory device. An obvious application is a magnetic version of a random access memory (RAM) device of the kind used in your computer. The advantage of magnetic random access memory (MRAM) is that it is „non-volatile‟ – information isn‟t lost when the system is switched off. MRAM devices would be smaller, faster, cheaper, use less power and would be much more robust in extreme conditions such as high temperature, or highlevel radiation or interference. The US electronics company Honeywell has already shown that arrays of linked MRAMS could be made to work. The potential market for MRAMS is worth 100 billion dollars annually. Over the past three years or so, researchers around the world have been working hard on a whole range of MRAM devices. A particularly promising device is the magnetic tunnel junction, which has two magnetic layers separated by an insulating metaloxide layer. Electrons can „tunnel‟ through from one layer to the other only when magnetisations of the layers point in the same direction, otherwise the resistance is high – in fact, 1000 times higher than in the standard spin valve. Even more interesting are devices that combine the magnetic layers with semiconductors like silicon. The advantage is that silicon is still the favourite material of the electronics industry and likely to remain so. Such hybrid devices could be made to behave more like conventional transistors. They could be used as non-volatile logic elements which could be reprogrammed using software during actual processing to create an entirely new type of very fast computing. The field of spintronics is extremely young and it‟s difficult to predict how it will evolve. New physics is still being discovered and new materials being developed, such as magnetic semiconductors, and exotic oxides that manifest an even more extreme effect called colossal magnetoresistance. What is certain is that the timespan from a breakthrough in fundamental physics to first commercial exploitation has been less than 10 years. The business opportunities for spintronics are still wide open. European research collaborations, some involving the UK, have a strong lead in developing the underlying physics and technology for this lucrative fledgling industry.
Fig 5. A general magnetic field sensor made of GMR multilayers ( iron-nickel with silver )
Page 14
Chapter 6 GMR SENSORS GMR sensors are already being developed in UK universities. They have a wide range of applications and the market is worth 8 billion dollars a year. Applications include: • Fast accurate position and motion sensing of mechanical components in precision and in robotics
engineering
• All kinds of automotive sensors for fuel handling systems, electronic engine control, antiskid systems, speed control and navigation • Missile guidance • Position and motion sensing in computer video games • Key-hole surgery and post-operative care
Fig 6. Spintronic sensor technology being tested on a Mercedes V8 engine at Oxford
Page 15
Chapter 7 Spin Valve GMR If the orientation of one of the magnetic layers can easily be changed by the presence of a magnetic field then the device will act as a filter, or ‗spin valve‘, letting through more electrons when the spin orientations in the two layers are the same and fewer when orientations are oppositely aligned. The electrical resistance of the device can therefore be changed dramatically.
Fig 7. Standard geometry for GMR based Spin Valves An electron ing through the spin-valve will be scattered more if the spin of the electron is opposite to the direction of the magnetisation in the FM layer.
Fig 8. GMR based Spin Valves for read head in hard drives Page 16
Chapter 8 Spintronic Devices
Spintronic devices are those devices which use the Spintronic technology. Spintronic-devices combine the advantages of magnetic materials and semiconductors. They are expected to be nonvolatile, versatile, fast and capable of simultaneous data storage and processing, while at the same time consuming less energy. Spintronic-devices are playing an increasingly significant role in high-density data storage, microelectronics, sensors, quantum computing and bio-medical applications, etc. Electronic Devices
Spintronic devices
1. Based on properties of charge of the electron
1. Based on intrinsic property spin of electron.
2. Classical property
2. Quantum property
4. Materials: conductors and semiconductors
4. Materials: ferromagnetic materials
5. Based on the number of charges and their energy
5. Two basic spin states; spin-up and spindown.
6. Speed is limited and power dissipation is high
6. Based on direction of spin and spin and spin coupling, high speed.
Some of the Spintronic devices are: Magnetoresistive Random Access Memory(MRAM) Spin Transistor Quantum Computer Spintronic Scanner
Page 17
Chapter 9 MRAM (Magnetoresistive Random Access Memory) An important spintronic device, which is supposed to be one of the first spintronic devices that have been invented, is MRAM. Unlike conventional random-access, MRAMs do not lose stored information once the power is turned off...A MRAM computer uses power, the four page e mail will be right there for you. Today pc use SRAM and DRAM both known as volatile memory. They can store information only if we have power. DRAM is a series of capacitors, a charged capacitor represents 1 where as an uncharged capacitor represents 0. To retain 1 you must constantly feed the capacitor with power because the charge you put into the capacitor is constantly leaking out. MRAM is based on integration of magnetic tunnel junction (MJT). Magnetic tunnel junction is a three-layered device having a thin insulating layer between two metallic ferromagnets. Current flows through the device by the process of quantum tunneling; a small number of electrons manage to jump through the barrier even though they are forbidden to be in the insulator. The tunneling current is obstructed when the two ferromagnetic layers have opposite orientations and is allowed when their orientations are the same. MRAM stores bits as magnetic polarities rather than electric charges. When a big polarity points in one direction it holds1, when its polarity points in other direction it holds 0. These bits need electricity to change the direction but not to maintain them. MRAM is non volatile so, when you turn your computer off all the bits retain their 1`s and 0`s.
Fig 9. 256 K MRAM Page 18
Chapter 10 SPIN TRANSISTORS Traditional transistors use on-and-off charge currents to create bits- the binary zeroes and ones of computer information. “Quantum spin field effect” transistor will use up-and-down spin states to generate the same binary data. One can think of electron spin as an arrow; it can point upward or downward; “spinup and spin-down can be thought of as a digital system, representing the binary 0 and 1. The quantum transistor employs also called “spin-flip” mechanism to flip an up-spin to a downspin, or change the binary state from 0 to 1. One proposed design of a spin FET (spintronic field-effect transistor) has a source and a drain, separated by a narrow semi conducting channel, the same as in a conventional FET. In the spin FET, both the source and the drain are ferromagnetic. The source sends spin-polarized electrons in to the channel, and this spin current flow easily if it reaches the drain unaltered (top). A voltage applied to the gate electrode produces an electric field in the channel, which causes the spins of fastmoving electrons to process, or rotate (bottom). The drain impedes the spin current according to how far the spins have been rotated. Flipping spins in this way takes much less energy and is much faster than the conventional FET process of pushing charges out of the channel with a larger electric filed.
Fig 10. In these devices a non magnetic layer which is used for transmitting and controlling the spin polarized electrons from source to drain plays a crucial role. For functioning of this device first the spins have to be injected from source into this non-magnetic layer and then transmitted to the collector. These non-magnetic layers are also called as semimetals, because they have very large spin diffusion lengths. The injected spins which are transmitted through this layer start precessing as illustrated in Figure before they reach the collector due to the spin-orbit coupling effect. Page 19
Vgg Collector
gate
Source
InAlAs
InGaAs
Fig.11 Spin polarized field effect transistor. Vg is the gate voltage. When Vg is zero the injected spins which are transmitted through the 2DEG layer starts processing before they reach the collector, thereby reducing the net spin polarization. Vg is the gate voltage. When Vg >> 0 the precession of the electrons is controlled with electric filed thereby allowing the spins to reach at the collector with the same polarization.
Hence the net spin polarization is reduced. In order to solve this problem an electric field is applied perpendicularly to the plane of the film by depositing a gate electrode on the top to reduce the spin-orbit coupling effect as illustrated in Figure 4. By controlling the gate voltage and polarity can the current in the collector can be modulated there by mimicking the MOSFET of the conventional electronics. Here again the problem of conductivity mismatch between the source and the transmitting layer is an important issue. The interesting thing would be if a Heusler alloy is used as the spin source and a semimetallic Heusler alloy as the transmitting layer, the problem of conductivity mismatch may be solved. For example from the Slater-Pauling curve Mt = Zt - 24, Heusler alloys with Mt >>0 can act as spin sources and alloys with Mt ~ 0 can act as semimetals. Since both the constituents are of same structure the possibility of conductivity
mismatch
may
be
less.
Traditional transistors use on-and-off charge currents to create bits—the binary zeroes and ones of computer information. ―Quantum spin field effect‖ transistor will use up-and-down spin states to generate the same binary data. One can think of electron spin as an arrow; it can point upward or downward; ―spin-up and spin-down can be thought of as a digital system, representing the Page 20
binary 0 and 1. The quantum transistor employs also called ―spin-flip‖ mechanism to flip an upspin to a downspin, or change the binary state from 0 to 1. One proposed design of a spin FET (spintronic field-effect transistor) has a source and a drain, separated by a narrow semi conducting channel, the same as in a conventional FET. In the spin FET, both the source and the drain are ferromagnetic. The source sends spinpolarized electrons in to the channel, and this spin current flow easily if it reaches the drain unaltered (top). A voltage applied to the gate electrode produces an electric field in the channel, which causes the spins of fast-moving electrons to process, or rotate (bottom). The drain impedes the spin current according to how far the spins have been rotated. Flipping spins in this way takes much less energy and is much faster than the conventional FET process of pushing charges out of the channel with a larger electric filed. One advantage over regular transistors is that these spin states can be detected and altered without necessarily requiring the application of an electric current. This allows for detection hardware that are much smaller but even more sensitive than today's devices, which rely on noisy amplifiers to detect the minute charges used on today's data storage devices. The potential end result is devices that can store more data in less space and consume less power, using less costly materials. The increased sensitivity of spin transistors is also being researched in creating more sensitive automotive sensors, a move being encouraged by a push for more environmentally-friendly vehicles A second advantage of a spin transistor is that the spin of an electron is semi-permanent and can be used as means of creating cost-effective non volatile solid state storage that does not require the constant application of current to sustain. It is one of the technologies being explored for Magnetic Random Access Memory (MRAM) Spin transistors are often used in computers for data processing. They can also be used to produce a computer's random access memory and are being tested for use in magnetic RAM. This memory is superfast and information stored on it is held in place after the computer is powered off, much like a hard disk.
Page 21
Chapter 11 Spintronic Scanner
Cancer cells are the somatic cells which are grown into abnormal size. The Cancer cells have different electromagnetic sample when compared to normal cells. For many types of Cancer, it is easier to treat and cure the Cancer if it is found early. There are many different types of Cancer, but most Cancers begin with abnormal cells growing out of control, forming a lump that's called a tumor. The tumor can continue to grow until the Cancer begins to spread to other parts of the body. If the tumor is found when it is still very small, curing the Cancer can be easy. However, the longer the tumor goes unnoticed, the greater the chance that the Cancer has spread. This makes treatment more difficult. Tumor developed in human body, is removed by performing a surgery. Even if a single cell is present after the surgery, it would again develop into a tumor. In order to prevent this, an efficient route for detecting the Cancer cells is required. Here, in this paper, we introduce a new route for detecting the Cancer cells after a surgery. This accurate detection of the existence of Cancer cells at the beginning stage itself entertains the prevention of further development of the tumor. This spintronic scanning technique is an efficient technique to detect cancer cells even when they are less in number. An innovative approach to detect the cancer cells with the help of Spintronics: The following setup is used for the detection of cancer cells in a human body: (a) Polarized electron source (b) Spin detector (c) Magnetic Field
Polarized electron source: A beam of electrons is said to be polarized if their spins point, on average, in a specific direction. There are several ways to employ spin on electrons and to control them. The requirement for this paper is an electron beam with all its electrons polarized in a specific direction. The following are the ways to meet the above said requirement: Photoemission from negative electron affinity GaAs Chemi-ionization of optically pumped meta stable Helium An optically pumped electron spin filter A Wein style injector in the electron source A spin filter is more efficient electron polarizer which uses an ordinary electron source along with a gaseous layer of
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Rb. Free electrons diffuse under the action of an electric field through Rb vapour that has been spin polarized in optical pumping. Through spin exchange collisions with the Rb, the free electrons become polarized and are extracted to form a beam. To reduce the emission of depolarizing radiation, N2 is used to quench the excited Rb atoms during the optical pumping cycle.
Spin detectors: There are many ways by which the spin of the electrons can be detected efficiently. The spin polarization of the electron beam can be analyzed by using: (a)Mott polarimeter (b)Compton polarimeter (c)Moller type polarimeter Typical Mott polarimeters require electron energies of ~100 kV. But Mini Mott polarimeter uses energies of ~25 keV, requiring a smaller overall design. The Mini Mott polarimeter has three major sections: the electron transport system, the target chamber, and the detectors. The first section the electrons enter is the transport system. An Einsel lens configuration was used here. Two sets of four deflectors were used as the first and last lens. The electrons next enter the target chamber. The chamber consists of a cylindrical target within a polished stainless steel hemisphere. A common material used for the high-Z nuclei target is gold. Low-Z nuclei help minimize unwanted scattering, so aluminum was chosen. Scattered electrons then exit the target chamber and are collected in the detectors. Thus there are many methods for detecting the spin polarization of electrons.
External Magnetic Field: An external magnetic field is required during this experiment. The magnetic field is applied after the surgery has undergone. First, it is applied to an unaffected part of the body and then to the surgery undergone part of the body. It is already mentioned that the magnetic field could easily alter the polarization of electrons.
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This technique using spintronics is suggested by us to identify tumor cells after surgery. The procedure for doing this experiment is as follows: Optical Spin Filter: After surgery and the removal of the tumor, the patient is exposed to a strong magnetic field. Now the polarized electron beam is applied over the unaffected part and spin orientation of electrons are determined using polarimeter. Then the same polarized beam is targeted over the affected part of the body and from the reflected beam, change in spin is determined. Based on these two values of spin orientation, the presence of tumor cells can be detected even if they are very few in number. Hence, we suggest this method for the detection purpose. A detailed view of this
innovative
approach
is
given
as
follows.
Spin Orientation of the unaffected part of the body: Applying Magnetic Field: When the magnetic field is applied to the unaffected part of the human body, the normal somatic cells absorbs the magnetic energy and retains it. Determinig the Spin orientation: When the electrons get incident on the cells the magnetic energy absorbed by the cells alters the spin orientation of the electrons. These electrons get reflected and it is detected by the Mott polarimeter. Then the change in spin orientation of the electrons is measured as Sx.
Spin Orientation of the surgery undergone part of the body: Applying Magnetic Filter: In the surgery undergone part of the body an external magnetic field is applied. The cancer cells which are present, if any, will absorb more magnetic energy than the normal cells since they differ in their electromagnetic pattern. Determinig the spin Orientation: Now an electron beam which is polarized is incident on the surgery undergone part of the body. The magnetic energy absorbed by the cancer cell alter the spin orientation of the electron beam. Since cancer cells absorb more magnetic energy, the change in orientation caused by them is also more. If no cancer cells are present the amount of change is equal to the previous case. The change in spin is measured by the polarimeter as Sy. Page 24
Inference: If the change in the spin in the unaffected part of the body is same as that of the surgery undergone part, i.e. If Sx=Sy Then, There are no cancer cells in the surgery undergone part of the body and all the cells have been removed by the surgery. If the change in spin in the unaffected part is not equal to the change caused by the surgery undergone part of the body, i.e. If Sx not equals Sy Then, There are some cancer cells in the surgery undergone part of the body and the cancer cells are not completely removed by the surgery. The steps involved are: 1) The patient is exposed to a strong magnetic field so that his body cell gets magnetized. 2) A beam of electrons with polarized spin is introduced on the unaffected part of the body and the change in spin is detected by a polarimeter. Let it be X 3) A beam of electrons with polarized spin is introduced on the part which had undergone surgery. And the corresponding change in spin be Y 4) If X - Y = 0, it indicates that cancer cells have been removed from the body, if not it indicates the presence of traces of cancer cells and it has to be treated again for ensuring complete safety to the patient.
Thus this technique efficiently identifies the presence of cancer cells in that part of the body that has undergone surgery to prevent any further development.
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CONCLUSION
Spintronics is one of the most exciting and challenging areas in nanotechnology, important to both fundamental scientific research and industrial applications. These spintronic-devices, combining the advantages of magnetic materials and semiconductors, are expected to be non-volatile, versatile, fast and capable of simultaneous data storage and processing, while at the same time consuming less energy. They are playing an increasingly significant role in highdensity data storage, microelectronics, sensors, quantum computing and bio-medical applications, etc. It is expected that the impact of spintronics to the microelectronics industry might be comparable to the development of the transistor 50 years ago. Today everyone already has a spintronic device on their desktop, as all modern computers use the spin valve in order to read and write data on their hard drive. It was followed immediately by the discovery of Tunneling Magnetoresistance (TMR) leading to the magnetic tunnel junction that has been utilized for the next generation computer memory known as Magnetic Random Access Memory (MRAM), another spintronic device for computers. Therefore, the initial driving force for spintronics has been the improvement of computer technology. At present the research has been concentrating on the fabrication of spin transistors and spin logics devices integrating magnetic and semiconductors, with the aim of improving the existing capabilities of electronic transistors and logics devices so that the future computation and thus the future computer could become faster and consume less energy. There are four main areas in spintronics: . 1) Understanding the fundamental physics, such as spin-dependant transports across the magnetic/ semiconductor interfaces and spin coherence length in semiconductors. 2) Synthesising suitable spintronic materials with Curie temperatures above room temperature, large spin polarisation at the Fermi level and matching conductivity between the magnetic and semiconductor materials. 3) Fabricating devices with nanometre feature sizes and developing new techniques for mass production. 4) Integrating spin-devices with current microelectronics and computing.
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REFERENCES 1. IEEE Digital Explore Library 2. School of Physics & Astronomy, University of Nottingham 3. Department of Physics and MARTECH , Florida State University 4. Department of Physics and Center for Advanced Photonic and Electronic Materials University at Buffalo ,The State University of New York 5. Research Councils UK www.rcuk.ac.uk 6. Engineering and Physical Sciences Research Council (EPSRC) www.epsrc.ac.uk 7. Particle Physics and Astronomy Research Council (PPARC) www.pparc.ac.uk 8. Council for the Central Laboratory of the Research Councils www.cclrc.ac.uk
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