Spintronics

Spintronics

1. Introduction:

Conventional electronicdevices rely on the transportof electrical charge carriers –electrons in a semiconductorsuch as silicon. Now, however,physicists are trying to exploitthe ‘spin’ of the electron ratherthan its charge to create a remarkable new generation of‘spintronic’ devices which willbe smaller, more versatile and more robust than those currentlymaking up silicon chips andcircuit 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.

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 fundamentalparticles have a property calledspin which can be orientatedin one direction or the other –called ‘spin-up’ or ‘spin-down’ –like a top spinning anticlockwiseor 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:

  1. In addition to their mass, electrons have an intrinsic quantity of angular momentum 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

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Fig.1. Electron spinning

  1. 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”.
  2. In a magnetic field, electrons with “spin up” and “spin down” have different energies.
  3. In an ordinary electronic circuit the spins are oriented at random and have no effect on current flow.
  4. 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.

3. Giant Magneto Resistance:

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 theelectrical resistancedepending on whether themagnetizationof adjacentferromagneticlayers are in a parallel or anantiparallelalignment. 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).GMRis aquantum mechanicalmagnetoresistanceeffect observed in thin film structures composed of alternatingferromagneticand non magnetic layers. The 2007Nobel Prize in physicswas awarded toAlbert FertandPeter Grünbergfor 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.

3.1 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 passes 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 pass 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 passes 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 calledspintronicswhich has been used extensively in theread headsof modernhard drivesand magneticsensors. 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 (CPP)Spin valveGMR 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.

Hard disk drive manufacturers have investigated magnetic sensors based on thecolossal magnetoresistanceeffect (CMR) and the giantplanar Halleffect. 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 commercialdisk read-and-write headsbecause researchers have not demonstrated the CMR or giant planar hall effects at temperatures above 150K.

Magnetocoupleris a device that uses giant magnetoresistance (GMR) to couple two electrical circuits galvanicly isolated and works from AC down to DC.

Vibrationmeasurement inMEMSsystems.

DetectingDNA or protein bindingto capture molecules in a surface layer by measuring thestray fieldfromsuperparamagneticlabel particles.

4. 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 non-volatile, 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.

Some of the Spintronic devices are

  • Magnetoresistive Random Access Memory(MRAM)
  • Spin Transistor
  • Quantum Computer
  • Spintronic Scanner

4.1 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.

4.2 Spin Transistor

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 1 before they reach the collector due to the spin-orbit coupling effect.

Fig. 2 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 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 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 noisyamplifiersto detect the minute charges used on today'sdata 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 storagethat 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 incomputersfor 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.

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
3. Controlled by an external electric field in modern electronics / 3. Controlled by external magnetic field
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 spin-down
6. Speed is limited and power dissipation is high / 6. Based on direction of spin and spin coupling, high speed

4.3 Quantum Computer: