Seminar Report 2011 Spintronics

Introduction

Spintronics (a neologism meaning "spin transport electronics", also

known as magnetoelectronics, is an emerging technology that exploits the intrinsic

spin of the electron and its associated magnetic moment, in addition to its

fundamental electronic charge, in solid-state devices. They rely completely on

magnetic moment of the electron. Electrons are spin-1/2 fermions and therefore

constitute a two-state system with spin "up" and spin "down". Electrons have a

property that they occupy only one quantum state at a given time. To make a

spintronic device, the primary requirements are a system that can generate a

current of spin-polarized electrons comprising more of one spin species—up or

down—than the other (called a spin injector), Spin process can be accomplished

using real external magnetic fields or effective fields caused by spin-orbit

interaction.

Dept of EEE 18 S.S.M Polytechnic, Tirur Seminar Report 2011 Spintronics

Advantages of spintronics

 Non-volatile memory.

 Performance improves with smaller devices.

 Low power consumption.

 Spintronics does not require unique and specialized semiconductors.

 Dissipation less transmission.

 Switching time is very less compared to normal RAM chips, spintronic

RAM chips will:

o Increase storage densities by a factor of three,

o Have faster switching and rewritability rates smaller.

Limitations

 Controlling spin for long distances.

 Difficult to Inject and Measure spin.

 Interfernce of fields with nearest elements.

 Control of spin in silicon is diffic.

Historical Perspective

The research field of spintronics emerged from experiments on spin-

dependent electron transport phenomena in solid-state devices done in the 1980s,

including the observation of spin-polarized electron injection from a

ferromagnetic metal to a normal metal by Johnson and Silsbee (1985), and the

discovery of giant magnetoresistance independently by Albert Fert and Peter

Grünberg. The origins can be traced back further to the ferromagnet

/superconductor tunneling experiments pioneered by Meservey and Tedrow, and

initial experiments on magnetic tunnel junctions by Julliere in the 1970s. The use

of semiconductors for spintronics can be traced back at least as far as the

theoretical proposal of a spin field-effect-transistor by Datta and Das in 1990.

Working

Electrons are spin-1/2 fermions and therefore constitute a two-state

system with spin "up" and spin "down". To make a spintronic device, the primary

requirements are a system that can generate a current of spin-polarized electrons

comprising more of one spin species—up or down—than the other (called a spin

injector), and a separate system that is sensitive to the spin polarization of the

electrons (spin detector). Manipulation of the electron spin during transport

between injector and detector (especially in semiconductors) via spin precession

can be accomplished using real external magnetic fields or effective fields caused

by spin-orbit interaction.

Spin polarization in non-magnetic materials can be achieved either

through the Zeeman Effect in large magnetic fields and low temperatures, or by

non-equilibrium methods. In the latter case, the non-equilibrium polarization will

decay over a timescale called the "spin lifetime". Spin lifetimes of conduction

electrons in metals are relatively short (typically less than 1 nanosecond) but in

semiconductors the lifetimes can be very long (microseconds at low temperatures),

especially when the electrons are isolated in local trapping potentials (for

instance, at impurities, where lifetimes can be milliseconds).

All spintronic devices act according to the simple scheme:

(1) Information is stored (written) into spins as a particular spin orientation (up or down). (2) The spins, being attached to mobile electrons, carry the information along a wire, and (3) The information is read at a terminal.

Giant Magnetoresistance (GMR)

The simplest method of generating a spin-polarised current in a metal

is to pass the current through a ferromagnetic material. The most common

application of this effect is a giant magnetoresistance (GMR) device. A typical

GMR device consists of at least two layers of ferromagnetic materials separated

by a spacer layer. When the two magnetization vectors of the ferromagnetic layers

are aligned, the electrical resistance will be lower (so a higher current flows at

constant voltage) than if the ferromagnetic layers are anti-aligned. This

constitutes a magnetic field sensor.

Two variants of GMR have been applied in devices:

 current-in-plane (CIP), where the electric current flows parallel to the

layers and

 current-perpendicular-to-plane (CPP), where the electric current flows

in a direction perpendicular to the layers.

Other metals-based spintronics devices:

 Tunnel Magnetoresistance (TMR), where CPP transport is achieved by using quantum-mechanical tunneling of electrons through a thin insulator separating ferromagnetic layers.

 Spin Torque Transfer, where a current of spin-polarized electrons is used to control the magnetization direction of ferromagnetic electrodes in the device.

MRAM uses magnetic storage elements. Tunnel junctions are used to

read the information stored in MRAM. Attempts were made to control bit writing

by using relatively large currents to produce fields. This proves unpractical at

nanoscale level. The spin transfer mechanism can be used to write to the magnetic

memory cells. Currents are about the same as read currents, requiring much less

energy.

MRAM Promises:  Density of DRAM  Speed of SRAM  Non-volatility like flash

Spin Transistor

Ideal use of MRAM would utilize control of the spin channels of the

current. Spin transistors would allow control of the spin current in the same

manner that conventional transistors can switch charge currents. Using arrays of

these spin transistors, MRAM will combine storage, detection, logic and

communication capabilities on a single chip. This will remove the distinction

between working memory and storage, combining functionality of many devices

into one Datta Das Spin Transistor. The Datta Das Spin Transistor was first spin

device proposed for metal-oxide geometry, 1989. Emitter and collector are

ferromagnetic with parallel magnetizations. The gate provides magnetic field.

Current is modulated by the degree of precession in electron spin.

Current Research

Ferromagnetic transition temperature in excess of 100 K Spin

injection from ferromagnetic to non-magnetic semiconductors and long spin-

coherence times in semiconductors. Ferromagnetism in Mn doped group IV

semiconductors. Room temperature ferromagnetism. Large magnetoresistance in

ferromagnetic semiconductor tunnel junctions.

Future Outlook

 High capacity hard drives, the future Plastic data storage.

 Magnetic RAM chips

 Spin FET using quantum tunneling

 Quantum computers

Applications

Motorola has developed a 1st generation 256 kb MRAM based on a

single magnetic tunnel junction and a single transistor and which has a read/write

cycle of under 50 nanoseconds (Ever spin, Motorola's spin-off, has since

developed a 4 Mbit version. There are two 2nd generation MRAM techniques

currently in development: Thermal Assisted Switching (TAS) which is being

developed by Crocus Technology, and Spin Torque Transfer (STT) on which

Crocus, Hynix, IBM, and several other companies are working.

Another design in development, called Racetrack memory, encodes

information in the direction of magnetization between domain walls of a

ferromagnetic metal wire.

Semiconductor-based spintronic devices

Ferromagnetic semiconductor sources (like manganese-doped

gallium arsenide GaMnAs), increase the interface resistance with a tunnel barrier,

or using hot-electron injection.

Spin detection in semiconductors is another challenge, which has

been met with the following techniques:

 Faraday/Kerr rotation of transmitted/reflected photons

 Circular polarization analysis of electroluminescence

 Nonlocal spin valve (adapted from Johnson and Silsbee's work with

metals)

 Ballistic spin filtering

The latter technique was used to overcome the lack of spin-orbit

interaction and materials issues to achieve spin transport in silicon, the most

important semiconductor for electronics. Because external magnetic fields (and

stray fields from magnetic contacts) can cause large Hall effects and

magnetoresistance in semiconductors (which mimic spin-valve effects), the only

conclusive evidence of spin transport in semiconductors is demonstration of spin

precession and dephasing in a magnetic field non-collinear to the injected spin

orientation. This is called the Hanle effect.

Spin Detection

Spin detection in semiconductors is another challenge, which has been met

with the following techniques:

 Faraday/Kerr rotation of transmitted/reflected photons.

 Circular polarization analysis of electroluminescence.

Depicting Spin Of Electrons Concerns

 To devise economic ways to combine ferromagnetic metals and

semiconductors in integrated circuits.

 To find an efficient way to inject spin-polarized currents, or spin

currents, into a semiconductor.

 To maximize the time period for spin current to retain its polarization in

a semiconductor. To make semiconductors that are ferromagnetic at

room temperature and don’t lose their property even at high temperature

 To minimize spin currents at boundaries between different

semiconductors so as to minimize the loss.

Applications

Applications such as semiconductor lasers using spin-polarized

electrical injection have shown threshold current reduction and controllable

circularly polarized coherent light output. Future applications may include a spin-

based transistor having advantages over MOSFET devices such as steeper sub-

threshold slope.

Motorola has developed a 1st generation 256 kb MRAM based on a

single magnetic tunnel junction and a single transistor and which has a read/write

cycle of under 50 nanoseconds.

There are two 2nd generation MRAM techniques currently in

development:

 Thermal Assisted Switching (TAS) which is being developed by Crocus Technology, and  Spin Torque Transfer (STT) on which Crocus, Hynix, IBM, and several other companies are working.

Semiconductor lasers using spin-polarized electrical injection have

shown threshold current reduction and controllable circularly polarized coherent

light output.

Spintronic Couplers

Using the same sputtering technology, it is possible to build a thin

film on-chip coil. A current in this coil, when combined with an on-chip GMR

magnetic sensor separated by an insulating layer, can couple a signal across the

insulator achieving galvanic isolation. Like the sensor, these components can be

combined with other semiconductor functions to produce a very high-speed digital

isolator. In a spintronic coupler, four GMR resistors form a Wheatstone bridge

(see Figure 7). A thin polymer dielectric barrier layer provides several thousand

volts of isolation from the input coil. A magnetic field proportional to the input

current signal is generated beneath the coil winding. The resulting magnetic field

flips the spin of electrons in the GMR resistors, changing their resistance. A

magnetic shield protects the sensor from external fields.

There are two spins (UP spin and DOWN Spin).This spintronic

scanning technique is an efficient technique used in the medical field to

detectcancer cells.

Cancer cells are easy to be identified only when they are large in

number. These cells when matured results in formation of tumor, which has to be

removed by surgery. After surgery there may be presence of even a single cancer

cell, which would result in growth of tumor in effected part of the body. The

spintronic scanning is an efficient technique to detect cancer cells even when they

are less in number.

A Patient is exposed to a strong magnetic field so that his body cell

gets magnetized. A beam of electrons with polarized spin is introduced on the

uneffected part of the body and the change in spin is detected by a polarimeter. A

beam of electrons with polarized spin is introduced on the part which had

undergone surgery.

The difference in spin of electrons when introduced to normal area

and abnormal area indicates whether 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

Conclusion

Spintronics is a technology with a fast track from the discovery of GMR and

MTJ materials to the incorporation of these materials in commercial devices.

Spintronics read heads dominate the hard-disk market. Magnetic sensors based on

spintronics are making inroads in markets where some combination of high

resolution, high sensitivity, small size, and low power are required. Digital data

couplers and displacing opto isolators in many applications and are making

inroads into new markets heretofore unavailable. MRAM devices are on the

horizon and offer the promise of laptop computers that do not need to boot up and

cell phones with increased battery time and increased capabilities.

Dept of EEE 18 S.S.M Polytechnic, Tirur