Hi-Tech College of Engineering and Technology

PAPER PRESENTATION

ON

SPINTRONICS

SUBMITTED TO:

HI-TECH COLLEGE OF ENGINEERING AND TECHNOLOGY

BY

N.SIVALALITHA

ECE-1/4

B.PURNIMA

ECE-1/4

CONTENTS

1. ABSTRACT

2. INTRODUCTION

3. METALS BASED SPINTRONIC DEVICES

4. OTHER METALS BASED SPINTRONICS DEVICES 4.1.APPLICATIONS

5. SEMICONDUCTOR-BASED SPINTRONIC DEVICES 5.1.APPLICATIONS

6. SOME MORE APPLICATIONS

7. CONCLUSION

8. REFERENCES

1. ABSTRACT:

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 et al. and Peter Grünberg et al. (1988). 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.

2. INTRODUCTION:

Spintronics (a neologism meaning "spin transport electronics"), also known as magnetoelectronics, is an emerging technology which exploits the intrinsic spin of electrons and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.

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 to have 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).

3.METALS-BASED SPINTRONIC DEVICES:

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: (1) current-in-plane (CIP), where the electric current flows parallel to the layers and (2) current-perpendicular-to-plane (CPP), where the electric current flows in a direction perpendicular to the layers.

4.OTHER METALS-BASED SPINTRONICS DEVICES:

Tunnel Magnetoresistance where CPP transport is achieved by using quantum-mechanical tunneling of electrons through a thin insulator separating ferromagnetic layers

The tunnel magnetoresistance effect (TMR), occurs when a current flows between two ferromagnets separated by a thin (about 1 nm) insulator. Then the total resistance of the device, in which tunneling is responsible for current flowing, changes with the relative orientation of the two magnetic layers. The resistance is normally higher in the anti-parallel case. The effect is similar to Giant Magnetoresistance except that the metallic layer is replaced by an insulating tunnel barrier.

It was discovered in 1975 by Michel Julliere, using iron as the ferromagnet and germanium as the insulator. This experiment was carried out at 4.2 K however, so it did not attract much practical attention.

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

Spin torque transfer writing technology is a technology in which data is written by re-orienting the magnetisation of a thin magnetic layer in a tunnel magnetoresistance (TMR) element using a spin-polarised current. An electrical current is generally unpolarised (consisting of 50% spin-up and 50% spin-down electrons), a spin polarised current is one with more electrons of either spin. By passing a current through a thick magnetic layer one can produce a spin polarised current.

At very small device scales it is possible that a spin polarised current can transfer its spin angular momentum to a small magnetic element. Spin torque transfer magnetic RAM (STT-MRAM) has the advantages of lower power-consumption and better scalability over conventional MRAM. Spin torque transfer technology has the potential to make possible MRAM devices combining low current requirements and reduced cost, however the amount of current needed to re-orient the magnetisation is, at present, too high for commercial applications and the reduction of this current density alone is the basis for a lot of current academic research in spin-electronics.

Hynix Semiconductor and Grandis formed a partnership in April 2008 to explore commercial development of STT-RAM technology.

4.1.APPLICATIONS:

The storage density of hard drives is rapidly increasing along an exponential growth curve, in part because spintronics-enabled devices like GMR and TMR sensors have increased the sensitivity of the read head which measures the magnetic state of small magnetic domains (bits) on the spinning platter. The doubling period for the areal density of information storage is twelve months, much shorter than Moore's Law, which observes that the number of transistors that can cheaply be incorporated in an integrated circuit doubles every two years.

MRAM, or magnetic random access memory, uses arrays of TMR or Spin torque transfer devices. MRAM is nonvolatile (unlike charge-based DRAM in today's computers) so information is stored even when power is turned off, potentially providing instant-on computing. Motorola has developed a 256 kb MRAM based on a single magnetic tunnel junction and a single transistor. This MRAM has a read/write cycle of under 50 nanoseconds. Another design in development, called Racetrack memory, encodes information in the direction of magnetization between domain walls of a ferromagnetic metal wire.

5. SEMICONDUCTOR-BASED SPINTRONIC DEVICES:

In early efforts, spin-polarized electrons are generated via optical orientation using circularly-polarized photons at the bandgap energy incident on semiconductors with appreciable spin-orbit interaction (like GaAs and ZnSe). Although electrical spin injection can be achieved in metallic systems by simply passing a current through a ferromagnet, the large impedance mismatch between ferromagnetic metals and semiconductors prevented efficient injection across metal-semiconductor interfaces. A solution to this problem is to use ferromagnetic semiconductor sources (like manganese-doped gallium arsenide GaMnAs), increasing 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-colinear to the injected spin orientation. This is called the Hanle effect.

5.1.APPLICATIONS:

Advantages of semiconductor-based spintronics applications are potentially lower power use and a smaller footprint than electrical devices used for information processing. Also, 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.

6.SOME MORE APPLICATIONS:

·  Spin pumping is a method of generating a spin current, the spintronic analog of a battery in conventional electronics.

·  spin transfer is the phenomenon in which the spin angular momentum of the charge carriers (usually electrons) get transferred from one location to another. This phenomenon is responsible for several important and observable physical effects.

·  Spinplasmonics is a field of nanotechnology combining spintronics and plasmonics

In a spinplasmonic device, light waves couple to electron spin states in a metallic structure.

Spinplasmonic devices potentially have the advantages of high speed, miniaturization, low power consumption, anUnlike semiconductor-based devices, smaller spinplasmonics devices are expected to be more efficient in transporting the spin-polarized electron current.d multifunctional

·  ADVANTAGES:

The various advantages of spintronics are as follows:

-spintronics does not require unique and specialised semiconductors, therefore it can be implemented or worked with common metals, such as copper, aluminium and silver.

- spintronics devices wouldwould consume less power compareed to conventional electronics, because the energy needed to change spin is a easy compared to energy needed to push charge around.

- since spins don’t change when power is turned turned off, the memory remains non-volatile.

·  DISADVANTAGES:
If an attempt were made to make magnetic RAM capable of retaining important data, it would be very difficult task to achieve. The primary reason being interference of fields with nearest element.

suppose the indvidual memory elements are adressed by flipping their spins up or down to yield the zeros and ones of binary computer logic. In that case, the common strategy of running current pulses through wires to induce magnetic fields to rotate the elements is flawed. This may happen because the fringe fields that are generated may interfere with neighbouring elements.

7.CONCLUSION:

Spintronics still remains to be far away from being the best friend and source of electronic industry. But in order to convert it into reality, many major manufactures have already started investigating magnetic RAM technology, and they are keeping their eyes on magnetic CPUs for the furure. As the development in spintronics is bound to bring a new era of semiconductor spintronics that could potentially transform the microelectronics industry. But more impotantly,with the magnetic storage industry currently accounting for billions range when power is turned off, the memory remains non-volatile.

8.REFERENCES:

1.  IBM RD 50-1 | Spintronics—A retrospective and perspective

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5.  http://prola.aps.org/pdf/PRB/v39/i7/p4828_1

6.  PII: 0370-1573(94)90105-8

7.  http://www.sciencedirect.com/science/article/B6TVM-46R3N46-10D/2/90703cfc684b0679356dce9a76b2e942

8.  Cookies Required

9.  http://www.sigmaaldrich.com/materials-science/alternative-energy-materials/magnetic-materials/tutorial/spintronics.html

10.  Phys. Rev. B 62 (2000): B. T. Jonker, Y. D. Park, B. R. Bennett, H. D. Cheong, G. Kioseoglou, and A. Petrou - Robust electrical spin injection

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12.  Phys. Rev. Lett. 90 (2003): X. Jiang, R. Wang, S. van Dijken, R. Shelby, R. Macfarlane, G. S. Solomon, J. Harris, and S. S. Parkin - Optical Detection of Hot-Electron

13.  Phys. Rev. Lett. 80 (1998): J. M. Kikkawa and D. D. Awschalom - Resonant Spin Amplification in

14.  Polarized optical emission due to decay or recombination of spin-polarized injected carriers - US Patent 5874749

15.  Electrical detection of spin transport in lateral ferromagnet-semiconductor devices: Abstract: Nature Physics