INTRODUCTION

            Spintronics is the portmanteau meaning of “spin transport electronics”. It refers to the study of the role of intrinsic spin of electron and associated magnetic moment in solid state physics and the devices that exploits the spin properties in addition to the fundamental electronic charge. For example, spin relaxation and spintransport in metals and semiconductors are under continuous research and study, not only being basic solid state issue but also for the already demonstrated potential that has in electronic technology. 

HISTORY

Spintronics emerged from discoveries in 1980s concerning spin-dependent electron transport phenomena in the solid-state devices. Milestones in the development of spintronics are given below.
  • 1985-observation of spin-polarized electron injection from a ferromagnetic metal to a normal metal by Johnson and Silsbee 
  • 1988 - Giant Magneto resistive Effect (GME) discovered by Albert Fert in France and Peter Gruenberg 
  • 1989 - IBM scientists made a string of key discoveries about the "giant magneto resistive" effect in thin-film structures 
  • 1997 - first GMR (Giant magneto resistive) Hard disk head introduced by IBM 
  • 2001 - University of Arkansas physicists have successfully injected a stream of electrons with identical spins into a semiconductor. 
  • 2002 - Plastic Shows Promise For Spintronics, Magnetic Computer Memory 
  • 2003 - Min Ouyang and David Awschalom of the University of California at Santa Barbara have transferred electron spins across molecular bridges between quantum dots for the first time. 
  • 2004-Jing Shi and colleagues at the University of Utah in the US have made the first organic spin valve. 
  • 2005-July - New Spintronic Speed Record - 2GHz MRAM devised and Discovery of 'Doping' Mechanism in Semiconductor Nanocrystals. 
  • 2006-January - Researchers at the University of Michigan created a computer chip based on the esoteric science of quantum mechanics. 
  • 2007- First breakthrough in applying spin-based electronics to silicon and Nobel prize for Giant Magneto resistance discovery 
  • 2009-Scientists proved the existence of a 'spin battery' June - Scientists Discover Magnetic Super atoms 
  • 2010- Thermo-Spintronics chips that run on heat are possible 
  • 2010 October - Researchers developed a way to reliably read the spin of a single electron 
  • 2011-January - Researchers manage to generate spin current in Graphene March - Researchers develop room-temperature spintronics transistors 
  • 2012-A new technology aims to improving the reliability of spintronics logic IC s
The spintronic properties of gallium manganese arsenide finally explained-New theory generalizes spin transfer torques, total angular momentum current, and mechanical torques 

THEORY

            Spin of the electron is an angular momentum intrinsic to the electron and which is different from the angular momentum due to its orbital motion. The electron’s spin is (1/2)ђ, which implies that the electron acts as a fermion by the spin-statistics theorem. Like orbital angular momentum, the electron spin has an associated magnetic moment and the magnitude of that moment is given by,
                                   M= ђ
            The magnetic and electronic properties of a material are affected by the combined action of the spins of their electrons. For example, a material with permanent magnetic moment is considered as a ferromagnet.

In most of the materials electron spins are equally present in both up and down state and the transport properties are independent of this. A spintronic device requires generation or manipulation of a spin polarized population of electrons which results in an excess of spin-up or spin-down electrons. A net spin polarization can be achieved either  by the exchange of energy present in a ferromagnet or through creating an equilibrium energy splitting between spin up and spin down such as putting a material in a large magnetic field(zeeman effect). The period of time in which a non-equilibrium population can be maintained is known as spin life time which is very short and researches are going on to extending this lifetime to relevant timescales.

Current efforts in designing and manufacturing spintronic devices involve two different approaches. The first is perfecting the existing GMR-based technology by either developing new materials with larger spin polarization of electrons or making improvements or variations in the existing devices that allow for better spin filtering. The second effort, which is more radical, focuses on finding novel ways of both generation and utilization of spin-polarized currents. These include investigation of spin transport in semiconductors and looking for ways in which semiconductors can function as spin polarizer’s and spin valves.

Spintronic devices can be divided into two 
  1. Metal based spintronic devices 
  2. Semiconductor based spintronic devices.

1. METAL BASED SPINTRONIC DEVICES

            When current is passed through a ferromagnetic material spin polarised current is generated in it. A device which utilizes this effect is Giant Magneto Resistance (GMR) device. It is a prototype which is already using in industry as a read head and a memory-storage cell. It consists alternating layers of ferromagnetic and non-magnetic metal layers. Depending on the relative orientation of the magnetization vectors in the magnetic layer, the device resistance changes from small (for parallel magnetization) to large (for ant parallel magnetization). That is, when the two magnetization vectors of the ferromagnetic layers are aligned the electrical resistance will be lower, so a higher current flows for a constant voltage than if the ferromagnetic materials are anti-aligned. This change in resistance, which is also known as the magneto resistance is used to sense the changes in magnetic fields, ie a magnetic field sensor.
Other metal based spintronic devices are:
  • Tunnel Magneto resistance(TMR),where CPP transport is achieved by using quantum-mechanical tunneling of electrons
  • Spin torque transfer, where a current of spin-polarized electrons is used to control the magnetization direction of ferromagnetic electrodes in the device.

APPLICATIONS

1. Read heads of modern hard drives are based on the GMR effect.
2. Magnetic sensors.
3. Racetrack memory

2. SEMICONDUCTOR-BASED SPINTRONIC DEVICES

            Current efforts in designing and manufacturing spintronic devices involve two different approaches. The first is perfecting the existing GMR-based technology by either developing new materials with larger spin polarization of electrons or making improvements or variations in the existing devices that allow for better spin filtering. The second effort, which is more radical, focuses on finding novel ways of both generation and utilization of spin-polarized currents. These include investigation of spin transport in semiconductors and looking for ways in which semiconductors can function as spin polarizer’s and spin valves. The importance of this effort lies in the fact that the existing metal-based devices do not amplify signals (although they are successful switches or valves), whereas semiconductor based spintronic devices could in principle provide amplification and serve, in general, as multi-functional devices. Perhaps even more importantly, it would be much easier for semiconductor-based devices to be integrated with traditional semiconductor technology.

Because external magnetic fields (and stray fields from magnetic contacts) can cause large Hall effects and magneto resistance 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.

APPLICATIONS

            1. Semiconductor Lasers
            2. Magnetic Tunnel Transistor

            In addition to the near-term studies of various spin transistors and spin transport properties of semiconductors, a long-term and ambitious subfield of spintronics is the application of electron and nuclear spins to quantum information processing and quantum computation. It has long been pointed out that quantum mechanics may provide great advantages over classical physics in physical computation. However, the real boom started after the advent of Shor's factorization algorithm and quantum error correction schemes. Among the many quantum computer hardwares that were proposed are the ones based on electron and nuclear spins. Obviously, the spins of electrons and spin-1/2 nuclei provide perfect candidates for quantum bits (qubits) as their Hilbert spaces are generally well-defined and their decoherence relatively slow.

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