Spintronics

Spintronics (a neologism for "spin-based electronics"), also known as magnetoelectronics, is an emerging technology which exploits the quantum spin states of electrons as well as making use of their charge state. The electron spin itself is manifested as a two state magnetic energy system.

The discovery of giant magnetoresistance in 1988 by Albert Fert et al. and Peter Grünberg et al. independently is considered as the birth of spintronics.

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Theory

Spintronics describes technology that makes use of the spin state of electrons. It can provide an extension to electronics.

Electrons exhibit the basic properties of spin, charge, and mass. When the intrinsic spin of an electron is measured, it is found in one of two spin states, which we denote as spin up and spin down. Since the Pauli Exclusion Principle dictates that the quantum-mechanical wavefunction of two paired fermions must be antisymmetric, no two electrons can occupy the same quantum state, implying that an entangled pair of electrons cannot have the same spin. There is generally a splitting of the spin-up and spin-down energy levels via the Zeeman effect, so electrons with their spins aligned with an external field are less energetic than electrons with their spins anti-aligned. Electrons absorb or emit photons (quanta of electromagnetic energy) to change valence orbits, and they lose spin coherence by interacting with mutually resonant photon frequencies, causing the electrons to spin flip by energy transfer, through mutual spin-orbit coupling, and through photon emission.

In order to make a spintronic device, the primary requirement is to have a system that can generate a current of spin polarized electrons, and a system that is sensitive to the spin polarization of the electrons. Most devices also have a unit in between that changes the current of electrons depending on the spin states.

The simplest method of generating a spin-polarised current is to inject 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, then an electrical current will flow freely, whereas if the magnetization vectors are antiparallel then the resistance of the system is higher.

Two variants of GMR have been applied in devices, current-in-plane where the electric current flows parallel to the layers and current-perpendicular-to-the-plane where the electric current flows in a direction perpendicular to the layers.

Applications

Spintronic devices are used in the field of mass-storage devices; recently (in 2002) IBM scientists announced that they could compress massive amounts of data into a small area, at approximately one trillion bits per square inch (1.5 Gbit/mm²) or roughly 1 TB on a single sided 3.5" diameter disc. The storage density of hard drives is rapidly increasing along an exponential growth curve. 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 in an integrated circuit doubles every eighteen months. Also the hard disk drives use a spin effect to function, the giant magnetoresistive effect (see below).

The most successful spintronic device to date is the spin valve. This device utilizes a layered structure of thin films of magnetic materials, which changes electrical resistance depending on applied magnetic field direction. In a spin valve, one of the ferromagnetic layers is "pinned" so its magnetization direction remains fixed and the other ferromagnetic layer is "free" to rotate with the application of a magnetic field.

When the magnetic field aligns the free layer and the pinned layer magnetization vectors, the electrical resistance of the device is at its minimum. When the magnetic field causes the free layer magnetization vector to rotate in a direction antiparallel to the pinned layer magnetization vector, the electrical resistance of the device increases due to spin dependent scattering. The magnitude of the change, (Antiparallel Resistance - Parallel Resistance) / Parallel Resistance x 100% is called the GMR ratio.

Devices have been demonstrated with GMR ratios as high as 200% with typical values greater than 10%. This is a vast improvement over the anisotropic magnetoresistance effect in single layer materials which is usually less than 3%. Spin valves can be designed with magnetically soft free layers which have a sensitive response to very weak fields (such as those originating from tiny magnetic bits on a computer disk), and have replaced anisotropic magnetoresistance sensors in computer hard disk drive heads since the late 1990s.

Future applications may include a spin-based transistor which requires the development of magnetic semiconductors exhibiting room temperature ferromagnetism. One possible material candidate is manganese doped gallium arsenide GaMnAs. The operation of MRAM or magnetic random access memory is also based on spintronic principles. Spintronics-based non-volatile 3D optical data storage has also been proposed.

Multiferroics which have properties of being able to change internal molecular geometry under electrostatic or electromagnetic influence are a hotbed of research at several universities.

See also

• Spin pumping
• Spin transfer
• Spinhenge@Home

Further reading

• Ultrafast Manipulation of Electron Spin Coherence. J. A. Gupta, R. Knobel, N. Samarth and D. D. Awschalom in Science, Vol. 292, pages 2458-2461; June 29, 2001.
• Spintronics: A Spin-Based Electronics Vision for the Future. S. A. Wolf et al, Science 294, 1488-1495 (2001)
• How to Create a Spin Current. P. Sharma, Science 307, 531-533 (2005)
• Search Google Scholar for highly cited articles with query: spintronics OR magnetoelectronics OR "spin based electronics"
• "Electron Manipulation and Spin Current". D. Grinevich. 3rd Edition, 2003.*

• IBM (2003)
• Wired: update on MRAMs, 2003 Jul
• Spintronics research targets GaAs.
• Spintronics at SUNY Albany's College of Nanoscale Science and Engineering
• Spintronics information community site