Spintronics (a neologism meaning "spin transport electronics"), also known as magnetoelectronics, is an emerging technology that exploits both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices.
An additional effect occurs when a spin-polarized current is induced. In these cases, the electron spin is quantized in the direction perpendicular to both the plane normal and the two-dimensional wave vector, thus splitting the energy band. This is called the Rashba effect.
Spintronics emerged from discoveries in the 1980s concerning spin-dependent electron transport phenomena in solid-state devices. This includes 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 of spintronics can be traced back even 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.
Electrons are spin-1/2 fermions, i.e. particles that obey Fermi–Dirac statistics and have half-integer spin.
The total magnetic moment of an electron is equal to the sum of its spin moment (on account of its spin about its own axis) and the orbital (on account of its orbit around nucleus of an atom).
It is found that the spin moment given by is twice as strong as the orbital moment.
The spin moment S is quantized and can be obtained from:
where is the spin angular momentum quantum number.
So the magnitude of the spin moment is always:
The orientation of the vector magnitude S is also restricted. Specifically, it’s projection onto a given axis is given by the spin projection quantum number:
To make a spintronic device, the primary requirements are, first, 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, secondly, a separate system 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). However 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).
Metal-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.
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.
Read heads of modern hard drives are based on the GMR or TMR effect.
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 (Everspin, 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.
There are Magnetic sensors using the GMR effect.
Semiconductor-based spintronic devices
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
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.
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.
Magnetic Tunnel Transistor: The magnetic tunnel transistor with a single base layer, by van Dijken et al. and Jiang et al., has the following terminals:
- Emiter (FM1): It injects spin-polarized hot electrons into the base.
- Base (FM2): Spin-dependent scattering takes place in the base. It also serves as a spin filter.
- Collector (GaAs): A Schottky barrier is formed at the interface. This collector regions only collects electrons when they have enough energy to overcome the Schottky barrier, and when there are states available in the semiconductor.
The magnetocurrent (MC) is given as:
And the transfer ratio (TR) is
MTT promises a highly spin-polarized electron source at room temperature.
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- Spintronics portal with news and resources
- RaceTrack:InformationWeek (April 11, 2008)
- Spintronics research targets GaAs.
- Spintronics at SUNY Albany's College of Nanoscale Science and Engineering
- IBM to use 'spintronics' to increase computer memory capacity (April 12, 2008)
- Semiconductor spintronics lab at University of Maryland
- Spintronics Tutorial
- Lecture on Spin transport by S. Datta (from Datta Das transistor) -Part 1 and Part 2
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