The first major breakthrough in spintronics was the discovery of the giant magnetoresistance (GMR) effect in 1988. Working independently, Albert Fert in France and Peter Grünberg in Germany found that in a material consisting of alternating layers of magnetic and nonmagnetic atoms a very small change in a magnetic field can produce a large change in electrical resistance. Employing advances in nanotechnology (see under micromechanics), they used chemical techniques that allowed them to make layers of different materials that were only a few atoms thick. The GMR effect was used in the development of data-storage devices that were physically smaller but allowed increasingly denser packing of the information content. The first commercial devices using the GMR effect, produced in 1997, had a 40-fold increase in data density when compared with conventional electronics. The technology is now used in computer storage, personal music players, PDAs, cell phones, and other devices that benefit from the increased size of readable memory. In a more sensitive effect, called tunneling magnetoresistance (TMR), an insulating material acts as a sandwich. Electrons can move through the sandwich by quantum tunneling. Another spintronic breakthrough product is magnetoresistive memory (MRAM), which uses electron spin to store information; while requiring less power than coventional magnetic storage technologies, it combines the density of DRAM (dynamic random access memory) with the speed of SRAM (static random access memory) and the nonvolatility of flash memory. In recognition of their contributions, Fert and Grünberg shared the 2007 Nobel Prize in physics.
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).
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:
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.
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:
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.
Frost & Sullivan Lauds Organic Spintronics' Active R&D Efforts that Have Placed it at the Forefront of Spintronics-Based Innovation for Sensing Applications.
Aug 17, 2009; Based on its recent analysis of the spintronics markets for sensor applications, Frost & Sullivan recognizes Organic...
Frost & Sullivan Lauds Organic Spintronics' Active R&D Efforts That Have Placed It at the Forefront of Spintronics-Based Innovation for Sensing Applications
Aug 19, 2009; Based on its recent analysis of the spintronics markets for sensor applications, Frost & Sullivan recognizes Organic...