Spin-transfer torque is an effect in which the orientation of a magnetic layer in a tunnel magnetoresistance or spin valve can be modified using a spin-polarized current.
Charge carriers (such as electrons) have a property known as spin which is a small quantity of angular momentum intrinsic to the carrier. An electrical current is generally unpolarized (consisting of 50% spin-up and 50% spin-down electrons); a spin polarized current is one with more electrons of either spin. By passing a current through a thick magnetic layer, one can produce a spin-polarized current. If a spin-polarized current is directed into a magnetic layer, angular momentum can be transferred to the layer, changing its orientation. This can be used to excite oscillations or even flip the orientation of the magnet. The effects are usually only seen in nanometer scale devices.
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Spin-transfer torque can be used to flip the active elements in magnetic random access memory. Spin-transfer torque random access memory, or STT-RAM, has the advantages of lower power consumption and better scalability over conventional MRAM which uses magnetic fields to flip the active elements. The name STT-RAM was first coined by Grandis, Inc. Spin-transfer torque technology has the potential to make possible MRAM devices combining low current requirements and reduced cost; however, the amount of current needed to reorient the magnetization is at present too high for most commercial applications, and the reduction of this current density alone is the basis for current academic research in spin electronics.[1]
Hynix Semiconductor and Grandis formed a partnership in April 2008 to explore commercial development of STT-RAM technology.[2][3] On August 1, 2011, Grandis announced that it had been purchased by Samsung Electronics for an undisclosed sum.[4]
Hitachi and Tohoku University demonstrated a 32-Mbit STT-RAM in June 2009.[5]
Other companies working on STT-RAM include Crocus Technology[6] and Spin Transfer Technologies.[7] In May 2011, Russian Nanotechnology Corp. announced an investment of $300 millions in Crocus Technology which will build an MRAM factory in Russia.