Introduction on Spintronics
According to the Oxford English Dictionary, spintronics is “A branch of physics concerned with the storage and transfer of information by means of electron spins in addition to electron charge as in conventional electronics”.
The research field of spintronics essentially took o® with the discovery of giant magnetoresistance (GMR, 2007 Nobel prize GrÄunberg and Fert) in the late 80s/early 90s, which led to a 100 times increase in the storage capacity of hard drives less than 10 years later. Since then it has differentiated into three closely related themes: i) ferromagnetic metal spintronics, ii) ferromagnetic semiconductor spintronics, and iii) paramagnetic semiconductor spintronics.
Spintronics is interesting from a fundamental point of view because spin is not a conserved quantity. Charge, on the other hand, is a conserved quantity in the sense that it always obeys a continuity equation: unless there is a source, the charge in a given volume element can only change by charge owing in and out of the element. No charge can appear or dissappear. The same is not true for spin. An electron with spin pointing in the, say, z-direction can encounter a magnetic impurity that changes its spin to the x-direction. So, the spin in the z-direction is not conserved in this example. Another interesting feature of spin currents is that they can in principle occur without any charge current. Consider the case that all the \up” electrons move in one direction, and that all the down” electrons move in the opposite direction.
In the case that the material is paramagnetic, i.e., nonmagnetic, there will be a pure spin current but no charge current. This situation occurs in the SPIN HALL EFFCT: a charge current lead to a pure spin current in the direction perpendicular to it. From the above it is clear that understanding spintronics requires understanding of ferro-magnetism and electron transport, and of combining them with this in mind, we cover the following main topics:
Spin transfer torques: interaction between magnetization and electron transport
Spin hall effect: effects of spin-orbit coupling.
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