Spin Injection–Extraction Processes in Metallic and Semiconductor Heterostructures

Abstract Spintronics is a discipline focusing on phenomena and devices essentially dependent on spin transport. We review recent theoretical and experimental advances in achieving large spin injection efficiency (polarization of current) and accumulated spin polarization. These include tunnel and giant magnetoresistance, (pseudo)spin‐torque and spin–orbit effects on electron transport in various heterostructures. We give a microscopic description of spin tunneling through oxide and modified Schottky barriers between a ferromagnet (FM) and a semiconductor (S). It is shown that in such FM–S junctions electrons with a certain spin projection can be efficiently injected into (or extracted from) S, while electrons with the opposite spin can accumulate in S near the interface. The derived criterion for efficient injection suggests that the barrier should be rather transparent (this is opposite to a criterion in the literature suggesting that it should be opaque). In degenerate semiconductors, extraction of spin can proceed at low temperatures. We mention a few novel spin‐valve ultrafast devices with small dissipated power: a magnetic sensor, a spin transistor, an amplifier, a frequency multiplier, a square‐law detector and a source of polarized radiation. We also discuss effects related to spin–orbital interactions, such as the spin Hall effect (SHE) and a recently predicted positive magnetoresistance accompanying SHE. Some esoteric devices such as ‘spinFET’, interacting spin logic and spin‐based quantum computing are discussed and problems with their realization are highlighted.