Spintronics

Spintronics uses electron spin and spin-dependent transport in electronic materials and devices. In a magnetic multilayer, the relative magnetization directions can change resistance because electrons of different spin scatter differently. Giant magnetoresistance and tunnel magnetoresistance turn this dependence into electrical readout. Spin accumulation instead describes a nonequilibrium imbalance of spin populations. The field connects quantum angular momentum to transport and information storage; spin polarization need not imply that the entire material is a permanent magnet.^[ref]

Tunnel magnetoresistance is the change in resistance of a magnetic tunnel junction when the relative magnetizations of its two electrodes change. A common convention iswhere P and AP denote parallel and antiparallel alignment. This connects quantum tunnelling to the spin dependence of electronic states in the electrodes.

In the Jullière model, assume spin-conserving tunnelling with spin-independent matrix factors, and let the density of states polarizations at the Fermi energy be $P_i=(n_{i\uparrow }-n_{i\downarrow })/(n_{i\uparrow }+n_{i\downarrow })$. Parallel alignment pairs matching spin channels, while antiparallel alignment pairs opposite channels. Hence $G_{\mathrm{P}}\propto 1+P_1{P}_2$ and $G_{\mathrm{AP}}\propto 1-P_1{P}_2$. Using $R=1/G$ givesThis is a consequence of the stated two-channel model, rather than a universal material law.

M. Jullière's 1975 experiment compared the conductance of iron–germanium–cobalt thin-film junctions with parallel and antiparallel magnetizations at low temperature. The thin separating layer, magnetic preparation of the films and conductance measurement together made relative magnetic orientation an experimentally controllable transport variable.^[ref]

The Jullière model describes spin-conserving tunneling between ferromagnetic electrodes using their spin-resolved density of states at the Fermi energy. If $P_i=(n_{i\uparrow }-n_{i\downarrow })/(n_{i\uparrow }+n_{i\downarrow })$ and the tunneling factors are spin independent, summing the two spin channels givesThe last equality uses $R=1/G$. For $P_1=P_2=0.4$, the predicted tunnel magnetoresistance is $0.32/0.84\approx 0.381$, or 38.1 percent.

The motivating experiment compared low-temperature conductance of iron–germanium–cobalt thin-film junctions after preparing parallel and antiparallel magnetizations. The thin separating layer permits quantum tunnelling, while changing magnetic alignment tests the two-channel prediction. Interface states, spin-dependent tunneling amplitudes and finite-bias effects are additional physics outside this simple model.^[ref]

Spin accumulation is a nonequilibrium imbalance between spin populations, often described by a difference of spin-dependent electrochemical potentials $\delta {\mu }_s={\mu }_{\uparrow }-{\mu }_{\downarrow }$. A spin-polarized injected current can create such an imbalance even in a nonmagnetic conductor. Diffusion spreads it while spin relaxation tends to remove it, defining characteristic time and length scales. A transverse magnetic field makes the spin orientation precess and can change a detector signal, the principle of a Hanle spin-precession measurement. This nonequilibrium population is distinct from equilibrium magnetization or a charge-density accumulation.