Magnetic semiconductors

Abstract

Magnetic properties are introduced into solids by paramagnetic ions. These are transition-metal ions of the iron series with a partially filled electronic 3d shell or rare-earth ions of the lanthanide series with an incomplete 4f shell. In magnetic semiconductors, they represent a cation component of the crystal, while in diluted magnetic semiconductors, they are a substitutional alloy component on the cation sublattice. The magnetic moments of the paramagnetic ions are coupled by different kinds of exchange interactions. Superexchange mediated by p states of anion ligands favors antiferromagnetism with antiparallel alignment of the magnetic moments, while double exchange and p-d exchange favor ferromagnetism with parallel alignment. Magnetic ordering is disturbed if the thermal energy exceeds the exchange energy; critical Curie and Néel temperatures exist for the transition from the paramagnetic high-temperature range to magnetically ordered respective ferromagnetic and antiferromagnetic regimes at lower temperature. The spin of carriers is utilized in spintronics for current control by creating a nonequilibrium spin polarization. When phase effects can be neglected, the transport of spin-polarized carriers is described by a two-current model assuming two independent channels of different spin projections. Spintronic structures typically comprise two ferromagnetic layers, which are separated by a thin nonmagnetic spacer layer. For insulating spacers and antiparallel aligned ferromagnetic moments a large difference in resistance of the two spin-polarized channels is found, referred to as tunneling magnetoresistance. Similarly a giant magnetoresistance is observed for a thin conductive spacer. The orientation of the ferromagnetic magnetization can be altered by spin-transfer torque exerted by a spin-polarized current. Diluted magnetic semiconductors serve as a valuable test bench for exploratory physics of spintronic devices.