The switching of magnetic layers is studied under the action of a spin current in a ferromagnetic metal/non-magnetic metal/ferromagnetic metal spin valve. We find that the main contribution to the switching comes from the non-equilibrium exchange interaction between the ferromagnetic layers. This interaction defines the magnetic configuration of the layers with minimum energy and establishes the threshold for a critical switching current. Depending on the direction of the critical current, the interaction changes sign and a given magnetic configuration becomes unstable. To model the time dependence of the switching process, we derive a set of coupled Landau-Lifshitz equations for the ferromagnetic layers. Higher order terms in the non-equilibrium exchange coupling allow the system to evolve to its steady-state configuration.PACS numbers:

The spin-polarized tunnel current and its connection to the interlayer exchange interaction is studied in ferromagnet-insulator-ferromagnet thin-film planar junctions out of equilibrium. Building on the nonequilibrium Keldysh formalism, it is possible to systematically include a contact interaction between localized spins and conduction electrons and extend previous treatments on spin currents and exchange interaction. In particular, a Landauer-type formula is derived for the spin current that explains the result found earlier ͓Schwabe, Wingreen, and Elliott, Phys. Rev. B 54, 12 953 ͑1996͔͒ that the exchange interaction between the ferromagnetic slabs increases in proportion to the slab width. Furthermore, switching is shown to occur between parallel and antiparallel coupling of the slabs for different applied biases under feasible experimental conditions.

Spin currents injected into magnetic thin films may noticeably change the magnetization of the films. To describe this effect, exchange coupled Landau-Lifshitz equations for the local magnetization and the spin-polarized charge carriers are combined with transport equations for charge and spin currents. For steady state transport one obtains two different instability conditions. Both conditions are supported by recent experimental data on current induced magnetization reversal and spin-wave excitations. No second ferromagnetic layer is needed for excitations due to spin transfer.

The tunnel magnetoresistance and its connection to the interlayer exchange interaction is studied in ferromagnet-insulator-ferromagnet junctions. Building on the nonequilibrium Keldysh formalism, we include a contact interaction between localized spins and conduction electrons and derive an expression for the tunnel current that depends on the magnetization of the layers as SL • SR in correspondence with the exchange coupling. From this expression we also obtain a formula for the tunnel magnetoresistance. At low bias and for systems where one can neglect size-quantization effects Julliere's formula is rederived. The temperature dependence of the dynamical parameters arises from the system of localized spins, its interaction with the conduction electrons, and the thermal reservoirs.

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