A quasiclassical theory of giant magnetoresistance in nanoscale point contacts between different ferromagnetic metals is developed. The contacts were sorted by three types of mutual positions of the conduction spin-subband bottoms which are shifted one against another by the exchange interaction. A model of linear domain wall has been used to account for the finite contact length. The magnetoresistance is plotted against the size of the nanocontact. In heterocontacts the magnetoresistance effect turned out to be not only negative, as usual, but can be positive as well. Relevance of the results to existing experiments on GMR in point heterocontacts is discussed.

We present a theoretical approach to calculate the spin-dependent current and tunnel magnetoresistance (TMR) in a double-barrier magnetic tunnel junction (DMTJ), in which the magnetization of the middle ferromagnetic metal layer can be aligned parallel or antiparallel in relation to the fixed magnetizations of the left and right ferromagnetic electrodes. The electron transport through the DMTJ is considered as a three-dimensional problem, taking into account all transmitting electron trajectories as well as the spin-dependent momentum conservation law. The dependence of the transmission coefficient and spin-polarized currents on the applied voltage is derived as an exact solution to the quantum-mechanical problem for the spin-polarized transport. In the range of the developed physical model, the resonant tunneling, nonresonant tunneling, and enhanced spin filtering can be explained; the simulation results are in good agreement with experimental data.

Based on model calculations we predict electric-field control of the spin transfer torque (STT) in magnetic tunnel junctions with ferroelectric barriers. We demonstrate that the bias dependence of the in-plane, T , and out-of-plane, T ⊥ , components of the STT can be dramatically modified by the ferroelectric polarization. In particular, the magnitude of the STT can be enhanced or suppressed by switching the polarization direction and in some cases the sign of STT can be toggled. The underlying mechanism is the combination of polarization-induced symmetry breaking and the interplay of the bias-induced and polarization-induced spin-dependent screening giving rise to a rich behavior of the electrostatic potential energy profile. These properties could lead to enhanced switching efficiency in STT-based devices and open a new avenue for applications of multiferroic devices.

We predict that the tunneling electroresistance effect is present at finite bias even in multiferroic tunnel junctions (MFTJs) with inversion symmetry. The effect is highly sensitive to the relative magnetization orientation in the electrodes. In addition, we demonstrate control of the bias-dependence of the tunneling magnetoresistance (TMR) in MFTJs via switching of the ferroelectric polarization of the barrier. The polarization induces a monotonic bias behavior in TMR which can be reversed by polarization switching. The magnitude of both effects is proportional to the polarization. The underlying mechanism is the inversion symmetry breaking due to the polarization combined with the interplay of the bias-induced and polarization-induced spin-dependent interfacial screening. These results expand the possibilities for the next-generation multifunctional devices.

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