The known methods for measuring the spin transport parameters in spin-valve structures are based on the Hanle effect, viz., electron spin precession in an external magnetic field and a decrease in the magnetoresistive signal. These methods make it possible to determine the spin relaxation time in the paramagnetic layer and the relative current polarization constant. We describe an alternative method of measuring in zero external magnetic field, which is based on the resonant increase in the magnetic susceptibility of the paramagnetic layer due to the paramagnetic resonance induced by the nonequilibrium magnetization due to the spin accumulation effect. The proposed method makes it possible to determine the absolute value of spin accumulation in a paramagnet, which can be used as a parameter for numerical solution of three-dimensional diffusion equations of spin transport.

The free layer in a nanoheterostructure with the magnetic tunnel junction (MTJ) usually has the shape of a thin disk with a diameter of several tens of nanometers and a thickness of several nanometers. For certain values of the current passing through such an MTJ structure, the magnetization of the free layer experiences the stationary precession caused by the compensation of precession energy dissipation by the spin-transfer effect. An important property of such an oscillator is the linear dependence of the oscillation frequency on the applied voltage. If the shape of the MTJ structure acquires ellipticity during its preparation, the magnetization oscillations become nonsinusoidal, and the voltage dependence of the frequency becomes more complicated. In this article, we consider an approximate expression for calculating the frequency of a monodomain nano-oscillator in the MTJ structure with a nonzero ellipticity, which has been obtained using the asymptotic method of solution of the Landau–Lifshitz equation with additional phenomenological transport terms. This expression is also compared with the results of numerical calculations and shows good agreement for small deviations of the MTJ structure shape from a symmetric disk.

The notion of spin accumulation and spin relaxation in the diffusive regime was introduced by Valet and Fert in 1993 to describe the current-perpendicular-to-plane (CPP) diffusive transport in metallic magnetic multilayers. This theory has been quite successful in explaining the giant magnetoresistance of magnetic multilayers in CPP geometry in the frequency range from DC to a few gigahertz. In this paper, we investigate the dynamic aspect of spin accumulation from the theoretical point of view when reaching the terahertz (THz) frequency range. The characteristic relaxation time of electron elastic scattering is typically in the femtosecond range. However, since spin accumulation results from a diffusion process involving a very large number of individual scattering events, the characteristic time of spin accumulation variation when the current and/or the magnetic configuration are varied can be significantly longer than that of the input signal, eventually reaching the picosecond range. In spintronic devices operating in the THz range such as those based on ferrimagnetic or antiferromagnetic materials, the spin accumulation amplitude and, correlatively, the device magnetoresistance can therefore depend on the actual device operating frequency. We investigate this question by extending the Valet and Fert theory in the time domain.

One of important characteristics of a magnetic tunneling junction (MTJ) nanoheterostructure that is used as magnetic random access memory (MRAM) cell and is switched by the spin transfer torque (STT) effect is its stability diagram, which determines the range of values of the external magnetic field and applied voltage under which a cell is in one of two stable states. To numerically construct such diagrams, one usually uses the solution of the Landau–Lifshitz dynamic equation in the single-domain approximation along with phenomenological constants that determine the values of spin torque in the material. In the present paper, the problem of spin-dependent electron transfer in an MTJ structure is considered, whose solution in the free-electron approximation allows one to calculate the values of spin torque for a given material in any magnetic configuration of the system and apply them to the integration of the Landau–Lifshitz equation. The method used allows one to more precisely reproduce the shape of the stability diagram and predict the critical values of the magnetic field and voltage necessary to switch an MTJ-based MRAM cell.

We analytically investigated the spin-dependent transport properties in a lateral spin-valve device comprising pinned ferromagnetic electrodes allowing the injection of a spin current in a spin conducting channel where spin orbit scattering takes place. This produces an inverse spin Hall (ISHE) voltage across the thickness of the spin conducting channel. It is shown that by adding an extra soft ferromagnetic electrode with rotatable magnetization along the spin conducting channel, the ISHE generated voltage can be magnetically modulated by changing the magnetization orientation of this additional electrode. The dependence of the ISHE voltage on the direction of magnetization of the ferromagnetic electrode with rotatable magnetization was calculated in various configurations. Our results suggest that such structures could be considered as magnetic field sensors in situations where the total thickness of the sensor is constrained such as in hard disk drive readers.

We study spin-dependent transport theoretically in structures with a barrier composed of magnesium oxide and ferromagnetic electrodes with a noncollinear orientation of magnetizations in the ferromagnetic layers. We consider the effects of tunnel magnetoresistance and of the spin torque in the two-band s-d model using the Keldysh technique of nonequilibrium Green's functions with the s-d hybridization taken into account. We investigate the role of a chromium layer as an additional spin filter for s electrons and the possibility of increasing the effects of tunnel magnetoresistance and of the spin torque for the resonance tunneling in the five-layer structure Fe|Cr|Fe|MgO|Fe.

Theoretical studies of spin dependent transport in Fe/Cr/MgO/Fe tunnel junctions with noncollinear alignment of magnetizations of metallic layers comprising these magnetic tunnel junctions are presented. Calculations are performed with use of nonequilibrium Green function technique in the framework of the Keldysh formalism. Electronic band structure of ferromagnetic electrodes is modeled within a two-band model with majority and minority states being s-like and d-like electrons, respectively. Furthermore, interfacial s-d hybridization is taken into account and calculated using perturbation corrections for the wave and Green functions. It is shown that in the presence of Cr layer at the Fe/MgO interface, the contribution from s-d hybridization to the total current is much stronger in the antiparallel magnetizations configuration compared to the parallel one leading to decrease in tunnel magnetoresistance values in agreement with earlier reports.

In this paper we present theoretical studies of spin dependent transport in Fe/Cr/MgO/Fe tunnel junctions with non-collinear alignment of magnetizations of metallic layers comprising these MTJs. Calculations are performed with use of non-equilibrium Green function technique in the framework of Keldysh formalism. WKB approximation is used for wave and Green functions in the trapezoidal barrier region under applied voltage. Electronic structure of ferromagnetic electrodes is modeled with two bands model, i.e. with majority s-electrons and minority d-holes. Furthermore, we introduce s-d hybridization by calculating the corresponding perturbation corrections for the wave and Green functions.

The Josephson effect in a superconductor-insulator-ferromagnet-superconductor structure with a stepwise change in the transparency of the superconductor/ferromagnet interface has been investigated within the formalism of the Usadel equations. It is shown that, at a certain thickness of the ferromagnet layer, this feature leads to the formation of a point contact with an anomalous dependence of the critical current on the external magnetic field.

We demonstrate a spin diode consisting of a semiconductor-free nanoscale Fe/MgO-based double tunnel junction. The device exhibits a near perfect spin-valve effect combined with a strong diode effect. The mechanism consistent with our data is resonant tunneling through discrete states in the middle ferromagnetic layer sandwiched by tunnel barriers of different spin-dependent transparency. The observed magnetoresistance is a record high>1000%, essentially making the structure an on/off spin switch. This, combined with the strong diode effect, approximately 100, demonstrates a new device principle, promising for memory and reprogrammable logic applications.

In homogeneous negative phase velocity media, the Doppler and Cherenkov-Vavilov effects and the refraction and pressure of light are anomalous: they are inverse with respect to the corresponding effects in conventional media. Using the geometrical optics approximation, it is shown that the optical Magnus effect in inhomogeneous negative phase velocity media is also anomalous. The effect is demonstrated by considering a metamaterial consisting of parallel amorphous ferromagnetic microwires in a magnetic field. The metamaterial proves to be a left-handed one in the realistic region of the electromagnetic spectrum. The optical properties of such a left-handed medium can be controlled by the external magnetic field.

The critical current in a Josephson junction with a weakly ferromagnetic layer of a transition-metal alloy is calculated. The Gor’kov equations are solved taking the s-d scattering in a ferromagnet into account. It is shown that consideration of this scattering, which results in the destruction of Cooper pairs, makes it possible to achieve good agreement with experiments which could not be explained by other models.

The helicoid that forms in an external magnetic field in the presence of a point defect is modeled in the framework of the Landau theory of second-order phase transitions. A general solution to the nonlinear problem retaining all terms in the Helmholtz free-energy functional is obtained by means of Green functions. The magnetization distribution in the plane perpendicular to the helicoid axis and to the external field is calculated.

In the framework of the Valet and Fert theory of current perpendicular to plane (CPP) giant magnetoresistance (GMR) in metallic multilayers [Valet and Fert, Phys. Rev. B 48, 7099 (1993)], the calculation of the CPP resistance and magnetoresistance has been generalized to any multilayered stacks including spin valves with synthetic free layers, laminated free and pinned layers, and dual spin valves. The theory takes into account bulk and interfacial spin-dependent scattering as well as spin flip in all layers. It also considers the effect of scattering at lateral edges of submicron multilayered pillars which can be viewed as a current in plane effect intruding on CPP transport. This latter effect plays a role when the diameter of the pillar becomes of the order of the elastic mean-free paths (i.e., below ∼30 nm). Based on the theory, a code has been developed to compute the CPP resistance and CPP magnetoresistance from the transport parameters of each material involved in the stack (spin-dependent resistivities, spin-dependent interfacial resistances, and spin-diffusion length in each layers). As examples, we compare the results of the calculations with various already published CPP experiments. In particular, we interpret experiments previously carried out on (NiFe/Cu/Co/Cu)N and (Co 6 nm/Cu/Co 1 nm/Cu)N multilayers in which the order of the layers in the stack had been shown to affect the CPP resistance and magnetoresistance, a property which could not be explained in a simple two-channel serial resistance model. We also investigate the influence of the thickness of the various layers and underline the key role of the spin diffusion length in these thickness variations. Unexpected predictions are made with this theory such as the existence of a maximum in CPP–MR as a function of the thickness of the antiferromagnetic pinned layer. This type of calculations should allow a faster optimization of CPP–GMR in metallic multilayers.

Quantum-statistical calculations are presented for the anomalous Hall effect in a magnetic sandwich with a tunnel junction across a thin dielectric spacer. The tunneling current flows across the junction perpendicular to the plane of the layers while the Hall component of the current lies in this plane. The Kubo formalism and the Green’s functions are used to calculate the contribution of skew scattering to the Hall conductivity. The classical size effect in the Hall conductivity of this structure is studied and two new effects are observed. One is associated with the dependence of the effective electric field in the magnet on the transparency of the dielectric potential barrier for electrons when the current flows perpendicular to the layers of the structure and may be called “ geometric”. The other occurs as a result of the influence of the strong electric field in the dielectric on the electron motion in the adjacent magnetic layers.

A quantum-statistical model using the Kubo formalism is proposed for describing the magnetoresistance of a multilayer structure with the current perpendicular to the plane of the layers. In particular, this model describes the case of noncollinear magnetization of consecutive ferromagnetic layers of the structure. Interference between electron wave functions with different directions of the spin projections onto the magnetization axis, which arises in the noncollinear configuration, is investigated along with the role of electron scattering, not only within the bulk of the layers, but also at their interfaces.

The results of an investigation of the magnetic and magnetooptical properties of Au/Cu-wedge/15-Å-NiFe sandwiches are reported. Oscillations of the equatorial Kerr effect as a function of the copper wedge thickness are observed. The period of these oscillations is found to be of the order of 5–6 Å. The experimentally observed oscillations of the equatorial Kerr effect are attributed to a quantum size effect.