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We calculate the response of a ferromagnet - antiferromagnet junction to a high-frequency magnetic field as a function of the spin-polarized current through the junction. Conditions are choused under which the response is zero in absence of such a current. It is shown that increasing in the current density leads to proportional increase in the resonance frequency and resonant absorption. A principal possibility is indicated of using ferromagnet - antiferromagnet junction as a terahertz radiation detector., Comment: 10 pages, 4 figures

Spin-polarized current effect is studied on the static and dynamic magnetization of the antiferromagnet in a ferromagnet - antiferromagnet nanojunction. The macrospin approximation is generalized to antiferromagnets. Canted antiferromagnetic configuration and resulting magnetic moment are induced by an external magnetic field. The resonance frequency and damping are calculated, as well as the threshold current density corresponding to instability appearance. A possibility is shown of generating low-damping magnetization oscillations in terahertz range. The fluctuation effect is discussed on the canted antiferromagnetic configuration. Numerical simulation is carried out of the magnetization dynamics of the antiferromagnetic layer in the nanojunction with spin-polarized current. Outside the instability range, the simulation results coincide completely with analytical calculations using linear approximation. In the instability range, undamped oscillations occur of the longitudinal and transverse magnetization components., Comment: 19 pages, 3 figures. arXiv admin note: substantial text overlap with arXiv:1106.3519

Spin-polarized current effect is studied on the static and dynamic magnetization of the antiferromagnet in a ferromagnet - antiferromagnet junction. The macrospin approximation is generalized to antiferromagnets. Canted antiferromagnetic configuration and resulting magnetic moment are induced by an external magnetic field. The resonance frequency and damping are calculated, as well as the threshold current density corresponding to instability appearance. A possibility is shown of generating low-damping magnetization oscillations in terahertz range. The fluctuation effect is discussed on the canted antiferromagnetic configuration., Comment: 14 pages, 1 figure

Energy efficiency in terahertz range is evaluated experimentally of a spin-injection oscillator based on a ferromagnetic rod-film structure with point contact between the components. Choice of the film material influences substantially the efficiency. A magnetic flux concentrator is used to improve the efficiency. It is found from the measurements that the quantum efficiency can exceed unity. The latter indicates substantial contribution of stimulated radiative transitions., Comment: 8 pages, 5 figures

The stability is analyzed of the magnetic junction collinear configurations against small fluctuations under amplitude-modulated current with CPP mode. High spin injection is assumed. Under parametric resonance conditions, with the modulation frequency twice the precession frequency, instability is possible of one, or another, or both the collinear configurations. When the dc component of the current density exceeds the instability threshold of the antiparallel configuration, the parametric instability is suppressed by nonparametric one which is induced by the dc current. The parametric instability manifests itself as lowering the threshold of the dc current density in presence of the high-frequency current, such an effect has been observed in experiments repeatedly., Comment: 9 pages, 2 figures

Electromagnetic radiation of 1 - 10 THz range has been found at room temperature in a structure with a point contact between a ferromagnetic rod and a thin ferromagnetic film under electric current of high enough density. The radiation is due to nonequilibrium spin injection between the structure components. By estimates, the injection can lead to inverted population of the spin subbands. The radiation power exceeds by orders of magnitude the thermal background (with the Joule heating taking into account) and follows the current without inertia., Comment: 8 pages, 5 figures

A magnetic tunnel junction Fe3O4/MgO/Fe with (001) layer orientation is considered. The junction magnetic energy is analyzed as a function of the angle between the layer magnetization vectors under various magnetic fields. The tunnel magnetoresistance is calculated as a function of the external magnetic field. In contrast with junctions with unidirectional anisotropy, a substantially lower magnetic field is required for the junction switching., Comment: 7 pages, 3 figures

Current flowing is studied in magnetic junctions consisting of a ferromagnetic metal (FM), antiferromagnetic conductor (AFM) and a nonmagnetic metal closing the electric circuit. The FM layer with high anisotropy and pinned spins of the magnetic atoms in the lattice acts as a spin injector relative to the AFM layer. To obtain resulting magnetization in the AFM layer, magnetic field is applied, which may be varied to control the magnetization. The spin-polarized current from the FM layer creates a torque and makes it possible to switch the magnetization. A possibility is shown to lower the threshold current density by the orders of magnitude by means of the magnetic field., Comment: 10 pages, 1 figure

Two channels of the sd exchange interaction are considered in magnetic junctions. The first channel describes the interaction of transversal spins with the lattice magnetization. The second one describes the interaction of longitudinal spins with magnetization. We show the longitudinal channel leads to a number of significant effects: 1) drastic lowering of the current instability threshold down to three (or even more) orders of magnitude; 2) creation of large enough distortion of equilibrium due to current driven spin injection leading to inversion of energy spin subband populations and laser-like instability in THz frequency range at room temperature. External magnetic field may tend to lower additionally the instability threshold due to the proximity effect of purely magnetic reorientation phase transition. This effect demonstrates the new properties: the giant magnetoresistance (GMR) becomes strongly current dependent and the exchange switching becomes of very low threshold. We derived some matching condition that should be satisfied to achieve high spin injection level. Some characteristic quantities were appeared in the condition. We investigated also the junctions having variable lateral dimensions of the layers, for example, a ferromagnetic rod contacting with a very thin ferromagnetic film. Large enhancement of the current density may appear near the contact region leading to the spin injection luminescence., Comment: 13 pages, 7 figures

The results are presented of a numerical simulation of the switching magnetic junction by a spin-polarized current pulse under applied magnetic field with the current density and field below the threshold values. A possibility is shown of the switching with controllable time delay relative to the current pulse., Comment: 7 pages, 3 figures

The stability is analyzed of the equilibrium configurations of a magnetic junction with a free layer that has cubic symmetry and two anisotropy axes in the layer plane. Different variants of the switching between various configurations are considered. A possibility is shown of the substantial lowering of the threshold current density needed for the switching. Numerical simulation is made of the switching dynamics for various configurations., Comment: 14 pages, 7 figures

Properties are discussed of a ferromagnetic junction of the type "rod contacting with film". Very high current density of the order of $10^9$ A/cm$^2$ may be achieved in the contact region. We show it can lead to inversion of population of the spin energy subbands. Spin injection depends strongly on the direction of the current (forward or backward). We prepared experimentally a rod-to-film structure and investigated high density current flowing through it. Current dependent radiation has been observed by means of a THz receiver. In particular, the radiation becomes different for forward and backward currents. It shows the radiation includes not only heating but also non-thermal (spin-injection) effects., Comment: 10 pages, 9 figures

The distribution is calculated of the electron spin polarization under current-driven spin injection from a probe to a ferromagnetic film. It is shown that the main parameters determining difference of the spin polarization from the equilibrium value are the current density and the spin polarization of the probe material, while the relation between the probe diameter and the spin diffusion length influences the result very weakly, to a certain extent. A possibility is shown of reaching inverse population of the spin subbands at distances from the probe boundary comparable with the spin diffusion length., Comment: 8 pages, 2 figures

A tunnel magnetic junction is considered with magnetic hard and magnetic soft layers of cubic symmetry. The magnetic switching is analyzed of the layers by a magnetic field perpendicular to the initial magnetizations. In such a situation, an additional peak of the TMR ratio appears at the magnetic field value lower substantially than the anisotropy energy of the soft layer., Comment: 7 pages, 3 figures

Current-induced switching is considered in a magnetic junction. The junction includes pinned and free ferromagnetic layers which work in the regime of the high spin injection. It is shown that in such a regime the exchange magnetization reversal threshold can be lowered up to two times when the axes of the layers are noncollinear., Comment: 5 pages, 1 figure

An effect is considered of alternating (high-frequency) current on the spin-valve type magnetic junction configuration. The stability with respect to small fluctuations is investigated in the macrospin approximation. When the current frequency is close to the eigenfrequency (precession frequency) of the free layer, parametric resonance occurs. Both collinear configurations, antiparallel and parallel ones, can become unstable under resonance conditions. The antiparallel configuration can become unstable under non-resonant conditions, also. The threshold current density amplitude is of the order of the dc current density switching the magnetic junction., Comment: 8 pages

Nonequilibrium electron spin polarization is calculated under spin injection from one ferromagnet to another in magnetic junction. It is shown that the nonequilibrium spin polarization can be comparable with equilibrium one if the material parameters are chosen appropriately. This leads to lowering the threshold current density necessary for the junction switching and opens a perspective to creating THz laser based on the spin-polarized current injection., Comment: 6 pages, 1 figure. Corrected Typos. Submitted to Physica status solidi B

Phenomenological approach is developed in the theory of spin-valve type ferromagnetic junctions to describe exchange switching by current flowing perpendicular to interfaces. Forward and backward current switching effects are described and they may be principally different in nature. Mobile electron spins are considered as being free in all the contacting ferromagnetic layers. Joint action of the following two current effects is investigated: the nonequilibrium longitudinal spin-injection effective field and the transverse spin-transfer surface torque. Dispersion relation for fluctuations is derived and solved for a junction model having spatially localized spin transfer torque: depth of the torque penetration into the free layer is assumed much smaller than the total free layer thickness. Some critical value of the well known Gilbert damping constant is established for the first time. Spin transfer torque dominates in the instability threshold determination for small enough damping constants, while the spin-injection effective field dominates for high damping. Fine interplay between spin transfer torque and spin injection is necessary to provide a hysteretic behavior of the resistance versus current dependence. The state diagram building up shows the possibility of non-stationary (time dependent) nonlinear states arising due to instability development. Calculations lead to the instability rise time values of the order of 0.1 ns. Spin wave resonance frequency spectrum softening occurs under the current growing to the instability threshold. Magnetization fluctuations above the threshold rise oscillating with time for low damping, but rise aperiodically and much more rapid for high damping., Comment: 16 pages, 7 figures. Corrected typos. Submitted to Phys. Rev. B

Boundary conditions are derived that determine the penetration of spin current through an interface of two non-collinear ferromagnets with an arbitrary angle between their magnetization vectors. We start from the well-known transformation properties of an electron spin wave functions under the rotation of a quantization axis. It allows directly find the connection between partial electric current densities for different spin subbands of the ferromagnets. No spin scattering is assumed in the near interface region, so that spin conservation takes place when electron intersects the boundary. The continuity conditions are found for partial chemical potential differences in the situation. Spatial distribution of nonequilibrium electron magnetizations is calculated under the spin current flowing through a contact of two semi-infinite ferromagnets. The distribution describes the spin accumulation effect by current and corresponding shift of the potential drop at the interface. These effects appear strongly dependent on the relation between spin contact resistances at the interface., Comment: 10 pages, 2 figures. Submitted to J. Magn. Magn. Mater. Corrected typos

We consider theoretically a current flowing perpendicular to interfaces of a spin-valve type ferromagnetic metallic junction. For the first time an effective approach is investigated to calculate a simultaneous action of the two current effects, namely, the nonequilibrium longitudinal spin injection and the transversal spin-transfer surface torque. Dispersion relation for fluctuations is derived and solved. Nonlinear problem is solved about steady state arising due to instability for a thick enough free layer., Comment: 9 pages

Joint influence of two effects, namely, nonequilibrium spin injection by current, and current induced surface torque, on spin-valve type ferromagnetic metallic junctions is considered theoretically. The CPP configuration is assumed. The consideration is based on solving a coupled set of equations of motion for the mobile electron and lattice magnetizations. Boundary conditions are derived from the total magnetization flux continuity condition. A dispersion relation is derived for current dependent spin-wave fluctuations. The fluctuations become unstable under current density exceeding some threshold value. Joint action of the longitudinal spin injection and the torque lowers the instability threshold. The spin injection softens spin wave frequency near the threshold and can pin magnetization at the injecting contact. The pinning rises under the current increasing, so that the appearance of new spin-wave resonance lines can be observed., Comment: 25 pages, 4 figures

A possibility is discussed of observing spin injection effect on the ferromagnet domain structure by means of resonant excitation of the domain wall oscillations by a spin-polarized ac injection current. The natural frequency of the domain wall oscillations in a thin ferromagnetic film with parallel anisotropy is calculated. Amplitude of the domain wall forced oscillations excited by the spin-polarized ac current is determined. Then effect of such oscillations on the current is considered and appearance of nonlinear phenomena such as rectification of the ac current and second harmonic generation is predicted., Comment: 7 pages, 2 figures

The magnetic reversal by spin-polarized current of a magnetic junction consisting of two ferromagnetic layers and a nonmagnetic spacer in between is considered. Initially, the free layer is magnetized antiparallel to the pinned layer by an external magnetic field. Under current flowing, a nonequilibrium spin polarization appears in the free layer. The interaction between the injected spins and the lattice leads to instability of the antiparallel orientation at the high enough current density and to switching the free layer to a state with magnetization parallel to one in the pinned layer. If the free layer thickness and the external magnetic field strength are large enough, then a nonuniform switching is favorable, so that only a part of the free layer near the injector switches. Such a switching is accompanied with appearance of a domain wall between the switched and non-switched regions. The domain wall can oscillate around the equilibrium position with some natural frequency depending on the external magnetic field., Comment: 14 pages, 4 figures, corrected typos, added authors' addresses

Magnetic junction is considered which consists of two ferromagnetic metal layers, a thin nonmagnetic spacer in between, and nonmagnetic lead. Theory is developed of a magnetization reversal due to spin injection in the junction. Spin-polarized current is perpendicular to the interfaces. One of the ferromagnetic layers has pinned spins and the other has free spins. The current breaks spin equilibrium in the free spin layer due to spin injection or extraction. The nonequilibrium spins interact with the lattice magnetic moment via the effective s-d exchange field, which is current dependent. Above a certain current density threshold, the interaction leads to a magnetization reversal. Two threshold currents are found, which are reached as the current increases or decreases, respectively, so that a current hysteresis takes place. The theoretical results are in accordance with the experiments on magnetization reversal by current in three-layer junctions Co/Cu/Co prepared in a pillar form., Comment: 9 pages,2 figures, PDF; corrected typos

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., Comment: 8 pages, 2 figure. Submitted to Phys. Rev. B