Экспериментально исследована генерация электромагнитных волн ТГц-диапазона током в планарных структурах, из двух ферромагнитных пленок. Ток пронизывает оба слоя при контакте пленок с тонким медным стержнем. Первый слой � инжектор, он поляризует спины носителей тока; второй � рабочий слой, в нем создается инверсия спиновой населенности и генерация ТГц-волн. При токе 350 мА длина волны излучения 16.4 ТГц, мощность 10 мВт при комнатной температуре.

Spin-polarized current can tilt magnetic sublattices in an antiferromagnetic layer of a ferromagnet-antiferromagnet junction without imposing any external magnetic field. Thus, the current can induce magnetization in the antiferromagnetic layer. This effect seemingly took place in experiments on the observation of terahertz emission at the precession frequencies of current-induced magnetization. In this work, the mechanisms of motion of the induced magnetization, as well as the possibility of detection and generation of radiation emitted during such motion, are suggested and discussed. The notion of spin-injection antiferromagnetic resonance is introduced.

The spin-injection emission of the system that consists of a nonmagnetic conducting rod and two-layer planar heteroepitaxial structure formed by thin metal and magnetite films is experimentally studied. The electromagnetic emission in the terahertz range is demonstrated for the first time due to the creation of the nonequilibrium population of the spin energy subbands (relatively low current densities of about 105 A/cm2 are needed at room temperature).

The effect of spin-polarized current on the steady-state magnetization and oscillations of antiferromagnet magnetization in a ferromagnetic-antiferromagnetic magnetic junction is analyzed. The macrospin approximation is generalized to describe antiferromagnets. The canted configuration of the antiferromagnet and the resultant magnetic moment are produced by the application of an external magnetic field. The resonance frequency, damping, and threshold current density corresponding to the emergence of instability are calculated. The possibility of generating weakly damped magnetization oscillations in the terahertz range is demonstrated. The effect of fluctuations on the canted configuration of the antiferromagnet is discussed.

A ferromagnet-antiferromagnet junction in the presence of an in-plane magnetostatic field and a spin-polarized current flowing perpendicularly to the junction layers is considered. In the macrospin approximation, the system of nonlinear equations describing the dynamics of magnetization of the antiferromagnet layer in such a junction is numerically solved with allowance for the spin torque transfer and the spin equilibrium disturbance caused by spins injected by the current into the antiferromagnet. It is shown that, when the current exceeds a certain threshold, the magnetization becomes instable and that, beyond the instability region, the results are in complete agreement with the theory using linearization in small deviations from the equilibrium. It is found that the development of instability causes switching from the antiparallel configuration to the parallel one and that, in the instability region, nondecaying oscillations of the longitudinal and transverse components of the antiferromagnet magnetization are formed.

The response of a ferromagnet-antiferromagnet junction to a high-frequency magnetic field is calculated as a function of the spin-polarized current density through the junction. Conditions are chosen 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.

The stability is analyzed of the magnetic junction collinear configurations against small fluctuations under amplitude-modulated current with current-perpendicular-to-plane 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 direct 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 direct current. The parametric instability manifests itself as lowering the threshold of the direct current density in the presence of the high-frequency current; such an effect has been observed in experiments repeatedly.

Two channels of the s-d exchange interaction are considered in magnetic junctions. The first channel describes the interaction of transversal spins with the lattice magnetization. The second channel describes the interaction of longitudinal spins with the magnetization. We show that 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 sufficiently large distortion of equilibrium due to the current- driven spin injection leading to inversion of populations of the energy spin subbands and laser-like instability in the THz frequency range at room temperature. External magnetic field is likely to tend to additionally lower the instability threshold due to the proximity effect of purely magnetic reorientation phase transition. This effect demonstrates 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 appeared in the condition, namely, the so called "spin resistances" , where the a number of a layer in the junction. For a three-layer junction ( ), the matching condition is , where the number corresponds to the main functional layer. We investigated also the junctions having variable lateral dimensions of the layers, for example, a ferromagnetic rod contacting with a very thin ferromagnetic film. A large enhancement of the current density can appear near the contact region, leading to the spin injection luminescence.

The process of magnetic junction switching by a spin-polarized current pulse in the presence of an external magnetic field has been numerically simulated at the current densities and magnetic fields below the corresponding threshold values for separate effects. It is established that the switching can be performed with controlled delay relative to the current pulse.

A tunnel magnetic junction is considered with magnetic hard and magnetic soft layers of cubic symmetry. The magnetic switching of the layers is analyzed for a magnetic field perpendicular to the initial magnetizations. In such a situation, an additional peak in the tunnel magnetoresistance ratio appears at the magnetic field value that is substantially lower than the anisotropy field of the soft layer.

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.

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.

The current flow is investigated in the structures consisting of a ferromagnetic metal (FM) an antiferromagnetic (AFM) conductor and a nonmagnetic metal that closes the electric circuit. In such a structure, the FM layer is characterized by a relatively strong anisotropy and fixed lattice spins and serves as the injector of spins into the AFM layer. In the presence of a relatively strong external magnetic field, the resulting magnetization is induced in the AFM layer and the magnitude of the magnetization can be controlled. It is demonstrated that the polarized current from the FM layer generates a torque and allows the exchange switching of the magnetization vector. The distinctive feature of the effect lies in the fact that the switching threshold with respect to current can be decreased by several orders of magnitude due to the dependence of the magnetization on the external field

The energy efficiency of the spin-injection emitter based on the rod-film ferromagnetic structure with a point contact between the components in the terahertz frequency range is experimentally estimated. The film material substantially affects the efficiency. A concentrator of magnetic flux is proposed for an increase in the efficiency. The experimental estimations allow the quantum yield of the emitter that is greater than unity. This result indicates significant contribution of the stimulated radiative transitions.

Electromagnetic radiation of 1–10 THz range is observed 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 components of the structure. 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 taken into account) and follows the current without inertia.

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.

The stability of equilibrium configurations of magnetic junctions with a free layer of cubic anisotropy with two axes in the layer plane has been analyzed. Variants of switching between different configurations have been considered. It has been demonstrated that the possibility exists of considerably decreasing the threshold current density necessary for switching. Numerical simulation of the dynamics of switching between different configurations has been performed.

The distribution of the spin polarization of conduction electrons is calculated under the conditions for the spin injection from a probe to a ferromagnetic film in the presence of current. It is demonstrated that the main parameters that determine the deviation of the spin polarization from equilibrium are the current density and spin polarization of the probe material, whereas the ratio of the probe diameter to the spin diffusion length weakly affects the result in certain ranges. The population inversion of the spin subbands can be reached at distances of about spin diffusion length from the probe-film interface.

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.

Properties are discussed of a ferromagnetic junction in the form of a rod contacting with a film. Very high current density of the order of j ∼ 109 A/cm2 may be achieved in the contact region that can lead to inversion of population of the spin energy subbands. The inversion 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, the radiation intensity being different for forward and backward currents. It shows that the radiation includes not only thermal contribution but also non-thermal (spin-injection) one.