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The observation of the stimulated terahertz radiation in magnetic junctions at the room temperature resulting from the current injection of nonequilibrium spins is reported. Contacts between two ferromagnetic layers and between a ferromagnetic and an antiferromagnetic layer are investigated. As the current is increased, first, a narrow isolated radiation peak at terahertz frequencies is observed, which is analogous to the laser peak; then, the peak broadens and its intensity increases by more than an order of magnitude.

The possibility of generation of terahertz electromagnetic waves by the current in a planar structure composed of metalferromagnetic nanoparticles imbedded into an antiferromagnetic medium, is considered.

We report on apparently the first observation of terahertz stimulated radiation in magnetic junctions due to nonequilibrium spins injected by current at room temperatures. The contact between ferromagnetic layers is investigated and so the contact between ferromagnetic and antiferromagnetic layers. The current flowing in the structure creates an inversed population of spin energy levels. After a placing of such a structure in a resonator positive feedback and stimulated emission appears. When current is growing, we observe the relatively narrow (in frequencies) peak - analogous to laser generation but at terahertz frequencies. For the higher currents we observe the picture of electromagnetic turbulence.

Fundamentally new devices for the generation of terahertz waves due to the effect of the spin-polarized current in ferromagnetic and antiferromagnetic metal structures (magnetic junctions) are considered. The spin injection and the sd exchange and its effect on the interaction with electromagnetic waves play the main role in such devices. The effects that are induced by the polarized current in the antiferromagnetic materials (exchange instability, induced magnetization, and spin-injection antiferromagnetic resonance) are discussed. The principles and effects of the generation of the terahertz electromagnetic oscillations are considered, and the characteristics of the experimental prototypes of the corresponding devices are analyzed.

The magnetostatic field distribution in a structure formed from a ferromagnetic (FM) rod and thin FM film is numerically calculated. The effective terahertz radiation was previously discovered in such a structure in the presence of a current. It is shown that nanodimensional sections of the boundary between the lateral surface of the rod and the surface of the film can be a source of radiation. On these sections, the magnetostatic field changes its direction, a circumstance that provides for conditions for radiative transitions of electrons.

The stimulated emission of terahertz radiation in magnetic junctions at room temperature resulting from the current injection of nonequilibrium spins is observed. Contacts between two ferromagnetic layers as well as between a ferromagnetic and an antiferromagnetic layer are investigated. The current produces the population inversion of spin energy levels. As the system is placed in a cavity, positive feedback and stimulated emission appear. The development of a dominant emission peak and onset of electromagnetic turbulence are observed.

The terahertz region of the electromagnetic spectrum (approximately 0.3–30 THz) is still insufficiently mastered primarily because of the absence of compact and controllable emitters (oscillators) and receivers (detectors) reliably operating in this range in a wide temperature range, including room temperature. The corresponding recent studies in this field, which were supported by the Russian Foundation for Basic Research, have been reviewed. New physical effects have been proposed and principles of the operation of terahertz devices based on these effects have been implemented. These effects refer to the physics of ferromagnetic and/or antiferromagnetic conducting layers assembled in micro- and nanostructures, which are called magnetic junctions. These effects are as follows: the formation of a quasiequilibrium distribution of current-injected electrons over the energy levels and the possibility of inverted population of levels, induction of the macroscopic magnetization by a spin-polarized current in an antiferromagnetic layer in the absence of external magnetic field, the appearance of current-induced contribution to antiferromagnetic resonance, and the experimental observation and study of the properties of terahertz radiation in ferromagnet-ferromagnet and ferromagnet-antiferromagnet junctions.

The spin-injection emission in a ferromagnetic junction at terahertz frequencies is theoretically analyzed. It is demonstrated that the efficiency of the emission caused by the sd-exchange interaction of the injected spin in which the electromagnetic field is involved strongly depends on the orientation angle of the magnetization of the active region relative to the magnetization direction of the injecting region. The fact that the calculated radiation frequency and power are close to the experimental results shows that the sd-exchange emission must be taken into account in the interpretation of the observed terahertz emission in magnetic junctions.

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 change in the direction of the circumferential lattice magnetization by using the spin-polarized current is found by magnetic force microscopy to be possible for square epitaxial Fe (001) microstructures in the basic state. Such a change of the magnetization occurs when applying the magnetic saturation field to the structure with the subsequent switching off of the field accompanied by passing the current with the density above a certain threshold on the order 1012 A/m2 through the structure. In the basic state of the structure, the change of the magnetization depends not only on the current density but also on the spin polarization and the region of the current flow. The effect of the spin-polarized current on the circumferential magnetization of the structure, which is related to the injection of spins by the current, is discussed.

Current flow in the structure consisting of a hard-magnetic ferromagnetic rod (hardened steel) with a diameter of up to 100 μm that contacts a thin (10–100 nm) ferromagnetic (FeMn) film is studied at room temperature. Effective magnetization is induced in the FeMn film in the presence of the spin-polarized current in spite of the absence of the intrinsic magnetization. The terahertz radiation that is detected in this work is caused by the precession of the induced magnetization. The emission is interpreted using the recent theoretical predictions on the effect of the sd exchange on the lattice dynamics of antiferromagnetic layer.

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.

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 taking into account) and follows the current without inertia. Efficiency of the oscillator depends strongly on the material used and quantum efficiency may exceed the unity. It means the stimulated radiation processes play an important role.

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 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.

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.

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

Results of numerical simulation of switching of a magnetic transition induced by a spin-polarized current pulse in the presence of an external magnetic field for a current density and a field strength that are lower than the corresponding threshold values are presented. The magnetic field regenerates the system and drives it to the threshold of an orientation phase transition, which simplifies exchange switching induced by a current pulse. This switching is characterized by the same locality as the exchange interaction, i.e., it is local at atomic lengths. It is demonstrated that there exists the possibility of switching with a controllable delay with respect to the current pulse.

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 possibility of resonance absorption in the terahertz range caused by the sd-exchange interaction at the incidence of an electromagnetic wave on a ferromagnetic metal has been predicted. The absorption coefficient has been calculated. It has been shown that the resonance frequency is determined by the magnetization of a ferromagnet and the absorption coefficient additionally depends on the orientation of the magnetization with respect to the plane of polarization of the wave.

The generation of electromagnetic THz waves by a current is experimentally investigated in planar structures comprised of two ferromagnetic films. When the films come into contact with a thin copper rod, the current is applied to both layers. The first layer, which plays the role of an injector, polarizes the spins of the current carriers. In the second working layer, the conditions are created for spin population inversion and the generation of THz waves. At a current of 350 mA, the radiation wavelength is 16.4 THz and a power of 10 mW is reached at room temperature.

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.

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.

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.

The dependence of the anisotropic magnetoresistance (AMS) of the single-crystal Fe(001) film grown with the aid of the pulsed laser evaporation on the direction and magnitude of the external magnetic field is analyzed. Three ranges of magnetic fields with different types of variation in AMS with the direction of magnetic field are revealed. The discontinuities of resistance and the hysteresis are demonstrated in a certain range of magnetic fields. The anisotropy field is estimated using the experimental results.

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.

The current in a system formed by a conducting microprobe and a thin-film layered structure is experimentally studied. The possibility of a stable current flow with a density of up to 3 × 106 A/cm2 is demonstrated. It is found that, in order to attain an operation mode with a high current density, a sufficiently strong current (∼200 mA) must be first transmitted through the sample. The current distribution in the studied structure is calculated. The theoretical distribution is found to be in agreement with experimental results.

A spin valve with two pinned ferromagnetic layers sandwiching a free ferromagnetic layer with a thickness smaller than the spin diffusion length in the same layer and than the domain wall thickness is considered. The instability conditions are determined for various mutual orientations of the magnetization of the layers. The possibility of a considerable decrease in the instability threshold due to joint action of spin-polarized current and an external magnetic field is indicated. It is shown that in addition to collinear states, a nonequilibrium noncollinear state can exist, into which the system is switched after exceeding the instability threshold.

A review is given on the exchange switching of ferromagnetic metallic junctions under the effect of a low threshold current. A dramatic (orders of magnitude) threshold current reduction is achieved under conditions that include the dominance of the current-driven nonequilibrium spin injection, the optimum relation between spin resistances of the layers, and the application of an external magnetic field near the reorientation phase transition point.

We study the passage of transverse current through a ferromagnetic nanojunctions, viz., a layered nanostructure of the spin-valve type containing two ferromagnetic layers separated by a spacer that prevents exchange coupling between the layers in the absence of current, but does not affect spin polarization of the current. The conditions for a high level of injection of spins by current are derived at which the concentration of injected nonequilibrium spins can reach or even exceed their equilibrium concentration. In such conditions, a number of new effects are observed. The threshold of exchange switching by current is lowered by several orders of magnitude due to matching of spin resistances of the layers. The application of an external magnetic field in the vicinity of the orientation phase transition additionally lowers this threshold. This leads to multistability, in which one value of the current corresponds to two (or more) stable noncollinear orientations of magnetization, and switching itself becomes irreversible. A methodical feature of this research is that the calculation is performed in the so-called macrospin approximation, which is in good agreement with most of known experiments. In this approximation, the equations of motion taking into account the torque as well as spin injection are derived for the first time and solved.

A perpendicular current that flows through a ferromagnetic nanojunction (a layered nanosized spinvalve structure containing two ferromagnetic metal layers separated by a spacer) is studied. The spacer is thick enough to prevent the exchange coupling of the layers in the absence of the current. On the other hand, the spacer is sufficiently thin and does not affect the spin polarization in the presence of the current. Conditions for a relatively high current-induced spin injection are found. Under such conditions, the concentration of the injected nonequilibrium spins can be equal to or even greater than the equilibrium concentration and several new effects are observed: The threshold of the exchange switching by the current decreases by several orders of magnitude owing to the matching of the spin resistances of the layers, the threshold additionally decreases in the presence of an external magnetic field in the vicinity of the orientational phase transition, and multistability emerges (several stable noncollinear orientations of magnetization correspond to a single value of the current). Switching appears to be irreversible.

It is shown that, in ferromagnetic nanojunctions, i.e., conducting layered structures containing no less than two ferromagnetic layers, a high level of spin injection by a perpendicular current can be achieved. In this case, the structures exhibit fundamentally new interesting properties, specifically, an irreversible magnetization switching and a bistability.

The nonequilibrium spin polarization of electrons is calculated under the conditions of spin injection by current passing from one ferromagnetic film to another in a magnetic junction. It is shown that the nonequilibrium spin concentration may substantially increase with an appropriate choice of parameters of the films and operation conditions. This leads to a decrease in the threshold current density required for switching the magnetic junction by several orders of magnitude.

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., 10 pages, 2 figures. Submitted to J. Magn. Magn. Mater. Corrected typos

The spin flux matching conditions that should be satisfied at the boundaries between contacting layers of a magnetic junction to ensure the inverse population of spin subbands at experimentally achievable current densities of −107–108 A/cm2 are determined. The essence of the conditions is that an efficient spin injection takes place at the input boundary, whereas the injection at the output boundary is blocked. The fulfillment of the matching conditions for contacting layers with antiparallel magnetization orientations leads to stability in magnetic fluctuations for any forward currents.

The theory of exchange current switching in ferromagnetic nanojunctions is developed. Included in a junction are two contacting layers: a layer with fixed lattice magnetization and a free layer. The theory takes into account two concurrent effects induced by a polarized current in the junction: (1) transfer of transverse spins from the current carriers directly to the magnetic lattice and (2) injection of longitudinal spins into the free layer and the subsequent generation of a nonequilibrium sd-exchange field. A vector condition that requires continuity of the total spin flux of the electrons and the lattice is applied at the layer boundaries. The equation of motion that meets the vector boundary condition is derived for the magnetization values. The obtained solution exhibits specific features characteristic of inverse switching: Switching occurs when electrons are moving in an efficient injection field and, at the same time, involves transfer of torque to the lattice.

The exchange switching of spin valves by an inverse current can be explained by the interaction of the charge carriers with the spin-injection effective magnetic field. Such an interaction gives rise to transverse spin components, which are transferred to the magnetic lattice and cause its instability and switching. The spin-injection field is produced by longitudinal spin components, but it opens up a channel for the transverse spin transfer to the lattice. The spin transfer to the lattice and the switching occur in the free layer of the spin valve.

A stationary state of a spin-valve magnetic junction under the conditions of current-induced instability is studied. It is shown that switching is nonuniform if the switchable free-layer thickness exceeds the longitudinal spin diffusion length. The given-voltage mode is considered. The angle between the magnetization vectors of adjacent magnetic layers in the switched state is found as a function of the applied voltage. The current-voltage (I-V) characteristic of a switched junction is calculated. It is predicted that, if the relative magnetoresistance is sufficiently high, switching exhibits hysteretic behavior leading to a bistable I-V characteristic.

Mechanisms are determined for homogeneous current-induced switching in ferromagnetic metallic junctions of the spin-valve type. The spin flux is perpendicular to the junction surface and is directed from the pinned layer to the free one. The switching is due to the joint action of two mechanisms: (i) current-induced surface torque and (ii) current-induced injection of nonequilibrium longitudinal spins into the free-layer bulk. Both mechanisms lead to the instability and reorientation of magnetization; however, only the injection mechanism can stabilize the switching.