P. E. Zilberman

Spin-injection terahertz radiation in magnetic junctions

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.

Disturbance of spin equilibrium by current through the interface of noncollinear ferromagnets

Boundary conditions are derived that determine the penetration of spin current through an interface of two noncollinear 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.

Effect of current on magnetization oscillations in the ferromagnet-antiferromagnet junction

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.

Macrospin in ferromagnetic nanojunctions

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.

Generation of terahertz waves by a current in magnetic junctions

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.

Current-induced resonance in ferromagnet-antiferromagnet junctions

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.

Surface spin-transfer torque and spin-injection effective field in ferromagnetic junctions: Unified theory

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.

Moving of domain walls by spin polarized current

Summary form only given. On the base of a phenomenological theory we considered here a model of a three layer magnetic junction, consisting of two ferromagnetic layers (electrodes) separated by a nonmagnetic ultrathin layer (spacer). One of the electrodes has a magnetization pinned parallel to interfaces along z-axis and the other one is free and may contain a magnetic domain structure. An electrical current may be present that transports carriers through the spacer from the pinned layer to the free one. No spin-dependent surface scattering exists and current affects the magnetic state of the free layer due to injection of the spins only. We found a solution of continuity equations for carriers having up and down spins that satisfy boundary conditions at interfaces of the free electrode. Current dependent density and magnetization of the injected spins were calculated. This allows us to find the current dependent energy of exchange interaction between carriers and localized spins (s-d interaction). Experiments were conducted with three-layer tunnel junctions of a type Ni/S/Fe, where S is a spacer of diamond-like carbon or polyimide films with thickness /spl sim/8 nm and cross section /spl sim/100 /spl mu/m/sup 2/. The observations from the theoretical and experimental studies described here may be considered as a manifestation of domain walls moving by a spin-polarized current.

Magnetoelastic waves in ferromagnet plates and films

The explosive development of investigations of magnetization waves in ferrite layers (plates and films) that occurred in recent years resulted in significant deepening and expansion of our conceptions of the effects of the interaction between such magnetization waves and elastic waves. The possibility of synchronization and hybridization of magnetization waves and elastic waves was shown in theory and experimentally for phase velocities of these waves in the direction of propagation along the ferrite layer exceeding the sound speed in the ferrite by one to two orders. At such phase velocities the inhomogeneous exchange energy often turns out to be insubstantial for magnetization wave propagation, and only the dipole energy is substantial. Therefore, we speak substantially about a new kind of magnetoelastic waves, the result of hybridization of pure magnetodipole and elastic waves. Such waves have been designated “fast magnetoelastic waves.” A brief survey of the theoretical, experimental, and applied investigations of fast magnetoelastic waves is contained in this paper. In the section “Theory” we present and discuss the fundamental equations, boundary conditions, and approximations necessary for the computation of fast magnetoelastic wave properties and the effects accompanying their propagation. The results of such computations currently known are presented. In the section “Experiments” we describe the fundamental requirements imposed on the specimens and the measurement methodology to detect and investigate the properties of fast magnetoelastic waves. Results of measurements executed at the present time as well as the prospects for further development in this direction are discussed.

sd-Exchange electron spin resonance in a ferromagnetic metal

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.