A. I. Panas

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.

Current-induced inverse population of spin subbands in magnetic junctions

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.

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.

Current-induced spin injection from probe to ferromagnetic film

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 irreversible magnetization switching and bistability effects in ferromagnetic junctions

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.

Current in a system formed by a microprobe and a thin-film layered structure

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-injection terahertz generator based on the metatransition monolithic structure

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.

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.

Mode pulling and competition in a system with chaotic dynamics

Abstract The classical effects of mode pulling and switching in oscillatory systems with two or more degrees of freedom are shown to acquire qualitatively new features when a system with regular dynamics is replaced by systems with strange attractors.

Spin-Injection Stimulated Radiation of Terahertz Waves in Magnetic Junctions (“Twaser” Action)

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.