G. M. Mikhailov

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.

Analysis of the frequency characteristics of the spin-injection emitter in the terahertz range using a diffraction grating

A method for the measurement and identification of the radiation frequencies of the spin-injection oscillator in the terahertz frequency range using a diffraction grating is developed and experimentally implemented. For the system consisting of a ferromagnetic rod (Fe) and nanosized ferromagnetic film (Fe3O4 with a thickness of up to 100 nm), the thermal and dynamic components of the radiation that is induced by the current flow are separated. The diffraction peaks at a frequency of about 9.6 THz and a band of 37% at a level of 3 dB and at a frequency of 43.5 THz are demonstrated. It is shown that the intensity of each signal is significantly higher than the thermal background level.

Terahertz emission in the ferromagnetic-antiferromagnetic structure

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.

sd-Exchange emission in ferromagnetic 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-injection radiation of terahertz waves in ferromagnetic structures

We propose and discuss the mechanisms of terahertz sd exchange radiation during transmission of the spin-polarized current in a structure from a thin steel rod pressured to an ultrathin ferromagnetic film. The two mechanisms considered are quantum transitions between the quasi-Fermi levels in the film near the rod and precession of injected spins around the film magnetization.

Spin-injection stimulated emission of terahertz waves in magnetic junctions

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.

Detection and generation of submillimeter and terahertz modes in ferromagnet-antiferromagnet 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.

Efficiency of the terahertz spin-injection emitter

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.

MAGNETIC EPITAXIAL NANOSTRUCTURES FROM IRON AND NICKEL

Magnetoresistance of a planar single crystalline Ni nanostructures has been measured at room and liquid helium temperatures. Bending GMR effect increases from 4% under diffusive to more than 200% under ballistic electron transport in cross-type nanostructures. Magnetoresistance measured for longitudinal and transverse magnetic fields are qualitatively different and explained by a nonsymmetrical magnetic domain structure in the central part of the cross.

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.