Yu. V. Kobljanskyj

Spin-transfer torque based damping control of parametrically excited spin waves in a magnetic insulator

The damping of spin waves parametrically excited in the magnetic insulator Yttrium Iron Garnet (YIG) is controlled by a dc current passed through an adjacent normal-metal film. The experiment is performed on a macroscopically sized YIG(100 nm)/Pt(10 nm) bilayer of 4 × 2 mm2 lateral dimensions. The spin-wave relaxation frequency is determined via the threshold of the parametric instability measured by Brillouin light scattering spectroscopy. The application of a dc current to the Pt film leads to the formation of a spin-polarized electron current normal to the film plane due to the spin Hall effect. This spin current exerts a spin transfer torque in the YIG film and, thus, changes the spin-wave damping. Depending on the polarity of the applied dc current with respect to the magnetization direction, the damping can be increased or decreased. The magnitude of its variation is proportional to the applied current. A variation in the relaxation frequency of ±7.5% is achieved for an applied dc current density of...

Microwave absorption properties of permalloy nanodots in the vortex and quasi-uniform magnetization states

When the in-plane bias magnetic field acting on a flat circular magnetic dot is smaller than the saturation field, there are two stable competing magnetization configurations of the dot: the vortex and the quasi-uniform (C-state). We measured microwave absorption properties in an array of non-interacting permalloy dots in the frequency range 1–8GHz when the in-plane bias magnetic field was varied in the region of the dot magnetization state bi-stability. We found that the microwave absorption properties in the vortex and quasi-uniform stable states are substantially different, so that switching between these states in a fixed bias field can be used for the development of reconfigurable microwave magnetic materials.

Effective microwave ferrite convolver using a dielectric resonator

A highly effective microwave convolver with extended frequency range is proposed and experimentally tested. This convolver uses nonlinear interaction of two dipolar spin wave (or backward volume magnetostatic wave) signals contrapropagating from the ends in a ferrite film placed inside an open dielectric resonator. The output signal, proportional to the convolution of the input signals, is received by the resonator, and is transmitted to a load through a regular waveguide. The proposed convolver does not have a strict upper limit for signal frequency, and at a signal frequency of 4.67 GHz has a record value of bilinearity coefficient of B=−11.7 dB W.

Nonlinear amplification and compression of envelope solitons by localized nonstationary parametric pumping

Amplification of envelope solitons by localized parametric pumping has been investigated. The possibility of extremely large single-soliton amplification (above the theoretical limit for an ideal linear amplifier) due to signal compression is predicted theoretically and demonstrated experimentally for backward volume magnetostatic waves in yttrium–iron–garnet films. In the case of a strongly localized nonstationary pumping signal, compression occurs due to the amplification of a part of the signal. In the case of a quasiuniform parametric pumping signal compression results from the development of collective oscillations of parametrically coupled spin waves. A single-soliton amplification gain of 17 dB has been obtained experimentally.

Microwave signal processing using dipole-exchange spin waves

The possibility of the application of dipole-exchange spin waves (DESW) in ferrite (yttrium-iron garnet or YIG) films for microwave signal processing is investigated. The short-wavelength DESW were excited as a result of scattering of an input long-wavelength dipolar spin wave pulse on inhomogeneities in the YIG film.

Parametric interaction of dipolar spin wave solitons with localized electromagnetic pumping

A new nonlinear method of effective amplification of single envelope solitons is discussed. This method combines amplification of a signal pulse with its simultaneous compression. It was found that the maximum single-soliton amplification gain can be two times larger than the pulse compression rate, and for large compression rates can significantly exceed the theoretical limit for an ideal linear amplifier. The proposed method was experimentally tested on envelope solitons of dipolar spin waves (backward volume magnetostatic waves), propagating in yttrium-iron garnet films and parametrically interacting with localised and pulsed electromagnetic pumping. The single-soliton amplification gain of 17 dB was obtained. The compression of spin wave pulses was realised both by amplification of only the central part of the pulse, and as a result of collective oscillations of parametrically coupled spin waves.

Formation of Bose–Einstein magnon condensate via dipolar and exchange thermalization channels

Thermalization of a parametrically driven magnon gas leading to the formation of a Bose–Einstein condensate at the bottom of a spin-wave spectrum was studied by time- and wavevector-resolved Brillouin light scattering spectroscopy. Two distinct channels of the thermalization process related on dipolar and exchange parts of a magnon gas spectrum are clearly determined. It has been found that the magnon population in these thermalization channels strongly depends on applied microwave pumping power. The observed magnon redistribution between the channels is caused by the downward frequency shift of the magnon gas spectrum due to the decrease of the saturation magnetization in the course of injection of parametrically pumped magnons.