G. A. Melkov

Excitation of self-localized spin-wave bullets by spin-polarized current in in-plane magnetized magnetic nanocontacts : A micromagnetic study

Abstract : It was shown by micromagnetic simulation that a current-driven in-plane magnetized magnetic nanocontact, besides a quasilinear propagating Slonczewski spin-wave mode, can also support a nonlinear self-localized spin-wave bullet mode that exists in a much wider range of bias currents. The frequency of the bullet mode lies below the spectrum of linear propagating spin waves, which makes this mode evanescent and determines its spatial localization. The threshold current for the excitation of the self-localized bullet is substantially lower than for the linear propagating mode, but finite-amplitude initial perturbations of magnetization are necessary to generate a bullet in our numerical simulations, where thermal fluctuations are neglected. Consequently, in these simulations the hysteretic switching between the propagating and localized spin-wave modes is found when the bias current is varied.

Parametric interaction of magnetostatic waves with a nonstationary local pump

A solution is obtained for the general problem of the nonstationary interaction of backward volume magnetostatic waves in films of yttrium-iron garnet with local parametric pumping. In the case of a large pump region, l≫λ, where λ is the wavelength of the backward volume magnetostatic waves, the problem reduces to a system of truncated equations for two packets of counter propagating waves. In the opposite case, l<λ, the exact problem of parametric interactions of the eigenmodes of a ferrite film (both counterpropagating and in the same direction) is solved numerically. Both cases are studied experimentally and good qualitative and quantitative agreement is obtained with the theory. For the first time, the reversal of a wave front and the time reversal of the shape of backward volume magnetostatic wave pulses are observed and a change in the propagation time for the peak of the signal pulse and a reduction in its width owing to pumping are recorded. Two operating regimes are identified for a nonstationary parametric backward volume magnetostatic wave amplifier with local pumping, which differ in the ratio of the duration of the pump pulse to the transit time for the wave through the local pump region, and the effect of the parametric excitation of two-dimensional spin waves on the interaction of backward volume magnetostatic waves with a local nonstationary parametric pump is determined.

Nano-structured magnetic metamaterial with enhanced nonlinear properties

Nano-structuring can significantly modify the properties of materials. We demonstrate that size-dependent modification of the spin-wave spectra in magnetic nano-particles can affect not only linear, but also nonlinear magnetic response. The discretization of the spectrum removes the frequency degeneracy between the main excitation mode of a nano-particle and the higher spin-wave modes, having the lowest magnetic damping, and reduces the strength of multi-magnon relaxation processes. This reduction of magnon-magnon relaxation for the main excitation mode leads to a dramatic increase of its lifetime and amplitude, resulting in the intensification of all the nonlinear processes involving this mode. We demonstrate this experimentally on a two-dimensional array of permalloy nano-dots for the example of parametric generation of a sub-harmonic of an external microwave signal. The characteristic lifetime of this sub-harmonic is increased by two orders of magnitude compared to the case of a continuous magnetic film, where magnon-magnon relaxation limits the lifetime.

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(100u2009nm)/Pt(10u2009nm) bilayer of 4u2009×u20092u2009mm2 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.

Quantum coherence due to Bose-Einstein condensation of parametrically driven magnons

The room-temperature kinetics and thermodynamics of the magnon gas driven by microwave pumping has been investigated by means of the Brillouin light scattering (BLS) technique. We show that for high enough pumping powers the quantum relaxation of the driven gas results in a quasi-equilibrium state described by the Bose–Einstein statistics with a nonzero chemical potential. Further increase of the pumping power causes a Bose–Einstein condensation in the magnon gas documented by an observation of the magnon accumulation at the lowest energy level. Using the sensitivity of the BLS to the coherence degree of the scattering magnons, we confirm the spontaneous emergence of coherence of the magnons accumulated at the bottom of the spectrum, if their density exceeds a critical value.

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=−11.7u2009dBu200aW.

Microwave spectral analysis by means of nonresonant parametric recovery of spin-wave signals in a thin magnetic film

We report on the storage and nonresonant parametric recovery of microwave signals carried by a dipolar surface spin-wave pulse in a thin ferrimagnetic film. The information about the intensity of the spectral components of the signal within a narrow frequency band is saved due to the excitation of a dipolar-exchange standing spin-wave mode across the film thickness and is afterward restored by means of parametric amplification of this mode. The intensity of the restored signal measured for varying shifts between the signal carrier frequency and half of the pumping frequency, which is equal to the frequency of the standing mode, reveals information about the entire frequency spectrum of the input microwave signal.

Monochromatic microwave radiation from the system of strongly excited magnons

A conversion of a broadband microwave energy accumulated by a system of strongly excited magnons into a monochromatic microwave is demonstrated. The mechanism is based on recently discovered room-temperature Bose–Einstein condensation of nonequilibrium magnons. A realization of an electronically tunable microwave generator pumped by an incoherent broadband sources and the achievable linewidth of the monochromatic radiation are discussed.

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.