Microwave detectors based on the spin-torque diode effect

The spin-transfer torque (STT) effect provides a new method of manipulation of magnetization in nanoscale objects. The STT effect manifests itself as a transfer of spin angular momentum between the parallel magnetic layers separated by a nonmagnetic spacer and traversed by a dc electric current. The transfer of the spin angular momentum from one layer to another could result in the excitation of the microwave-frequency magnetization dynamics in one of the magnetic layers. On the other hand, when a magnetization dynamics is excited in a magnetic layered structure by an external microwave signal both the structure electrical resistance and current through the structure will acquire microwave components resulting in the appearance of a rectified dc voltage on the magnetic structure. This “spin-torque diode effect” can be used for the development of ultra-sensitive spintorque microwave detectors (STMD). Below we present a brief review of our recent work on the general properties of STMDs, analyze the performance of the “resonance-type” and “threshold-type STMD” and consider the possible applications for such microwave detectors

Dark solitons in ferromagnetic chains with first- and second-neighbor interactions

We study the ferromagnetic spin chain with both first- and second-neighbor interactions. We obtained the condition for the appearance and stability of bright and dark solitons for arbitrary wave number inside the Brillouin zone. The influence of the second-neighbor interaction and the anisotropy on the soliton properties is considered. The scattering of dark solitons from point defects in the discrete spin chain is investigated numerically.Comment: 7 pages,5 figure

Coherent Excitation of Heterosymmetric Spin Waves with Ultrashort Wavelengths

In the emerging field of magnonics, spin waves are foreseen as signal carriers for future spintronic information processing and communication devices, owing to both the very low power losses and a high device miniaturization potential predicted for short-wavelength spin waves. Yet, the efficient excitation and controlled propagation of nanoscale spin waves remains a severe challenge. Here, we report the observation of high-amplitude, ultrashort dipole-exchange spin waves (down to 80 nm wavelength at 10 GHz frequency) in a ferromagnetic single layer system, coherently excited by the driven dynamics of a spin vortex core. We used time-resolved x-ray microscopy to directly image such propagating spin waves and their excitation over a wide range of frequencies. By further analysis, we found that these waves exhibit a heterosymmetric mode profile, involving regions with anti-Larmor precession sense and purely linear magnetic oscillation. In particular, this mode profile consists of dynamic vortices with laterally alternating helicity, leading to a partial magnetic flux closure over the film thickness, which is explained by a strong and unexpected mode hybridization. This spin-wave phenomenon observed is a general effect inherent to the dynamics of sufficiently thick ferromagnetic single layer films, independent of the specific excitation method employed

Nonlinear Microwave Phenomena in Ferrite Films : Basic Physics and Applications (Invited)

A short overview of the theory and applications of nonlinear spin wave phenomena in ferrite films at microwave frequencies is given. A method of classical Hamiltonian formalism for spin waves in bulk ferrimagnets in combination with the theory of spin wave spectrum for thin magnetic films is used for theoretical description of nonlinear spin wave phenomena in films. Comparative roles of three-wave and four-wave interaction processes in the nonlinear dynamics of spin waves in films are discussed. A local character of spin wave excitation by the magnetic field of a strip-line antenna is taken into account. Several examples of nonlinear microwave signal processing devices based on the nonlinear dynamics of spin waves in ferrite films (frequency-selective limiter, signal-to-noise enhancer, convolver, and a soliton delay line) are presented

Dipolar localization of quantized spin-wave modes in thin rectangular magnetic elements

Dipole-exchange spectrum of quantized spin wave modes of a tangentially magnetized rectangular thin-film magnetic element is calculated using the method of tensorial Green’s functions. The strong inhomogeneity of the internal bias magnetic field along the magnetization direction leads to the localization of spin wave modes either at the edges (exchange localization) or at the center (dipolar localization) of the element. The mode intensity distributions along the other in-plane direction are determined by the dipolar boundary conditions and have the usual cosinusoidal form. The approximate theory developed in the paper gives a quantitative description of the resonance fields and qualitative description of the spatial distributions of quantized spin wave modes in a thin square permalloy element (in plane sizes 50×50 μm2, thickness 0.1 μm) recently observed by space-resolved Kerr spectroscopy. The theory shows also that the mode localization in this case (in the element center) is of the dipolar nature

Nonlinear Diffraction of Magnetostatic Waves in Ferrite Films

Two-dimensional nonlinear diffraction of magnetostatic waves (MSW) in ferrite films is studied numerically using the model of nonlinear parabolic equation. This equation is derived by a new method from a bilinear relation similar to the Lorentz lemma. It is shown, that in the framework of this model the evolution of two-dimensional backward volume MSW pulse leads to a collapse in the case of an infinite film, while in a film waveguide of a finite width the collapse can be avoided, and the pulse amplitude can be stabilized. Dynamics of modulation instability of both backward volume and forward volume MSW under continuous excitation is also investigated

Theory of magnetization dynamics in a dual-free-layer spin-torque nano-oscillator with isotropic free layers

The analytical theory of magnetization dynamics in a dual-free-layer spin-torque nano-oscillator (DFL STNO) operating in the absence of a bias magnetic field is developed. The theory is based on a system of simplified dynamic equations written in collective variables and yields an analytic expression for the upper dc current density threshold of stable magnetization dynamics (generation) in the DFL STNO. This threshold is caused by a dipolar coupling between the free layers and can be controlled by choosing an appropriate geometry of the DFL STNO structure

Observation of spatiotemporal self-focusing of spin waves in magnetic films

The first observation of spatiotemporal self-focusing of spin waves is reported. The experimental results are obtained for dipolar spin waves in yttrium-iron-garnet films by means of a newly developed space- and time-resolved Brillouin light scattering technique. They demonstrate self-focusing of a moving wave pulse in two spatial dimensions, and formation of localized two-dimensional wave packets, the collapse of which is stopped by dissipation. The experimental results are in good qualitative agreement with numerical simulations

Linear and Nonlinear Diffraction of Dipolar Spin Waves in Yttrium Iron Garnet Films Observed by Space- and Time-Resolved Brillouin Light Scattering,

A new advanced space- and time-resolved Brillouin light scattering (BLS) technique is used to study diffraction of two-dimensional beams and pulses of dipolar spin waves excited by strip-line antennas in tangentially magnetized garnet films. The new technique is an effective tool for investigations of two-dimensional spin wave propagation with high spatial and temporal resolution. Linear effects, such as the unidirectional exci-tation of magnetostatic surface waves and the propagation of backward volume magnetostatic waves (BVMSW) in two preferential directions due to the non-collinearity of their phase and group velocities are investigated in detail. In the nonlinear regime stationary and non-stationary self-focusing effects are studied. It is shown, that non-linear diffraction of a stationary BVMSW beam, having a finite transverse aperture, leads to self-focusing of the beam at one spatial point. Diffraction of a finite-duration (non-stationary) BVMSW pulse leads to space-time self-focusing and formation of a strongly localized two-dimensional wave packet (spin wave bullet). Numerical modeling of the diffraction process by using a variational approach and direct numerical integration of the two-dimensional non-linear Schrödinger equation provides a good qualitative description of the observed phenomena