Radiation Of Spin Waves Research Articles

Excitation and propagation of spin waves in non-uniformly magnetized waveguides

The characteristics of spin waves in ferromagnetic waveguides with non-uniform magnetization have been investigated for situations where the shape anisotropy field of the waveguide is comparable to the external bias field. Spin-wave generation was realized by the magnetoelastic effect by applying normal and shear strain components, as well as by the Oersted field emitted by an inductive antenna. The magnetoelastic excitation field has a non-uniform profile over the width of the waveguide because of the non-uniform magnetization orientation, whereas the Oersted field remains uniform. Using micromagnetic simulations, we indicate that both types of excitation fields generate quantised width modes with both odd and even mode numbers as well as tilted phase fronts. We demonstrate that these effects originate from the average magnetization orientation with respect to the main axes of the magnetic waveguide. Furthermore, it is indicated that the excitation efficiency of the second-order mode generally surpasses that of the first-order mode due to their symmetry. The relative intensity of the excited modes can be controlled by the strain state as well as by tuning the dimensions of the excitation area. Finally, we demonstrate that the nonreciprocity of spin-wave radiation due to the chirality of an Oersted field generated by an inductive antenna is absent for magnetoelastic spin-wave excitation.

Spin Wave Radiation by a Topological Charge Dipole.

The use of spin waves (SWs) as data carriers in spintronic and magnonic logic devices offers operation at low power consumption, free of Joule heating. Nevertheless, the controlled emission and propagation of SWs in magnetic materials remains a significant challenge. Here, we propose that skyrmion-antiskyrmion bilayers form topological charge dipoles and act as efficient sub-100 nm SW emitters when excited by in-plane ac magnetic fields. The propagating SWs have a preferred radiation direction, with clear dipole signatures in their radiation pattern, suggesting that the bilayer forms a SW antenna. Bilayers with the same topological charge radiate SWs with spiral and antispiral spatial profiles, enlarging the class of SW patterns. We demonstrate that the characteristics of the emitted SWs are linked to the topology of the source, allowing for full control of the SW features, including their amplitude, preferred direction of propagation, and wavelength.

Spin-wave radiation from vortices in 3He−B

We consider a vortex line in the B phase of superfluid $^3$He under uniformly precessing magnetization. The magnetization exerts torque on the vortex, causing its order parameter to oscillate. These oscillations generate spin waves, which is analogous to an oscillating charge generating electromagnetic radiation. The spin waves carry energy, causing dissipation in the system. Solving the equations of spin dynamics, we calculate the energy dissipation caused by spin wave radiation for arbitrary tipping angles of the magnetization and directions of the magnetic field, and for both vortex types of $^3$He-B. For the double-core vortex we also consider the anisotropy of the radiation and the dependence of the dissipation on twisting of the half cores. The radiated energy is compared with experiments in the mid-temperature range $T \sim 0.5 T_c$. The dependence of the calculated dissipation on several parameters is in good agreement with the experiments. Combined with numerically calculated vortex structure, the radiation theory produces the order of magnitude of the experimental dissipation. The agreement with the experiments indicates that spin wave radiation is the dominant dissipation mechanism for vortices in superfluid $^3$He-B in the mid-temperature range.

Attraction, merger, reflection, and annihilation in magnetic droplet soliton scattering

The interaction behavior of solitons are defining characteristics of these nonlinear, coherent structures. Due to recent experimental observations, thin ferromagnetic films offer a promising medium in which to study the scattering properties of two-dimensional magnetic droplet solitons, particle-like, precessing dipoles. Here, a rich set of two-droplet interaction behaviors are classified through micromagnetic simulations. Repulsive and attractive interaction dynamics are generically determined by the relative phase and speeds of the two droplets and can be classified into four types: (1) merger into a breather bound state, (2) counterpropagation trapped along the axis of symmetry, (3) reflection, and (4) violent droplet annihilation into spin wave radiation and a breather. Utilizing a nonlinear method of images, it is demonstrated that these dynamics describe repulsive/attractive scattering of a single droplet off of a magnetic boundary with pinned/free spin boundary conditions, respectively. These results explain the mechanism by which propagating and stationary droplets can be stabilized in a confined ferromagnet.

Spin wave excitation patterns generated by spin torque oscillators

Spin torque nano-oscillators (STNO) are nanoscale devices that can convert a direct current into short wavelength spin wave excitations in a ferromagnetic layer. We show that arrays of STNO can be used to create directional spin wave radiation similarly to electromagnetic antennas. Combining STNO excitations with planar spin waves also creates interference patterns. We show that these interference patterns are static and have information on the wavelength and phase of the spin waves emitted from the STNO. We describe a means of actively controlling spin wave radiation patterns with the direct current flowing through STNO, which is useful in on-chip communication and information processing and could be a promising technique for studying short wavelength spin waves in different materials.

Magnetic nano-oscillator driven by pure spin current

With the advent of pure-spin-current sources, spin-based electronic (spintronic) devices no longer require electrical charge transfer, opening new possibilities for both conducting and insulating spintronic systems. Pure spin currents have been used to suppress noise caused by thermal fluctuations in magnetic nanodevices, amplify propagating magnetization waves, and to reduce the dynamic damping in magnetic films. However, generation of coherent auto-oscillations by pure spin currents has not been achieved so far. Here we demonstrate the generation of single-mode coherent auto-oscillations in a device that combines local injection of a pure spin current with enhanced spin-wave radiation losses. Counterintuitively, radiation losses enable excitation of auto-oscillation, suppressing the nonlinear processes that prevent auto-oscillation by redistributing the energy between different modes. Our devices exhibit auto-oscillations at moderate current densities, at a microwave frequency tunable over a wide range. These findings suggest a new route for the implementation of nanoscale microwave sources for next-generation integrated electronics.

Vortex Viscosity in Magnetic Superconductors Due to Radiation of Spin Waves

In type-II superconductors that contain a lattice of magnetic moments, vortices polarize the magnetic system inducing additional contributions to the vortex mass, vortex viscosity, and vortex-vortex interaction. Extra magnetic viscosity is caused by radiation of spin waves by a moving vortex. Like in the case of Cherenkov radiation, this effect has a characteristic threshold behavior and the resulting vortex viscosity may be comparable to the well-known Bardeen-Stephen contribution. The threshold behavior leads to an anomaly in the current-voltage characteristics, and a drop in dissipation for a current interval that is determined by the magnetic excitation spectrum.

Spin-transfer dynamics in spin valves with out-of-plane magnetized CoNi free layers

We have measured spin transfer-induced dynamics in magnetic nanocontact devices having a perpendicularly magnetized Co/Ni free layer and an in-plane magnetized CoFe fixed layer. The frequencies and powers of the excitations agree well with the predictions of the single-domain model and indicate that the excited dynamics correspond to precessional orbits with angles ranging from zero to 90 degrees as the applied current is increased at a fixed field. From measurements of the onset current as a function of applied field strength we estimate the magnitude of the spin torque asymmetry parameter lambda ~ 1.5. By combining these with spin torque ferromagnetic resonance measurements, we also estimate the spin wave radiation loss in these devices.

Radiation of spin waves from the open end of a microscopic magnetic-film waveguide

We have studied experimentally the radiation of spin waves from a permalloy-film microwaveguide into a continuous permalloy film. We show that due to a strong mismatch of the spin-wave spectrum caused by a variation in the demagnetizing field at the interface between the waveguide and the film, a frequency interval exists, where spin waves experience total reflection from the junction penetrating into the permalloy film in a tunnelinglike manner. At frequencies above this interval, complex frequency-dependent radiation patterns were observed characterized by a preferential radiation direction appearing due to the intrinsic anisotropy of the spin-wave dispersion characteristics in the film.

Radiation of spin waves by a domain wall of uniaxial highly anisotropic ferromagnet in the precessional dynamical mode under the action of a DC magnetic field

In this work, we have carried out a numerical integration of Landau-Lifshitz equations for a non-dissipative highly anisotropic uniaxial ferromagnet in the region of the nonparametric threshold radiation of spin waves by a domain wall (DW) moving in the precessional dynamical mode in a strong dc field. The deviation of the calculated values of the forward velocity of a DW is different from zero because of the backward reaction of the field of radiation on it from the known analytical results increases with decreasing constant of uniaxial anisotropy and with approaching the acting field the threshold value. Special attention is given to the asymmetry of the field of radiation relative to the DW plane depending on the direction of the DW velocity.

Linear and nonlinear spin-wave dynamics in macro- and microscopic magnetic confined structures

We present an investigation of the propagation of spin waves in macro- and microscopic laterally confined magnetic structures and the influence of direct spin-polarized current on spin-wave radiation by nanocontacts. For detection of spin waves the space-resolved Brillouin light scattering technique is used. Nonlinear effects modifying the spin-wave propagation and eigenmodes are studied experimentally and analysed theoretically. We show that the spin-polarized current changes the effective damping constant of the ferromagnet. The dependence of the damping constant is linear for one direction of the current, in agreement with existing theoretical models. For the opposite direction of the current the dependence is strongly nonlinear showing saturation.

Effect of spin-polarized electric current on spin-wave radiation by spin-valve nanocontacts

Using a spatially resolved microfocus Brillouin light scattering technique with spatial resolution of 250nm, the authors have studied radiation of spin waves by a spin-valve nanocontact into a continuous underlying magnetic film. They have found that a spin-polarized electric current flowing through the contact affects the intensity of radiated spin waves. They show that for one of the directions of the current, the spin-wave intensity is noticeably increased, whereas for the other direction, spin waves are suppressed. They associate the findings with an amplification of spin waves due to spin-torque-transfer phenomena.

Strong Radiation of Spin Waves by Core Reversal of a Magnetic Vortex and Their Wave Behaviors in Magnetic Nanowire Waveguides

We conducted micromagnetic numerical studies on the strong radiation of spin waves (SWs) produced by the magnetic-field-induced reversal of a magnetic vortex core, as well as their wave behaviors in magnetic nanowires. It was found that the radial SWs can be emitted intensively from a vortex core in a circular dot by virtue of localized large torques employed at the core, and then can be injected into a long nanowire via their contact. These SWs exhibit wave characteristics such as propagation, reflection, transmission, interference, and dispersion. These results offer a preview of the generation, delivery, and manipulation of SWs in magnetic elements, which are applicable to information-signal processing in potential SW devices.

Radiation of spin waves from magnetic vortex cores by their dynamic motion and annihilation processes

We report on micromagnetic simulation results of radiation of strong spin waves from the cores of magnetic vortices driven by their dynamics motion or the annihilation of a vortex-antivortex pair in a rectangular shaped magnetic thin film. Such strong spin-waves are distinguished from spin wave modes typically excited in patterned magnetic elements. The spin wave excitation with relatively low frequencies of 0–22 GHz are associated with the shape of an element, a magnetization configuration, and an applied magnetic field, while dominating spin waves in the higher frequencies of 22–96 GHz are driven by either the motion or annihilation of vortex cores present in the confined element. The latter case yields much higher amplitudes than the former does. It is found that large torques applied at the local area of the vortex cores, driven by the large exchange fields in the core region during their dynamic motion and collapse, induce a rapid energy dissipation into the surrounding areas through the spin-wave excitation and subsequent propagation. In addition, it is found that the strong spin waves radiated by the dynamic evolution processes of the vortex cores propagate well into a long stripe-shaped magnetic wire. Such traveling spin waves can be applicable for a new generation of magnetic logic devices.

Two-dimensional patterns of spin-wave radiation by rectangular spin-valve elements

The radiation of spin waves by rectangular spin-valve elements into a surrounding magnetic film has been studied experimentally using a novel micro-focus Brillouin light scattering setup allowing for spatially resolved measurements of the intensity of dynamic magnetization with a resolution better than 300nm. The investigated elements have lateral dimensions of 1.3×2.3μm2 and consist of a 10nm thick Co80Fe20 and a 5nm thick Ni81Fe19 layer separated by a 4nm thick Cu spacer layer. It was found, that the elements radiate spin waves at frequencies corresponding to their laterally quantized spin-wave eigenmodes. Two-dimensional distributions of intensities of spin waves radiated by different eigenmodes were recorded outside the element. It was shown, that the radiation patterns consist of several rays intersecting each other and forming spots where the amplitude of variable magnetization locally increases. A theoretical model qualitatively explaining the observed radiation patterns has been suggested.

Dissipative Currents in SuperfluidHe3Weak Links

We calculate the current-pressure relation for pinholes connecting two volumes of bulk superfluid 3He-B. The theory of multiple Andreev reflections, adapted from superconducting weak links, leads to a nonlinear dependence of the dc current on pressure bias. In arrays of pinholes one has to take into account oscillations of the texture at the Josephson frequency. The associated radiation of spin waves from the junction leads to an additional dissipative current at small biases, in agreement with measurements.

Radiation of spin waves by a single micrometer-sized magnetic element

Dynamic magnetic properties of a single micrometer-sized magnetic element consisting of a permalloy and a partially patterned CoFe layer separated by an intervening Cu spacer layer have been studied by means of a micro-focus Brillouin light scattering setup, which allows for local measurements of the magnetization dynamics on the submicrometer scale. It is shown that quantized spin-wave modes excited in the magnetic element act as radiation sources for spin waves in the surrounding magnetic film. It is found that the intensities of spin waves excited by different quantized modes follow different distance laws when traveling away from the region of excitation.

Textural Effects and Spin-Wave Radiation in Superfluid3He Weak Links

The Josephson coupling between two volumes of superfluid 3He-B also couples the phase difference to the spin-orbit texture of the order parameter. As a result, the equilibrium configuration of the macroscopic texture depends on the phase difference. If the junction is biased by a pressure head, the Josephson oscillations of the supercurrent are accompanied by an oscillation of the texture close to the junction. This leads to a radiation of spin waves from the weak link, and thus contributes to the dissipative dc currents flowing through it. PACS numbers: 67.57.De, 67.57.Fg, 67.57.Np.

Classical dependence of the domain wall velocity on the magnetic field in garnet ferrite films with orthorhombic magnetic anisotropy

The dependence of the domain wall velocity V on the acting magnetic field H is investigated for bismuth-containing single-crystal garnet ferrite films with orthorhombic magnetic anisotropy. It is shown that this dependence includes both the initial linear portion and a saturation portion and exhibits a complex behavior. This behavior is explained within the model of domain wall motion with spin wave radiation.

Excitation of spin waves by an electric current

The excitation of spin waves in an unbounded ferromagnetic film by a direct spin-polarized current distributed over a small area is treated macroscopically. The derived critical threshold current for excitation has two additive terms: The first arises from radiation of spin waves and is constant. The second arises from local viscous dissipation and varies in proportion to damping coefficient, external field, and area. An application of these predictions modifies the existing interpretation of experiments by Tsoi and collaborators employing currents flowing through point contacts.