Magnetized Yttrium Iron Garnet Film Research Articles

Spin-wave self-imaging: Experimental and numerical demonstration of caustic and Talbot-like diffraction patterns

Extending the scope of the self-imaging phenomenon, traditionally associated with linear optics, to the domain of magnonics, this study presents the experimental demonstration and numerical analysis of spin-wave (SW) self-imaging in an in-plane magnetized yttrium iron garnet film. We explore this phenomenon using a setup in which a plane SW passes through a diffraction grating, and the resulting interference pattern is detected using Brillouin light scattering. We have varied the frequencies of the source dynamic magnetic field to discern the influence of the anisotropic dispersion relation and the caustic effect on the analyzed phenomenon. We found that at low frequencies and diffraction fields, the caustics determine the interference pattern. However, at large distances from the grating, when the waves of high diffraction order and number of slits contribute to the interference pattern, the self-imaging phenomenon and Talbot-like patterns are formed. This methodological approach not only sheds light on the behavior of SW interference under different conditions but also enhances our understanding of the SW self-imaging process in both isotropic and anisotropic media.

Inverse “Foldover” Resonance in an Yttrium Iron Garnet Film

Nonlinear magnetic resonance is studied in an in-plane magnetized yttrium iron garnet (YIG) film. For YIG films magnetized perpendicular to the plane, the effect referred to as the foldover resonance is well known. It arises because the precession frequency increases with the deviation of the magnetization. When the field is reduced, the frequency of the precession remains resonant because the demagnetizing field decreases with the deviation of the magnetization. The signal disappears when the radio frequency pump power is insufficient to maintain a nonequilibrium state of the system. In the in-plane magnetized yttrium iron garnet film, the precession frequency decreases with an increase in the pump amplitude. Accordingly, the foldover effect arises under an increase in the field. The fundamental difference is that the precession in the latter case should be unstable with respect to the decay into spin wave modes. The deviation angles of magnetization of about 10° are reached, and the rate of decay of the uniform precession into spin waves, which depends on the deviation angle of the magnetization, is measured. This study opens up another way of achieving the magnon density corresponding to the formation of its Bose–Einstein condensate.

Confinement of Bose–Einstein magnon condensates in adjustable complex magnetization landscapes

Coherent wave states such as Bose–Einstein condensates (BECs), which spontaneously form in an overpopulated magnon gas even at room temperature, have considerable potential for wave-based computing and information processing at microwave frequencies. The ability to control the transport properties of magnon BECs plays an essential role in their practical use. Here, we demonstrate the spatiotemporal control of the BEC density distribution through the excitation of magnon supercurrents in an inhomogeneously magnetized yttrium iron garnet film. The BEC is created by microwave parametric pumping and probed by Brillouin light scattering spectroscopy. The desired magnetization profile is prepared by heating the film with optical patterns projected onto its surface using a phase-based wavefront modulation technique. Specifically, we observe a pronounced spatially localized magnon accumulation caused by magnon supercurrents flowing toward each other originating in two heated regions. This accumulation effect increases the BEC lifetime due to the constant influx of condensed magnons into the confinement region. The shown approach to manipulate coherent waves provides an opportunity to extend the lifetime of freely evolving magnon BECs, create dynamic magnon textures, and study the interaction of magnon condensates formed in different regions of the sample.

Magneto-optical imaging of coherent spin dynamics in ferrites.

The explosive development of quantum magnonics is associated with the possibility of its use as macroscopic quantum systems. In particular, they can find an application for quantum computing processors and other devices. The recently discovered phenomenon of magnon Bose-Einstein condensation and coherent precession of magnetization can be used for these purposes. Our letter describes a method for the optical observation of the coherently precessing magnetization in conditions when the concentration of magnons reaches the value necessary for their quantum condensation. The investigations were conducted in the out-of-plane magnetized yttrium iron garnet films. The required magnon density was achieved by magnetic resonance technique. The magneto-optical imaging method provides such important parameters of the coherent spin dynamics as the amplitude and phase distributed all over the sample. It should become an indispensable read-out tool for the upcoming quantum technologies based on the magnon Bose-Einstein condensation.

Features of the Interaction of a Magnon Bose—Einstein Condensate with Acoustic Modes in Yttrium Iron Garnet Films

The interaction of a magnon Bose—Einstein condensate (mBEC) in a perpendicularly magnetized yttrium iron garnet film with high-order acoustic modes appearing at the sizes of an yttrium iron garnet substrate has been studied experimentally. Narrow energy absorption lines of the mBEC have been observed at the coincidence of frequencies. The minimum relaxation of the mBEC has been detected under acoustic antiresonance conditions when the precession frequency is exactly in the middle between acoustic resonance modes. This discovery will allow the optimization of coherence times when fabricating quantum magnonic devices.

Phase reciprocity of spin-wave excitation by a microstrip antenna

Using space-, time-, and phase-resolved Brillouin light-scattering spectroscopy we investigate the difference in phase of the two counterpropagating spin waves excited by the same microwave microstrip transducer. These studies are performed both for backward volume magnetostatic waves and magnetostatic surface waves in an in-plane magnetized yttrium iron garnet film. The experiments show that for the backward volume magnetostatic spin waves which are reciprocal and excited symmetrically in amplitude there is a phase difference of associated with the excitation process and thus the phase symmetry is distorted. On the contrary, for the magnetostatic surface spin waves which are nonreciprocal and unsymmetrical in amplitude the phase symmetry is preserved there is no phase difference between the two waves associated with the excitation. Theoretical analysis confirms this effect.

Waveguide-light-spin-wave interaction in nonuniformly magnetized yttrium iron garnet films

The magneto-optic interaction of waveguide light (WL) with spin waves (SW) in nonuniformly magnetized yttrium iron garnet (YIG) films was investigated both experimentally and theoretically. The measurements were carried out on YIG film of thickness 3.8 µm at the optical wavelength of 1.15 µm and in the SW frequency band of 3-4 GHz. It is shown that for collinear WL-SW interaction, a spatial nonuniformity of the magnetizing field can provide SW beam focusing and improve WL-SW interaction efficiency by a factor of an order of magnitude. Field nonuniformity in the transverse direction results in a spatial separation of regions of WL interaction with SW of different frequencies. For orthogonal geometry of the WL-SW interaction the artificial field nonuniformity leads to an increase in the Bragg diffraction angles.

Self-generation of fundamental dark solitons in magnetic films

The self-generation of single, fundamental, stable, spin-wave-envelope black and gray dark solitons has been realized for the first time. These solitons were generated from microwave magnetostatic surface waves (MSSWs) propagated in an in-plane magnetized yttrium iron garnet film MSSW delay line in a resonant ring. These fundamental dark solitons were made possible by a new and general filtering technique for a high gain nonlinear resonant ring. The amplitude and phase profiles together with the power spectra of the self-generated microwave pulses confirm their fundamental dark soliton nature.

Extraordinary power and modulation dependence of the auto-oscillation frequency in tangentially magnetized YIG films

The power dependence of auto-oscillations (AO) in yttrium–iron–garnet (YIG) film within a cavity is studied experimentally. We demonstrate that the dependence of the AO frequency on the pumping power is not always positive as commonly observed and calculated. We find DC magnetic field regions where the power dependence is negative. This behavior is extraordinary as it happens under pumping powers very close to the threshold, where the power dependence is supposed to be positive. Besides we demonstrate that the dependence of the AO frequency on the modulation amplitude of the DC magnetic field is not always negative as might be expected. At certain regions of the field we observe a positive dependence. This positive dependence on the modulation amplitude is consistent with the negative power dependence.

Observation of self-generation of dark envelope solitons for spin waves in ferromagnetic films

We have performed experiments in which self-generation of dark envelope solitons for spin waves at microwave frequencies were obtained. Stable self-generation of dark solitons was observed in a ring consisting of tangentially magnetized yttrium iron garnet film and a microwave signal amplifier.

Origin of the fine structure of subsidiary absorption in tangentially magnetized YIG films

Parametric excitation of spin waves (SWs) under conditions of perpendicular pumping was investigated in epitaxial films of yttrium iron garnet (YIG). The related microwave power absorption was measured at the frequency 9.1 GHz in the magnetic field range 1200–1600 Oe. A series of absorption peaks closely spaced on the magnetic field scale were observed near the threshold of first-order Suhl instability of spin waves. The peaks spaced by 0.2–0.7 Oe originate from the shallow ripple of the butterfly curve. This is the first observation of the fine structure at subsidiary absorption in YIG films. The peak spacing was measured as a function of dc magnetic field H, film thickness d, and in-plane sample dimensions L for square samples. In the field range 1300–1400 Oe the spacing did not depend on H and was inversely proportional to L in the range of 0.6–3 mm. No thickness dependence was found in the range d=3–45 μm. Explanation is given in terms of standing spin waves set in the film plane and related to the exchange part of SW dispersion curves in ferrite film.

Parallel pumping fine structure at 9.4 GHz for in-plane magnetized yttrium iron garnet thin films

A fine structure in the parallel pumping spin wave instability absorption has been observed. The data were obtained at 9.4 GHz on a narrow linewidth in-plane magnetized yttrium iron garnet film with a magnetic layer thickness of 15.9 μm. The standard butterfly curve of the spin wave instability threshold microwave field amplitude hcrit versus external static magnetic field Hext was constructed from hcrit determinations obtained by sweeping the static field at different microwave power levels and measuring the change in microwave loss. For values of Hext below the minimum, hcrit position, the loss versus Hext profiles showed a characteristic fine structure previously observed only in spheres. The fine structure spacing was on the order of 5 Oe and the square of the spacing decreased linearly with increasing static field. These fine structure results are in accord with predictions from theory for critical modes at one-half the pump frequency, with the critical mode wave vectors aligned perpendicular to the film plane, and with the wave numbers for these modes quantized as standing spin wave modes for a thin film. Analysis of the fine structure data gives a spin wave exchange stiffness parameter Dfs=(6.0±0.9)×10−9 Oe cm2, which agrees within experimental error with the value D=(5.4±0.2)×10−9 Oe cm−2 obtained from Brillouin light scattering measurements on YIG spheres. The standing wave critical modes are explained in terms of cooperative volume dipole-dipole and shape demagnetizing induced ellipticity for the modes.

Backward volume wave solitons in a yttrium iron garnet film (invited) (abstract)

Solitons are nonlinear propagating wave pulses that preserve their shape without spreading. Recently, microwave envelope solitons have been observed in thin yttrium iron garnet (YIG) films for magnetostatic surface wave and forward volume wave configurations. In this work, we report the first observation of microwave envelope of magnetostatic backward volume waves (MSBVW) for an in-plane magnetized YIG film with waves propagating parallel to the magnetization direction. Both soliton profiles and the nonlinear peak power response were observed. The experiments were conducted using a microstrip magnetostatic wave delay line structure. A single crystal YIG film of 7.2 μm thickness, 2 mm×15 mm in size, and with unpinned surface spins was magnetized in-plane by a static external field of 1343 Oe along the long 15 mm edge. The 10 GHz ferromagnetic resonance linewidth of the film was 0.6 Oe. Square microwave pulses with pulse widths from 2 to 200 ns and a carrier frequency of 5.78 GHz were launched parallel to the field through a microstrip antenna. The output signal was received by a second microstrip antenna placed 4 mm downstream and analyzed in the time domain with a microwave transition analyzer. Envelope soliton behavior evident from the time resolved waveforms was observed for various input-power/pulse-width combinations. At relatively low power levels, one sees a broad output signal with a peak power increasing linearly with the input power. As the input power is increased above some threshold, a sharp soliton pulse emerges and the peak power increases more rapidly with input power than in the low power regime. The threshold varies with pulse width, as expected for solitons. A further increase of the input power produces multiple soliton profiles and a corresponding drop in the peak power. These results clearly demonstrate the existence of MSBVW solitons in YIG films. Dr. J. D. Adam of Westinghouse is acknowledged for providing the YIG films. This work was supported in part by the National Science Foundation, Grant No. DMR-8921761 and by the U. S. Army Research Office, Grant No. DAAL03-91-G-0327.

Spin-wave envelope solitons in thin ferromagnetic films (invited)

The process of spin-wave (SW) envelope soliton formation and propagation has been studied experimentally in thin yttrium-iron-garnet (YIG) films at microwave frequencies. The formation of solitons has been observed in perpendicularly and tangentially magnetized YIG films in the spectral regions of high dispersion near the ‘‘dipole gaps’’ in the dipole-exchange SW spectrum. With the increase of the input pulse duration or/and amplitude, the multisoliton regime of SW propagation and the regime of SW modulational instability have been observed. With a further increase of input power the spectrum of SW modulational instability became more complicated and a SW strange attractor was formed through the destruction of two-frequency quasiperiodic motion. The theoretical interpretation of the observed nonlinear SW phenomena is given in the framework of nonlinear Schrodinger equation (NSE) model taking into account peculiarities of the thin-film dipole-exchange SW spectrum. The results of numerical calculations of SW envelope profiles performed using NSE model are compared to the results of the experiments.