Yttrium Iron Garnet Sphere Research Articles

Control of spin-wave instability threshold in YIG sphere

Spin-wave instability threshold under multiple drive excitations is studied in an yttrium iron garnet sphere at 4.2 K . Both a static magnetic field and a parallel RF field are applied along the [1 1 1] direction. Another perpendicular RF field excites magnetostatic modes through the first-order Suhl process. Interactions between spin-waves and magneostatic modes cause a change of instability thresholds.

Chaotic Dynamics During Slow Relaxation Process in Magnon Systems

Experimental evidence of chaotic dynamics during a slow relaxation process with very low frequency magnetization auto-oscillations in magnon systems is presented. A ferromagnetic resonance experiment using a sphere of pure yttrium iron garnet (YIG) at the X-band pumping frequency within the coincidence regime reveals a slow relaxation process with a long tail after a stepwise drop in external pumping power. The relaxation process is found to follow a power-law with time as a transient process involving a transition of local dynamics from a quasi-periodic state to a chaotic state as the collapse of torus.

Quantitative Millimetre Wavelength Spectrometry

Quantitative Millimetre Wavelength Spectrometry

Spin-wave alternating periodic–chaotic dynamics

A description is given of pulsed ferromagnetic subsidiary resonance experiments in a YIG sphere at room temperature. Beyond the Suhl instability threshold, evidences were found for the elusive alternating periodic–chaotic sequence. The spin-wave self-oscillations have recently been characterized through one-dimensional return maps. Here it is shown that the observed scenario is also in good qualitative agreement with results from numerical simulations with a standard two-mode microscopic model.

Microwave interferometer for steady-state plasmas

Standard single frequency, “fringe-counting,” microwave interferometers are of limited use for steady-state plasma experiments. We have constructed a swept frequency microwave interferometer, similar to a classic zebra-stripe interferometer, optimized for electron density measurements in steady-state plasma experiments. The key element in the system is a frequency doubled YIG oscillator capable of sweeping from 20 to 40 GHz. As the source frequency is swept, the sum of the reference and plasma leg signals exhibits a series of beats. Both the frequency shift and phase shift of the beat pattern due to the addition of plasma in one leg of the interferometer is used to determine the line-integrated electron density.

Characterization of spin-wave dynamics near homoclinic orbits through one-dimensional mapping

A description is given of pulsed high-power ferromagnetic resonance experiments in a YIG sphere at room temperature. Beyond the Suhl instability threshold, evidences were found for the alternating periodic-chaotic sequence. The chaotic spin-wave self-oscillations were characterized through return maps obtained from the local maxima in the observed transient-free time series. As a result, a simple one-dimensional mapping scheme that exhibits the basic features of the observed transition is presented.

Magnetically tunable superconducting resonators and filters

Compact, low-loss, tunable filters are needed for overload protection in the front end of many microwave-frequency systems. We have demonstrated magnetically tunable superconducting resonators and filters comprising microwave circuits coupled to ferrite substrates in monolithic structures using niobium at 4 K and hybrid resonator structures using YBCO at 77 K. A three-pole 1% bandwidth filter with 10-GHz center frequency and 1-dB insertion loss is described. In these devices the tunability results from changes in the magnetization of the ferrite rather than changes in the ferrimagnetic resonance frequency as in conventional YIG filters. Tunability data plotted us a function of magnetic field are fitted to the hysteresis theory developed previously and indicate that a tuning range of 13% is achievable. We have demonstrated switching times of less than one microsecond in structures incorporating closed magnetic paths in the form of a circular toroid.

Observation of mixed-mode oscillations in spin-wave experiments

High-power ferromagnetic resonance experiments in a 1-mm-diam yttrium iron garnet sphere, driven at 8.9 GHz at room temperature, reveal evidence for a universal scenario not yet observed in this system, namely, the periodic–chaotic sequence. Well above the first-order Suhl instability threshold (P/PC∼6 dB), low-frequency (f∼200 kHz) auto-oscillations consisting of large-amplitude peaks followed by n small undulations (mixed-mode oscillations) have been observed. Both periodic and aperiodic trajectories were detected with n=1, 2, 3, and 4, which are reminiscent of the so-called homoclinic chaos. A discussion is made on the basis of a standard two-mode model.

Ferromagnetic resonance imaging of Co films using magnetic resonance force microscopy

Lateral one-dimensional imaging of cobalt (Co) films by means of microscopic ferromagnetic resonance (FMR) detected using the magnetic resonance force microscope (MRFM) is demonstrated. A novel approach involving scanning a localized magnetic probe is shown to enable FMR imaging in spite of the broad resonance linewidth. We introduce a spatially selective local field by means of a small, magnetically polarized spherical crystallite of yttrium iron garnet (YIG). Using MRFM-detected FMR signals from a sample consisting of two Co films, we can resolve the ∼20 μm lateral separation between the films. The results can be qualitatively understood by consideration of the calculated spatial profiles of the magnetic field generated by the YIG sphere.

The Onset of Auto-Oscillations of Parametrically Excited Spin-Wave Pairs

The parallel pumping of spin waves in an yttrium iron garnet (YIG) sphere placed in a microwave cavity has been performed by X -band microwaves. The onset and the principal frequency of auto-oscillations above the threshold of spin-wave instability were studied at various magnetic fields. We found anomalous field dependence of auto-oscillations frequency in the low field side of the butterfly curve, where the threshold of parallel pumping instability exhibits no anomaly. This result shows that the higher order nonlinear relaxation process of parametrically excited magnons becomes important in a nonequilibrium system.

Spin-wave experiments: Simultaneous pumping revisited

Confirming theoretical predictions more than 30 years old, we show that the parametric instability threshold of microwave parallel-pumped spin waves in YIG spheres is reduced by the simultaneous application of an orthogonal RF field at half the pumping frequency. As a function of the orthogonal field, the reduction first increases with the square of the field, as predicted, and saturation is critically reached at the second-order Suhl instability threshold. In addition, we show that the auto-oscillation threshold is also affected by the orthogonal field, but not its amplitude nor its frequency.

Dynamics of transients in yttrium-iron-garnet.

Yttrium-iron-garnet (YIG) is an important technological material used in microwave devices. In this paper we use dual microwave (1-4 GHz) drives to study the dynamical bifurcation behavior of magnetostatic and spin-wave modes in YIG spheres and rectangular films. The samples are placed in a dc magnetic field and driven by cw and pulse-modulated microwave excitations at magnetostatic mode frequencies. A second microwave drive applied to the sample excites additional spin-wave modes that can interact with those arising from the original excitation and thereby affect the transmission characteristics at the primary frequency. We find a significant decrease in transmission of the primary when the secondary frequency is tuned to approximately half that of the primary drive. This decrease is observed both in the steady state behavior and in the initial overshoot transient associated with pulse modulation of the primary excitation. Results such as these are often treated by extending linear theory to include higher order interaction terms. Herein we present a simple dynamical model that reproduces results that qualitatively resemble the experimental data. (c) 1997 American Institute of Physics.

Stochastic Resonance in Chaotic Spin-Wave Dynamics

Institut fu¨r Festk¨orperphysik, Technische Hochschule Darmstadt, Hochschulstrase 6, D–64289 Darmstadt, Germany(January 31, 1997)We report the first experimental observation of noise–freestochastic resonance by utilizing the intrinsic chaotic dynam-ics of the system. To this end we have investigated the effectof an external periodic modulation on intermittent signals ob-served by high power ferromagnetic resonance in yttrium irongarnet spheres. Both the signal–to–noise ratio and the resi-dence time distributions show the characteristic features ofstochastic resonance. The phenomena can be explained bymeans of a one–dimensional intermittent map. We presentanalytical results as well as computer simulations.PACS numbers: 05.45.+b, 05.40.+j, 75.30.DsI. INTRODUCTION

Dynamic control of instability thresholds in yttrium-iron-garnet

Yttrium-iron-garnet (YIG) is an important material used in microwave limiters and other devices. In this work, we use dual microwave (1–4 GHz) drives to study the behavior of magnetostatic and spin-wave modes in YIG spheres and rectangular films. The samples are placed in a dc magnetic field and driven by cw and pulse-modulated microwave excitations at magnetostatic mode frequencies. A second microwave drive applied to the sample excites additional spin-wave modes that can interact with those arising from the original excitation and thereby affect the transmission characteristics at the primary frequency. For both films and spheres, we observe a significant decrease in transmission of the primary when the secondary frequency is tuned to approximately half that of the primary drive. This decrease is observed both in the steady state behavior and in the initial overshoot transient associated with pulse modulation of the primary excitation.

Auto-Oscillations and Chaos for Microwave Signals in Ferrite Single Crystals

The recent progress of nonlinear dynamics has stimulated a renaissance of high-power ferromagnetic resonance experiments on ferrites, both under the aspects of basic research and application. We report on resonance experiments in yttrium iron garnet spheres probing chaotic scenarios above the first-order Suhl threshold. Different techniques have been proposed to change the observed irregular auto-oscillations of magnetization into regular motion. The delayed feedback control method of Pyragas is studied both in theory and experimental application.

Delayed bifurcations at the first-order Suhl threshold

In this paper, we present both theoretical and experimental analysis of delayed dynamic bifurcations as could be observed by ferromagnetic resonance (FMR) on an yttrium iron garnet (YIG) sphere. In Sec. 1 the static bifurcation behaviour of the first-order Suhl instability is analyzed in terms of a two-mode model, which represents the simplest possible description of this process. In Sec. 2 our analytical results are extended to the dynamic case by means of numerical simulations, including the influence of noise. We present our experimental results and compare them with the obtained theoretical predictions.

On the measurement of SSB noise figure using sideband cancellation

A new method for determining the single-sideband noise figure of a device from double-sideband measurements is proposed which eliminates the need for tunable filters and hence greatly reduces the complexity and cost of performing accurate noise figure measurements. This paper compares results obtained to those obtained using a traditional single-sideband measurement system with a tuneable filter and provides a full error analysis of the new technique. The method also offers considerable possibilities for future use at higher (microwave) frequencies where YIG filters and image reject mixers are not readily available.

On-off intermittency in spin-wave instabilities.

A particular case of chaos-to-chaos intermittency identified as on-off intermittency has experimentally been observed in a yttrium iron garnet sphere at high power ferromagnetic resonance. The theoretically predicted power law scaling with the exponent $\ensuremath{-}1$ has been detected in the dependencies of the mean ``laminar'' phase length on both static magnetic field and pumping microwave power. In the distribution of the laminar lengths a $\ensuremath{-}3/2$ power law was observed close to the onset of intermittency.

Critical modes at the first-order Suhl instability in YIG spheres

The FMR fine structure of the first-order Suhl instability has been investigated in YIG spheres at room and helium temperature. Several series of closely spaced resonances were observed and analyzed in terms of spherical modes. Different types of critical modes resulting in contrasting dynamic behaviours, can be attributed to competing damping mechanisms.

Controlling unstable periodic orbits in a spin-wave experiment using the OGY scheme

Dynamic instabilities of magnetization in YIG spheres were studied by high-power ferromagnetic resonance at the first-order Suhl instability. By means of an analog feedback technique implementing the controlling scheme of Ott, Grebogi and Yorke, we suppressed the chaotic FMR signal and stabilized a periodic oscillation by applying a very small time-dependent feedback signal of less than 10−3 of the applied microwave power.