G. Wiese

Multistability and chaos by parametric excitation of longwave modes in a YIG sphere

Spin-wave instabilities in spheres of yttrium iron garnet have been studied by ferromagnetic resonance within the coincidence regime of the first-order Suhl instability (1.8–3.4 GHz), i.e. both the uniform mode and pairs of spin-waves were simultaneously excited on resonance. From the characteristic behaviour above the threshold, three different regimes of resonance fields (and frequencies) could be distinguished: For low fields (640–680 Oe) and for high fields (950–1210 Oe) the amplitude of magnetization remains time independent and only at high input power becomes suddenly chaotic. In the intermediate regime (680–950 Oe) a very complex multistable behaviour occurs and a variety of oscillations and sequences of bifurcations are observed. We discuss our experimental results in terms of a multi-mode model which, beyond spin-waves, considers the excitation of longwave modes with wavelengths in the order of sample dimension. Magnetostatic modes and spin-waves can be described within a unified picture by introducing “spherical spin-waves” which are classified by symmetry. A new mechanism based on the indirect excitation of the magnetostatic (4,3,0) mode is introduced. Numerical simulations show that this mechanism can explain both the multistability and the complex dynamic behaviour of the system.

Possible 4f7 5d1 Ground State of Gd Impurities in the Mixed-Valence Compound SmB6, Observed with Electron Spin Resonance

In SmB6 single crystals we observe a very anomalous ESR spectrum of Gd below 5 K. As far as we know, this is the first Gd spectrum, which cannot be explained by the electronic configuration Gd3+ 4f7. We discuss the appearance of the anomalous spectrum below 5 K by a trapped conduction electron, forming a formal divalent Gd 4f7 5d1 ground state. Above 6 K, where the conduction band of SmB6 becomes populated, we observe the usual Gd3+ Dysonian resonance as in conventional metals. The Hamiltonian of the 4f7 5d1 configuration, including a dynamic Jahn-Teller effect of the 5d electron, describes the complicated Gd spectrum reasonably well. The question, why SmB6, until now, seems to be the only host in which Gd3+ attracts a further electron, remains open.

Parametric excitation of spherical modes in ferromagnetic spheres by perpendicular and parallel pumping

The fine structures of the threshold of subsidiary absorption and parallel pumping, observed on highly polished YIG spheres in 1975 by W. Jantz et al., are discussed with a model which uses instead of plane spin-waves the true modes regarding the shape of the sphere. These spherical modes are magnetostatic modes taking additionally the exchange interaction into account. At the closely spaced resonances of the fine structure of parallel pumping single spherical modes with angular-momentum quantum numberm=0 are excited, whereas at the resonances of subsidiary absorption pairs of spherical modes with big angular-momentum quantum numbers (|m|≈1000) satisfying the selection rulem+m′=1 are excited. The analysis of the fine structure of subsidiary absorption shows that in regimes where the plane wave approximation predicts the excitation of spin-waves with formalk→0, modes withk≈1.5·105 cm−1 have the lowest threshold. This discrepancy is discussed with the effect of surface pit scattering, which increases the threshold of modes withk < 2π/λpit, where λpit is a typical size of the surface pits. Consequently modes withk≈22π/λpit instead ofk→0 have the lowest threshold.

Temperature-Induced Nonlinearity at Parametrically Excited Spin Waves

Parametrically excited spin waves in magnetic spheres of yttrium iron garnet give rise to complex multistable behaviour, which occurs for special experimental conditions at room temperature, but vanishes at helium temperature. This is in contrast to what one expects from the decreasing damping of the spin waves with temperature, which should result in an even more complex multistability at low temperature. We conclude that, in addition to the well-known nonlinear dipolar couplings, a temperature-induced nonlinearity occurs, which is only effective at higher temperature. This novel effect in solid-state physics can be qualitatively understood from the special form of the Hamiltonian of this system.

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

Abstract 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.

Chaotic Dynamics in Spin-Wave Instabilities

Ferromagnetic samples excited by strong microwave fields show a variety of nonlinear phenomena. We report on magnetic resonance experiments in yttrium iron garnet (YIG) probing spin-wave instabilities above the first-order Suhl threshold. A very complex multistable behaviour and various types of auto-oscillations and sequences of bifurcations are observed, ending up in chaos. The experimental behaviour can be interpreted in terms of a multimode-model taking into account both spin waves and magnetostatic modes.

Fine structure and critical modes at the first-order Suhl instability in YIG spheres

The FMR fine structure of the first-order Suhl instability in YIG spheres, previously observed by Jantz and Schneider (1975), has been re-investigated both for the conditions of coincidence and subsidiary absorption (3 and 9 GHz) from room temperature down to 1.6K. The observed fine structure consists of several series of closely spaced resonances, indicating a critical wave vectork≠0 even for half the pumping frequency lower than the 45° spin-wave branch, which is in contrast to the prediction of Suhls theory. The spacing of these resonances was analyzed in terms of spherical modes. We were able to determine the indices and the effective wave numbers (≈105 cm−1) of the critical modes. The very different dynamic behaviour observed at room temperature and at 1.6K can be attributed to a competition of different damping mechanisms—mainly surface pit scattering and three-magnon confluence processes — and their dependences on field, frequency, and temperature.

Temperature-induced multistability in spin-wave turbulence

Abstract Parametrically excited spin-waves in magnetic spheres of yttrium iron garnet give rise to complex multistable behaviour occuring for special experimental conditions at room temperature, but vanishing at helium temperature. We conclude that, in addition to the well-known nonlinear dipolar couplings, a novel “temperature-induced” nonlinearity occurs, which is only effective at higher temperature.

Multistability and Chaos in Transverse-Pumped Spin-Wave Instabilities

Spin wave instabilities in YIG spheres have been studied by FMR within the coincidence regime of the first-order Suhl instability. A complex multistable behaviour is observed, resulting in various types of auto-oscillations, sequences of bifurcations, intermittency and chaos. Our experimental results are discussed in terms of a multimode-model considering both spin-waves and magnetostatic modes.