Michinobu Mino

Strange Attractor of Chaotic Magnons Observed in Ferromagnetic (CH3NH3)2CuCl4

By parallel pumping experiments in ferromagnetic (CH 3 NH 3 ) 2 CuCl 4 chaotic auto-oscillations of magnon amplitude are observed at the pumping frequency of 9.39 GHz and the temperature of 1.65 K. Strange attractors constructed in three-dimensional phase space from time series data of chaotic oscillations display properties of stretching and folding of the sheet of trajectories. Fractal dimensions of strange attractors are obtained by the method of the correlation integral as a function of driving microwave power. With increasing driving power fractal dimension increases from 1.6 to 3.4. The largest Lyapunov exponent is obtained as 0.43±0.05 by using empirical return maps. The chaotic state arises directly from period-two oscillations without cascade of period doubling bifurcations.

Neutron Scattering Studies on Spin-Peierls Material CuGeO3

Crystal structure and phonon dynamics up to 150 meV of the spin-Peierls material CuGeO 3 have been investigated by neutron scattering experiments. We found a large lattice anomaly in the a-b plane, vertical to the one-dimensional antiferromagnetic spin chain, associated with additional diffraction peaks in the b * -axis at temperatures much higher than the spin-Peierls transition temperature. The phonon modes of the chain oxygens at 100 meV show consistent evolution with diffraction. Therefore, we conclude that the structural evolution of the chain oxygens in the a-b plane, which mediates the spin-spin interaction along the c -axis, drives the spin-Peierls transition.

Quantum effect in the high-field phase of CsCuCl3

Abstract A small magnetization jump at Bc = 12.5 T in CsCuCl3 found in 1978 has been a mystery for a long time. In this compound, the magnetization was supposed to be linear because of smooth tilt of the magnetic moments toward the field direction for B || c-axis. Then a jump was completely unexpected by the molecular field model. Recently, Shiba proposed a model where the 120°-structure at low field changes to the coplanar type above Bc due to a quantum effect. This is the first example which shows a quantum effect in high fields very clearly in a magnetic system with strong frustration. This structure is confirmed by high field neutron diffraction and high field submillimeter wave ESR. But the plateau observed in the magnetization for B ⊥ c is still an open question.

Neutron diffraction of CsCuCl3 in high fields

Abstract Neutron diffraction measurements of triangular lattice CsCuCl 3 have been performed in a pulsed high magnetic field up to 14T. For an applied field parallel to the c -axis and below B c = 10T at 7 K, Bragg reflection is observed at (1/3 1/3 0.085) showing a long period helical spin alignment along the c -axis and a triangular umbrella-like structure in the c -plane. The intensity of this reflection drastically changes at B c and a new reflection at (1/3 1/3 0) appears. These results well explain a new type of magnetic phase transition which cannot be understood by the classical theory. When B is perpendicular to the c -axis more complicated results are observed.

Characterization of Chaotic States of Parallel-Pumped Magnons in Yttrium Iron Garnet

Parallel pumping experiments in YIG magnons have been performed at 4.2 K. Above the spin-wave instability threshold, chaotic auto-oscillations are observed. From time-series data, return maps, fractal dimensions, positive Lyapunov exponents, phase space portraits of strange attractors and two-dimensional Poincare sections are obtained. The largest Lyapunov exponent, which describes the degree of chaos, is obtained as 0.34. The Poincare sections of the phase space trajectories exhibit the stretching and holding which are characteristics of strange attractors.

Fractal structure of strange attractor at the subsidiary resonance in yttrium iron garnet

Transverse pumping at the first-order Suhl instability in YIG has been performed at a pumping frequency of 3.3 GHz at 4.2 K. Above the spin-wave instability threshold, period-doubling cascades and chaotic auto-oscillations are observed. From time-series data, return maps, fractal dimensions, phase space portraits of strange attractors, two-dimensional Poincare sections, and singularity spectrum f (α) are obtained. The Poincare sections of the phase space trajectories exhibit the stretching and folding which are characteristics of strange attractors. At the onset of chaos, the f (α) spectrum is consistent with the universal one.

Relaxation of labyrinth domain structure of ferromagnetic thin film under field cycles

The relaxation of a two-dimensional labyrinth pattern of magnetic domain structure in a uniaxial garnet thin film is studied. The usually observed domains of the labyrinth structure contain many defects. The labyrinth structure changes to a parallel-stripe structure under magnetic field cycles. In this evolution, defects of the domain pattern disappear. We have introduced a number of magnetic domain junctions as characteristic values of a domain pattern. A magnetic domain junction is defined as a point where magnetic domains in the same direction intersect like a three-forked road. This is one type of defect in the domain pattern. It is found that the number of junctions decreases with the power law as a function of the number of ac magnetic field cycles.

Magnetic structure of DyAg in high magnetic field studied by neutron diffraction

We have carried out neutron-diffraction experiments on the metamagnetic compound DyAg which has two phase transitions at 7.6 T and 10.2 T in a field along the [001] direction. Some models have been proposed according to the magnetization process. By observing the intensity variation of magnetic Bragg peaks, we finally reached a unique model, and this differs from all of the proposed models so far.

Parametric pair of waves as a nonlinear oscillator

We considered the process of parametric excitation of spin waves and magnetoelastic waves in magnetoordered crystals in a microwave resonator. From our analysis it follows that for the k and - k wave pair there is a single-mode realization (i.e. the pair can be represented as one oscillator). The theory of parametric resonance of waves constructed in terms of these collective variables provides a new perspective on some old spin-wave experiments and helps explain the recently obtained experimental data.

Quasiperiodic and chaotic oscillations of magnon systems driven by modulated pumping field

Abstract Parallel-pumping experiments in YIG have been performed with modulated pumping fields. Above the spin-wave instability threshold, frequency locking states and a quasiperiodic route to chaos are observed. In a quasiperiodic state, an obtained attractor is a torus. Above a transition to a chaotic state, a breakup of the torus occurs. And a positive largest Lyapunov exponent, which is one of quantitative measures of chaos, is obtained in the chaotic regime.