A. F. M. Arts

Electrical conduction and19F NMR of solid solutions La1−xBaxF3−x

Abstract The electrical conductivity and19F NMR spin-spin relaxation of single crystals of the anion-deficient solid solutions La1−xBaxF3−x (0 × 0.095) have been investigated. In the temperature dependence of the conductivity a knee is observed, which is ascribed to exchange of fluoride ions between inequivalent sublattices. This is confirmed by spin-spin relaxation in pure LaF3 and the solid solutions. Self-diffusion coefficients derived from the conductivity are in agreement with those from the spin-lattice relaxation time T1ϱ in the rotating frame, indicating an absence of associates (BaLa VF)x above room temperature.

Fluorine mobility in La1−xBaxF3−x (0 < x < 0.1) studied by nuclear magnetic resonance

Abstract The fluorine mobilities in pure LaF 3 and LaF 3 doped with up to 10 mole % Ba have been studied by observing the 19 F NMR relaxation times T 2 and T 1ϱ versus temperature in the range 200 to 1050 K. The data are consistent with a model of two fluorine sublattices with distinct correlation times and coupled by exchange. The distinction becomes less apparent with concentration. T 1 appears to be dominated by relaxation associated with the impurities and dopant ions.

Neutron-scattering study of the randomly mixed antiferromagnet with competing anisotropies K2CoxFe1-xF4

The phase diagram of the two-dimensional (2D) randomly mixed antiferromagnet with orthogonal competing anisotropies K2CoxFe1-xF4 has been examined with elastic neutron scattering, in particular for x=0.27. For this composition the system supports long-range order of the axial spin components in 3D below a regular second-order transition at TN=64.4+or-0.6K. The transverse components also order below TN, but in 2D and over limited distances only. Below TL=27+or-2K the transverse components acquire 3D order by reorientation, establishing an oblique antiferromagnetic phase in four domains. The lower transition is first order.

Rb2Cu1-xCoxF4, a two-dimensional Ising spin glass

We report on the dc susceptibility of the insulating spin glass Rb2Cu1−xCoxF4, with x=0.22. This model compound is a realization of the two‐dimensional short‐range Edwards–Anderson model, which has competing nearest‐neighbor Ising interactions. Below a frequency‐dependent freezing temperature spin‐glass behavior is observed for longitudinal spin components only. The nonlinear part of the susceptibility is found to excellently obey static Tc=0 scaling with β=0.0±0.1 and Δ=3.2±0.2, in conformity with the Monte Carlo results for the two‐dimensional Edwards–Anderson model, Tc=0, β=0, and Δ=3.5±0.5. These findings thus provide experimental evidence for the lower critical dimensionality for Ising spin glasses to exceed 2.

Phonon avalanches in an acoustic cavity

Abstract We have observed stimulated emission of resonant acoustic phonons by Zeeman-split E ( 2 E ) doublets in dilute ruby following local population inversion by selective optical pumping. The phonons generated reverberate in the crystal acting as an acoustic cavity, the phonon beam being amplified upon each passage through the active zone. The frequency spread of the avalanche is found to be 55 MHz .

COHERENT PHONON AVALANCHES

Abstract Avalanches of resonant acoustic phonons are observed following population inversion of the Zeeman-split E ( 2 E) doublet in dilute ruby by selective optical pumping. In confined geometries only a single stage of stimulated emission is seen, whereas longer crystals form an acoustic cavity allowing multiple passage of the phonon pulse through the active zone. Coherent equations of motion for the lattice displacement and the spin Bloch vector excellently account for these results.

Brillouin scattering by localized acoustic modes in silicon‐oxynitride–fused‐silica optical waveguides

Brillouin scattering connecting guided optical modes is used to study the acoustic waves in a silicon‐oxynitride–fused‐silica double layer deposited onto a Si substrate. The frequencies of the acoustic modes as derived from the acoustic wave equation are found to reproduce those of the experiment. The scattering intensities are in agreement with a calculation based on the coupled‐waveguide‐mode formalism with due account of the elasto‐optic effect and the corrugation of the surface and silicon‐oxynitride–silica interface. Finally, the sound velocities and Pockels coefficients of the silicon‐oxynitride layer are determined.

The fluorine ionic motion in tysonite-structured solid solutions

Abstract We report on 19F pulse-NMR experiments (T2, T1ϱ and T1) on solid solutions La1−xBaxF3−x(0

Aging in two-dimensional random-exchange Ising ferromagnets

Abstract Domain growth in the two-dimensional random-exchange Ising ferromagnet Rb2Cu0.89Co0.11F4 is studied using ac and dc magnetization measurements. From a scaling analysis of the time and frequency dependences of the nonequilibrium part of the ac susceptibility the ferromagnetic domain size R(t) is found to grow according to R(t) α ⦜ ln t]1/gy , with ψ = 1 4 .

Magnetic susceptibility of the double-layer antiferromagnet K3Mn2F7

Abstract Susceptibility measurements are presented on a single crystal of the quadratic double-layer antiferromagnet K3Mn2F7. At low temperatures, fitting to renormalized spin-wave theory yields J/kB = −7.62 ± 0.15 K from χβ, and J/kB = −7.7 ± 0.2 K from χ||. In the paramagnetic regime, however, J/kB = −8.04 ± 0.24 K by use of the high-temperature expansion. Short-range correlations above TN = 58 ± 1 K are found to be intermediate between the single-layer (2D) and infinite-layer (3D) structures.