An observed exponential decay in the electron paramagnetic resonance linewidth of the nearly classical two-dimensional antiferromagnet as a function of temperature is shown to be the result of solitons interacting with magnons, and provides an experimental confirmation of these excitations. The temperature dependence of the linewidth is calculated using the dynamic spin correlation function derived from soliton-magnon scattering in the Born approximation. Data in the critical regime for (n-propylammonium) 2 tetrachloromanganese (II) are presented and compared to the theory.

It is shown that the introduction of a very small amount of nonmagnetic impurities into the magnetic sites of a classical two-dimensional antiferromagnet creates a new type of static (impuritypinned) soliton that affects the Arrhenius, exp͑2E͞T͒, temperature-dependent electron paramagnetic resonance linewidth by drastically changing the parameter E. Data just above the transition temperature for ͑C 3 H 7 NH 3 ͒ 2 M x Mn 12x Cl 4 confirm the existence of these impurity-pinned solitons.[S0031-9007(98)05498-2] PACS numbers: 75.10. Hk, 75.40.Gb, 76.30.Fc Two-dimensional magnetic systems support interesting nonlinear excitations including solitons and vortices. For the two-dimensional (2D) isotropic ferromagnetic Belavin and Polyakov [1] obtained these solitonlike solutions (BP solitons) from topological considerations. The energy of this excitation is found to be independent of the soliton size resulting from scale invariance of the continuum Heisenberg Hamiltonian. The significance of these excitations was recognized early in connection with the critical properties of 2D magnets. For example, in [1] it was shown that the existence of large localized excitations will cause the correlation length to remain finite at any nonzero temperature as expected from the Mermin-Wagner theorem [2].Recently we have shown [3,4] that BP solitons dominate the thermodynamics in the fluctuation region immediately above the Néel temperature of a large class of nearly classical 2D antiferromagnets. Experimentally this is observed as an Arrhenius behavior of the temperature-dependent electron paramagnetic resonance (EPR) linewidth in layered manganese systems which was first predicted by Waldner [5,6]. In [3,4] the EPR linewidth was calculated from the dynamic spin correlation function with the time dependence from the solitonmagnon interaction; moveover, it was shown that the calculated linewidth matched the observed Arrhenius behavior.In this Letter we show that a new type of soliton pinned to a nonmagnetic impurity will form, and this pinned soliton has a lower energy than a large pinned soliton with a corresponding larger density in the lattice. This lowering of energy for the impurity solitons occurs simply because of elimination of exchange bonds at the impurity, which is a significant effect in the small and a negligible effect in the large impurity solitons. Because of this energy difference, the smaller pinned soliton will dominate the BP soliton in the fluctuation region. In order to relate these small impurity-pinned solitons to experimental data, we first obtain the temperature-dependent EPR linewidth resulting from these structures as a function of impurity concentration. This calculation shows that there will be large changes in the temperature dependence of the EPR linewidth as the impurity concentration is varied in a small (less than 1%) range. Finally, this effect is observed by EPR measurements on manganese compounds with nonmagnetic impurities where the calculated impurity dependence is indeed observed.We begin with...

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The mixed crystal Rb~-"(NH4)"HzAs04 (RADA) has been investigated for several values of x(x =0, 0.12, 0.15, and 0.20) by measuring the complex dielectric permittivity along the a tetragonal axis in the temperature range from 3 to 300 K and frequency range from 1 Hz to 30 kHz. We find coexistence of ferroelectric and proton-glass order in RADA for x =0.12 and probably at x 0.15. The long-range ferroelectric order is not destroyed by the onset of proton-glass ordering. This behavior is similar to that of other proton-glass systems and of the magnetic Ising glass systems reported by Wong etal. in Feo»Mg0. 45C12 and by Yoshizawa etal. in Fe Mn]-Ti03.

The dc conductivity and ac dielectric susceptibility of normal and deuterated lithium hydrazinium sulfate have been measured over a wide temperature range at frequencies up to 9.33 GHz. Over a very large temperature and frequency range the real and imaginary parts of the susceptibility are very large (up to &' = &" =10) and vary with frequency somewhat as f This unusual behavior is shown to result from the nearly one-dimensional protonic conductivity and its extreme sensitivity to barriers caused by local structural defects. Etching studies indicate that the crystal is not ferroelectric, implying that the apparent hysteresis loops result from saturation of the ac conduction.