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...

Layered manganese-halide compounds exhibit quasi-two-dimensional magnetic behavior in the temperature region immediately above the ordering temperature where soliton excitation can be experimentally detected as an Arrhenius, exp(E/T), temperature-dependent electron paramagnetic resonance linewidth, where E is the soliton energy. When a nonmagnetic impurity such as magnesium is introduced into the Mn lattice, experimental linewidth data indicate that the excitation energy is dramatically reduced and the temperature range over which Arrhenius behavior is observed is widened to include higher temperatures. These effects occur for very small (less than 1%) impurity concentrations.Quasi-two-dimensional (2D) magnetic systems can be physically realized as layered metal-halide compounds of the form (C n H 2n+1 NH 3 ) 2 MX 4 , where M is a transition metal ion and X is Cl or Br. For the case with M = Mn(II) and X = Cl these compounds are nearly-classical (the Mn ion has a spin of 5/2) spin systems with long-range antiferromagnetic order. However, as the ordering temperature is approached from above there is a range of a few Kelvins (referred to as the fluctuation region) where short-range order effects within the metal-halide layer become important. It is in this relatively narrow temperature range where the system exhibits 2D antiferromagnetic correlations and the corresponding 2D nonlinear excitations, or solitons, can be experimentally studied and compared with theoretical calculations of magnetic quantities. From the theoretical point of view it is known that the classical 2D magnet with an isotropic exchange interaction supports soliton excitations. The form and energy of these excitations was obtained by Belavin and Poyakov [1] from simple topological considerations indicating that the energy is E s ¼ 4pJS 2 , where J is the exchange constant and S is the value of the spin. Topology also dictates that these are vortex-like excitations, but to avoid a singularity at the excitation center, the magnetization rotates through an angle of 180 going radially out from the center. In the following we refer to these as Belavin-Polyakov (BP) excitations. It is expected that these structures should be present in layered magnetic compounds, and indeed, early EPR measurements [2, 3] and recent research [4,5] has shown that solitons rather than magnons are the dominant contribution to the temperature dependence of the electron paramagnetic resonance (EPR) linewidth in the fluctuation region. Experimentally this is noticed as an Arrhenius, exp E s =T ð Þ, temperature dependence where E s is the soliton excitation energy. In this work the effect of nonmagnetic impurities interacting with solitons on the EPR linewidth is investigated. There are various interesting effects that can be attribu-

The title compound, (C 4 NH 10 )CuCl 3 , is shown to exist in two phases, the previously reported R phase and a new β phase. DSC studies indicate that the β phase transforms to the R phase at 93 °C. The β phase is stable at the room temperature, with the R phase metastable with respect to the β phase at room temperature. Crystals of the β phase are monoclinic, C2/c, with a ) 17.327(3) Å, b ) 8.360(2) Å, c ) 12.005(2) Å, and β ) 100.92 (3)°with Z ) 8 for F ) 1.883 g/cm 3 . The structure contains chains of bibridged Cu 2 Cl 6 2dimers running parallel to the c direction. This is in contrast to the previously reported R form, which contains uniform chains of face-shared octahedra. The chains in the R phase lie parallel to the unique (monoclinic) axis. Thus, no crystallographic relationships exist between the two compounds and the phase transition must be first order in nature. A unique feature of the phase transition is a decrease in volume as the crystal is heated through the transition. This is a result of the formation of the more compact chains of face-shared octahedra in the high-temperature phase. Magnetic susceptibility studies of the β phase are indicative of competing ferromagnetic and antiferromagnetic coupling, with the onset of long-range order at 7.5 K. The data are interpreted in terms of a ladder chain consisting of ferromagnetic dimers coupled into antiferromagnetic chains through short Cl‚‚‚Cl contacts.

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Recent theoretical and experimental work confirm the existence of solitons in the nearly classical two-dimensional (2D) Heisenberg antiferromagnet. Previous ac susceptibility measurements on (n-propyl ammonium)2MnCl4 or (PAMC), a 2D spin 5/2 antiferromagnet AFM shows that the susceptibility is strongly dependent on a weak magnetic field (≈0.2 Oe) at temperatures below TC. These data provided the motivation for a calculation of the field dependence of the static soliton structure factor. This is done by assuming the small field dependent term as a perturbation around the static soliton solution. The dynamic part of the correlation function is assumed to result from soliton–magnon scattering in the Born approximation. These field-dependent and dynamic results are combined to obtain the dynamic field-dependent correlation function.