Abstract

RbFeCl3·2H2O consists of layers of weakly coupled linear chains with an interlayer exchange which is another order of magnitude smaller. The low-temperature magnetic behaviour is well described with the 2D S′ = 12 rectangular Ising model. Mössbauer spectra are presented from a broad region around the Néel temperature, which is 12.08 K. Towards the ordering temperature an increasing line broadening is evident. Above the ordering temperature a paramagnetic and a magnetic phase apparently coexist. It will be shown that a model for a distribution of ordering temperatures does not describe the observed spectra well. The possible occurrence of superparamagnetic effects is discussed. Although this mechanism cannot be excluded, an even better explanation can be found within the relaxation model of Blume for a fluctuating hyperfine field, arising from an effective S′ = 12 doublet. The general features of the observed relaxation phenomena are well reproduced by this model. We attribute the decrease in the fluctuation rate of this effective field to critical slowing down of the electron spin correlations.

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