Spin fluctuations in a spin-1 paramagnet with easy-planar anisotropy: Application to praseodymium

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Abstract
The effects of fluctuations in an S=1 Heisenberg system with strong easy-planar crystal-field anisotropy are considered in the paramagnetic phase. The Green functions are derived from their equations of motion. The decoupling procedure properly accounts for single-site correlation. The spin-correlation functions are determined self-consistently by equations which reduce to the ones derived in the correlated-effective-field approximation of ME Lines (1974, 1975) in the limits of high temperatures or small anisotropy. The linewidths deduced for the magnetic excitations compare with the results of Bak (1975), determined by a diagrammatic expansion of the Green functions to first order in 1/Z. Except for the introduction of the finite lifetime of the excitations, the application of the theory to the case of Pr leads to only minor adjustment of the 'effective' molecular-field properties.