Studies of a polymeric chromium phosphinate: Structure and static magnetic properties

The semicrystalline polymer, poly(chromium bismethylphenyldioctyl-phosphinate), PCrP-C, contains $S=\frac{3}{2}$ chromium ions coupled by Heisenberg antiferromagnetic exchange ($\frac{J}{k}=-2.5\mathrm{}$ K) along the linear molecule. We present susceptibility and specific-heat data on pressed sheet samples. X-ray diffraction shows that there is preferential alignment of the molecules in the plane of the sheets, and allows an analysis of the resulting magnetic anisotropy. There is an easy plane perpendicular to the molecular axis and the anisotropy parameter ($\frac{D}{k}\simeq -0.03\mathrm{}$ K) is such that a crossover from Heisenberg to $\mathrm{XY}$ behavior is observed as the temperature is lowered. There is no evidence for three-dimensional ordering above 20 mK. The effect of an applied magnetic field is to induce a flopped spin alignment in the one-dimensional short-range-ordered regime. We present a calculation, based on a transfer-matrix technique to include both magnetic field and anisotropy within a classical Heisenberg model, giving good semiquantitative agreement with the experimental data.