CsNiF3: Ferromagnetic chains with X-Y like anisotropy

We have measured the specific heat and the susceptibility of powdered samples of CsNiF₃, a compound which can be considered as consisting of nearly isolated ferromagnetic chains with a large uniaxial anisotropy, which favours the plane perpendicular to the chain-axis. For the intrachain exchange constant a value J/k=(8.3±0·8)K has been deduced from the susceptibility. Using this we were able to separate the magnetic contribution from the measured specific heat. The magnetic part shows a broad maximum at kT_max/J=1.8 and a small anomaly at kT_c/J=0.32, the latter reflecting the transition to 3-dimensional order. Only about 13 per cent of the magnetic entropy is gained below T_c.