Anomalous magnetic susceptibility and relaxation effects in the one-dimensional antiferromagnet K2FeF5above the Neel temperature

Mossbauer measurements of induced hyperfine fields have been used to obtain information on the magnetic susceptibility of single crystals of K 2 FeF 5 . At temperatures well above the Neel temperature the behaviour corresponds to the normal isotropic paramagnetic susceptibility of the Fe 3+ ions. At temperatures just above the Neel temperature the induced fields observed indicate that the susceptibility is both field dependent and slightly anisotropic. In addition, for fields applied along the magnetic easy axis, the Mossbauer spectra show considerable line broadening effects at temperatures just above the Neel temperature. This behaviour can be related to the correlation of the spins in one dimension above the Neel temperature and to spin relaxation effects associated with the formation of three-dimensionally ordered clusters as the magnetic transition is approached.