Hysteresis and relaxation behavior in diluted A2Fe(1-x)In(x)Cl(5)center dot H2O (A=K,Rb)

Low-anisotropy antiferromagnetic compounds of the general formula A(2)Fe(1-x)In(x)Cl(5) . H2O (A = K,Rb) exhibit a net spontaneous magnetization below their transition temperature (T-N) when they are cooled in very small applied magnetic fields. This magnetization is observed when the easy axis of the system is aligned parallel to the field and is found to saturate at fields of a few Oe. Hysteresis loops M vs H have been obtained at several temperatures for single crystals of samples with different concentrations x. For low values of x the loops are not symmetric and show a series of well-defined jumps. This behavior suggests that a structure of domains is present, the jumps being associated with avalanche-type movements of the antiferromagnetic domain walls. The coercive fields are of the order of a few hundreds of Oe and for similar x are lower in the Rb-based samples than in the K ones. When the applied field is removed below T-N some of the samples of the dilute rubidium derivative show relaxation effects. The drop in M after the field is removed is two orders of magnitude larger than the drop expected from the susceptibility term (chi parallel to H) The origin of the net magnetization in this and in other dilute antiferromagnets has not yet been established but net domain wall magnetization of antiferromagnetic domains is suggested as the most consistent mechanism.