Anomalous magnetic phase diagrams in the site-diluted Heisenberg antiferromagnets, A2Fe1-xInxCl5•H2O (A = Rb, K)

The effect of the substitution of diamagnetic ions for paramagnetic ones in the magnetic phase diagrams of the low-anisotropy antiferromagnets A(2)Fe(1-x)In(x)Cl(5). H2O (A = Rb, K) is investigated. In the region where the spin-flop (SF) transition occurs, the consequences of dilution are manifested as the appearance of a structure of secondary transition lines and a substantial enhancement of the transition width. In the SF region a multiple-peck structure is observed in the ac susceptibility measurements which is associated with the secondary transition lines. This behaviour is discussed in terms of several mechanisms proposed previously. When the sample is cooled in applied fields below H-SF(T) we observe the presence of a remanent magnetization (M-r) in the antiferromagnetic (AF) phase. Such magnetization was previously found in these solid solutions at very low fields (a few Oe). Here we also find that M-r follows a temperature dependence that is independent of the concentration x and is the same for the K and Rb derivatives.