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 A2Fe1-xInxCl5·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-peak 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 HSF(T) we observe the presence of a remanent magnetization (Mr) 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 Mr follows a temperature dependence that is independent of the concentration x and is the same for the K and Rb derivatives.The research at the University of Zaragoza was supported by CICYT grants MAT92-896, MAT94-0043 and MAT97-951. The research at the University of Sao Paulo was supported by grants from the CNPq and FAPESP, grant numbers 96/12051-1 and 96/06208-5 and by FINEP—Financiadora de Estudos e Projetos and the CNPq-RHAE programme. The cooperation between the University of Sao Paulo and the University of Zaragoza was supported ˜ by the cooperative grants CSIC-CNPq and CCInt-USP. One of us (Javier Campo) acknowledges a doctoral fellowship from the Diputacion Foral de Navarra.Peer Reviewe