Effects of cation site disorder on the magnetic properties of the chain compounds Ca3CoXO6 (X = Co, Rh)

Abstract

Ca3CoXO6 (X = Co, Rh) is a low-dimensional system of materials with (CoXO6)a chains separated by Ca ions and packed in a triangular fashion. For Ca3Co2O6, each chain has CoO6 octahedra alternating with CoO6 trigonal prisms by sharing the triangular face; in the case of Ca3CoRhO6, most of the Co ions in the octahedral sites are replaced by Rh ions. We have studied Ca3CoXO6 (X = Co, Rh) by carrying out thorough measurements of its magnetic properties, high-resolution neutron powder diffraction, and X-ray absorption spectroscopy. Ferromagnetic ordering along the chain's direction (c-axis) appears at T (c1) similar to 25 K in Ca3Co2O6 while it occurs at a much higher temperature similar to 90 K in Ca3CoRhO6. The magnetic order in the packing ab-plane, which results from the interaction between neighboring ferromagnetic chains, is found to change from a partiallydisordered antiferromagnetic to a ferrimagnetic and then to a ferromagnetic one under an applied magnetic field in both Ca3Co2O6 and Ca3CoRhO6. For Ca3CoRhO6 with a higher T (c1), a controllable parameter, that is, the degree of the Co-Rh cation site ordering exists within a chain, which affects the magnetic properties in a major way. This cation site order can be controlled experimentally by varying the rate of cooling to room temperature after the final sintering in the sample's synthesis procedures. In the samples air-quenched at a cooling rate of several hundred K/min, T (c1) was decreased to similar to 80 K from 90 K for normal samples with a 2-K/min cooling rate, and the ferrimagnetic region in the H -T phase diagram shrunk. In contrast, the Tc1 of the ones that were slow cooled at a cooling rate of similar to 0.5 K/min was increased to a higher temperature of similar to 100 K, and the ferrimagnetic region in the H - T phase diagram correspondingly became enlarged. For comparison, Ca3Co2O6 samples with different cooling rates were also investigated, and their magnetic properties were found to be independent of the synthesis conditions. We discuss the present results in terms of the disorder, frustration, and dimensionality of the Ca3CoXO6 system.