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)∞ 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 ~ 25 K in Ca₃Co₂O₆ while it occurs at a much higher temperature ~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 ~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 ~0.5 K/min was increased to a higher temperature of ~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.