Low temperature crystallographic and magnetic study of LaCoO 3

Abstract

The structural and magnetic properties of LaCoO 3 between 300 and 4·2°K are studied. Previous investigators have usually reported the existence of a local maximum in the susceptibility at 90°K in this material, but a recent single crystal study by Naiman and coworkers indicated no such phenomenon. We find that the magnetic properties in chemically pure materials are strongly dependent on the method of preparation. Thus we have been able to obtain samples exhibiting a susceptibility maximum, and then remove it by additional regrinding and renring. Samples both with and without a susceptibility maximum were studied using neutron diffraction. Our results indicate that both sample types have R 3̄ c symmetry at room temperature and below. In addition, neither type exhibits any long-range bulk magnetic order at 4·2°K. This result is in accord with that of Koehler and Wollan. Both sample types were studied with a vibrating-coil magnetometer. An anomalously rapid increase of magnetic moment with decreasing temperature was observed below 40°K in a field of 10 kOe. The presence of a small ferromagnetic component was established by the presence of hysteresis at 4·2°K, a trace of this hysteresis remaining at 77δK. The magnetic moment, as measured in 200 Oe, was found to decrease gradually with increasing temperature above 4·2°K. The absence of any abrupt drop indicates the remanent moment to be due to isolated regions of a magnetic phase dispersed in a non-magnetic matrix. We conclude that variations in the size and number of these regions are the cause of the discrepancies between observed magnetic properties of different samples of LaCoO 3, that they are virtually unavoidable because of near-degeneracy with the bulk diamagnetic phase, and that their presence precludes definitive measurement of the low-temperature susceptibility of LaCoO 3.