Successive Phase Transitions of LuBaCo₄O₇ with Kagome and Triangular Lattices

Abstract

Magnetic, thermal and neutron diffraction measurements have been carried out on single-crystal samples of LuBaCo₄O₇ with kagome and triangular lattices of Co spins. Anomalies of the magnetic susceptibility have been found at temperatures T_a1∼70 K and T_a2∼50 K. We have also found a second-order structural transition at T_c1∼165 K and a first-order structural transition at T_c2∼105 K (T_c2′∼80 K) with increasing (decreasing) temperature. Two groups of magnetic diffuse scattering, corresponding to the short-range correlations with the commensurate (C) and incommensurate (IC) periods, have been found to exist below the structural transition temperature T_c2 (T_c2′). The reflections with the C and IC periods originate from the moments on the kagome and triangular lattices, respectively. At 10 K, the magnetic correlation length is estimated to be ∼100 Å. The structural changes of LuBaCo₄O₇ and YBaCo₄O₇ differ significantly, which seems to induce the differences in their magnetic properties.