Vibronic superexchange in single-crystal LaMn1-xGaxO3 -: art. no. 184423

The temperature dependence of the thermal conductivity, low-field/high-field magnetization, and magnetic ac susceptibility under different hydrostatic pressures have been measured on single-crystal samples of LaMn1-xGaxO3. The data reveal a spin-glass phase in low magnetic fields and a ferromagnetic phase in high fields H greater than or equal to2 kOe. Orbital fluctuations suppress the phonon component and the spin-glass phase suppresses the spin-wave component of the thermal conductivity kappa (T); the x = 0.5 sample has a kappa (T) like that of a glass insulator. On the other hand, pressure stabilizes long-range antiferromagnetic order. The data demonstrate a competition between magnetic and elastic energies; in the x = 0.5 sample, regions with three-dimensional orbital fluctuations and ferromagnetic (180 degrees - phi) Mn(III)-O-Mn(III) vibronic superexchange interactions coexist with regions in which orbital ordering gives rise to an antiferromagnetic interaction between (001) planes.