Field-frequency mapping of the electron spin resonance in the paramagnetic and antiferromagnetic states of LaMnO₃

Electron-spin resonance has been studied in a stoichiometric single crystal of LaMnO₃ in the temperature range of 4.2 K-250 K. LaMnO₃ exhibits an extensively studied antiferromagnetic (AF) transition at T_N = 141K. The spin resonance was detected both in a fixed-frequency microwave setup with various fixed frequencies (75 GHz, 150 GHz, and 225 GHz) in field-sweep mode or in a broad frequency band instrument at various fixed fields. The latter facility covers the far-infrared range with a lower cutoff frequency of 4 cm (120 GHz). Magnetic fields up to 14 T have been applied along the crystallographic b direction (the easy-axis direction in the AF state). The field dependence of the resonance is fully mapped and the results at low temperatures are well described by the AF resonance theory of Kittel and Keffer modified for the weak ferromagnetism. Deviations from the theory are evident at temperatures close to T_N. In the paramagnetic state the spin resonance is broad, and in addition to the expected paramagnetic signal around g = 2, a temperature-dependent anomalous resonance has been also observed.