Antiferromagnetic Resonance in MnCl2·4H2O over the Temperature Range 0.32-1.62°K

Antiferromagnetic and paramagnetic resonance experiments have been performed on Mn${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O ($T_N=1.62\mathrm{}°$K) in the temperature range 1.02 to 4.2°K at 3-cm microwave wavelengths and in the temperature range 0.32 to 4.2°K at 1.2-cm microwave wavelengths. Temperatures below 1°K were achieved with liquid-${\mathrm{He}}^3$ refrigeration. In the antiferromagnetic state, resonance on the critical-field hyperbola is observed in accordance with the Nagamiya-Yosida theory for orthorhombic anisotropy. The low-field (below critical field) antiferromagnetic resonance behavior, both on and off axis, is also in reasonably good agreement with the theory. Above the critical field, the resonances deviate substantially from the theoretical predictions. This is expected because the Nagamiya-Yosida theory is valid only when the external magnetic field is small compared to the exchange field, and this condition is not satisfied in Mn${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O for the high-field resonances. At temperatures below ∼0.6°K, the low-field resonance which occurs with 1.2-cm microwaves and with the external magnetic field parallel to the intermediate anisotropy axis deviates sharply from the theory. Also, below 0.6°K, an additional resonance having some properties similar to critical-field resonance appears at a magnetic-field value lower than the critical field. These low-temperature phenomena may be associated with a modification in the magnetic structure below ∼0.6°K.