Magnetic Phase Diagram of MnF2 from Ultrasonic and Differential Magnetization Measurements

The magnetic phase diagram of MnF₂, in the H-T plane, is determined in magnetic fields up to 200 kG directed along the [001] and [100] directions. The magnetic phase transitions appear as anomalies in the ultrasonic attenuation and/or the differential magnetization. Near the various second-order phase transitions, the attenuation of longitudinal sound waves exhibits λ anomalies, whereas near the spin-flop transition (which is a first-order transition) the ultrasonic attenuation exhibits a sharp spike and/or an abrupt increase, depending on the mode of propagation. The spin-flop transition is accompanied by a spike in the differential magnetic moment. The Néel temperature is T_N=(67.33+/-0.03)°K, and the triple point for H∥[001] is at T₃=(64.9+/-0.1)°K and H₃=119+/-2 kG. The field at the spin-flop transition (for H∥[001]) increases monotonically with temperature from 92+/-1.5 kG at 4.2°K to 119+/-2 kG at the triple point. The curvature, at T_N, of the antiferromagnetic-paramagnetic boundary with H∥[001] is d²TdH²=-(3.2+/-0.2)×10^{- 10}°K/G². The curvature, at T_N, for the antiferromagnetic-paramagnetic boundary with H∥[100] is smaller by about an order of magnitude. The various phase boundaries are compared with the predictions of the molecular-field theory and other theoretical models.