Magnetic Phase Diagram of MnF2from Ultrasonic and Differential Magnetization Measurements

The magnetic phase diagram of Mn${\mathrm{F}}_2$, 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 $\lambda$ 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 $T_N=(67.33\mathrm{}\pm 0.03)°$K, and the triple point for H∥[001] is at $T_3=(64.9\mathrm{}\pm 0.1)°$K and $H_3=119\mathrm{}\pm 2$ 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 $\frac{d^{2}T}{d{H}^2}=-(3.2\mathrm{}\pm 0.2)\times {10}^{-10\mathrm{}}°$K/${\mathrm{G}}^2$. 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.