Crossover behavior of the magnetic phase boundary of the low-anisotropy antiferromagnet KNiF3

Abstract

The ordering temperature $T_c$ of the low-anisotropy cubic antiferromagnet KNi${\mathrm{F}}_3$ was measured as a function of magnetic field $H$, up to 180 kOe. At any fixed $H$, $T_c$ was determined from the $\lambda$ anomaly in the thermal-expansion coefficient. The transition temperature $T_c$ increased monotonically with increasing $H$. At the maximum field of 180 kOe, $T_c$ was 0.18 K higher than the Néel temperature $T_N=246.4\mathrm{}$ K. The observed phase boundary $T_c\mathrm{}\left(H\right)\mathrm{}$ is in qualitative agreement with that predicted by Fisher, Nelson, and Kosterlitz for the fully isotropic three-dimensional Heisenberg antiferromagnet. The increase of $T_c$ with increasing $H$ is attributed to a crossover from a Heisenberg-like transition at $H=0\mathrm{}$ to an $\mathrm{XY}$-like transition at finite $H$. Magnetostriction data, taken below $T_N$, suggest a tetragonal lattice distortion of order ${10}^{-4\mathrm{}}$ at $T\ll T_N$.