Sign of the Nearest-Neighbor Exchange Interaction and its Derivative in GdAs

The sign of the nearest-neighbor exchange interaction $J_1$ and its logarithmic derivative $\frac{\partial \ln J_1}{\partial r}$ has been determined for the intermetallic type-II antiferromagnet GdAs. The ${\mathrm{As}}^{75}$ nuclear-magnetic-resonance Knight-shift data in the paramagnetic state of GdAs were used to yield a value for the Curie-Weiss temperature $\theta \simeq -35\mathrm{}°$K. Combining this value with the Néel temperature $T_N\simeq 25°$K and using the molecular field theory gives $J_1=-0.08\mathrm{}°$K and $J_2=-0.40\mathrm{}°$K. The measurement of the sign of $\frac{\partial \ln J_1}{\partial r}$ in GdAs was performed using low-temperature x-ray powder-diffraction techniques in order to study the exchange-striction-induced trigonal lattice distortion for $T<\mathrm{}{T}_N$. The observed trigonal distortion was found to be a compression along the [111] direction of the NaCl-type GdAs crystal structure. From the sign of the trigonal distortion, it is concluded that $\frac{\partial \ln J_1}{\partial r}$ is negative. Similar data for GdP suggest that $\frac{\partial \ln J_1}{\partial r}$ is also negative for this compound.