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

The sign of the nearest-neighbor exchange interaction J₁ and its logarithmic derivative ∂lnJ₁∂r has been determined for the intermetallic type-II antiferromagnet GdAs. The As⁷⁵ nuclear-magnetic-resonance Knight-shift data in the paramagnetic state of GdAs were used to yield a value for the Curie-Weiss temperature θ-35^∘K. Combining this value with the N\'eel temperature TN25^∘K and using the molecular field theory gives J₁=-0.08^∘K and J₂=-0.40^∘K. The measurement of the sign of ∂lnJ₁∂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<TN. 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 ∂lnJ₁∂r is negative. Similar data for GdP suggest that ∂lnJ₁∂r is also negative for this compound.