Neutron scattering investigation of the phase transitions in uranium arsenide

A number of neutron elastic scattering experiments are reported on single crystals of the compound UAs. This has the NaCl crystal structure, orders antiferromagnetically with the type-I structure (AF-I) at $T_N\sim 126$ K, and exhibits a first-order phase transition to the type-I $A$ structure at 63.5 K. The most interesting behavior can be studied via the critical (diffuse) scattering near $T_N$. Above $T_N$ the critical scattering is anisotropic and centered about a wave vector that suggests a tendency to order with an incommensurate structure, but then at $T_N$ suddenly orders with the commensurate type-I ordering. We present a detailed analysis by writing down a Hamiltonian in which the strong cubic anisotropy is included, and the obtain the $\overrightarrow{\mathrm{q}}$-dependent susceptibility via a mean-field calculation. The resulting parameters show that the $z-z$ exchange interaction within the (001) planes is ∼40 as strong as the $x-x$ or $y-y$ coupling between the spins. Moreover, the competition within the interactions leads to the system experiencing "frustration" in deciding between AF-I or a sinusoidal spin arrangement, so that the system is in the vicinity of a Lifshitz point. The strong cubic anisotropy is interpreted as arising from bonding effects between the $5f$ wave functions and the anion $p$ orbitals.