Neutron Study of the Magnetic Structures and Phase Transitions in RCuAs$_2$ (R=Pr, Nd, Tb, Dy, Ho, Yb)

Neutron diffraction and inelastic scattering studies have been carried out on polycrystalline samples of the above titled materials as a function of temperature and applied magnetic field to determine the magnetic structures, order parameters, and overall spin dynamics. The space group of these compound is P4/nmm with typical (tetragonal) lattice parameters a $\sim$ 3.9 \AA\ and c $\sim$ 10.0 \AA. PrCuAs$_{2}$ develops commensurate magnetic order at T$_{N}$=6.4 K with an ordered moment of 1.24 $\mu _{B}$ oriented along the c-axis. The ordering wave vector is (0,0,1/2), with a $\Gamma _{3}^{1}$ representation. NdCuAs$_{2}$ orders at T$_{N}$=3.54 K with the same ordering wave vector (with the $\Gamma _{10}^{2}$ or $\Gamma _{9}^{2}$ representations), but with the moment direction in the a-b plane along [110] of magnitude 3.5 $\mu _{B}$. TbCuAs$_{2}$ and HoCuAs$_{2}$ exhibit incommensurate order at T$_{N}$=9.1 and 4.0 K, respectively. The results for the inelastic scattering will be discussed.