SOFT PHONON BEHAVIOR AND MAGNETISM AT THE LOW-TEMPERATURE STRUCTURAL PHASE-TRANSITION OF LA1.65ND0.35CUO4

We report a neutron scattering investigation of a La1.65Nd0.35CuO4 single crystal which undergoes a second order structural transition with decreasing temperature from a high temperature orthorhombic phase (space group Bmab) to a distinct low temperature orthorhombic phase (space group Pccn) at T(s) = 76 K. The transition is induced by the softening of the phonon mode that involves rotations of the tilt axis of the CuO6 octahedra. Antiferromagnetic long range order is established below T(N) = 316 K, and a hysteretic reorientation to a non-collinear spin structure occurs below T(s). The low energy spin dynamics of La1.65Nd0.35CuO4 above T(s) are closely similar to those of stoichiometric La2CuO4; at 100 K the gaps for in-plane and out-of-plane polarized spin waves are 2.3 meV and 5 meV, respectively. Out-of-plane polarized magnons are little affected by the structural transition, but the gap for in-plane polarized magnons increases markedly below T(s).