Soft phonon behavior and magnetism at the low temperature structural phase transition of La_1.65Nd_0.35CuO₄

We report a neutron scattering investigation of a La_1.65Nd_0.35CuO₄ 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 noncollinear spin structure occurs below T_s. The low energy spin dynamics of La_1.65Nd_0.35CuO₄ 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.