Low-energy Nd spin waves in noncollinear Nd 2CuO 4

Abstract

The low-energy Nd spin waves for the noncollinear antiferromagnet (AF) Nd 2CuO 4 are derived within the context of a RPA model for the eight sublattice AF structure. The dispersion of spin wave modes is given for the whole Brillouin zone and for arbitrary uniaxial anisotropy. The structure factors are determined for total momentum transfer in the tetragonal plane. This theory was successful in interpreting and analyzing single-crystal neutron scattering results for Nd 2CuO 4. The results show that: (1) The noncollinear Nd order is enforced by a ferromagnetic CuNd exchange. (2) There are acoustic and optical bands of a total width ⋍ 0.6 meV showing that NdNd exchange is of equal importance as the CuNd exchange. (3) Doping with Ce leads to an optical mode softening at x ⋍ 0.1 with signifies the onset of a magnetic instability of the Nd structure; this is the likey reason for the onset of a high specific-heat γ value in the Ce doped crystals.