LOW-ENERGY ND SPIN-WAVES IN NONCOLLINEAR ND2CUO4

The low-energy Nd spin waves for the noncollinear antiferromagnet (AF) Nd2CuO4 are derived within the context of a RPA model for the eight s ublattice AF structure. The dispersion of spin wave modes is given for the whole Brillouin zone and for arbitrary uniaxial anisotropy. The s tructure factors are determined for total momentum transfer in the tet ragonal plane. This theory was successful in interpreting and analyzin g single-crystal neutron scattering results for Nd2CuO4. The results s how that: (1) The noncollinear Nd order is enforced by a ferromagnetic Cu-Nd exchange. (2) There are acoustic and optical bands of a total w idth similar or equal to 0.6 meV showing that Nd-Nd exchange is of equ al importance as the Cu-Nd exchange. (3) Doping with Ce leads to an op tical mode softening at x similar or equal to 0.1 which signifies the onset of a magnetic instability of the Nd structure; this is the likel y reason for the onset of a high specific-heat gamma value in the Ce d oped crystals.