FIELD-DEPENDENCE OF ND SPIN-WAVES IN ND2CUO4

The noncollinear (NC) antiferromagnet Nd2CuO4 exhibits a transition to a collinear (C) structure at a critical field of 0.75 T. This transit ion is described within an exchange model for the Cu and Nd moments wh ich is largely determined by zero-field neutron scattering results. Fu rthermore, a previously developed theory of Nd spin waves is extended for applied fields along [1 1 0]. It is shown that the NC to C transit ion leads to significant changes in the spin wave spectrum, notably at the X- and M-points of the Brillouin zone, where a strong increase of mode splitting is observed. A suggestion on the origin of a possible lifting of fourfold degeneracy in zero field is given. (C) 1998 Elsevi er Science B.V. All rights reserved.