MAGNETIC-STRUCTURE AND SPIN DYNAMICS OF THE PR AND CU IN PR2CUO4

Neutron-scattering techniques have been used to study the magnetic structure and spin dynamics of the Pr and Cu spins in Pr2CuO4. In the ordered state the Cu spin-wave velocity c has been determined to be 0.85 0.08 eV, which corresponds to an in-plane nearest-neighbor exchange constant J=130 13 meV. A spin-wave gap of 5 meV has been observed, corresponding to a reduced anisotropy constant =(J-Jxy)/J of 2×10-4. In the paramagnetic regime the evolution of the Cu spin-correlation length with temperature is adequately described by the renormalized classical theory for the quantum nonlinear sigma model. For the Pr ions, significant dispersion is observed for the first excited-state crystal-field level, directly demonstrating that there are Pr-Pr exchange interactions both within the a-b plane as well as along the c axis. These interactions along the c axis must be mediated through the CuO planes which are also involved in superconductivity in these cuprate materials. A singlet-doublet magnetic exciton model, with Pr-Pr Heisenberg exchange terms as large as 0.8 meV, provides a good quantitative description of the measured dispersion relations. The temperature dependence of the magnetic excitons also can be qualitatively understood with this theory if the exchange terms are modified by a temperature-dependent renormalization factor. The zero-field ordered moment at low temperatures for the Cu is determined to be 0.40 0.02 B, in good agreement with results reported by other groups. However, field-dependent diffraction measurements suggest that the correct Cu spin structure is the noncollinear one, where spins in adjacent layers along the c axis are orthogonal, rather than the collinear structure assumed by other groups. This noncollinearity is also reflected in the configuration of the small induced moments (0.08 0.005 B) that develop at low temperatures on the Pr ions. The magnetic-field temperature phase diagram for the case of an applied field along the [11 0] direction reveals that the spin-rotation energy increases rapidly with decreasing temperature from 200 K down to 4.5 K. © 1995 The American Physical Society.