Neutron-scattering techniques have been used to study the magnetic structure and spin dynamics of the Pr and Cu spins in ${\mathrm{Pr}}_2$${\mathrm{CuO}}_4$. In the ordered state the Cu spin-wave velocity c has been determined to be 0.85±0.08 eV Å, 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 ${\mathrm{\alpha }}_{\mathrm{\parallel }}$=(J-${\mathit{J}}^{\mathit{x}\mathit{y}}$)/J of ∼2×${10}^{\mathrm{-}4}$. 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${\mathrm{\mu }}_{\mathit{B}}$, 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${\mathrm{\mu }}_{\mathit{B}}$) 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.

• Received 5 October 1994

DOI:https://doi.org/10.1103/PhysRevB.51.5824