Magnetic-resonance modes for Eu2-xGdxCuO4, a compound with coupled paramagnetic-weak-ferromagnetic excitations.

Abstract

The magnetic-resonance modes for a three-sublattice-coupled paramagnetic--weak-ferromagnetic system are derived in the molecular-field approximation. The model is proposed to describe the microwave-absorption experiments in the Gd cuprates below the ordering temperature of the Cu magnetic moments. Two of the sublattices are associated with the Cu moments and the third describes the ${\mathrm{Gd}}^{3+}$ paramagnetic lattice. An isotropic antiferromagnetic interaction is considered to couple the Cu moments with an antisymmetric exchange being responsible for the weak ferromagnetism. Two separate components are considered for the magnetic anisotropy of the Cu moments: a strong out-of-plane term (coincident with the ${\mathrm{CuO}}_{2}$ planes), and a smaller in-plane anisotropy. The Cu and Gd magnetic moments are assumed to interact through an isotropic ferromagnetic exchange interaction. Static and dynamic corrections to the paramagnetic and weak-ferromagnetic modes are described. The former are due to internal fields originated in the Cu-Gd interaction. The dynamic corrections are important for some orientations of the applied magnetic field for which the modes become degenerate. Anticrossing and strong mode mixing is observed in this case. The predictions of the model are compared with the available experimental data. It is shown that both static and dynamic corrections are needed in order to account for the anisotropy of the observed microwave absorptions. A consistent description of the complex magnetic resonance behavior of the Gd cuprates is obtained within the proposed model.