Coupled weak ferromagnetic-paramagnetic excitations in (Gd 0.5Eu0.5)2CuO4

The electron paramagnetic resonance spectra of Gd3+ in single crystals of (Gd0.5Eu0.5)2CuO4 show an anomalous anisotropy for temperatures below the magnetic ordering of the Cu ions (T approximately=280 K). The authors have performed a detailed experimental study of this feature, and they discuss their results in terms of a model that assumes a weak Heisenberg interaction between Cu and Gd moments. The authors show that that anisotropy is a consequence of the dynamic coupling of the Gd3+ paramagnetic mode with a weak ferromagnetic mode originating in the Cu magnetic moments