Different Gd3+ sites associated with magnetic ordering and structural distortions in Eu2CuO4:Gd3+ observed via electron-paramagnetic-resonance measurements

We have measured the EPR spectrum of dilute Gd3+ ions substituting for Eu3+ in Eu2CuO4. Each one of the resonance lines shows a splitting into two or more weaker lines below TN 215 K. The simultaneous appearance of a low-field microwave-absorption signal is attributed to the onset of long-range antiferromagnetic ordering with a weak ferromagnetic component. The splitting of the EPR spectrum is associated with different Gd sites in the magnetically ordered structure. A reduction of the local symmetry of the rare-earth sites below TN is described in terms of an internal magnetic field lying in the ab plane and additional crystal-field terms related to local crystallographic distortions. Dipolar and exchange contributions to the internal field are discussed. Possible random static displacements of the oxygen atoms in the CuO2 planes and their role in the development of the weak ferromagnetism and the presence of internal fields at the Gd sites are also analyzed. A strong dependence of the Gd EPR spectrum and the microwave absorption on the magnetic history of the samples has been observed, suggesting the formation of magnetic domains or a glassy state.