Weak ferromagnetism and Raman scattering in Eu 2− xPr xCuO 4

Abstract

Magnetization and Raman experiments performed in single crystals of Eu 2− x Pr x CuO 4 (0≲ x≲1) are interpreted in terms of local distortions within the CuO 2 planes. We find that samples grown in Pt crucibles contain Pt impurities, that are partially responsible for these distortions. We observed: (i) the appearance of two forbidden Raman modes ( f RM) and (ii) a weak-ferromagnetic (WF) component when the crystals are field cooled in the ab-plane. The WF component, the onset temperature where 3D long-range WF is first observed, T WF, the coercive field, H C, and the intensity of the f RM decrease as the Pt content decreases. For crystals with a similar Pt content, the above quantities are larger for samples with smaller lattice unit cell volume, V. The crystal with the highest Pt content and smallest V ( x=0) shows, for T→0, an average canted Cu moment of 5.5(3)×10 −3 μ B/Cu. The intensity of the f RM decrease with T, suggesting that the number of distortions decrease as V expands. This is consistent with recent neutron diffraction results in Eu 2CuO 4, where a tetragonal to orthorhombic phase transition was observed at T≲200 K. We concluded that the Pt impurities and/or the reduction of V beyond a critical value, V c, are the origin of these orthorhombic distortions which in turn are responsible for the f RM and WF observed in these compounds.