Weak ferromagnetism and magnetic phase transitions in Gd$_2$CuO$_4$

TL;DR

This study uses polarized neutron scattering to investigate magnetic structures and phase transitions in Gd₂CuO₄, revealing temperature-dependent cross-overs in interlayer interactions and the origin of weak ferromagnetism.

Contribution

It provides a detailed model of magnetic structures and phase transitions in Gd₂CuO₄, highlighting the temperature-driven changes in interlayer coupling and magnetic order.

Findings

Sharp intensity minima at T_{c1} and T_{c2} indicate cross-overs in interlayer interactions.

Weak ferromagnetism arises from Cu moment rotations linked to structural distortions.

Magnetic structures are modeled for both zero-field and field-induced phases.

Abstract

We report a polarised neutron study of the magnetic structures and phase trasitions in \gdc\ in low magnetic fields. These experiments have been complemented by integrated intensity measurements with unpolarised neutrons in zero field. Polarised neutron flipping ratio measurements have been made with magnetic fields $H = 0.05$, 0.10 and 0.5 T in the temperature range 4-20 K. These have enabled us to deduce that the anomalous temperature behaviour of the coherent magnetic scattering from the Cu sublattice, which shows sharp intensity minima at \Tcn1 $\approx 18$K and \Tcn2 $\approx 8$ K, is due to cross-overs in the sign of the interaction between strongly coupled, weakly ferromagnetic, CuO$_2$ layers. At \Tcn1\ the coupling changes from ferromagnetic to anti-ferromagnetic and long-range order between layers is temporarily lost. \Tcn2\ is the temperature at which the Gd moments order and a further reorganisation of the interlayer order takes place. The weak ferromagnetism of the CuO layers is found to be due to a small rotation of the Cu moments in the same direction as that in which their coordinating oxygen squares rotate in the tetragonal to orthorhombic distortion of the crystal structure. Further analysis of the flipping ratio measurements has enabled us to model the magnetic structures of the zero-field and the field-induced phases of Gd$_2$CuO$_4$.