Magnetic and electronic Co states in layered cobaltate GdBaCo2O5.5-x

TL;DR

This study investigates the electronic and magnetic states of Co ions in GdBaCo2O5.5, revealing orbital order, specific spin states, and a non-collinear magnetic structure through advanced x-ray techniques.

Contribution

It provides detailed insights into the orbital and spin states of Co ions and their relation to structural distortions in GdBaCo2O5.5, which was not previously characterized.

Findings

Co3+ ions are in HS state at pyramidal sites and LS state at octahedral sites.

Orbital order is associated with the metal-insulator transition.

Magnetic moments are aligned in the ab plane with a non-collinear structure.

Abstract

We have performed non-resonant x-ray diffraction, resonant soft and hard x-ray magnetic diffraction, soft x-ray absorption and x-ray magnetic circular dichroism measurements to clarify the electronic and magnetic states of the Co3+ ions in GdBaCo2O5.5. Our data are consistent with a 3+ Py Co HS state at the pyramidal sites and a 3+ Oc Co LS state at the octahedral sites. The structural distortion, with a doubling of the a axis (2ap x 2ap x 2ap cell), shows alternating elongations and contractions of the pyramids and indicates that the metal-insulator transition is associated with orbital order in the t2g orbitals of the 3+ Py Co HS state. This distortion corresponds to an alternating ordering of xz and yz orbitals along the a and c axes for the 3+ Py Co . The orbital ordering and pyramidal distortion lead to deformation of the octahedra, but the 3+ Oc Co LS state does not allow an orbital order to occur for the 3+ Oc Co ions. The soft x-ray magnetic diffraction results indicate that the magnetic moments are aligned in the ab plane but are not parallel to the crystallographic a or b axes. The orbital order and the doubling of the magnetic unit cell along the c axis support a non-collinear magnetic structure. The x-ray magnetic circular dichroism data indicate that there is a large orbital magnetic contribution to the total ordered Co moment.