Magnetic and electronic Co states in the layered cobaltate GdBaCo2 O5.5-x

Abstract

We have performed nonresonant 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 GdBaCo2 O5.5. Our data are consistent with a Co Py 3+ high-spin (HS) state at the pyramidal sites and a Co Oc 3+ low-spin (LS) state at the octahedral sites. The structural distortion with a doubling of the a axis (2 ap ×2 ap ×2 ap 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 Co Py 3+ HS state. This distortion corresponds to an alternating ordering of xz and yz orbitals along the a and c axes for the Co Py 3+. The orbital ordering and pyramidal distortion lead to deformation of the octahedra but the Co Oc 3+ LS state does not allow an orbital order to occur for the Co Oc 3+ 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 noncollinear magnetic structure. The x-ray magnetic circular dichroism data indicate that there is a large orbital magnetic contribution to the total ordered Co moment. © 2008 The American Physical Society.