Magnetic Structure of CaBaCo4O7: Lifting of Geometrical Frustration towards Ferrimagnetism

TL;DR

This study uncovers how significant structural distortion and charge ordering in CaBaCo4O7 lift geometrical frustration, leading to a unique ferrimagnetic order despite the triangular lattice topology.

Contribution

It reveals the detailed magnetic structure and charge ordering in CaBaCo4O7, demonstrating how structural distortions enable ferrimagnetism in a geometrically frustrated lattice.

Findings

CaBaCo4O7 exhibits the largest distortion in the 114 series.

Charge ordering with Co2+ and mixed valence states is observed.

Ferrimagnetic structure involves triple chains with specific coupling patterns.

Abstract

CaBaCo4O7 represents a new class of ferrimagnets whose structure is built up of CoO4 tetrahedra only, similarly to other members LnBaCo4O7 of the 114 series, forming an alternate stacking of kagome and triangular layers. Neutron powder diffraction reveals, that this compound exhibits the largest distortion within the 114 series, characterized by a strong buckling of the kagome layers. Differently from all other members it shows charge ordering, with Co2+ sitting on two sites (Co2, Co3) and mixed valent cobalt Co3+/Co2+L sitting on two other sites (Co1, Co4). The unique ferrimagnetic structure of this cobaltite at 4 K can be described as the assemblage of ferrimagnetic triple chains (Co1 Co2 Co3) running perpendicular to the kagome layers, ferromagnetically coupled within the layers, and antiferromagnetically coupled with a fourth cobalt species Co4. The lifting of the geometrical frustration towards ferrimagnetism, which appears in spite of the triangular topology of the cobalt lattice, is explained by the very large structural distortion, charge ordering phenomena and large cobalt valence compared to other LnBaCo4O7 oxides.