Structural, magnetic and electronic properties of CaBaCo4-xMxO7 (M= Fe, Zn)

TL;DR

This study investigates how substituting Fe and Zn into CaBaCo4O7 affects its structure, magnetic order, and electronic states, revealing site preferences, valence states, and the impact on magnetic properties and local disorder.

Contribution

It provides new insights into the site-specific substitution effects on the structural and magnetic properties of CaBaCo4O7, including valence states and local disorder mechanisms.

Findings

Fe substitution suppresses ferrimagnetic order, inducing antiferromagnetism.

Zn substitution leads to magnetic glass behavior due to disrupted magnetic interactions.

Iron is in a 3+ state, zinc in 2+, affecting the Co oxidation state and local structure.

Abstract

The effect of substituting iron and zinc for cobalt in CaBaCo$_4$O$_7$ has been investigated using neutron diffraction and x-ray absorption spectroscopy. The orthorhombic distortion present in the parent compound CaBaCo$_4$O$_7$ decreases with increasing the content of either Fe or Zn. The samples CaBaCo$_3$ZnO$_7$ and CaBaCo$_{4-x}$Fe$_x$O$_7$ with $x \leq 1.5$ are metrically hexagonal but much better refinements in the neutron diffraction patterns are obtained using an orthorhombic unit cell. The two types of substitution have opposite effects on the structural and magnetic properties. Fe atoms preferentially occupy the sites at the triangular layer. Thus, the replacement of Co by Fe supresses the ferrimagnetic ordering and CaBaCo$_{4-x}$Fe$_x$O$_7$ samples are antiferromagnetically ordered with a new propagation vector k=(1/3,0,0). However, the Zn atoms prefer occupying the Kagome layer, which is very detrimental for the long range magnetic interactions giving rise to a magnetic glass. The oxidation state of iron and zinc is found to be 3+ and 2+, respectively, independently of the content. Therefore, the average Co oxidation state changes accordingly with the Fe$^{3+}$ or Zn$^{2+}$ doping. Also, x-ray absorption spectroscopy data confirms the different preferential occupation for both Fe and Zn cations. The combined information obtained by neutron diffraction and x-ray absorption spectroscopy indicates that cobalt atoms can be either in a fluctuating Co$^{2+}$/Co$^{3+}$ valence state or, alternatively, Co$^{2+}$ and Co$^{3+}$ ions being randomly distributed in the lattice. These results explain the occurrence of local disorder in the CoO$_4$ tetrahedra obtained by EXAFS. An anomaly in the lattice parameters and an increase in the local disorder is observed only at the ferrimagnetic transition for CaBaCo$_4$O$_7$ revealing the occurrence of local magneto-elastic coupling.