X-ray absorption and emission spectroscopy study of Mn and Co valence and spin states in TbMn$_{\rm 1-x}$Co$_{\rm x}$O$_3$

TL;DR

This study uses advanced x-ray spectroscopy to analyze how Mn and Co valence and spin states evolve in TbMn$_{1-x}$Co$_x$O$_3$, revealing changes in magnetic order and structure with doping.

Contribution

It provides detailed insights into the valence and spin state transitions of Mn and Co in TbMn$_{1-x}$Co$_x$O$_3$ using XAS and XES, highlighting the impact on magnetic properties.

Findings

Mn$^{3+}$ to Mn$^{4+}$ transition with doping

Co$^{2+}$ HS to Co$^{3+}$ LS transition

Collapse of ferromagnetism at high Co doping

Abstract

The valence and spin state evolution of Mn and Co on TbMn$_{\rm 1-x}$Co$_{\rm x}$O$_3$ series is precisely determined by means of soft and hard x-ray absorption spectroscopy (XAS) and K$\beta$ x-ray emission spectroscopy (XES). Our results show the change from Mn$^{3+}$ to Mn$^{4+}$ both high-spin (HS) together with the evolution from Co$^{2+}$ HS to Co$^{3+}$ low-spin (LS) with increasing $\rm x$. In addition, high energy resolution XAS spectra on the K pre-edge region are interpreted in terms of the strong charge transfer and hybridization effects along the series. These results correlate well with the spin values of Mn and Co atoms obtained from the K$\beta$ XES data. From this study, we determine that Co enters into the transition metal sublattice of TbMnO$_3$ as a divalent ion in HS state, destabilizing the Mn long range magnetic order since very low doping compositions (${\rm x} \le 0.1$). Samples in the intermediate composition range ($0.4 \le {\rm x} \le 0.6$) adopt the crystal structure of a double perovskite with long range ferromagnetic ordering which is due to Mn$^{4+}$-O-Co$^{2+}$ superexchange interactions with both cations in HS configuration. Ferromagnetism vanishes for ${\rm x} \ge 0.7$ due to the structural disorder that collapses the double perovskite structure. The spectroscopic techniques reveal the occurrence of Mn$^{4+}$ HS and a fluctuating valence state Co$^{2+}$ HS/Co$^{3+}$ LS in this composition range. Disorder and competitive interactions lead to a magnetic glassy behaviour in these samples.