X-ray absorption and emission spectroscopy study of Mn and Co valence and spin states in TbMn1-xCoxO3

Abstract

The valence and spin state evolution of Mn and Co on TbMn1-xCoxO3 series is precisely determined by means of soft and hard x-ray absorption spectroscopy (XAS) and Kβ x-ray emission spectroscopy (XES). Our results show the change from Mn to Mn both high-spin (HS) together with the evolution from Co HS to Co low-spin (LS) with increasing 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 Kβ XES data. In this paper, we determine that Co enters into the transition metal sublattice of TbMnO as a divalent ion in HS state, destabilizing the Mn long-range magnetic order since very low doping compositions (x≤0.1). Samples in the intermediate composition range (0.4≤x≤0.6) adopt the crystal structure of a double perovskite with long-range ferromagnetic ordering which is due to Mn-O-Co superexchange interactions with both cations in HS configuration. Ferromagnetism vanishes for x≥0.7 due to the structural disorder that collapses the double perovskite structure. The spectroscopic techniques reveal the occurrence of Mn HS and a fluctuating valence state Co HS/Co LS in this composition range. Disorder and competitive interactions lead to a magnetic glassy behavior in these samples.