X-ray Absorption Studies on the Single Crystal SrFeO2.86
S. H. Hsieh1, R. S. Solanki1*, K. Saravanan1, Y. C. Shao1, Y. F. Wang1, S. H. Lee1, C. H. Yao1, C. H. Du1, H. T. Wang2, J. W. Chiou3, H. M. Tsai4, C. W. Pao4, J. F. Lee4, H. J. Lin4, W. F. Pong1
1Department of Physics, Tamkang University, Tamsui, Taiwan
2Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
3Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan
4National Synchrotron Radiation Research Center, Hsinchu, Taiwan
* presenting author:Ravindra Singh Solanki, email:ravin.so85@gmail.com
Recently, magnetic materials showing colossal magnetoresistance (CMR) are subject of intense research in comparison to materials exhibiting giant magnetoresistance (GMR) due to their possible applications in the next generation of magnetic data storage read heads. It is reported that phase separation and orbital ordering play an important role in determining the CMR of transition-metal oxides. But the detailed dependence of CMR on the spin, charge, orbital and structural degree of freedom is still not evident. A very large negative MR of about 90% in the single crystal of SrFeO2.85 related to the charge and magnetic ordering transition temperature near 70K has been observed. To understand the mechanism responsible for the large negative MR, we investigate the electronic and atomic properties of SrFeO2.86. The high quality single crystals of SrFeO2.86 were prepared by floating zone method. Temperature dependence of magnetization and resistivity measurements indicate a first order phase transition around 70K, which is consistent with previous studies. Temperature-dependent x-ray absorption near-edge structure (XANES), x-ray linear dichroism (XLD), and extended x-ray absorption fine structure (EXAFS) were performed at the Fe K-edge, L3,2-edge and O K-edge. Fe K-edge spectra are primarily associated with the Fe 1s→4p transition and a comparison of this spectra with FeO/Fe2+, Fe3O4/Fe2+,3+, and Fe2O3/Fe3+ K-edge spectra confirm that Fe ion in our sample is in a valence state of Fe3+,4+. The major feature of Fe L3,2-edge spectra are associated with Fe 2p→3d transition. These spectra are split into L3 and L2 regions due to spin-orbit interaction. The crystal-field of oxygen octahedra at L3 and L2 split 3d band into eg and T2g states. The eg states consist of 3d3z2-r2 and 3dx2-y2 orbitals. The XLD spectra from Fe L3,2-edge probe directly the preferential occupation of the 3d orbital, also called orbital ordering. This ordering strongly influences the bonding between transition metal ions and oxygen and thereby the magnetic and electronic properties of the compound. Our XLD results illustrate that at room temperature and at 30K, Fe eg orbitals 3d3z2-r2, are preferentially occupied however in the thermal hysteresis region i.e. between ~40-70K, 3dx2-y2 orbitals are preferentially occupied. Thus, during heating of the sample, in the transition region, there is a charge transfer phenomenon. Temperature dependence of the Fe K-edge EXAFS studies shows that the Fe-O bond length only changes in the ab-plane around transition temperature. Our XLD results along with EXAFS studies suggest that the charge transfer during thermal hysteresis is induced by lattice distortions of the FeO6 octahedra in SrFeO2.86.

Keywords: Colossal Magnetoresistance, X-ray Absorption Spectroscopy, X-ray Linear Dichroism