Local Structure of Amorphous EuO–TiO₂ Thin Films Probed by X‐Ray Absorption Fine Structure

Abstract

We have investigated the local structure of amorphous oxides in EuO–TiO2 system to understand the mechanism that gives rise to curious ferromagnetic properties recently observed in the system. X‐ray absorption spectroscopy has been performed on amorphous EuTiO3 and Eu2TiO4 thin films prepared by the pulsed‐laser‐deposition method. The Ti K‐edge X‐ray absorption near‐edge structure (XANES) and extended X‐ray absorption fine structure (EXAFS) data analyses reveal that the coordination number of Ti4+ in the amorphous thin films is around 4, which is in sharp contrast to their crystalline counterparts, where Ti4+ ions are octahedrally coordinated by oxide ions. It is therefore inferred that Ti4+ acts as a network‐forming cation in the random network structure of the amorphous oxides. From the Eu L3‐edge EXAFS data analyses, it is found that the coordination number of Eu2+ is much lower and the nearest Eu–O bond length is shorter in the amorphous thin films than in the crystalline counterparts. The amorphization‐induced enhancement of ferromagnetic interactions observed in the EuO–TiO2 system is discussed in terms of the local environment around Eu2+ ions.

Ferromagnetic amorphous oxides in the EuO-TiO2system studied by the Faraday effect in the visible region and the x-ray magnetic circular dichroism at the EuM4,5andKawamoto1, Fujita2, Akamatsu3 et al. 2013Phys. Rev. BSelf Cite7420View full textAdd to dashboardCite

“…5(a) and 5(c)] is characterized by an intense peak and a satellite structure, with an energy separation of ∼10 eV; for example, the major peak and satellite for the Eu L 3 edge are observed at 6972 and 6981 eV, respectively, as indicated by the vertical dashed lines. According to our previous Eu L 3 edge XAS study, 16 the major peak and satellite are attributable to the contributions from Eu 2+ and Eu 3+ ions, respectively. To estimate the relative contributions of Eu 2+ and Eu 3+ , the XAS for Eu L 3 edge was deconvoluted by the superposition of two Lorentzians and two arctangent functions, according to the procedure used by Godart et al 43 for the analysis of mixed valence in rare-earth intermediate compounds.…”

“…This idea is in agreement with the fact that the amorphization of EuTiO 3 crystal results in a decrease in the coordination number of Eu 2+ (12 in perovskite EuTiO 3 vs about 6 in a-ETO) as demonstrated by our previous analysis of x-ray absorption fine structure. 16 Namely, the reduction of coordination number of Eu 2+ upon amorphization makes the crystal field acting on Eu 2+ stronger in the amorphous phase than in the crystalline counterpart, resulting in the smaller 4f -5d energy separation for the former.…”

“…Recent x-ray absorption fine structure studies revealed that the local structure of Eu 2+ in a-ETO and a-2ETO is similar to that in EuO with rock-salt-type structure rather than their crystalline counterparts. 16 Such a local environment of Eu 2+ is known to be closely related to the magnitude of an indirect ferromagnetic exchange interaction often observed in Eu 2+ -based oxides, which involves an Eu 4f electron excited to the Eu 5d band where it interplays with another Eu 4f spin on the neighboring Eu 2+ ion. 17,18 The coupling constant of the indirect exchange interaction is proportional to J intra b 2 /U 2 f d , where J intra is the intra-atomic exchange constant between the Eu 4f and 5d states, b is the transfer integral between the neighboring Eu ions, and U f d is the energy difference between the Eu 4f and 5d levels.…”

TiO2/Eu3+ Thin Films with High Photoluminescence Emission Prepared by Electrophoretic Deposition from Nanoparticulate Sols

“…sol–gel,16–19 hydrothermal,20,21 and solvothermal22 synthesis, etc.). These nanopowders can be used for the preparation of the thin films by different deposition techniques such as dipping,17,18 low‐pressure hot target magnetron sputtering,23 or pulsed laser deposition 24. Previous works25 dealing with the preparation of glass‐like coatings on metallic substrates have unequivocally demonstrated that the use of EPD enabled the preparation of coatings with a greater thickness and increased density and uniformity than those obtained by simply dipping.…”

Ferromagnetism induced by lattice volume expansion and amorphization in EuTiO3 thin films

“…Details of the procedure of measurement and analysis of EXAFS were described elsewhere. 30 Variation of magnetization with temperature and magnetic field was measured by using a superconducting quantum interference device (SQUID).…”