Signature of structural distortion in optical spectra of YFe2O4 thin film

Abstract

We report structural, optical, and electro-optical properties of polycrystalline YFe2O4 thin films, deposited on (0001) sapphire substrates using the electron-beam deposition technique. The optical spectra of a 120 nm YFe2O4 show Fe d to d on-site and O 2p to Fe 3d, Y 4d, and Y 5s charge-transfer electronic excitations. Anomalies in the temperature dependence data of the charge-transfer excitations and the splitting of the 4.46 eV charge-transfer peak strongly suggest a structural distortion at 180 ± 10 K. Evidence of such a structural distortion is also manifested in the surface resistance versus temperature data. In addition, the YFe2O4 thin film at low temperatures shows strong electro-optical properties, as high as 9% in the energy range of 1 - 2.5 eV, for applied electric fields up to 500 V.cm−1.

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“…Various trivalent ions are selected as R ion in reported films: Lu, [18][19][20][21][22][23][24][25] Yb, [26][27][28] Tm 17 and In. 29 The change in R ions is unlikely to affect intrinsically as well as for bulk.…”

“…29 The change in R ions is unlikely to affect intrinsically as well as for bulk. These films are fabricated on various substrates: Al 2 O 3 (001), 18,21,25,26,28 YSZ(111), 17,19,24,27 ZnO(001), 29 MgO(111), 20,22,23 MgAl2O4(111) 23 and 6H-SiC(001). 23 Also various deposition techniques are tried: pulsed laser deposition (PLD), 17,18,20,21,24,27,29 electron beam deposition (EB), 19,25,26 molecular beam epitaxy (MBE), 22,23 and rf magnetron sputtering.…”

“…As for the temperature dependence, the electric property shows anomaly around T N and three-dimensional charge-ordering temperature, T CO . 21,24,26 Figure 5 shows the temperature dependence of resistance of the film. 21 The discontinuous change and kink are observed at T CO and T N in R-T curve respectively.…”

“…The optical property is reported in refs. 19,23,[25][26] Figure 7 shows representative absorption spectra of LuFe 2 O 4 film as a function of photon energy at 78 and 300K. 19 The peaks are observed around 1.6 eV, 2.4 eV and 3.3 eV, corresponding to the Fe 2+ d to d on-site transition and the O 2p to Fe 3d charge-transfer transition, estimated from first-principles electronic structure calculations.…”

“…However, the film fabrication of this material is still a developing research due to the difficulty in selecting substrates and keeping the stoichiometory, which changes various properties significantly. 15, 16 13 reports have been published by 2017, [17][18][19][20][21][22][23][24][25][26][27][28][29] following the first report by M. Seki et al in 2010. 17 In this paper, the film growth and electric, magnetic and optical properties of the films are briefly reviewed.…”