Electronic Excitations and Optical Properties of RFeO$_{\mathrm{3}}$ (R $=$ Lu and Yb) Thin Films

We present structural, electronic, and optical properties of multiferroic RFeO$_{\mathrm{3}}$ (R $=$ Lu and Yb) thin films, deposited on single crystal sapphire and YSZ substrates under the various growth conditions using RF Magnetron Sputtering. Growth temperature and annealing are found to be critical to stabilize hexagonal RFeO$_{\mathrm{3}}$ thin films. Based on the X-ray diffraction data, annealing above \textasciitilde 900 $^{\mathrm{0}}$C has been found to change the crystal structure of RFeO$_{\mathrm{3}}$ from a metastable hexagonal to an orthorhombic structure. Optical spectroscopy in the 0.5-6.5 eV range has been used to study the optical and electronic excitations of the RFeO$_{\mathrm{3}}$ thin films. The electronic excitations dominated by Fe d to d on-site as well as O 2p to Fe 3d, R 6s, and R 5d charge-transfer transitions for hexagonal and orthorhombic RFeO$_{\mathrm{3}}$ are distinctly different, consistent with the excitations from the FeO$_{\mathrm{5}}$ and FeO$_{\mathrm{6}}$ building blocks for hexagonal and orthorhombic RFeO$_{\mathrm{3}}$, respectively. Further, the optical spectra exhibit strong temperature dependence with an anomaly at the magnetic transition, indicating a structural distortion. We'll also present the magnetic measurements data on the RFeO$_{\mathrm{3}}$ thin films.