Synthesis and magnetic properties of double-perovskite oxide ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$ thin films

We have investigated epitaxial structures and physical properties of double-perovskite ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$ films grown by pulsed-laser deposition. The films showed magnetic hysteresis at low temperatures regardless of the degree of Mn/Fe order and the saturation magnetization became smaller for the higher Mn/Fe-order films. The x-ray absorption and x-ray magnetic circular dichroism measurements revealed antiferromagnetic coupling between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Fe}}^{3+}$ ions, resulting in ferrimagnetism of double-perovskite ${\mathrm{La}}_2{\mathrm{MnFeO}}_6$. A band structure was established based on the synchrotron radiation photoemission and optical spectra. The optical gap ranged 1.1 eV above the valence state dominated by the Mn $3d e_g$ orbitals, which is similar to the charge-transfer insulating state in the end member compound of ${\mathrm{LaMnO}}_3$.

Figure 1

XRD analyses of the LMFO thin films. (a) Out-of-plane XRD patterns measured with Cu $K{\alpha }_1$ radiation for LFMO films grown at $950{}^{\circ }\mathrm{C}$ and $750{}^{\circ }\mathrm{C}$ in $P_{{\text{O}}_2}=100$ mTorr. The arrows indicate the double-perovskite superlattice reflections. Magnification around the STO (111) reflection is shown at the right side. (b) PLD growth conditions of LFMO films mapped in a $T_g\text{−}{P}_{{\text{O}}_2}$ diagram. The colors represent degree of the Mn/Fe order estimated from the XRD peak intensity ratio between the LMFO (111) superlattice and LMFO (444) fundamental peaks. (c) Symmetric reflection profiles taken with synchrotron light source normalized by the intensity of the (222) peaks for LFMO films grown at $950{}^{\circ }\mathrm{C}$ and $750{}^{\circ }\mathrm{C}$ at $P_{{\text{O}}_2}=100$ mTorr. The solid bars indicate intensities for the ideal double-perovskite LMFO calculated without any atomic displacement of ${\mathrm{La}}^{3+}$ and ${\mathrm{O}}^{2-}$ ions.

Figure 2

$M\text{−}H$ curves of the LMFO films and disordered polycrystal taken at 5 K. For the films, the magnetic field was applied along the [$\overline{1}10$] direction of the STO substrate.

Figure 3

XAS and XMCD spectra of the LMFO films taken at 10 K. (a) Fe $2p$ XAS spectra. The XAS spectra of ${\mathrm{Fe}}^{2+}$ (FeO) and ${\mathrm{Fe}}^{3+} \left(\mathrm{LaFeO}{}_3\right)$ are also shown as Refs. [24, 25]. (b) Mn $2p$ XAS spectra. The XAS spectra of ${\mathrm{Mn}}^{3+} \left(\mathrm{LaMnO}{}_3\right)$ and ${\mathrm{Mn}}^{4+} \left(\mathrm{MnO}{}_2\right)$ are also shown as Refs. [26]. (c) Fe $2p$ and (d) Mn $2p$ XMCD spectra. The XMCD spectra were normalized by the integrated intensity of each XAS spectra.

Figure 4

Electronic structures of the high-order LMFO film. The green and orange colors in panels (a) and (c) correspond to the Mn and Fe derived states, respectively. (a) Valence band, O $1s$ XAS, and pDOS of Mn and Fe $3d$ obtained by the resonant photoemission techniques. (b) Absorption spectrum of the LMFO film. The optical conductivities of the end member compounds are also plotted as in Ref. [29]. (c) Schematic energy diagram of Mn and Fe $3d$ states.