Stabilization of Various Polymorph in Multiferroic ScFeO₃ Film

Polar iron oxides, which exhibit both ferroelectricity and (anti)ferromagnetism, are multiferroic materials, which promise potential application for a new type of memory device. Moreover, due to the strong magnetic interaction between Fe³⁺ cations, a higher magnetic order temperature in iron oxides is expected.<br/>It is well known that a perovskite structure is thermodynamically stable in <i>Re</i>³⁺FeO₃(Re = Rare earth element). While hexagonal <i>Re</i>FeO₃(h-<i>Re</i>FeO₃) with YMnO₃-type structure which exhibits ferroelectricity and weak-ferromagnetism is metastable. AlFeO₃ and GaFeO₃ have a polar κ-Al₂O₃structure which is a sesquioxide structure. When we consider crystal structures vs ionic radius of <i>A</i>³⁺ in <i>A</i>³⁺FeO₃, ScFeO₃ is located at the cross point of various kinds of crystal structures.<br/>Thin film fabrication techniques have contributed to the development of novel functional electronic materials and devices. Epitaxial films deposited on single crystal substrates suffering from interfacial strain due to a mismatch in the lattice constants between the film and the substrate allow stabilizing metastable phases.<br/>In this study, we attempted to stabilize various crystal structures in the film form by selecting the appropriate substrates or using a buffer layer and explored new polar iron oxides.<br/>ScFeO₃ films were deposited on various substrates by pulsed laser deposition (PLD) technique. The crystal structure of films was characterized by X-ray diffraction (XRD) and scanning transmission electron microscope (STEM).<br/>We stabilized five crystal structures: <i>κ</i>-Al₂O₃-, spinel-, corundum-, YMnO₃-, and bixbyite-type structures. XRD and HAADF-STEM were used to identify the phases. Four of the structures were obtained for the first time: <i>κ</i>-Al₂O₃-, spinel-, corundum-, and YMnO₃-type ScFeO₃. Since <i>κ</i>-Al₂O₃- and YMnO₃-type structures are polar, we investigated their ferroelectricity and magnetic properties.<br/>The YMO-type ScFeO₃ film on a perovskite electrode showed a ferroelectric <i>P</i>-<i>E</i> hysteresis loop with Pr ~ 4.9 μC/cm²_.<i>P</i>_s of <i>h</i>-ScFeO₃ was simply calculated using the displacement of the Sc ion evaluated from the HAADF-STEM image. The obtained <i>P</i>_s was ~8.7 μC/cm², which is consistent with the observed value. HAADF-STEM observations also revealed that at the domain boundary, the Sc displacement pattern of up–center–down was found, indicating a nonpolar region due to its correspondence to the <i>P-3c </i>phase pattern, which was previously reported in an ErFeO₃ epitaxial film. In addition, the domain boundary was not straight compared with a proper ferroelectric perovskite. Magnetic measurements confirmed weak ferromagnetism with <i>T</i>_N = 195 K. The T_N value is the highest in hexagonal <i>Re</i>FeO₃ and <i>Re</i>MnO₃.<br/>The <i>κ</i>-Al₂O₃- type ScFeO₃ film on a Nb doped SrTiO₃ substrate showed a ferroelectric <i>P</i>-<i>E</i> hysteresis loop with Pr ~ 4 μC/cm²_. Although the spontaneous polarization of <i>ε</i>-Fe₂O₃ calculated using Berry’s phase is 21 <i>μ</i>C cm⁻², the observed value is five times smaller than the theoretical value. This discrepancy can be correlated with the three in-plane domains. During polarization switching, because two close-packed oxygen layers shift in opposite directions along the <i>a</i>-axis (i.e., the in-plane direction in the <i>ε</i>-Fe₂O₃ film), this motion is suppressed by collision among the three in-plane domains. Magnetic measurements revealed the weak ferromagnetism with <i>T</i>_C ~ 295 K.