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We have proposed ReMnO3 (Re:rare earth) thin films as a new candidate for nonvolatile memory devices. In this letter, we report on fabrication of (0001) YMnO3 films on (111)MgO, (0001)ZnO:Al/(0001) sapphire, and (111)Pt/(111)MgO using rf magnetron sputtering. We succeeded in obtaining (0001) epitaxial YMnO3 films on (111) MgO and (0001)ZnO:Al/(0001) sapphire substrate, and polycrystalline films on (111)Pt/(111)MgO. The dielectric properties of the epitaxial and polycrystalline YMnO3 films are almost the same. The dielectric permittivities of both films are smaller than those reported for YMnO3 single crystal.

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YMnO(3) is a multiferroic material in which ferroelectric and antiferromagnetic ordering can coexist. We have studied a YMnO(3) bulk crystal in detail by Raman scattering in a wide temperature range of 15-1200 K, with comparison to a previous experiment at room temperature and a theoretical prediction for Raman-active phonon modes. In the low-temperature ferroelectric phase, the observed phonon spectra showed anomalous temperature variation at the Néel temperature, T(N)∼80 K, suggesting a coupling between the spin and phonon systems below T(N). Furthermore, spectra for the high-temperature paraelectric phase, reported here for the first time, showed a sudden change at the Curie temperature T(C)>900 K, suggesting an abrupt structural phase change from the ferroelectric to the paraelectric phase.

Ferroelectric properties of YMnO3 epitaxial films were studied. The ferroelectric properties of epitaxially grown (0001) YMnO3 films on (111)Pt/(0001)sapphire (epi-YMO/Pt) with an excellent crystallinity were compared to (0001)-oriented poly crystalline films on (111)Pt/ZrO2/SiO2/Si. The epi-YMO/Pt had saturated polarization–electric-field (P–E) hysteresis loops, with a remanent polarization (Pr) of 1.7 μC/cm2 and a coercive field (Ec) of 80 kV/cm. The fatigue property showed no degradation up to 1010 measured cycles. These results suggested that the YMnO3 epitaxial films were suitable ferroelectric material for the ferroelectric-gate field-effect transistors. Consequently, epitaxially grown (0001)YMnO3 films on epitaxial Y2O3/Si (epi-YMO/Si) were fabricated. The epi-YMO/Si capacitor had almost equivalent crystallinity compared to epi-YMO/Pt. It was recognized that the epi-YMO/Si capacitor exhibited the ferroelectric type C–V hysteresis loop with the width of the memory window of 4.8 V, which was almost identical to the value of twice coercive voltage of the P–E hysteresis loops of the epi-YMO/Pt. A retention time exceeding 104 s was obtained in the epi-YMO/Si capacitor.