| Research Abstract |
Ferromagnetic transition metal oxides, including perovskite manganites, represent the most promising materials for use as devices controlling magnetic states by an electric field at high temperature with high efficiency. This is because these materials possess a strong intrinsic relationship between charge and magnetism, showing ferromagnetism above room temperature by adjustment of carrier filling, in addition, particular magnetoelectric properties such as a colossal magnetoresistance phenomenon. We have constructed field-effect transistor structures that consist of a ferromagnetic (La, Ba)MnO_3 channel and a ferroelectric PbZr_<0.2>Ti_<0.8>O_3 gate insulator with the aim of controlling the metal-insulator transition at room temperature by applying an electric field. Investigations have revealed that the transition temperature changed from 280.5 K to 283.0 K (±26 μC/cm_2) for the La_<0.85>Ba_<0.15>MnO_3 channel layer. These shifts, which are linearly proportional to the magnitude of f
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erroelectric remnant polarization, are induced by the accumulation charge due to the electric field. The change in magnetism using an electric field was also evaluated by measuring the magnetocircular dichroism (MCD) signal. As a result, a reversible control of ferromagnetism was archived at room temperature.
Fe_<3-x>Mn_xO_4 (x=0, 0.1, or 0.5 : FMO) and Fe_<3-x>Zn_xO_4 (x=0, 0.5, and 0.9 : FZO) epitaxial thin films with very high T_c as new tunable ferromagnetic semiconductor were successfully prepared using a pulsed-laser deposition technique. It was confirmed that these films could be fabricated at higher substrate temperatures. FMO and FZO films showed spin polarization of the carrier at RT, as confirmed by anomalous Hall coefficient measurements. Their electrical conductivity and magnetic properties were systematically modulated with changing Mn or Zn substitution without lowering mobility. The HX-PES measurements confirmed the systematic change of the valence band electronic structures These characteristics allowed for the construction of functional oxide heterostructure-based spin devices, namely ferromagnetic electrical field effect transistors with ferroelectric gate Pb (Zr, Ti)O3 and ferromagnetic diode with Nb-doped SrTiO_3, working at room temperature.
The original nano-fabrication techniques (Mo-nano mask atomic force microscope (AFM) nanolithography, Mo lift off AFM ＆ nano-imprint lithography) was developed to construct robust nano-structures with atomically flat surface of ferromagnetic perovskite manganites, spinel ferrite down to 30nm in size. We found (La, Pr, Ca)MnO_3 channel structures with 500 nm width showed an extremely sharp metallic transition from charge ordered insulator to ferromagnetic metal at low temperature. Such extremely sharp metallic transition can be accounted for electronic phase separation phenomena. Furthermore, we successfully constructed nano-sized side gate FET structure in new high T_c ferromagnetic oxide semiconductors of (Fe, Mn)_3O_4 and (Fe, Zn)_3O_4. Less
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