Showing papers by "Shigeo Mori published in 2007"

Magnetic and dielectric properties of RFe2O4, RFeMO4, and RGaCuO4 (R=Yb and Lu, M=Co and Cu)

Abstract: The magnetic and dielectric properties have been investigated for the isostructural oxides of $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$, $R\mathrm{Fe}M{\mathrm{O}}_{4}$, and $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_{4}$ ($R=\mathrm{Yb}$ and Lu, $M=\mathrm{Co}$ and Cu). The magnetization measurements for $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ showed ferrimagnetic ordering at $\ensuremath{\sim}250\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This system also exhibited large dielectric constants of $\ensuremath{\sim}10\phantom{\rule{0.2em}{0ex}}000--30\phantom{\rule{0.2em}{0ex}}000$ at around room temperature, which is attributable to the charge-ordering-induced ferroelectricity, as was proposed in our recent paper. The magnetic transition temperatures are lowered to $\ensuremath{\sim}45--90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for $R\mathrm{Fe}M{\mathrm{O}}_{4}$. Magnetic ordering is not found for $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_{4}$. ac magnetic susceptibility measurements indicate that magnetic ordering becomes short-ranged by the substitution at the Fe site. The overall characteristic behavior of the magnetic properties is explained in terms of the change of a spin value as well as the dilution of magnetic interactions. Although the ac dielectric measurements show the existence of polar regions in each material, the dielectric constants below $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ become smaller in the order of $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$, $R\mathrm{Fe}\mathrm{Co}{\mathrm{O}}_{4}$, $R\mathrm{Fe}\mathrm{Cu}{\mathrm{O}}_{4}$, and $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_{4}$. From the analysis of the dielectric dispersion, the distribution of the fluctuation time of polar regions is wider in $R\mathrm{Fe}\mathrm{Co}{\mathrm{O}}_{4}$, $R\mathrm{Fe}\mathrm{Cu}{\mathrm{O}}_{4}$, and $R\mathrm{Ga}\mathrm{Cu}{\mathrm{O}}_{4}$ than in $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$; a coherent motion of polar regions is suppressed in the substituted systems. By comparison to the results from the magnetic measurements, the dielectric properties are discussed in connection with a charge transfer between the transition-metal $3d$ orbitals, consistently with the proposed mechanism of the ferroelectricity in $R{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$.

Coercivity and nanostructure in magnetic spinel Mg(Mn,Fe)2O4

Abstract: When Fe ions in the ferrimagnetic cubic MgFe2O4 are replaced by Jahn-Teller (JT)-active Mn ions, the structure evolves with two-step processes. For example, the quenched cubic MgMn1.5Fe0.5O4 becomes tetragonal and JT distorted with slow cooling. However, with further slow cooling, the clustering tendency of JT-distorted Mn ions induces the formation of a checkerboard nano-self-assembly consisting of Mn-rich (tetragonal, paramagnetic) and -poor (cubic, ferrimagnetic) rods. This morphological evolution accompanies a drastic modification of ferrimagnetic properties, e.g., the magnetic coercivity changes by ∼25. The nanocheckerboard assembly with ferrimagnetic nanorods with large shape anisotropy can be a platform for ultra high-density memory devices.

Doping Effect on Ferroelectric Microstructure in YMn1 − x Ti x O3

Abstract: We have investigated microstructures related to anomalous ferroelectric properties such as the magnetocapacitance effect in magnetic ferroelectric compounds YMn 1 − x Ti x O 3 and (Y,A)Mn 0.825 Ti 0.175 O 3 (A = La, Ca, Zr) mainly by the transmission electron microscopy. It is found that the size of the ferroelectric domains in YMn 1 − x Ti x O 3 depends strongly on the Ti concentration (x) and the size is estimated to be about 10–20 nm in the x = 0.175 compound. In addition, structural change from the ferroelectric P6 3 cm phase to the rhombohedral one was found in the x = 0.40 compound. On the other hand, characteristic diffuse scattering elongating along both the [001] and [110] direction was found in (Y,Zr)Mn 0.825 Ti 0.175 O 3 , which is similar to that found in the x = 0.175 compound. It is revealed that the partial substitution in YMnO 3 breaks the long-ranged FE ordering and structural fluctuation is induced, which should be responsible for the anomalous FE behaviors in these compounds.

Ferroelectricity from Valence Ordering in RFe2O4

Abstract: We report a polar ordering of electrons in RFe 2 O 4 , which consistently explains the ferroelectric property of this material. Pyroelectric current measurement, low frequency AC dielectric constants, resonant X-ray scattering diffraction experiment and the consideration for the charge frustration, consistently lead the presence of ferroelectricity originated from the ordering of Fe 3+ and Fe 2+ . The ferroelectric material originated from the polar electron distribution and free from the ionic displacement may potentially have great possibilities in future applications and fundamental researches.

Electronic Ferroelectricity from Charge Ordering in RFe 2 O 4

Abstract: We report a novel ferroelectricity arise from the polar ordering of Fe3+ and Fe2+ in a mixed valence triangular lattice oxide LuFe2O4, where the electric polarization do not arise from the ionic displacement. The polar ordering of Fe3+ and Fe2+ was confirmed with a resonant X-ray scattering study in SPring-8. The origin of such ordering is the competitive interaction between Fe3+ and Fe2+ in the triangular lattice, i.e. the charge frustration. The polar superlattice of Fe3+ and Fe2+ develops below 350 K where the electric polarization appears. The ferroelectricity arising from the polar charge ordering or the polar electron distribution may have a great potential for the future application of ferroelectrics.