Kosuke Fujiwara

Electric spontaneous polarization in YbFe2O4

We report the observation of the electric spontaneous polarization in YbFe2O4, one candidate “electronic ferroelectric,” where ferroelectricity originates from the polar charge order. Though we have proposed that in LuFe2O4 polar charge ordering of iron ions having different valence states give rise to ferroelectricity, some reports questioned not only the electronic origin but also the existence of the ferroelectricity itself in this system. In response to this, we show a direct macroscopic evidence of the existence of the ferroelectricity by a clear P-E hysteresis loop of YbFe2O4 belonging to the same system as that of LuFe2O4. Although further investigations are required about the electronic origin, we report the direct observation of the macroscopic electric polarization.

Possible charge order structure of stoichiometric YbFe2O4

ABSTRACT We report a possible charge ordering model of triangular mixed valence material YbFe2O4. It was found that a crystal synthesized with high stoichiometry on iron vacancy shows new and simple extinction rule in the superlattice reflections. The Laue class of the diffraction signal is expressed as 2/m. With the consideration of the charge neutrality and the superlattice signal of (1/3 1/3 0), we derived 5 possible charge ordering models of this material.

Observation of Flux-Grown α-Fe2O3 Single Crystal at the Morin Transition by 57Fe Synchrotron Radiation Mössbauer Diffraction

The Morin transition of a high-quality flux-grown single crystal of α-Fe2O3 was studied by 57Fe synchrotron radiation Mossbauer diffraction. The measured rocking curves and Mossbauer spectra gave direct evidence that the coexistence of magnetic phases during the Morin transition occurred over a wide temperature range. The complex magnetic structure at the phase coexistence induced a large magnetostrictive distortion in the α-Fe2O3 crystal surface. In contrast, however, when the antiferromagnetic phase was dominant at low temperatures, the distortion disappeared, and the initial high crystal perfection was recovered. The spectral line shapes were discussed in terms of the interference between electronic and nuclear scatterings.

Crystal-Site-Selective Spectrum of Fe3O4 Obtained by Mössbauer Diffraction

We have succeeded for the first time in obtaining a crystal-site-selective Mossbauer spectrum of Fe3O4 using the Mossbauer diffractometer at SPring-8 BL11XU. In order to extract the nuclear resonant scattering, which provides the crystal-site-selective emission spectrum, reflections having a Bragg angle near 45° were used. The 666 and 10 10 0 reflection spectra reveal only B- and A-site spectra, respectively. Mossbauer diffraction spectroscopy enables us to measure the crystal-site-selective spectrum and to determine the precise hyperfine structure. This technique provides new possibilities for studying the local crystallographic and magnetic structure on iron sublattices in various multisite materials.

Crystal-Site-Selective Spectrum of Fe3BO6 by Synchrotron Mössbauer Diffraction with Pure Nuclear Bragg Scattering

We have succeeded in obtaining the crystal-site-selective spectra of the collinear antiferromagnet Fe3BO6 using a synchrotron Mossbauer diffractometer with pure nuclear Bragg scattering at SPring-8...

Coherent X-ray diffraction for domain observation II

ABSTRACT The CXD method have been shown to be applicable to the crystal characterization as well as the domain observation. The present CXD technique could evaluate the crystal coherence length up to 10 μm and the wide variety of samples were chosen in this study. We have shown how the ideal Bragg reflection deforms as the disorder and the defects increases and form the speckle patterns, the typical scattering patterns of CXD, by changing the samples. Finally we have successfully observed the μm/sub-μm domain arrangements in LuFe2O4/(1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (x = 0.09), where it is hard to be observed by the conventional diffraction techniques.

Ferroelectric and magnetic properties for nano particles of multiferroic YbFe2O4

ABSTRACT Size effect and lattice strain effect for charge ordered structure and magnetic properties were investigated on the nano particles of geometrically spin and charge frustrated system YbFe2O4. The lattice strain of the nano particle was controlled with the ratio of Yb and Fe (2.00 ≤ Fe/Yb ≤ 2.04) in liquid synthesizing method. It was found that the lattice strain is not affected by the magnetic transition temperature, however the anomalous behavior at a lower magnetic transition (TLT) with an increment of the Fe / Yb ratio was detected. The development of charge ordering was two-dimensional and magnetic ordering goes down for all samples in spite of the variation of the local lattice strain. This is the first report of the coupling of charge and spin ordering to the size effect in this material. These results suggest that the discussion on the electric polarization induced from the charge ordering in this material need to care the long-range interaction like surface effect.