Search

Perovskite-type manganites exhibit various interesting phenomena arising from complex interplay among spin, charge, orbital, and lattice degrees of freedom. One such example is the keen competition between phases with different spin/charge/orbital orders. Keen competition between antiferromagnetic metal and orbital-ordered insulator is found in the slightly electron-doped regime near Mn4+ state which is stabilized by the high oxygen-pressure condition. Another one is the emergence of ferroelectricity either induced by the magnetic ordering or independently of the magnetic ordering. As the respective examples, perovskite-type YMnO3 and Sr1−xBaxMnO3 are discussed. In the YMnO3, the ferroelectric lattice distortion associated with the E-type spin order is observed for the first time. Displacement-type ferroelectricity with off-center magnetic ions is discovered for Sr0.5Ba0.5MnO3, which shows both large polarization value and strong coupling between ferroelectricity and magnetism.

A very small capacitor bank and miniature pulsed magnets have been developed to perform high-field synchrotron radiation experiments. A 20 T pulsed field can be generated using an energy of 1 kJ with typical pulse duration about 1 ms . Electron spin resonance experiments in millimeter wave range and high-field X-ray diffraction experiments have been done successfully.

We have performed resonant and non-resonant X-ray scattering on GdB 6 to investigate the two successive phase transitions at T N = ∼ 15 K and T * = ∼ 10 K . Below T N , new superlattice reflection at the wave vector [ 1 4 1 4 1 2 ] , which is the same wave vector determined by the epithermal neutron diffraction, has been observed by X-ray scattering. In the temperature region between T * and T N , it was confirmed by the polarization analyses that the superlattice reflection is due to magnetic X-ray scattering. On the other hand, interestingly, the superlattice reflection below T * is mainly due to Thomson scattering. Unlike behavior expected from usual magnetoelastic effects, the wave vector of the lattice distortion is identical to that of the magnetic structure below T * .

Gd substitution in DyB2C2 enhances the antiferromagnetic (AFM) interaction and weakens the antiferroquadrupolar (AFQ) interaction. In Dy0.8Gd0.2B2C2, two successive transitions occur. By performing resonant X-ray scattering measurements, we confirmed the AFQ ordering temperature T Q = 18.0 K and the Neel temperature T N = 19.5 K ; T N is higher than T Q in Dy0.8Gd0.2B2C2, in contrast to DyB2C2. There exists an AFM phase between T Q and T N , and the magnetic structure is described by k = [ 1 0 0 ] . This kind of AFM phase is also observed in HoB2C2 and TbB2C2, although the magnetic structures of these compounds are long-periodic ones.

We have performed a resonant magnetic X-ray scattering study on UNiGa 5 which shows an antiferromagnetic order below T N = 86 K. Resonant features are observed at both U M 4 -edge and Ga K-edge, even though Ga is normally a nonmagnetic atom. We consider that the resonance at the Ga edge originates from the orbital polarization of Ga 4p electrons which is induced by the hybridization with the U 5f electrons.

Investigation on a high-Tc superconductor, La1.85Sr0.15CuO4, was performed in the wide range of Q-E space by means of neutron scattering and synchrotron radiation. The structure as well as the phonon dynamics showed a clear evolution at Tc. We found a spontaneous strain and phonon softening, which proves a strong coupling between lattice and superconductivity.

We have investigated the field-induced coexistent phase of antiferroquadrupolar (AFQ) and antiferromagnetic order in TbB 2 C 2 by resonant X-ray scattering. Incident energy dependences of forbidden and superlattice reflections show that the field-induced AFQ order in TbB 2 C 2 is structurally the same one as in the AFQ-ordered phase of DyB 2 C 2 and HoB 2 C 2 at zero magnetic field.

We have revealed the relationship between the orbital ordering and the magnetism in the perovskite Ti oxides RTiO3 (R; rare earth elements) by using resonant X-ray scattering (RXS) near the K absorption edge of Ti. We observed the temperature dependence of the resonant signal in SmTiO3. The resonant signal hardly changed at the magnetic phase transition. Besides, the energy spectra at Q = ( 0 0 3 ) and the full width at half maximum (FWHM) for the 2 θ - ω scan are unchanged. Combining with the behavior that the periodicities of the azimuthal angle dependence for GdTiO3 and SmTiO3 are identical at each scattering vector, it is concluded that the change of the magnetism from ferromagnetic (FM) to antiferromagnetic (AFM) has little effect on the orbital ordering in RTiO3.

It has been theoretically expected that critical phenomena of helimagnetism is drastically different from those of collinear antiferromagnetism due to the existence of chiral degree of freedom. In order to verify this prediction, we carried out X-ray magnetic scattering measurements on a simplest helimagnet β-MnO2 using a synchrotron radiation. The critical exponent β was estimated at 0.25±0.05 from the temperature dependence of the scattering intensity. This value is consistent with the predicted value 0.253 for chiral system rather than with 0.346 for collinear system.

An anomalous pressure effect on the magnetic susceptibility of a spin-Peierls substance, MEM-[TCNQ] 2 , is observed. As the pressure increases, the susceptibility at low temperatures increases as if free magnetic spins are produced and it overwhelms the spin-Peierls transition. In order to clarify the pressure effect, the crystal structure of the substance under pressure has been studied by using synchrotron radiation X-rays. The reflection resulting from the superlattice of the spin-Peierls state below 18 K disappears under pressure and the crystal structure is significantly affected even at room temperature.

The 29 Si-NMR measurements on potassium-loaded zeolite LTA (K x /K 12 -Al 12 Si 12 O 48 ), where x is loaded potassium atoms per α -cage, were carried out. It was found that the NMR line-shape in the paramagnetic state is essentially the same for x =4–7, implying that the distribution and amplitude of the magnetic moment are quiet similar to each other. Possible distribution of the magnetic moment and the origin of the magnetic interactions are discussed.