We have investigated the spin/orbital phase diagram in the perovskite orthovanadate RVO 3 ͑R = Eu, Y, Dy, and Ho͒ by measurements of magnetization, dielectric constant, specific heat, Raman scattering spectra, and x-ray diffraction, focusing on the interplay between the V 3d spin and the 4f moment of the R ion. The thermally induced phase transition between the C-type spin/G-type orbital ordered state and the G-type spin/ C-type orbital ordered state is observed for Eu 1−x Y x VO 3 ͑x = 0 -0.52͒ without 4f moment. By comparing this phase diagram with the spin/orbital ordering in TbVO 3 , it is evident that the critical competition between the C-type spin/G-type orbital ordered phase and the G-type spin/C-type orbital ordered one depends not only on the GdFeO 3 -type lattice distortion but also on the presence of the 4f moment of the R ion. The magnetic field induced phase transition of the spin/orbital ordering is achieved concomitantly with polarizing R 4f moments for DyVO 3 and HoVO 3 . For DyVO 3 , the G-type spin/C-type orbital ordered phase is switched to the C-type spin/G-type orbital ordered one by applying a moderate magnetic field around 3 T. By contrast, the G-type spin/C-type orbital ordering is rather favored under the magnetic field in HoVO 3 . The results cannot be uniquely explained in terms of the exchange interaction between the V 3d spin and the R-ion 4f moment. The coupling of the R 4f moment polarization with the lattice distortion tied with the orbital ordering of the V 3d sublattice may also be relevant to this field induced phase transition.

The critical phase competition between different spin-orbital-ordered states has been investigated for the DyVO3 single crystal. As temperature is lowered, the compound exhibits a reentrant spin and orbital ordering (SO and OO) transition: C-->G-->C type for SO and G-->C-->G type for OO. It was found that a magnetic field also drives the phase transition from C to G for OO and concomitantly from G to C for SO, the latter of which is coupled with the metamagnetic transition of the Dy 4f moments. The mechanism of this novel magnetic-field-induced orbital switching is discussed.

Magnetic states in RVO 3 (R = Y, Tb) were investigated using low-temperature and high-pressure neutrondiffraction techniques, and the pressure-temperature magnetic phase diagrams were clarified up to 6.2 GPa. We elucidated that, on application of pressure, the magnetic ground state changes from C-type spin ordering (C-SO) to G-type spin ordering (G-SO); this corresponds to the orbital ground-state switching from G-type orbital ordering (G-OO) to C-type orbital ordering (C-OO). It is also interesting that the G-SO transition occurs simultaneously with the C-OO transition, but not with the G-OO one. When the transition temperature of C-OO exceeds that of G-OO under high pressure, the G-OO phase vanishes completely, and a simultaneous spin and orbital order-disorder phase transition occurs. Such a transition is the first case in perovskite-type transition-metal oxides.

No abstract

The orbital ordering in perovskite-type vanadium oxides, RVO 3 (R: rare earth), has been investigated by resonant X-ray scattering (RXS) near the V K-edge energy. The G-type orbital order, C-type orbital order and orbital disorder phases are elucidated on the basis of the azimuthal-angle and polarization dependence of the RXS signal reflecting the orbital ordering. IntroductionResonant X-ray scattering (RXS) is becoming one of the most powerful tools for studying orbital states in strongly correlated electron systems. Therefore, it is important to understand the origin of RXS signals to determine orbital states; the RXS signal reflects not only the orbital state, but also the structural characteristics (Nakao et al., 2006, and references therein). In RVO 3 , the two valence electrons of V 3+ ions occupy the nearly triply degenerate t 2g orbitals. Hence, the physical properties of RVO 3 are coupled with the orbital and spin states. Two types of orbital orderings have been reported in RVO 3 systems. One is C-type orbital order (C-OO), with an antiferroic arrangement of d xy d yz and d xy d zx in the ab plane and a ferroic arrangement along the c-axis. The other phase is G-type orbital order (G-OO), with an antiferroic arrangement in all the three orthogonal directions. These orbital orderings have been studied by RXS (Noguchi et al., 2000). However, there are some unclear points relating to the interpretation of the observed RXS. In this study, we have systematically investigated RXS near the V K-edge energy on the basis of model calculations and have revaluated the orbital orderings in RVO 3 .