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abstracts
BiFeO3 is arguably one of the most interesting multiferroic materials with both ferroelectric and magnetic transition above room temperature. Moreover, it has one of the highest polarization values, near 90 microC/cm2. The other interest lies in the cycloid magnetic structure with an unusually long-period of 620 Å. One of the key questions when it comes to the fundamental understanding of BiFeO3 is how the structure evolves as a function of temperature, especially across the cycloid transition, and whether there is any structural evidence of the most sought-after magnetoelectric coupling. The other principal question concerns with a full spin Hamiltonian necessary to describe the cycloid structure and the other fascinating physical properties. In order to answer these two questions, we have carried out high-resolution structural and dynamics studies using both neutron and synchrotron facilities on powder and single crystal samples. In this talk, we will highlight the structural evidence of the magnetoelectric coupling and high-field quenching of the cycloid above 20 Tesla from high-precision neutron scattering studies [1a,b,c]. We will also present inelastic neutron scattering data, from which we succeeded in establishing the full spin Hamiltonian [2-3].
abstracts
Glazer tilting system with tilting and rotation of oxygen octahedron, can describe ABO3 perovskite structure effectively. In highest symmetry, Pm-3m(No. 221) crystal structure is a0a0a0 without tilting and rotation. If temperature is lower, the different atomic radius of A and B causes tilting and rotation of BO6 octahedron. Glazer tiling notation of Pbnm(No. 62, cab lattice) orthorhombic structure is a-a-c+ with antiphase tilting along [110]cubic and in-phase rotation along [001]cubic for neighboring octahedron. SrRuO3 is rare example of itinerant ferromagnetic among 4d oxides. It shows zero thermal expansion, so called Invar effect below ferromagnetic transition(Tc=165 K). Otherwise, paramagnetic CaRuO3 has same Pbnm crystal structure without magnetic transition. To understand Invar effect and ferromagnetism of SrRuO3, We carried out high resolution Time-of-flight powder neutron diffraction using SuperHRPD beamline in J-PARC, with the best resolution Δd/d=0.03% of backscattering bank. Itinerant ferromagnetic SrRuO3 shows 50 fetometer increase of <Ru-O> mean bond below ferromagnetic transition while paramagnetic CaRuO3 shows decrease of <Ru-O> and follows well by the usual thermal expansion. For SrRuO3, Glazer tilting with deformation of RuO6 octahedron explains Invar effect and why lattice a is larger than lattice b in Pbnm structure. The increased <Ru-O> mean bond is considered as coupled order parameter with ferromagnetic transition. The band width of CaRuO3 is almost constant in the whole temperature range whereas ones of SrRuO3 decrease at low temperature. Then more localized Ru 4d orbitals probably contribute ferromagnetic transition.
