In the search for topological phases in correlated electron systems, materials with 5d transitionmetal ions, in particular, the iridium-based pyrochlores A2Ir2O7, provide fertile grounds. Several novel topological states have been predicted but the actual realization of such states is believed to critically depend on the strength of local potentials arising from distortions of the IrO6 cages. We test this hypothesis by measuring with resonant inelastic x-ray scattering the electronic level splittings in the A=Y, Eu systems, which we show to agree very well with ab initio quantum chemistry electronic-structure calculations for the series of materials with A=Sm, Eu, Lu, and Y. We find, however, that not distortions of the IrO6 octahedra are the primary source for quenching the spin-orbit interaction, but longer-range lattice anisotropies which inevitably break the local cubic symmetry.

We find the realization of large converse magnetoelectric (ME) effects at room temperature in a magnetoelectric hexaferrite Ba0.52Sr2.48Co2Fe24O41 single crystal, in which rapid change of electric polarization in low magnetic fields (about 5 mT) is coined to a large ME susceptibility of 3200 ps/m. The modulation of magnetization then reaches up to 0.62μ(B)/f.u. in an electric field of 1.14 MV/m. We find further that four ME states induced by different ME poling exhibit unique, nonvolatile magnetization versus electric field curves, which can be approximately described by an effective free energy with a distinct set of ME coefficients.

Strong spin-lattice coupling in condensed matter gives rise to intriguing physical phenomena such as colossal magnetoresistance and giant magnetoelectric effects. The phenomenological hallmark of such a strong spin-lattice coupling is the manifestation of a large anomaly in the crystal structure at the magnetic transition temperature. Here we report that the magnetic N é el temperature of the multiferroic compound BiFeO 3 is suppressed to around room temperature by heteroepitaxial misfi t strain. Remarkably, the ferroelectric state undergoes a fi rst-order transition to another ferroelectric state simultaneously with the magnetic transition temperature. Our fi ndings provide a unique example of a concurrent magnetic and ferroelectric transition at the same temperature among proper ferroelectrics, taking a step toward room temperature magnetoelectric applications.

We have investigated the structural, electronic, and magnetic properties of the pyrochlore iridates Eu2Ir2O7 and Pr2Ir2O7 using a combination of resonant elastic x-ray scattering, x-ray powder diffraction, and resonant inelastic x-ray scattering (RIXS). The structural parameters of Eu2Ir2O7 have been examined as a function of temperature and applied pressure, with a particular emphasis on regions of the phase diagram where electronic and magnetic phase transitions have been reported. We find no evidence of crystal symmetry change over the range of temperatures (∼6 to 300 K) and pressures (∼0.1 to 17 GPa) studied. We have also investigated the electronic and magnetic excitations in single crystal samples of Eu2Ir2O7 and Pr2Ir2O7 using high resolution Ir L3-edge RIXS. In spite of very different ground state properties, we find these materials exhibit qualitatively similar excitation spectra, with crystal field excitations at ∼3-5 eV, spin-orbit excitations at ∼0.5-1 eV, and broad low-lying excitations below ∼0.15 eV. In Eu2Ir2O7 we observe highly damped magnetic excitations at ∼45 meV, which display significant momentum dependence. We compare these results with recent dynamical structure factor calculations.

Using magnetoelectric measurements and neutron diffraction, we investigated multiferroic properties of Co 2 Y hexaferrite Ba 0.3 Sr 1.7 Co 2 Fe 12 O 22 in zero and finite magnetic field (H ⊥ c). Upon zero-field cooling, a longitudinal heliconical magnetic structure was observed below 280 K, which involves incommensurate planar helical ordering. When the magnetic field was applied perpendicular to its c axis, electric polarization was observed and the incommensurate ordering was replaced by a commensurate one as commonly observed in other hexaferrites. Electric polarization remained at its maximum during field reversal at 10 K, which indicates that magnetic anisotropy within the basal planes stabilizes the field-induced electric polarization.