Magnetoelectric coupling effect in Multiferroic Cu2V2O7 and Topological metal Bi1−xSbx (x ∼ 0.04)

Abstract

We synthesize and investigate strongly correlated materials by using the fundamental characterization tools such as magnetic property and electric transport measurement system. To perform the comprehensive investigation of exotic magnetoelectric materials, we found the microscopic origin and information about the cupric divanadate Cu2V2O7 and topological non trivial metal Bi1−xSbx (x ∼ 0.04). Single crystals of orthorhombic polar oxide α-Cu2V2O7 with space group Fdd2 are synthesized and their physical properties are measured. Neutron powder diffraction is also performed on a poly-crystal sample to extract the magnetic structure. The ground state is shown to be weakly ferromagnetic, that is, collinearly antiferromagnetic in the a-direction with a small remanent magnetization in the c-direction. When an external magnetic field is applied in the c-direction, further spin canting, accompanied by the induced electric polarization, occurs. It is demonstrated that the magnetoelectric effect in α-Cu2V2O7 is adequately described if spin-dependent p-d hybridization due to spin-orbit coupling as well as magnetic domain effects are simultaneously taken into account. We discuss the implication of the present result in the search for materials with multiferroicity and/or magnetoelectricity. The Weyl semimetal is a hopeful system to apply on the future spintronic devices. Antimony doped bismuth alloy,BixSb1−x, allows us to reach the critical point which is a boundary between the topological non trivial insulator and trivial band insulator. About 3-4% Sb doped case, electron and hole bands are touched at a point, becomes a Dirac metal. When external magnetic fields are turned on, degenerated Dirac bands are split to two Dirac bands along the applied field direction. These Dirac nodes have different chirallity and generate anormalous current when electric and magnetic fields are applied parallel direction. Thus, the negative value of longitudinal magneto resistance appears and this has been regarded as a clue of the existence of Weyl phase. However, we found more concrete measurement method to justify Weyl state by using measurement of non-linear dependence on the longitudinal magnetoresistance. Comparing with transverse magnetiresistance, other possibilities are ruled out which originate from extrinsic effects such as Joule heating and Schottky junction problem. Semi classical Boltzmann transport theory with topological aspect is introduced to explain the nonlinearlity. We find a quadratic dependence of conductivity with respect to E and B, reveals only in the presence of the Weyl phase.