Current control of spin helicity and nonreciprocal charge transport in a multiferroic conductor

In multiferroics, electronic polarity (P) and magnetization (M) show the inherently strong P-M coupling when P is induced by cycloidal spin modulation [Y. Tokura, Science 312, 1481 (2006)]. Cycloid has the rotation sense of freedom, clockwise (CW) or counterclockwise (CCW), termed the spin helicity, characterized by the sign of vector spin chirality χ_v=S_ixS_j. While magnetic insulators with well-defined electronic polarization have conventionally been studied, we extend the scope of such a multiferroic state of spin origin to conductors. In multiferroic insulators, mutual control of P and M is realized via the magnetoelectric (ME) effect. On the other hand, its counterpart in multiferroic conductors is here termed magnetopolar (MP) effect. There are two kinds of MP effect: electrical magnetochiral effect (eMChE) and electrical magneto-toroidal effect (eMToE). The electric current, instead of electric field, couples with the magneto-chirality χ_v・M and the toroidal moment T=PxM, respectively. We performed the current control of spin helicity in a centrosymmetric helimagnetic metal YMn₆Sn₆ and its detection through nonreciprocal resistivity (NRR). An enhancement of NRR is furthermore observed by the spin-cluster scattering via <χ_v>.