Current Control of Spin Helicity and Nonreciprocal Charge Transport in a Multiferroic Conductor.

Abstract

A multiferroic state with both electronic polarity (P) and magnetization (M) shows the inherently strong P-M coupling when P is induced by cycloidal (Néel-wall like) spin modulation. The sign of P is determined by the clockwise or counterclockwise rotation of spin, termed the spin helicity. Such a multiferroic state is not limited to magnetic insulators but can be broadly observed in conductors. Here, the current control of the multiferroics is reported in a helimagnetic metal YMn₆Sn₆ and its detection through nonreciprocal resistivity (NRR). The underlying concept is the coupling of the current with the toroidal moment <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mi>T</mi> <mo>∼</mo> <mi>P</mi> <mo>×</mo> <mrow></mrow> <mi>M</mi> <mo>∼</mo> <mo>(</mo> <mover><mi>q</mi> <mo>̂</mo></mover> <mo>×</mo> <mrow></mrow> <msub><mi>χ</mi> <mi>v</mi></msub> <mo>)</mo> <mo>×</mo> <mrow></mrow> <mi>M</mi></mrow> <annotation>$\bm{T}\sim \bm{P}\ensuremath{\times{}}\bm{M}\sim (\widehat{\bm{q}}\ensuremath{\times{}}{\bm{\chi}}_{v})\ensuremath{\times{}}\bm{M}$</annotation></semantics> </math> as well as with the magneto-chirality χ_v · M, where <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mspace></mspace> <mover><mi>q</mi> <mo>̂</mo></mover> </mrow> <annotation>$\hspace*{0.28em}\widehat{\bm{q}}$</annotation></semantics> </math> and χ_v are the unit modulation wave vector and the vector spin chirality, respectively. An enhancement of NRR is furthermore observed by the spin-cluster scattering via χ_v and its fluctuation. These findings may pave the way to an exploration of multiferroic conductors and the application of the spin-helicity degree of freedom as a state variable.