Fractional Spin Ferroelectric and Sliding Spin Current in Magnetic Sliding Ferroelectrics

Abstract

We investigate the fractional spin ferroelectric (FSFE) in magnetic sliding ferroelectrics (SFEs), where ferroelectric switching is characterized not only by the reversal of the out-of-plane electric polarization but also by a variation of fractional in-plane spin electronic polarization. We show that interlayer sliding in FSFEs can naturally lead to a symmetry-protected pure spin current, termed the sliding spin current here. The underlying mechanism is that, during switching, the contributions of valence electrons and ions to the in-plane charge transfer cancel each other, whereas the in-plane spin transfer, which stems solely from valence electrons, persists, leading to a pure spin current. We demonstrate our ideas in various material candidates, including $H$-stacked bilayer CrI$_3$, whose few-layer form has been experimentally confirmed to be a magnetic SFE, and $R$-stacked bilayers $2H$-V$X_2$ ($X=$ S, Se, Te), which have been experimentally synthesised. For a typical switching time of about $1$ ns, the estimated spin-current densities for bilayer CrI$_3$ and V$X_2$ reach $10^9 (\hbar/2e)\mathrm{A/m^2}$ and $10^8 (\hbar/2e)\mathrm{A/m^2}$, respectively. This means that by applying a periodic out-of-plane electric field, a significant alternating spin current can be generated in magnetic SFEs. Thus, our findings propose a compelling new mechanism for the all-electrical generation of pure spin current, and predict concrete realistic materials for experimental verification.