Ferroelectricity induced by ferriaxial crystal rotation and spin helicity in aB-site-ordered double-perovskite multiferroicIn2NiMnO6

Abstract

We have performed dielectric measurements and neutron diffraction experiments on the double perovskite ${\text{In}}_{2}{\text{NiMnO}}_{6}$. A ferroelectric polarization, $P\ensuremath{\simeq}30\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2},$ is observed in a polycrystalline sample below ${T}_{N}=$ 26 K where a magnetic phase transition occurs. The neutron diffraction experiment demonstrates that a complex noncollinear magnetic structure with ``cycloidal'' and ``proper screw'' components appears below ${T}_{N}$, which has the incommensurate propagation vector $\mathbit{k}=({k}_{a},0,{k}_{c};{k}_{a}\ensuremath{\simeq}0.274,{k}_{c}\ensuremath{\simeq}\ensuremath{-}0.0893)$. The established magnetic point group ${21}^{\ensuremath{'}}$ implies that the macroscopic ferroelectric polarization is along the monoclinic $b$ axis. Recent theories based on the inverse Dzyaloshinskii-Moriya effect allow us to specify two distinct contributions to the polarization of ${\text{In}}_{2}{\text{NiMnO}}_{6}$. One of them is associated with the cycloidal component, ${\mathbit{p}}_{1}\ensuremath{\propto}{\mathbit{r}}_{ij}\ifmmode\times\else\texttimes\fi{}{({\mathbit{S}}_{i}\ifmmode\times\else\texttimes\fi{}{\mathbit{S}}_{j})}_{\ensuremath{\perp}}$, and the other with the proper screw component, ${\mathbit{p}}_{2}\ensuremath{\propto}[{\mathbit{r}}_{ij}\ifmmode\cdot\else\textperiodcentered\fi{}{({\mathbit{S}}_{i}\ifmmode\times\else\texttimes\fi{}{\mathbit{S}}_{j})}_{||}]\mathbit{A}$. The latter is explained by coupling between spin helicity and ``ferriaxial'' crystal rotation with macroscopic ferroaxial vector $\mathbit{A}$, characteristic of the $B$-site ordered perovskite systems with out-of-plane octahedral tilting.