Chirality of structure and magnetism in the magnetoelectric compound ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$

Small-angle diffraction of polarized neutrons and resonant contribution to diffraction of synchrotron radiation have been applied to prove chirality of the crystal and magnetic structures of the magnetoelectric insulator ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$. Similarly to other chiral magnets with $P{2}_{1}3$ crystal structure the corresponding chiralities are linked to each other via the phenomenological Dzyaloshinskii-Moriya interaction. The crystal and magnetic structures for ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ have the same chirality as is observed for MnSi, Mn${}_{1-x}$Fe${}_x$Si, and MnGe. However, the combination of chiralities is opposite to that proposed from a recent theoretical consideration. The chiral link between structure and magnetism previously found for metallic compounds is also proved to exist for an insulator (${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$), allowing us to conclude that the conducting electrons play no role in a possible common microscopic mechanism of this specific magnetolattice interaction.

Figure 1

Maps of the polarized SANS intensities of MnSi [(a), (b)] at $T=15$ K and ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ [(c), (d)] at $T=35$ K for the polarization $P_0$ along the guide field [(a), (c)] and opposite to it [(b), (d)]. The applied magnetic field is $B=80$ mT for MnSi and 50 mT for ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$.

Figure 2

Intensity of ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ at $\mathbf{q}\parallel {\mathbf{P}}_0$ as a function of the magnetic field $B$ at $T=35$ K. The error bars are smaller than points.

Figure 3

The magnetic chiral fraction $\gamma$ as a function of the relative magnetic field $B/B_{C2}$ for MnSi (top) and ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ (bottom). The 1 and $-1$ values correspond to the enantiopure state with left and right handedness, respectively. $\tau =(T_c-T)/T_c$, MnSi data are adopted from Ref. [29].

Figure 4

View of a [O${}_2$Cu${}_7$]${}^{10+}$ dimer (a) with the magnetic moments (b) adopted from Refs. [13, 35]. These dimers form a spiral along the axis $\left(111\right)$ (c). The 3D framework made by the dimers along $\left(110\right)$ (d).