Control of chirality of transition-metal monosilicides by the Czochralski method

The crystallographic and magnetic chirality of the B20 (space group P2${}_1$3) solid solutions Fe${}_{1-x}$Co${}_x$Si and pure MnSi grown by the Czochralski method has been studied by means of the single crystal x-ray diffraction and the small-angle diffraction of polarized neutrons. To test the possibilities to control the crystal handedness, first, we have grown a series of MnSi crystals and one series of Fe${}_{1-x}$Co${}_x$Si samples with $x=0,0.10,0.15,0.20,0.25,0.30,0.50$ using a right-handed crystal as a seed. Second, we have grown another series of MnSi and Fe${}_{1-x}$Co${}_x$Si samples again of different concentrations using a left-handed crystal as a seed. In $90\%$ of the cases the grown sample has been found to be enantiopure and to inherit the crystallographic chirality of its seed crystal. In $10\%$ of the cases undefined circumstances flip the chirality over for the next progeny or produce a racemic sample. The magnetic chirality of these compounds has been shown to be rigorously related to their crystallographic handedness, but in the opposite way for MnSi and for Fe${}_{1-x}$Co${}_x$Si.

Figure 1

Dependence of the helix wave vector $k$ and the chirality $\gamma$ on the Co concentration $x$ for Fe${}_{1-x}$Co${}_x$Si.

Figure 2

View of a B20 cubic crystal (space group P2${}_1$3) along the $\langle 001\rangle$ (a), (b) and $\langle 111\rangle$ (c), (d) axes for two chiral configurations: (a), (c) right-handed with $u_{\mathrm{Si}}=0.164$ and $u_{\mathrm{Me}}=0.862$ and (b), (d) left-handed with $u_{\mathrm{Si}}=0.846$ and $u_{\mathrm{Me}}=0.138$ [here $u$ is the parameter in expression Eq. (1)]. The black spirals help to identify the sense of the skewing.

Figure 3

Fe${}_{1-x}$Co${}_x$Si samples grown from the right-handed (P) seed (series a), from the left-handed (P) seed (series b), and from another right-handed (P) seed (series c). The structural handedness determined in the experiment is denoted as (SR) for right-handed crystals and as (SL) for left handed crystals.

Figure 4

Series of the MnSi samples grown from the left-handed (P) seed (series a) and from the right-handed (P) seed (series b).