Ternary alloys Co-Zn-Mn with $\beta$-Mn-type structure exhibit helimagnetic order and skyrmion lattice states in a low-Mn-concentration region. Recent studies on ${\mathrm{Co}}_{10-\frac{x}{2}}{\mathrm{Zn}}_{10-\frac{x}{2}}{\mathrm{Mn}}_x$ (CZM) revealed that the magnetic modulation period and the paramagnetic-to-helimagnetic transition temperature are reduced with increasing Mn concentration $x$ [Tokunaga et al., Nat. Commun. 6, 7638 (2015)] and that the helimagnetic order is replaced with a spin-glass state in a high-$x$ region [Karube et al., Sci. Adv. 4, eaar7043 (2018)]. To understand the microscopic mechanism of the Mn substitution effect, we have performed crystal structure analyses of a series of CZM samples with $x=4,5,6,7,8,10,12,$ and 16 by neutron powder diffraction. We present a model for the site occupancies of Co, Zn, and Mn in the unit cell, which has two independent crystallographic sites, specifically $8c$ and $12d$ sites. Zn atoms are always accommodated in the $12d$ sites, and Co atoms prefer the $8c$ sites, although some of them also exist in the $12d$ sites in a low-$x$ region. Mn atoms mainly prefer the $12d$ sites but also share the $8c$ site with Co atoms in the region of $x\ge 4$. A comparison between the site occupancies and the $x-T$ magnetic phase diagram suggests that the amount of Mn in the $8c$ sites is relevant to the emergence of the spin-glass state.

• Received 15 May 2019

DOI:https://doi.org/10.1103/PhysRevB.100.064407