${\mathrm{Co}}_x{\mathrm{Zn}}_y{\mathrm{Mn}}_z$ ($x+y+z=20$) compounds crystallizing in the chiral $\beta$-Mn crystal structure are known to host skyrmion spin textures even at elevated temperatures. As in other chiral cubic skyrmion hosts, skyrmion lattices in these materials are found at equilibrium in a small pocket just below the magnetic Curie temperature. Remarkably, ${\mathrm{Co}}_x{\mathrm{Zn}}_y{\mathrm{Mn}}_z$ compounds have also been found to host metastable nonequlibrium skyrmion lattices in a broad temperature and field range, including down to zero field and low temperature. This behavior is believed to be related to disorder present in the materials. Here, we use neutron and synchrotron diffraction, density functional theory calculations, and dc and ac magnetic measurements to characterize the atomic and magnetic disorder in these materials. We demonstrate that Co has a strong site preference for the diamondoid $8c$ site in the crystal structure, while Mn tends to share the geometrically frustrated $12d$ site with Zn, due to its ability to develop a large local moment on that site. This magnetism-driven site specificity leads to distinct magnetic behavior for the Co-rich $8c$ sublattice and the Mn on the $12d$ sublattice. The Co-rich sublattice orders at high temperatures (compositionally tunable between 210 and 470 K) with a moment around $1{\mu }_B/\mathrm{atom}$ and maintains this order to low temperature. The Mn-rich sublattice holds larger moments (about $3{\mu }_B$) which remain fluctuating below the Co moment ordering temperature. At lower temperature, the fluctuating Mn moments freeze into a reentrant disordered cluster-glass state with no net moment, while the Co moments maintain order. This two-sublattice behavior allows for the observed coexistence of strong magnetic disorder and ordered magnetic states such as helimagnetism and skyrmion lattices.

• Received 15 October 2018

DOI:https://doi.org/10.1103/PhysRevMaterials.3.014402