Deciphering structural and magnetic disorder in the chiral skyrmion host materials Co$_x$Zn$_y$Mn$_z$ ($x+y+z=20$)

TL;DR

This study investigates the atomic and magnetic disorder in Co$_x$Zn$_y$Mn$_z$ compounds, revealing site preferences and magnetic behaviors that influence skyrmion stability and coexistence of magnetic states.

Contribution

It provides detailed insights into the atomic site occupancy and magnetic interactions in Co-Zn-Mn compounds, linking disorder to skyrmion hosting capabilities.

Findings

Co prefers the 8c site, Mn shares the 12d site with Zn.

Co sublattice orders at high temperatures with stable moments.

Mn moments fluctuate and freeze into a cluster-glass state at low temperatures.

Abstract

Co$_x$Zn$_y$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, CoxZnyMnz compounds have also been found to host metastable non-equilibrium 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 100K to 470K) with a moment around 1 $\mu_B$/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.

Full text

Supplemental Material: Deciphering structural and magnetic disorder in the chiral skyrmion host materials

CoxZnyMnz ($x+y+z=20$)

Joshua D. Bocarsly

jdbocarsly@mrl.ucsb.edu

Materials Department and Materials Research Laboratory, University of California, Santa Barbara, California 93106, United States

  

Colin Heikes

Center for Neutron Research, National Institute of Standards and Technology,

Gaithersburg, Maryland 20899, United States

  

Craig M. Brown

Center for Neutron Research, National Institute of Standards and Technology,

Gaithersburg, Maryland 20899, United States

  

Stephen D. Wilson

Materials Department and Materials Research Laboratory, University of California, Santa Barbara, California 93106, United States

  

Ram Seshadri

Materials Department and Materials Research Laboratory, University of California, Santa Barbara, California 93106, United States

I Additional diffraction patterns

II Magnetization vs. temperature for CoxZnyMnz samples

III Sample size dependence of precursor anomaly in Co8Zn9Mn3