The Effects of Doping on the Magnetic Transition Regions of the Skyrmion Hosting Material, Cu₂OSeO₃

The Cu₂OSeO₃ material system has gained a lot of interest over the last 10-15 years because it is the only multiferroic insulator known to host magnetic skyrmions. This material falls into a B20 class which has a non-centrosymmetric cubic crystal lattice and crystallises in a P2₁3 space group. Most materials in this class, such as MnSi, FeGe, and Fe_1-xCo_xSi, despite having different compositions, have been observed to host magnetic skyrmions.¹ This suggests that the role of the crystal structure might influence the capability of the material to host skyrmions. The skyrmion formation is a result of the four different Cu²⁺sites, which have a 3-down 1-up spin arrangement with Ferromagnetic (FM) and Antiferromagnetic (AFM) super-exchange interactions being present.² The lack of inversion symmetry in the corner shared O-Cu₄ tetrahedra result in an appreciable Dzyaloshinskii–Moriya interaction (DMI) between Cu²⁺ sites; this competes with the super-exchange interactions leading to spin canting that underpins the formation of helical/conical spin textures (depending on temperature and applied magnetic field strength).³However, due to these systems' complex spin and charge interactions, the underlying quantum-mechanical processes behind these phenomena are still not fully understood. Further experiments are required; as such, this project aims to provide details on the roles played by the crystal structure that may enable the magnetic structures observed in different conditions.<br/><br/>For this project, the focus is on the relationship between the crystal structure and the magnetic structures observed in the Cu₂OSeO₃ material system. By manipulating the magnetic Cu sites and non-magnetic Se sites, the spin interactions between the magnetic sites will change, resulting in changes to the interacting Heisenberg and Dzyaloshinskii-Moriya interactions that are known to form skyrmions. These interactions are highly localised between magnetic ions, which makes them sensitive to spatial changes. Non-magnetic doping of substituting Te into the Se sites results in chemical pressure and expansion of the unit cell, which changes the spacing between atoms and the subtle crystal distortions should have an impact on these interactions. While magnetic doping by substituting Co²⁺ into the Cu²⁺ sites should have a large impact on these magnetic interactions due to the Co²⁺ ion having a stronger moment than Cu²⁺. This, in turn, should change the nature of the formation of magnetic skyrmions in this material. Understanding the role that subtle distortion to the magnetic network plays on the formation and stability of topological spin structures will be vital in designing materials for spintronics. This might, in fact, provide a guide in developing ambient condition spintronics materials, in which their absences have hindered the technological application of these topological spin structures.<br/><br/>We will present data from high-resolution synchrotron powder XRD and neutron powder diffraction to present the effects of magnetic and non-magnetic doping on both the crystal and magnetic structure of the material. Along with small-angle neutron scattering to confirm the presence of the helical and skyrmion phases. While in-depth SQUID magnetometry measurements were carried out to see the effects of doping on magnetisation. This collection of data aims to build an understanding of how the crystal structure plays a more important in the formation of skyrmions than is currently believed in literature.<br/> <br/>1. N. Kanazawa <i>et al.</i>, <i>Adv Mater.</i> <b>29</b>(25), (2017).<br/>2. J-W.G. Bos <i>et al.</i>, <i>Phys. Rev. B</i>, <b>78</b>(9), [094416] (2008)<br/>3. S. Seki <i>et al.</i>, <i>Science</i> <b>336</b>, 198 (2012).