Helical Magnetism and Structural Properties of β-Mn Structure Co(x)Zn(1-x) Thin Films

Thin films of polycrystalline β-Mn CoZn have been successfully grown on amorph-ous Si/SiO2 substrates by co-deposition sputtering from elemental targets. By this method a range of as-deposited Co(x)Zn(1−x) compositions were produced, which were annealed post-growth to reveal a splitting into two key phase elements. This splitting is into stoichiometrically correct β-Mn at the substrate across almost all com-positions except compositional ratio extremes, capped by a layer of ZnO from the surfactance of zinc through the film. Both of these layers preferentially order along their easy axis, this being〈221〉in the isotropic displacement corrected P4132/P4332 (β-Mn) CoZn, and〈002〉in ZnO. The β-Mn CoZn hence has polar order to the crystallites, which typically range on the order of 100–200 nm in lateral dimensions. Magnetic measurements show that the CoZn films deposited at ratios tuned to give the optimal volume fraction of β-Mn have an elevated TC up to ∼420 K, otherwise the TC typically appears to be 394–404 K in more cobalt deficient samples, and steadily increasing from ∼400 K with increasing cobalt content in cobalt rich samples. The samples have a saturating magnetisation of 16 emu/cc below the ideal volume fraction of β-Mn, which has a saturating magnetisation typically of 120 emu/cc, before again steadily increasing above this with increasing cobalt content. The magnetic behaviour is all consistent with the observations of compositionally dependent crystallite formation from growth studies, and both saturating magnetisation and TC of the optimal volume fraction of β-Mn are close to bulk values. Using polarised neutron reflectometry and fitting techniques in Refl1D a helical magnetic structure was determined to exist in a polycrystalline Co10Zn10 thin filmsample of ∼110 nm thickness. The helical wavelength is found to be ∼175 nm, which is in good agreement with literature of bulk studies, this helical structure is seen to unwind from 5 mT up to 700 mT, becoming indistinguishable from a simple collinear ferromagnetic state around 47 mT at 300 K. Crucially however, is the extension of the helical region from just below the TC at 380 K all the way down to room-temperature at 300 K, this is approximately 4× better than in bulk studies and extends the temperature bounds on any potential skyrmion phase. The potential for robust room-temperature skyrmion structures in these films with a wide temperature band in these materials is incredibly promising for development of skyrmion devices. In conclusion, the scope of study and improvement yet to be made for this class of alloy in thin films promises to be a rich and rewarding avenue, but direct observation of these topological structures is still required to fully attribute the measured effects and progress further.