Growth and Characterization of β-Mn Structured CoZn Thin Films

Thin films of polycrystalline β-Mn structure CoZn have been grown on thermally oxidized Si substrates by co-sputtering from elemental targets followed by annealing. A range of films grown with variable Co deposition power and fixed Zn deposition power were produced, so as to vary the proportions of the two elements reaching the substrate, which were annealed post-growth. Whilst all films exhibited a (221) β-Mn structure CoZn texture in X-ray diffraction, transmission electron microscopy showed that the composition with the highest integrated intensity for that Bragg peak contained large voids and was covered by a thick ZnO cap owing to being Co-deficient overall. CoZn films deposited at ratios tuned to give the optimal volume fraction of β-Mn were continuous, with crystallites up to 200 nm in size, with a much thinner ZnO cap layer. Magnetic measurements show that such optimal CoZn films have a Curie temperature TC ∼ 420 K and saturation magnetization of 120 emu/cm3, properties close to those reported for bulk crystals. Excess Co beyond this point leads to a significant retained ferromagnetism above the Curie point of the CoZn, suggesting Co-rich aggregates. The β-Mn structure is chiral (P4132/P4332 space group) and is known to give rise to a Dzyaloshinkii-Moriya interaction (DMI) that stabilizes room-temperature skyrmions in the bulk. Our thin films are thus a potential materials platform, compatible with planar processing technology, for magnetic skyrmions arising from a bulk DMI.