Copper oxide selenite

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Copper oxide selenite, Cu2OSeO3, is an insulating inorganic compound that is piezoelectric and piezomagnetic. It becomes ferrimagnetic below about 58 K and, notably, is the only insulating material known to host magnetic skyrmions (as of 2021).

Structure and magnetism: It has a cubic, slightly distorted pyrochlore structure built from Cu4O and SeO3 units. In each Cu4 tetrahedron, three Cu2+ spins point in one direction while the fourth points the opposite way, giving ferrimagnetic order. A Dzyaloshinskii-Moriya interaction leads to helical spin textures and skyrmions at low magnetic fields. When a magnetic field increases, the spin pattern evolves from helical to conical to a skyrmion lattice, and finally to a field-polarized ferrimagnetic state.

Growth: Polycrystals form by heating a 2:1 mixture of CuO and SeO2 at 600 °C for 12 hours in vacuum. High-quality olive-green single crystals (~4 mm) can be grown by chemical vapor transport using NH4Cl as the transport agent (NH4Cl sublimes at 340 °C to give NH3 and HCl).

Properties: Thermal conductivity peaks around 9 K at about 400 W/(m·K). The magnetization damping constant is about 1×10^-4 at 5 K, only four times larger than the very low-damping material YIG, making Cu2OSeO3 attractive for high-frequency electronic applications. The magnetic transition temperature and spin textures depend strongly on the applied magnetic field.