Ferromagnetic behavior of non-stoichiometric ZnS microspheres with a nanoplate-netted surface

Abstract

Undoped ZnS microspheres with a size of 4–5 μm were produced using a hydrothermal method with different ratios of Zn and S precursors. Structural and morphological measurements show that the sphalerite ZnS microspheres have a cavity surface self-assembled with nanoplates with a thickness of 20–30 nm. Experimentally measured magnetic hysteresis curves for the undoped ZnS microspheres clearly indicate ferromagnetic behavior at room temperature with a saturation magnetization M_s = 3.66 and 1.566 memu g⁻¹ for an atomic ratio of Zn to S equal to 0.966 and 1.32, respectively. The calculations based on density functional theory within the generalized gradient approximation + Hubbard U (GGA + U) approach demonstrated that the ZnS (111) surface with Zn vacancies produces a ferromagnetic state with a magnetic moment per unit cell of 2.0 μ_B; the defective ZnS (111) surface with mixed Zn and S vacancies has a reduced magnetic moment of 1.12 μ_B because of the structural reconstruction, while the defective ZnS (111) surface with only S vacancies is non-magnetic. The observed weak ferromagnetism for the ZnS microspheres can be ascribed to the Zn vacancies and the cavity surface; the latter results in a large number of unsaturated bonds for the S and Zn atoms at the interfacial and surface regions. These studies will be helpful for understanding the d⁰ ferromagnetism.