Seeded growth of ferrite nanoparticles from Mn oxides: observation of anomalies in magnetic transitions

Abstract

A series of magnetically active ferrite nanoparticles (NPs) are prepared by using Mn oxide NPs as seeds. A Verwey transition is identified in Fe₃O₄ NPs with an average diameter of 14.5 nm at 96 K, where a sharp drop of magnetic susceptibility occurs. In MnFe₂O₄ NPs, a spin glass-like state is observed with the decrease in magnetization below the blocking temperature due to the disordered spins during the freezing process. From these MnFe₂O₄ NPs, MnFe₂O₄@Mn_xFe_1−xO core–shell NPs are prepared by seeded growth. The structure of the core is cubic spinel (Fdm), and the shell is composed of iron–manganese oxide (Mn_xFe_1−xO) with a rock salt structure (Fmm). Moiré fringes appear perpendicular to the 〈110〉 directions on the cubic shape NPs through the plane-matched epitaxial growth. These fringes are due to the difference in the lattice spacings between MnFe₂O₄ and Mn_xFe_1−xO. Exchange bias is observed in these MnFe₂O₄@Mn_xFe_1−xO core–shell NPs with an enhanced coercivity, as well as the shift of hysteresis along the field direction.