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@MnxFe1-xO core-shell NPs are prepared by seeded growth. The structure of the core is cubic spinel (Fd (3) over barm), and the shell is composed of iron-manganese oxide (MnxFe1-xO) with a rock salt structure (Fm (3) over barm). Moire 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 MnxFe1-xO. Exchange bias is observed in these MnFe2O4@MnxFe1-xO core-shell NPs with an enhanced coercivity, as well as the shift of hysteresis along the field direction.