Nonmagnetic Mg2+-induced Cation Occupation and Magnetic Properties of Magnetite Nanocrystals

Understanding the substitution on the cation occupation situation of spinel ferrites is of great significance for tailoring their magnetic properties and expanding the applications. As a necessary and constant element in human body, nonmagnetic Mg2+ with good biocompatibility can induce softer magnetism and larger DC hysteresis loss, which is beneficial for biomedicine. However, the occupancy of Mg2+ in Fe3O4 is still unclear. Herein, Mg-doped magnetite (MgxFe3-xO4, 0 ≤ x ≤ 0.4) nanocrystals were fabricated through thermal decomposition. The well-dispersed MgxFe3-xO4 nanocrystals show irregular hexagonal shape with a particle size of about 13–17 nm, indicating that Mg2+ substitution has ignorable influence on the particle size. Magnetic investigations reveal that all the MgxFe3-xO4 nanocrystals show soft ferrimagnetic feature with small coercivities at room temperature. The saturation magnetization values decrease first and then slightly increase, by improving the doping amount of Mg2+. Compared to Fe3O4 nanocrystals (Tv ≈ 108 K), the slight improved Verwey transition in MgxFe3-xO4 nanocrystals (Tv ≈ 114 K) indicates that Mg2+ may affect the electronic transition between Fe2+ and Fe3+ in octahedral sublattices. Mössbauer spectra show that Mg2+ is existed in both the tetrahedral and octahedral sites, but it prefers to occupy the octahedral sites. The proportion of Mg2+ in tetrahedral sites becomes larger by increasing the doping amount. This work shows that nonmagnetic Mg2+ can induce the cation redistribution and tailor the magnetic properties of spinel ferrites.