Size-dependent magnetic properties of Mn–Zn ferrite nanoparticles synthesized by chemical solution method

Mn–Zn ferrite (Mn_0.6Zn_0.4Fe₂O₄) nanoparticles were synthesized using a modified sol–gel method at lower annealing temperature (minimum to 300 ℃). The experimental results and analysis indicate that these nanoparticles exhibit excellent soft magnetic properties with the ultra-high saturation magnetization (Ms) up to 45.1 emu/g, demonstrating the significant potential applications for biomedical industry, electronic devices, and wastewater decontamination, etc. More interestingly, Mn_0.6Zn_0.4Fe₂O₄ nanoparticles exhibit unique size-dependent magnetic property, where the saturation magnetization (Ms) decreases with the decreasing crystalline size. Furthermore, there exists a linear proportional relationship between Ms and the reciprocal of the crystalline size (1/D). A novel theoretical model is subsequently proposed in the paper to elucidate the size-dependent magnetic property. Such magnetic nanoparticles composed of small-sized nanocrystalline can be analogized to a “core–shell” structure, in which the surface or interfacial state increases rapidly in proportion to the overall system. The magnetic moments are irregularly arranged in the “shell” that consists of abundant surface or interfacial states, so-called ‘magnetic disorder shell’. On the contrary, the ferrite “core” is a ferrimagnetically ordered region with a regular arrangement of magnetic moments, which is called as ‘magnetic order core’. With the decrease of crystalline size, the proportion of the ‘magnetic disorder shell’ increases. As a result, the magnetic property of the whole ferrite nanoparticle is weakened. It is believed that the present works contribute to the advancement of comprehending the correlation between the structure and property in magnetic nanoparticles.