Regulation of the morphology and magnetic properties of ZnFe ₂ O ₄ assemblies via magnetic fields

Abstract In this study, ZnFe 2 O 4 was synthesized at 200°C via a solvothermal pathway using acetilacetonate salts of zinc and iron as raw materials in self-developed magnetic fields of varying intensities. The following phenomena were observed. During the synthesis process, applying a magnetic field causes spherical assemblies with micrometer-scale diameters to transform into coarse chain-like aggregates exhibiting a length-to-diameter ratio of approximately 4.8. As the strength of the magnetic field increases, the ZnFe 2 O 4 particle size gradually decreases. The synthetic magnetic field causes the magnetization value of ZnFe 2 O 4 to increase or decrease. This is due to the interaction between surface spins and bulk spins, and this interaction is regulated by the particle size. The zero-field-cooling (ZFC) curves measured under a 100-Oe magnetic field obey the Curie-Weiss law in the high-temperature region. The effective magnetic moments of the superparamagnetic particles obtained through fitting are μ sp = 2.53×10 4 μ B (ZnFe 2 O 4 prepared under the condition without magnetic field), μ sp = 1.69×10 4 μ B (ZnFe 2 O 4 prepared under the condition of two magnets), and μ sp = 1.85×10 4 μ B (ZnFe 2 O 4 prepared under the condition of four magnets). The estimated magnetic particle sizes are 11.4, 10.6, and 9.9 nm, respectively, which are larger than the corresponding sizes (8.6, 7.2, and 6.2 nm) obtained from electron microscopy, indicating that the magnetic moments of adjacent particles tend to be parallel. This work reports a new approach that enables the preparation of magnetic particles with clean surfaces as well as tunable sizes, morphologies, and properties simply by adjusting the magnetic field strength without the need for any additives or templates, thus broadening their potential for various applications.