Regulation of morphology and magnetic properties of ZnFe₂O₄ assemblies via magnetic fields

ZnFe₂O₄ was synthesized at 200 °C via a solvothermal pathway using acetylacetonate 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 ZnFe₂O₄ particle size gradually decreases. The synthetic magnetic field causes the magnetization value of ZnFe₂O₄ 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 (1 Oe = 79.5775 A⋅m⁻¹) 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 × 10⁴μ_B (ZnFe₂O₄ prepared under the condition without magnetic field), μ_sp = 1.69 × 10⁴μ_B (ZnFe₂O₄ prepared under the condition of two magnets), and μ_sp = 1.85 × 10⁴μ_B (ZnFe₂O₄ prepared under the condition of four magnets). The estimated magnetic particle sizes are 11.4 nm, 10.6 nm, and 9.9 nm, respectively, which are larger than the corresponding sizes (8.6 nm, 7.2 nm, 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.