Synthesis and Electron Spin Resonance Study of Zn0.7Ni0.3RE0.1Fe1.9O4 (RE = Gd, Sm, Dy) Nanostructured Ferrites

Abstract

The glycol-thermal technique synthesized Zn_0.7Ni_0.3RE_0.1Fe_1.9O₄ (RE = 0, Gd, Sm, Dy) nanostructured ferrites with fine particles in the 8–13 nm range. X-ray diffraction (XRD) data confirmed a single-phase cubic spinel structure with no impurity peaks for the samples investigated. High-resolution transmission electron microscopy (HrTEM) showed nearly spherical particle images for the pure Zn_0.7Ni_0.3Fe₂O₄. HrTEM images for Zn_0.7Ni_0.3RE_0.1Fe_1.9O₄ (RE = Gd, Sm, or Dy) rare-earth-substituted fine particles revealed agglomerated particles with irregular shapes. Reduced particle sizes of rare-earth-substituted Zn-Ni compared to undoped compounds have been explained based on electron–electron repulsion of 4f electrons. Electron spin dynamics were investigated by electron spin resonance (ESR) measurements. A broad ESR signal with the highest intensity has been observed for the Sm-doped compound (Zn_0.7Ni_0.3Sm_0.1Fe_1.9O₄) with a particle size of about 8 nm. Broadening of the signal revealed stronger magnetic dipole interactions and narrower signals for Gd- and Dy-doped compounds indicate stronger superexchange interactions. The slightest ESR signal observed for the Gd-based (Zn_0.7Ni_0.3Gd_0.1Fe_1.9O₄) compound has been attributed to high spin–orbit coupling and the paramagnetic nature of Gd ions disrupting magnetic resonance. The evolution of the magnetic parameters, such as signal line width, intensities, and g-factors, as a function of the type of rare-earth ions, has been presented and correlated with the outer 4f electron number of rare-earth ions and particle size. Rare-earth-doped Zn_0.7Ni_0.3RE_0.1Fe_1.9O₄ (RE = Gd, Sm, Dy) nanostructured ferrites exhibit enhanced magnetic and structural properties, critical for advanced applications such as magnetic separation and biomedical imaging. This study demonstrates the influence of rare-earth doping on ESR parameters, particle size, and magnetization, providing insights into the role of 4f electron configurations.