Nickel–Zinc Ferrite Nanoparticles for Hyperthermia: Preserving Superparamagnetism Across a Broad Range of Particle Sizes

Nickel-zinc ferrite (NZF) compounds, renowned for their mixed spinel structures, hold significant promise for diverse applications in high-frequency devices and biomedicine. This study utilizes solvothermal synthesis to produce NZF nanoparticles (NPs) with tunable diameters ranging from 40 to 300 nm. These NZF NPs exhibit polycrystalline structures, with crystallite sizes tailored to be approximately 8 nm, a pivotal factor in preserving their superparamagnetic (SPM) properties across a broad size spectrum. A combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, energy-dispersive X-ray spectroscopy (EDS), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray photoelectron spectroscopy (XPS) confirms the structures and compositions of the synthesized samples. Magnetometry measurements revealed consistent saturation magnetization (<i>M</i>_S) values and SPM features at room temperature across all samples, despite variations in particle size. These nanoparticles transition to a ferrimagnetic (FiM) or blocked state at approximately 150 K. Consequently, comparable specific absorption rate (SAR) values are achieved across the samples, which can be attributed to the uniformity in the size of the primary crystals (crystallite size ∼8 nm) within the polycrystalline structures of the NZF nanoparticles. Our study underscores the significance of controlling the crystallite sizes in polycrystalline ferrite nanoparticles, enabling the attainment of desirable SPM and hyperthermia responses across a wide size spectrum. It also delineates an effective methodology for producing NZF nanoparticles suitable for a variety of applications, including optoelectronic and high-frequency devices, photocatalysis, environmental remediation, sensor systems, and advanced biomedical technologies.