Spinel cobalt ferrite (CoFe2O4, CFO) nanoparticles (NPs) are a major focus of fundamental science and technological innovation due to their distinctive mix of magnetic, electrical, and chemical characteristics. CFO NPs have outstanding chemical stability, modest saturation magnetism (∼80 emu/g), a high Curie temperature (∼793 K), and significant magnetocrystalline anisotropy. These characteristics, further improved by cation substitution and surface functionalization, enable a wide range of applications. This review provides a comprehensive analysis of CFO NPs, covering their synthesis methods, physicochemical characterization, surface modifications, and diverse applications. We compare the environmental impact, scalability, yield, and particle size control of a variety of synthesis techniques, including co-precipitation, hydrothermal, sol-gel route, combustion method, microemulsion, thermal decomposition, electrochemical synthesis, polyol method, and green synthesis methods. The sustainable alternative of green synthesis, which employs plant- and microbe-mediated biosynthesis, is becoming increasingly important in the biomedical and environmental sectors. Furthermore, we explore advanced surface functionalization techniques that employ monomeric, inorganic, and polymeric stabilizers to improve the biocompatibility and stability of CFO NPs. The effects of cation substitution (such as transition metals and rare-earth dopants) on the physicochemical and magnetic properties of CFO NPs are examined in detail, addressing challenges like cost and stability in real-world applications. Moreover, the present review provides a detailed discussion correlating structural, morphological, magnetic, dielectric, optical, and electrical properties of CFO with synthesis methods and modifications. The traditional energy storage and conversion applications of CFO are comprehensively discussed. Additionally, the review highlights magnetic applications, biomedical applications (e.g., MRI contrast agents, magnetic hyperthermia, and biosensors), the role of CFO in electronics and optoelectronics, purification and catalysis applications, as well as advances in electromagnetic technologies. Emerging applications, including their roles in quantum computing, nanorobotics, tissue engineering, and bioimaging, are also discussed, emphasizing the cutting-edge potential of CFO NPs in multifunctional technologies. The objective of this review is to critically evaluate recent advancements, challenges, and future research directions to bridge the divide in understanding CFO NPs. This systematic evaluation establishes a strong foundation for researchers, allowing them to investigate novel applications of CFO NPs in both current and emerging technological domains.

Induced strain and magnetostrictive coefficient (λ₁₁) with the applied magnetic field (H) for different compositions of Co_1.1Fe_1.9−xEr_xO₄.

We report on the combined experimental and theoretical simulation results of lead-free ferroelectrics, Ba_(1-x)Ca_xTiO₃ (x = 0.0–0.3) and BaTi_(1-y)Zr_yO₃ (y = 0.0–0.2), synthesized by standard solid state reaction method. First principles density functional calculations are used to investigate the electronic structure, dynamical charges, and spontaneous polarization of these compounds. In addition, the structural, ferroelectric, piezoelectric, and dielectric properties are studied using extensive experiments. The X-ray diffraction and temperature dependent Raman spectroscopy studies indicate that the calcium (Ca) substituted compositions exhibit a single phase crystal structure, while zirconium (Zr) substituted compositions are biphasic. The scanning electron micrographs reveal the uniform and highly dense microstructure. The presence of polarization-electric field and strain-electric field hysteresis loops confirms the ferroelectric and piezoelectric nature of all the compositions. Our results demonstrate higher values for polarization, percentage strain, piezoelectric coefficients, and electrostrictive coefficient compared to those existing in the literature. For smaller substitutions of Ca and Zr in BaTiO₃, a direct piezoelectric coefficient (d₃₃) is enhanced, while the highest d₃₃ value (∼300 pC/N) is observed for BaTi_0.96Zr_0.04O₃ due to the biphasic ferroelectric behavior. Calculation of Born effective charges indicates that doping with Ca or Zr increases the dynamical charges on Ti as well as on O and decreases the dynamical charge on Ba. An increase in the dynamical charges on Ti and O is ascribed to the increase in covalency of Ti-O bond that reduces the polarizability of the crystal. A broader range of temperatures is demonstrated to realize the stable phase in the Ca substituted compounds. The results indicate enhancement in the temperature range of applicability of these compounds for device applications. The combined theoretical and experimental study is expected to enhance the current scientific understanding of the lead-free ferroelectric materials.