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Abstract: In this paper, we explore the magnetic, optical, and antibacterial properties of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{CoF}}{{{\text{e}}}_{2}}{{{\text{O}}}_{4}}$$\end{document}(CFO), \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{C}}{{{\text{o}}}_{{0.98}}}{\text{A}}{{{\text{g}}}_{{0.02}}}{\text{F}}{{{\text{e}}}_{2}}{{{\text{O}}}_{4}}$$\end{document}(CAFO), and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{C}}{{{\text{o}}}_{{0.98}}}{\text{T}}{{{\text{i}}}_{{0.02}}}{\text{F}}{{{\text{e}}}_{2}}{{{\text{O}}}_{4}}$$\end{document}(CTFO) and spinel ferrite nanocrystals. The XRD analysis of these crystals reveals a single-phase cubic structure of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Fd\bar {3}m$$\end{document}space group with a crystallite size of 45, 46, and 41 nm, respectively. The room temperature magnetic measurement shows that CTFO has the highest saturation and remanent magnetization, whereas CFO has the highest coercivity and magnetic squareness ratio. Optical properties of these samples have been taken in the range of 300–900 nm. The band gap is calculated using Tauc’s plot, and it decreases with the doping of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{A}}{{{\text{g}}}^{ + }}$$\end{document}and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{T}}{{{\text{i}}}^{{4 + }}}$$\end{document}ions. Antibacterial studies of these samples have been done by disc diffusion method for Gram-positive and Gram-negative bacteria, Staphylococcus aureus(S. aureus) and Escherichia coli(E. coli), respectively. The compositions have shown better antibacterial response for S. aureusover E. coli. The 200 mg/mL CTFO sample has shown the most effective result against S. aureusbacteria, with 18 mm diameter value of zone of inhibition.

This study is focused on the elastic properties, magnetocaloric effect, critical behavior, and spin interaction nature of the pristine and rare-earth-ion (Dy3+and La3+) doped Co0.7Zn0.3Fe2O4(CZFO) spinel ferrite nanoparticles. It also highlights an unusual dielectric behavior of the samples near the magnetic phase transition. The spinel cubic structure and nano sized grains (22–41 nm) of the samples were confirmed by an X-ray diffractometer and a transmission electron microscope, respectively. The elastic properties were analyzed using Fourier transform infrared spectroscope to determine the interatomic bond strength and thermal responses. Magnetic measurements reveal that the CZFO sample shows ferrimagnetic behavior, whereas the doped samples exhibit antiferromagnetic (AFM) behavior with second-order magnetic phase transition, below the transition temperature (TM). The Dy3+doped sample exhibits the larger values of saturation magnetization, remanent magnetization, magnetic coercivity, the change in maximum magnetic entropy (−ΔSMmax), and relative cooling power (RCP) as compared to CZFO. Critical parameters (β, γ, and δ) are estimated using Arrott plots, the Kouvel–Fisher method, and Widom scaling relation. The type of exchange interaction during magnetic phase transition is derived using renormalization group theory. The dielectric constant (ε′) exhibits an anomaly near TMfor all samples, below which ε′ increases with Tattributed to Maxwell–Wagner and Koop theory. Above TM, an unconventional negative ε′ behavior is observed, which can be interpreted well by the Lorentz model of dielectric resonance. These materials with large values of TM, ΔSMmax, and RCP appear to be promising candidates for high temperature magnetic refrigeration technology, whereas the coexistence of AFM and magneto-dielectric behaviors can be used in designing multifunctional and spintronic devices.