Citation: Bohra, M.; Battula, S.V.; Singh, N.; Sahu, B.; Annadi, A.; Singh, V. Competing Magnetic Interactions in Inverted Zn-Ferrite Thin Films. Magnetism 2022, 2, 168–178. https:// doi.org/10.3390/magnetism2020012 Academic Editor: Federico Spizzo Received: 31 March 2022 Accepted: 11 May 2022 Published: 17 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Competing Magnetic Interactions in Inverted Zn-Ferrite Thin Films Murtaza Bohra 1, *, Sai Vittal Battula 1 , Nitesh Singh 1 , Baidyanath Sahu 1 , Anil Annadi 1 and Vidyadhar Singh 2 1 École Centrale School of Engineering (MEC), Mahindra University, Hyderabad 500043, India; saivittal18238@mechyd.ac.in (S.V.B.); nitesh20pphy012@mahindrauniversity.edu.in (N.S.); baidyanathsahu@gmail.com (B.S.); anilannadi@gmail.com (A.A.) 2 Department of Physics, Jai Prakash University, Chapra 841301, India; vsraj47@gmail.com * Correspondence: murtaza.bohra@mahindrauniversity.edu.in Abstract: Zn-ferrite is a versatile material among spinels owing to its physicochemical properties, as demonstrated in rich phase diagrams, with several conductive or magnetic behaviors dictated by its cation inversion. The strength and the type of cation inversion can be manipulated through the various thermal treatment conditions. In this study, inverted Zn-ferrite thin films prepared from radio frequency magnetron sputtering were subjected to different in situ (in vacuum) and ex situ (in air) annealing treatments. The temperature and field dependence of magnetization behaviors reveal multiple magnetic interactions compared to its bulk antiferromagnet behavior. Using the magnetic component model, the different magnetic interactions can be explained in terms of superparamagnetic (SPM), paramagnetic (PM), and ferrimagnetic (FM) contributions. At low temperatures, the SPM and FM contributions can be approximated to the hard and soft ferrimagnetic phases of Zn-ferrite, respectively, which changes with the annealing temperature and sputter power. Distinct magnetic properties emanating from in situ annealing compared to the ex situ annealing were ascribed to the nonzero Fe 2+ /Fe 3+ ratio, leading to the different magnetic interactions. The anisotropy was found to be the key parameter that governs the behavior of annealed in situ samples. Keywords: magnetic interactions; cation inversion; nanocrystalline Zn-ferrite; annealing 1. Introduction Zn-ferrite (ZnFe 2 O 4 ) is a spinel ferrite with a chemical composition that guarantees abundant and relatively cheap production costs, in addition to its environmentally friendly nature [1–6]. Its spinel structure is relatively “open”, with many vacant crystallographic sites, which facilitates the insertion of (mobile) dopants that can expand the number of applications of such material [1,2]. Zn-ferrite is very sensitive to growth conditions that can produce different amounts of defects and grain boundary densities, off-stoichiometry effects in the Zn and Fe content, nonzero Fe 2+ /Fe 3+ ratios, and micro/nano strains, which eventually affect the overall properties of nanostructured Zn-ferrite [7–12]. Even though thermal annealing is a convenient way for fine-tuning magnetic properties by controlling their crystallite sizes, modifying their surfaces, and affecting their magnetic interactions, it can also have detrimental effects. For instance, upon annealing, the migration of Zn cations can lead to the formation of two different crystalline (hematite and magnetite) phases along with Zn-ferrite, which eventually hinders many technological applications [6,9,13,14]. The thermodynamics of the cation disorder and the dependence of the degree of inversion with the annealing temperature have been studied in various nanostructured Zn-ferrite, ranging from nanoparticles to nanocrystalline thin films [7,15,16]. The cation inversion can be presented as [Zn 1−x +2 Fe x +3 ] A [Zn x +2 Fe 2−x +3 ] B O 4 in the inverted Zn-ferrite’s tetrahedral (A) and octahedral (B) sites, in contrast to the bulk [Zn] A [Fe 2 ] B O 4 normal structure. Despite the fact that we know that the distribution of cations (Fe 3+ and Zn 2+ ) governs the physical Magnetism 2022, 2, 168–178. https://doi.org/10.3390/magnetism2020012 https://www.mdpi.com/journal/magnetism