Citation: Crisan, O.; Crisan, A.D.; Randrianantoandro, N. Temperature-Dependent Phase Evolution in FePt-Based Nanocomposite Multiple-Phased Magnetic Alloys. Nanomaterials 2022, 12, 4122. https://doi.org/10.3390/ nano12234122 Academic Editor: Julián María González Estévez Received: 8 November 2022 Accepted: 18 November 2022 Published: 22 November 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/). nanomaterials Article Temperature-Dependent Phase Evolution in FePt-Based Nanocomposite Multiple-Phased Magnetic Alloys Ovidiu Crisan 1 , Alina Daniela Crisan 1, * and Nirina Randrianantoandro 2 1 National Institute for Materials Physics, P.O. Box MG-7, 077125 Magurele, Romania 2 Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Faculté des Sciences & Techniques, Université du Maine Avenue Olivier Messiaen, CEDEX 09, 72085 Le Mans, France * Correspondence: alina.crisan@infim.ro Abstract: A quaternary Fe–Pt–Nb–B alloy has been fabricated by the melt spinning method with the purpose of the formation of crystallographically coherent multiple magnetic phases, emerging from the same metastable precursor, as well as to investigate the phase interactions and the influence of their coupling on magnetic performances. For this purpose, extended structural and magnetic investigations were undertaken by making use of X-ray diffraction, transmission electron microscopy, and 57 Fe Mössbauer spectroscopy, as well as magnetic measurements using SQUID magnetometry. It was documented that intermediate metastable phases formed during primary crystallization, in intermediate stages of annealing, and a growth-dominated mode was encountered for the secondary crystallization stage upon annealing at 700 ◦ C and 800 ◦ C where fcc Fe3Pt and fct Fe2B polycrystalline were formed. The Mössbauer investigations have documented rigorously the hyperfine parameters of each of the observed phases. The fcc A1 FePt phase was shown to exhibit a peculiar ferromagnetic transition, and this transition has been proven to occur gradually between 300 K and 77 K. The magnetic measurements allowed us to identify the annealing at 700 ◦ C as optimal for obtaining good magnetic features. Coercive field dependence shows similarities to the random anisotropy model for samples annealed at 500 ◦ C to 700 ◦ C which are nanocrystalline. These results show good perspectives for use in applications where different magnetic states are required at different operating temperatures. Keywords: magnetic multiphase materials; 57 Fe Mössbauer spectroscopy; melt spun ribbons 1. Introduction Recent developments in the field of renewable energies, autonomous vehicles, smart buildings, and other IoT household applications have seen an emerging need for novel, reliable magnets, and magnetic sensing, with on-chip integration capabilities. The mag- netic multiphase materials, derived from the Fe–Pt systems, have been under scrutiny for several years now, due to their good performance given by the combined effect of ordered microstructure and magnetic coupling between the different magnetic phases. In some applications, such as sensor integration into logic devices [1], FePt-based magnets have proven more reliable than classic Nd–Fe–B magnets in obtaining a defined magnetic state, depending on the needed operating temperature. Much research effort has been spent on developing nanocomposite FePt-based magnets that are highly corrosion-resistant and with a working temperature which is higher than the traditional Nd–Fe–B ones [2–5]. FePt usually crystallizes in a disordered face-centered-cubic fcc A1 structure which upon annealing, for quasi-equiatomic stoichiometry, transforms into an ordered face- centered-tetragonal fct phase with high coercivity and large magnetocrystalline anisotropy. It has been shown [2] that by suitable additions to the initial chemical composition, the FePt-based alloys may achieve a state where several magnetic phases co-exist and the overall magnetic behavior of the alloy can benefit from the interplay between the perfor- mances of the individual magnetic phases, powered by the magnetic coupling between Nanomaterials 2022, 12, 4122. https://doi.org/10.3390/nano12234122 https://www.mdpi.com/journal/nanomaterials