Contents lists available at ScienceDirect Materials Characterization journal homepage: www.elsevier.com/locate/matchar Structure and magnetic properties of highly coercive L1 0 nanocomposite FeMnPt thin films O. Crisan ⁎ , F. Vasiliu, A.D. Crisan, I. Mercioniu, G. Schinteie, A. Leca National Institute for Materials Physics, 077125 Magurele, Romania ARTICLE INFO Keywords: Nanocomposite magnets L1 0 phase Mossbauer spectroscopy Phase evolution FeMnPt thin films ABSTRACT Among the rare-earth-free systems that are currently investigated in search for novel permanent magnet solu- tions for various applications, with special emphasis on the magnets required to operate in extreme conditions, the FePt binary system, where the tetragonal hard magnetic L1 0 phase can be formed by suitable microstructure processing via annealing, has been extensively studied. A variation of this system, the ternary FeMnPt system, has been also recently shown to exhibit good permanent magnet behavior due to the suitable formation of the L1 0 phase. In addition to be likely to form the L1 0 phase as its parent binary system, the ternary FeMnPt benefits from the reduced costs due to the reduced amount of Pt and may exhibit particular magnetic structure due to the influence of the antiferromagnetic Mn. In the present work, we have employed a mixed sputtering technique, based on the use of both elemental and compound target for developing L1 0 FeMnPt thin films with specific structural features that triggers better magnetic performances in terms of coercivity and maximum energy products. The as-obtained films have been thermally annealed and characterized by means of transmission electron microscopy, X-ray diffraction, Mossbauer spectroscopy, magneto-optic Kerr effect (MOKE) and SQUID magnetometry. The aim is to correlate the Mn induced microstructural and lattice changes with the magnetic properties and to optimize the microstructure for an early formation of the ordered L1 0 phase and increased coercivity compared to the as-prepared, structurally disordered, face centred cubic initial state of the films. 1. Introduction There is a recent surge of interest for the rare-earth-free permanent magnets and, for this purpose, the systems where the tetragonal hard magnetic L1 0 phase is present or can be formed by suitable micro- structure processing are the best candidates. Among these systems, of particular interest is the FeMnPt ternary alloy. In addition to be likely to form the L1 0 phase as its parent binary system (FePt) the ternary FeMnPt exhibit particular magnetic structure due to the influence of the antiferromagnetic (AF) Mn. Previously published papers [1] have in- vestigated magnetic and structural properties of highly chemically or- dered epitaxial (Fe 1−x Mn x ) 50 Pt 50 thin films. The Mn addition have been shown [1] to cause a steady reduction of magnetocrystalline anisotropy and saturation magnetization due to the antiparallel alignment of Fe and Mn moments. In our previous work [2] we have shown that in heterogranular FeMnPt alloys, the Mn addition promotes early forma- tion of the hard magnetic phase and moreover the phase structure of the as-cast Fe 35 Mn 15 Pt 50 alloy ribbons consist of a single FeMnPt L1 0 phase with Mn substituting Fe in the L1 0 structure. Early studies on FeMnPt magnetic phase diagram [3] showed that for intermediate concentration of Mn in the alloy there are two magnetic components, one ferromagnetic (FePt) and another one antiferromagnetic (MnPt). Spin configurations [4] as well as spin-lattice interactions through di- rectional short-range order [5] have been theoretically and experi- mentally studied in FeMnPt thin films. More recent works [6] have dealt with nanofabrication approach to the FeMnPt thin films, in order to tailor FeMnPt dots with high uniaxial anisotropy. It is known that most of the magnetic properties are highly influenced by the micro- structural features of the ternary alloys, such as: grain (or domain) sizes of the hard magnetic L1 0 phase, or lattice distortions or ordering parameter c/a of the tetragonal L1 0 symmetry. Based on first principles calculations using density functional theory, Gruner and Entel [7] showed that in FeMnPt nanoalloys the addition of Mn effectively in- creases the stability of L1 0 phase due to multiple twinning morpholo- gies. On the other hand, it has been shown from fully relativistic computational methods that in the ferromagnetic phase small ad- mixture of Mn in FeMnPt alloys will increase the magnetocrystalline anisotropy energy [8]. For chemically prepared 4 nm sized particles, it has been shown that moderate addition of Mn is beneficial for the coercivity [9]. It has been interpreted that the presence of Mn in the https://doi.org/10.1016/j.matchar.2019.04.028 Received 19 February 2019; Received in revised form 23 April 2019; Accepted 23 April 2019 ⁎ Corresponding author. E-mail address: ocrisan@infim.ro (O. Crisan). Materials Characterization 152 (2019) 245–252 Available online 24 April 2019 1044-5803/ © 2019 Published by Elsevier Inc. T