Elsevier

Materials Characterization

Volume 152, June 2019, Pages 245-252
Materials Characterization

Structure and magnetic properties of highly coercive L10 nanocomposite FeMnPt thin films

https://doi.org/10.1016/j.matchar.2019.04.028Get rights and content

Highlights

  • Fe60Mn5Pt35 and Fe50Mn10Pt40 thin films show formation of highly ordered L10 phase regions after annealing at 550ºC.

  • The high degree of ordering is confirmed by the large value of c/a ordering parameter (0.97).

  • Evidence of several magnetic sublattices is revealed by Mossbauer spectroscopy.

  • Both L10 FePt and L10 FeMnPt phases, as proven by the hyperfine parameters of the Mossbauer sextets, are obtained.

  • The hysteresis loops show strong coercive fields, promising for future use as RE-free permanent magnets.

Abstract

Among the rare-earth-free systems that are currently investigated in search for novel permanent magnet solutions for various applications, with special emphasis on the magnets required to operate in extreme conditions, the FePt binary system, where the tetragonal hard magnetic L10 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 L10 phase. In addition to be likely to form the L10 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 L10 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 L10 phase and increased coercivity compared to the as-prepared, structurally disordered, face centred cubic initial state of the films.

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 L10 phase is present or can be formed by suitable microstructure processing are the best candidates. Among these systems, of particular interest is the FeMnPt ternary alloy. In addition to be likely to form the L10 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 investigated magnetic and structural properties of highly chemically ordered epitaxial (Fe1−xMnx)50Pt50 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 formation of the hard magnetic phase and moreover the phase structure of the as-cast Fe35Mn15Pt50 alloy ribbons consist of a single FeMnPt L10 phase with Mn substituting Fe in the L10 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 directional short-range order [5] have been theoretically and experimentally 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 microstructural features of the ternary alloys, such as: grain (or domain) sizes of the hard magnetic L10 phase, or lattice distortions or ordering parameter c/a of the tetragonal L10 symmetry. Based on first principles calculations using density functional theory, Gruner and Entel [7] showed that in FeMnPt nanoalloys the addition of Mn effectively increases the stability of L10 phase due to multiple twinning morphologies. On the other hand, it has been shown from fully relativistic computational methods that in the ferromagnetic phase small admixture 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 FePt L10 lattice may induce local strain that promotes earlier ordering. Coercivity increase may as well be linked to the local bonding of Ptsingle bondMn (or Fesingle bondMn) in the L10 structure, due to the AF nature of the Mn atoms [9]. Another first principles calculations study of FeMnPt L10 bulk alloy with Mn aligned either ferro or antiferromagnetically in the same atomic plane (or different in the AF case) with Fe atoms [10] showed that the substitution promotes several in-plane lattice values for a fixed amount of Mn and the change in the charge hybridization will impact into a magnetic anisotropy reduction for the AFM phase and enhancement of the FM alignment. It has also been shown that in epitaxial Fe-Mn-Pt thin films [11] there is an increase in the coercivity for 12 at.% Mn content in the FeMnPt films and this has been linked to sublattice ordering of ferromagnetically aligned Mn atoms as well as to the tetragonal distortion represented by the enhanced c/a ratio comparing to the parent FePt system.

In the present work, we have employed a mixed sputtering technique, based on the use of both elemental and compound target for developing L10 FeMnPt thin films with specific structural features, that triggers better magnetic performances in terms of coercivity and maximum energy products. The aim is to correlate the Mn induced microstructural and lattice changes with the magnetic properties and to optimize the microstructure for a maximized energy product.

Section snippets

Experimental

Direct current and Radio-frequency (RF) magnetron sputtering are versatile synthesis techniques suitable for high-yield deposition of films, layers, multilayers and other compounds with thicknesses ranging from few monolayers up to micron thick depositions. The sputtering facility used for the synthesis of the films is a Physical Vapor Deposition PVD multi-technique UHV system with cylindrical upper opening chamber from Intercovamex, furnished with four radially mounted magnetrons, driven by

XRD

The X-ray diffraction analysis has been performed with Cu Kα radiation (λ = 0.15402 nm) in grazing incidence geometry. The angular interval investigated was between 15° and 65° (in 2θ). All spectra taken have been fitted using a full-profile, Rietveld-type algorithm with MAUD (Materials Analysis Using Diffraction) software. The X-ray diffractograms of both as-cast and annealed samples are displayed in Fig. 1.

It can be seen that for the as-cast samples, the two diffractograms show rather similar

Discussion

Structural data have shown that slight variations of Mn content induce variations of the lattice parameters of the ordered phase. For instance, Hasegawa and Ishio [14] have studied crystalline structure and magnetic properties of some Fe1−xyMnxPty films. After annealing at 600 °C for 1 h, films with 0 ≤ x ≤ 0.4 exhibit L10 type ordered structure and the lattice parameter a increases from 3. 84 Å to 3.98 Å whereas the lattice parameter c decreases from 3.68 Å to 3.60 Å, with increasing Mn

Conclusions

Two ternary FeMnPt thin films with different stoichiometries, Fe60Mn5Pt35 and Fe50Mn10Pt40, have been synthesized by sputtering in a Physical Vapor Deposition high vacuum multichamber system. The as-deposited films have been subjected to suitable annealings in order to fully achieve the disorder-order transformation and occurrence of the hard magnetic L10 tetragonal phase. The structural characterization by means of high resolution electron microscopy and X-ray diffraction indicates the

Acknowledgements

Financial support from Romanian Ministry of Research and Innovation from project PN-III-P4-ID-PCE-2016-0833 and also EU Competitiveness Operational Programme POC Project P_37_697 (28/01.09.2016) “Boron- and rare-earths-based advanced functional materials” is gratefully acknowledged.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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