Nitriding-induced texture, ordering and coercivity enhancement in FePtAgB nanocomposite magnets

https://doi.org/10.1016/j.jmmm.2015.11.017Get rights and content

Highlights

  • FePtAgB melt spun ribbons exhibit direct formation of L10 phase.

  • Nitriding at 450 °C induces strong texture and enhanced degree of ordering in L10 FePt.

  • Combined effect of annealing and nitriding strongly enhances coercivity and (BH)max.

Abstract

FePt system attracts currently a great deal of interest for applications as future RE free permanent magnets. Among the key issues to be solved one may count the decreasing of the ordering temperature and improvement of magnetic behavior. For that purpose we have studied the effect of a nitriding post-synthesis procedure on the FePtAgB melt spun ribbons, aimed at refining the microstructure and enhancing the magnetic performances. Deep structural characterization by transmission electron microscopy, electron diffraction, energy dispersive X-ray spectroscopy and X-ray diffraction allowed us to observe the morphology and to correctly assign and identify the nature of the main granular phases observed. Nitriding procedure is shown to strongly enhance the (001) texturing and the degree of ordering of the L10 FePt phase, as well as largely increase of coercivity, compared to the as-cast state. These changes are interpreted in terms of Ag segregation towards intergranular region associated to N diffusion and creation of vacancies that favor consistently the process of ordering the FePt grains into the L10 tetragonal phase.

Introduction

Good properties of the FePt system rely on the disorder–order structural phase transformation of the equiatomic FePt, from a disordered face-centered-cubic fcc A1 symmetry to a highly ordered tetragonal L10 symmetry, the latter having enhanced magnetocrystalline anisotropy (107 J/m3) and high coercive field (1000 kA/m) [1]. Much effort was devoted to the study of systems derived from FePt and to the decrease of the relatively high ordering temperature Tord (550 °C in the case of nanoparticles). Similar studies have been performed on thin films, multilayers and nanogranular alloys [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Substitution with some other elements (for example Ag, Au, Sb [13], [14], [15]) has been considered and shown to lead to a decrease of Tord down to about 300 °C. Nevertheless, this triggers drastic alteration of magnetic properties (the maximum energy product (BH)max and saturation magnetization [16], [17]). The structural phase transformation from A1 to L10 has been previously studied on FePt systems [18], [19] via calorimetric measurements, while the effect of alloy composition on the thermodynamic and kinetic parameters [20] and of the alloying additions of Ag and Au [21] on the A1 to L10 transformation in FePt systems have also been evidenced. Magnetic properties improvements in FePt and FePtB systems by addition of Co and Au have also been observed [22], [23], [24]. The homogeneity of the FePt-based alloys in prepared melt-spun ribbons is an important challenge. To obtain a homogeneous FePt-based nanocomposite magnet in ribbon shape, metalloids, for example B, and refractory element such as Nb, that has the aim of limiting the grain growth during solidification, can be added to the alloy composition [25], [26], [27], [28]. Moreover, in analogy to the case of FePtAg thin films where addition of Ag has been proven to be beneficial in decreasing Tord [14], [29], [30], the replacement of Nb with Ag in the composition was considered [31]. In our previous papers, we have shown that a FePtAgB alloy obtained by melt-spinning exhibits direct formation of the L10 phase from the as-cast state without the need of post-synthesis annealing [31], [32]. Building on that foundation, the present paper proposes to study the influence of nitriding on the L10 phase on FePt-based alloys with different Ag content. The nitriding process leads to drastic modifications of microstructure and magnetic properties, as compared to their as-cast counterparts. Incorporation of a few at% N in binary FePt alloy films either by sputtering in N atmosphere or by ion implantation has been reported to result in a faster L10 ordering due to faster atomic diffusion and improvement of hard magnetic properties [33], [34], [35]. The release of N atoms from the FePt phase yields vacancies and voids and, apart from a pronounced effect on the kinetics of the disorder–order transformation FePt caused by diffusion mechanisms, it has been concluded [36] that N is incorporated mainly in the grain boundary region, which hinders grain growth and results in a smaller grain size. As the effect of the microstructure changes on the magnetic properties has been shown to be quite important [37], [38], [39], [40], [41]. Our work emphases the nitriding effect on the microstructure changes, based on an interfacial vacancies mechanism that, combined with the Ag-induced modifications [32], may explain the improvement of the ordering and the drastic changes of the magnetic behavior, as compared to the original as-cast samples.

Section snippets

Experimental

Two FePt–Ag–B alloys, having the nominal composition Fe48Pt28Ag6B18 (hereafter denoted Ag6) and Fe45Pt28Ag9B18 (Ag9), have been prepared by melt spinning, the synthesis method being described in [32]. The as-cast samples have then been subjected to a complex nitriding procedure in a Gas Reaction Controller unit, from Advanced Material Corporation (AMC), Pittsburgh, PA. The facility allows automatic steps of vacuum/purging with nitrogen and inert gases in the temperature range 0–500 °C and

Results and discussion

Fig. 1, Fig. 2 show typical TEM images of the microstructures observed in samples Ag9 and Ag6 whereas the insets provide the associated SAED patterns, respectively. Both TEM images show two distinct phases/types of grains, one with brighter contrast, denoted as minority phase Pmin, and another one with much darker (even black) contrast, denoted as majority phase Pmaj. For both phases, the grains are smaller (for Pmaj between 10 and 20 nm; for Pmin also between 10 and 20 nm) and a grain morphology

Conclusions

Nanocrystalline FePt-based ribbons with Ag addition that exhibit direct formation of the L10 phase in the as-cast state have been subjected to a nitriding process, consisting of annealing at 450 °C for 120 min under 100 bar N gas pressure, in order to observe the modifications induced in the microstructure and the magnetic behavior. Two FePtAgB samples with Ag content 6 and 9 at% respectively have been studied. TEM imaging and EDP analysis show that upon nitriding a homogeneous granular structure

Acknowledgments

Financial support from Romanian Ministry of Education by the funding agency UEFISCDI under the Grant IDEI 291/2011 is gratefully acknowledged.

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