Structure and magnetic properties of melt-spun Fe–Pt–B alloys with high B concentrations

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Highlights

  • Substitution of 5–10 at.% Pt for Fe in the Fe70B30 alloy increases the amorphous-forming ability.

  • L10-FePt and Fe2B phases are formed for melt-spun Fe70−xPtxB30 (x = 5–20) alloys after annealing.

  • Annealed Fe70−xPtxB30 (x = 10–20) alloys exhibit the characterizations of nanocomposite magnets.

Abstract

The phase evolution, thermal stability, structure and magnetic properties of the melt-spun Fe70−xPtxB30 (x = 0–20) alloys have been investigated. Addition of small amounts of Pt to the Fe70B30 alloy increases the amorphous-forming ability, leading to the formation of an amorphous phase in the alloys with x = 5 and 10. Further increase in Pt content results in the formation of the fcc-FePt + Fe2B or fcc-FePt + L10-FePt + Fe2B crystalline phases. After appropriate annealing, the homogeneous nano-sized structure consisting of hard L10-FePt and soft Fe2B magnetic phases were formed for the alloys with x = 10–20, which exhibited good hard magnetic properties. As the Pt content increase, the coercivity (iHc) of the L10-FePt/Fe2B nanocomposite alloys significantly increased, while the remanence (Br) decreased. The Br, reduced remanence (Mr/Ms), iHc, and energy product (BH)max of the alloys are in the range of 0.81–1.20 T, 0.76–0.84, 173.2–523.1 kA/m, and 64.0–88.3 kJ/m3, respectively.

Introduction

A nanocomposite structure comprising the highly anisotropic hard magnetic phase and soft magnetic phase has opened a way for a new generation of permanent magnets with enhanced hard magnetic properties [1], [2]. In the nanocomposite permanent magnets, the hard magnetic phase provides high coercive field, while the soft magnetic phase provides high saturation magnetization. The sizes of both the hard and soft magnetic phases are in nanoscale range so that the magnetic moments of adjacent grains are exchange-coupled, leading to an enhanced the hard magnetic properties [1], [2], [3]. In the last decade, the rare earth-based nanocomposite magnets have been widely studied. The common problems associated with the hard magnets were high corrosion rate and high susceptibility to oxidation. In this regard, Fe–Pt-based nanocomposite magnets got much attention [4], [5], [6] because the ordered tetragonal phase (L10-FePt) has an extremely high magnetocrystalline anisotropy (K = 7 MJ m3), attractive saturation magnetization (1.38 T), and high corrosion resistance. Recently, we succeeded in fabricating a new type of L10-FePt/Fe2B nanocomposite magnets with low Pt contents (18–24 at.%) by the melt spinning technique [7], [8]. These magnets were obtained from the Fe–Pt–B amorphous phase or mixed structure consisting of amorphous and fcc-FePt phases after subsequent annealing treatment, which resulted in the formation of hard L10-FePt and soft Fe2B magnetic phases. The L10-FePt/Fe2B nanocomposite permanent magnet has some advantages, e.g., (1) low Pt contents, (2) good hard magnetic properties, (3) high corrosion resistance, and (4) simple process for fabrication. Therefore, the nanocomposite magnets have attracted considerable attention in recent years, the effects of the alloying, annealing treatment and prepared process on the microstructure and magnetic properties of the Fe–Pt–B alloys have been studied [9], [10], [11], [12].

It is known that the magnetic properties of nanocomposite magnets are strongly dependent on their compositions and structure [13], [14]. Therefore, it is important to optimize the compositions and microstructure of the alloys for obtaining better magnetic properties. In addition, for extension of application fields, it is also necessary to further reduce Pt content without detriment to the magnetic properties of the L10-FePt/Fe2B magnets. We have investigated the effects of B concentration on the structure and magnetic properties of (Fe0.75Pt0.25)100xBx nanocomposite magnets alloys [15]. It was found that the higher B contents accelerate the formation of L10-FePt phase, leading to the increase in the coercivity (iHc) of the alloys. However, little is known about the effects of the compositions on the structure and magnetic properties of Fe–Pt–B alloys with high B contents. In this article, we report on the effects of the Pt content on the phase evolution, thermal stability, structure and magnetic properties of melt-spun Fe–Pt–B alloys with a high B concentration of 30 at.%. In addition, the structure and magnetic properties of the melt-spun alloys are also investigated after annealing treatment.

Section snippets

Experimental procedure

Appropriate amounts of pure Fe (99.99 mass%), Pt (99.9 mass%) and boron (99.5 mass%) were arc-melted in an argon atmosphere to obtain the Fe70xPtxB30 (x = 0–20) alloy ingots. The ingots were crushed into small pieces to accommodate the size of a quartz crucible used for melt spinning. The nozzle diameter of the crucible was ∼0.5 mm. Ribbon specimens were produced by melt spinning at the surface velocity of about 33 m/s in an argon atmosphere. Thermal stability was investigated under an Ar atmosphere

Results and discussion

Fig. 1 shows XRD patterns of the melt-spun Fe70xPtxB30 (x = 0, 5, 10, 15 and 20) alloy ribbons. Although the solidified structure consists of Fe2B and amorphous is formed for the Fe70B30 alloy, the addition of 5–10 at.% Pt causes the formation of a single amorphous phase through the suppression of precipitation of the Fe2B phase. However, the further increase in Pt content to 15 and 20 at.% results in the formation of crystalline phases, which are identified as fcc-FePt and Fe2B for alloy with x = 

Summary

Addition of small amounts of Pt to the Fe70B30 alloy increases the amorphous-forming ability, leading to the formation of an amorphous phase in the alloys with x = 5 and 10. Further increase in Pt content results in the formation of the crystalline phases. The fcc-FePt + Fe2B and fcc-FePt + L10-FePt + Fe2B were formed for the alloys with x = 15 and 20, respectively. After appropriate annealing, the nano-sized mixed structure consisting of hard L10-FePt and soft Fe2B magnetic phases were formed for the

Acknowledgements

This research was supported by the Natural Science Foundation of China (Grant Nos. 51171034 and 51271043), the Fundamental Research Funds for the Central Universities of China (Grant No. DUT11RC(3)29).

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