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Journal of Magnetism and Magnetic Materials 310 (2007) 2593–2595

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The effect of annealing time on the magnetic properties andmicrostructure of (Fe0.675Pt0.325)84B16 ribbons

C.W. Changa, H.W. Changb, C.H. Chiua, C.H. Chena, W.C. Changa,�, H. Ouyangc, C.C. Liuc

aDepartment of Physics, National Chung Cheng University, Ming-Hsiung, Chia-Yi, TaiwanbInstitute of Physics, Academia Sinica, Nankang, Taipei, Taiwan

cDepartment of Material Science and Engineering, National Chung Hsing University, Taichung, Taiwan

Available online 5 December 2006

Abstract

Effect of annealing time on the magnetic properties and microstructure of (Fe0.675Pt0.325)84B16 ribbon has been studied. For the as-

quenched ribbons (Vs ¼ 45m/s) annealed isothermally at 500 1C, iHc increases from 0.05 kOe for the quenched ribbons to the optimal

value of 7.5 kOe after 5 h annealing, but Br increases from 1.2 kG for the as-quenched ribbons to 7.5 kG after 5 h annealing at first, then

decreases when the annealing time is over 6 h. From thermal magnetic analyzer (TMA) and high resolution transmission electron

microscope (HRTEM), magnetically soft Fe2B and Fe3B were found to coexist with magnetically hard g1-FePt phase in the ribbons after

suitable annealing. The existence of sufficient fine Fe3B with Fe2B phases and the well exchange coupling effect between hard and soft

phases are the main cause of the improved remanence and magnetic energy product of the ternary (Fe0.675Pt0.325)84B16 ribbons.

r 2006 Elsevier B.V. All rights reserved.

PACS: 74.25.Ha; 61.82.Rx; 68.55.Nq; 75.30.Gw; 75.50.Ww

Keywords: Permanent magnets; Nanocomposites; Melt spinning; FePt; FePtB; Exchange coupling

Fe–Pt alloys have been a major interest for permanentmagnet researchers because the ordered tetragonal FePtphase shows extremely large magnetocrystalline anisotropy(7� 107 erg/cm3) and attractive saturation magnetization(13.8 kG) [1]. Recently, the excellent magnetic propertiescombining with high maximum energy product ((BH)max)of 14.0MGOe and high intrinsic coercivity (iHc) of 7.5 kOehave been achieved in (Fe0.675Pt0.325)84B16 ribbons [2],resulting from the finer grain size and the coexistence ofsufficient soft Fe2B and Fe3B grains with hard g1-FePtgrains. However, other studies [3,4], judging from theX-ray diffraction patterns, indicated that only soft Fe2Band g-FePt phases are coexisted with ordered hard g1-FePtphase in FePtB ribbons with similar composition. To date,no other evidence shows that magnetically soft Fe3B phaseexists in FePtB-tpye ribbons except for the result fromthermal magnetic analysis (TMA) [5]. In this paper, we first

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/j.jmmm.2006.11.152

onding author. Tel.: +886 5 2720411 61337;

2721091.

ddress: phywcc@ccunix.ccu.edu.tw (W.C. Chang).

focus on the effect of annealing time on the magneticproperties of (Fe0.675Pt0.325)84B16 ribbon. In addition, highresolution transmission electron microscopy (HRTEM) isemployed to examine the phases inside the matrix.Alloy ingot with nominal composition of (Fe0.675

Pt0.325)84B16 was prepared by arc melting. To ensuresufficient supercooling, melt-spun ribbons were producedfrom ingots then quenched at wheel speeds with 45m/s.The ribbons selected were annealed at 500 1C for 1–6 h forcrystallization and phase formation to improve thepermanent magnetic properties. The phase mixture andmagnetic phases of the ribbons were determined by both athermal gravimetric analyzer (TGA) with an externallyapplied magnetic field (conventionally referred to as‘‘TMA’’) and a high resolution transmission electronmicroscope (HRTEM). The ribbons were magnetized at apulse field of 40 kOe prior to the magnetic measurementwith a vibrating sample magnetometer (VSM) under amaximum magnetic field of 12 kOe.Table 1 lists the magnetic properties, including rema-

nence Br, coercivity iHc, reduced remanence ratio sr/s12kOe,

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Table 1

The magnetic properties of the (Fe0.675Pt0.325)84B16 ribbon annealed at

500 1C for 1–6 h

Br (kG) iHc (kOe) sr/s12kOe (BH)max (MGOe)

As quenched 1.2 0.05 0.10 —

500 1C 1h 3.4 2.5 0.59 5.6

500 1C 2h 7.9 5.2 0.78 9.8

500 1C 3h 9.0 7.3 0.84 12.3

500 1C 4h 9.1 7.4 0.84 12.4

500 1C 5h 9.4 7.5 0.85 14.0

500 1C 6h 9.2 7.3 0.84 12.6

0 200 400 600 800100 300 500 700 900

Mag

netiz

atio

n (a

rb. u

nit)

As quenchedγ-FePt

Temperature (°C)

500°C 6h

500°C 5h

500°C 4h

500°C 3h

500°C 2h

500°C 1h

Fe2B

γ1-Fe3B

γ1-FePt

Fig. 1. Presents TMA scans of the (Fe0.675Pt0.325)84B16 ribbon annealed at

500 1C for 1–6 h.

Fig. 2. High-resolution TEM images of the (Fe0.675Pt0.325)84B16 ribbon

annealed at 500 1C for 1–6 h.

C.W. Chang et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2593–25952594

and energy product (BH)max of the (Fe0.675Pt0.325)84B16

ribbon annealed at 500 1C for 1–6h. Clearly, with increas-ing annealing time Ta, Br and iHc increase from 1.2 kGand 0.1 kOe for the as-quenched ribbon to 9.4 kG and7.5 kOe after 5 h annealing, respectively, then decreasewhen Ta is above 5 h. The s12kOe and sr represent themagnetization under the applied magnetic field of12 kOe and the residue magnetization when the appliedfield is reduced to zero, respectively. Considerably highBr and reduced remanence ratio (sr/s12kOe) can beobtained when Ta is at 3 h, but they reach the maximumat Ta ¼ 5 h. The values of Br and reduced remanence ratio(sr/s12kOe) are higher than those of the ribbons with non-interacting single phase g1-FePt ribbons, which suggeststhat multiple magnetic phases with strong exchange-coupling effect might be existed in the studied ternaryFePtB ribbons.

To determine the magnetic phases in each alloy ribbons,TMA was firstly employed, which have been successfullyemployed for identifying the magnetic phases of nanocom-posite NdFeB ribbons [6]. Fig. 1 presents TMA scans of the(Fe0.675Pt0.325)84B16 ribbon annealed at 500 1C with variousannealing time. For the as-quenched ribbon, two mainmagnetically phases, disordered g-FePt and Fe2B wereobserved. As the ribbon annealed at 500 1C for 1 h, orderedg1-FePt appeared, but the volume fraction of it is extremelylow in the matrix. The peak of the ordered g1-FePt phaseincreases, but that of the disordered g-FePt phasedecreases, with increasing Ta. The ordered g-FePt phasefinally vanishes when Ta is above 3 h. Besides, an additionalphase, namely Fe3B, with the Curie temperature close to500 1C, was detected when Ta is over 2 h.

Since its TMA peak is embedded in the plateau of thescan between 480 and 520 1C, the direct phase distinctionbecame much difficult. Therefore, high-resolution TEM isemployed for more detail phase identification. Fig. 2 showsthe microstructure of (Fe0.675Pt0.325)84B16 alloy ribbon afterannealed 500 1C for 5 h analyzed by high resolutiontransmission electron microscopy. Obviously, multiplephases, namely, g1-FePt, Fe2B and Fe3B are all found topresent in this ribbon. The average grain sizes obtainedfrom TEM image were about 25 nm for g1-FePt, 10 nm forFe2B, and 20 nm for Fe3B phases, respectively.

The excellent permanent magnet properties have beenachieved in (Fe0.675Pt0.325)84B16 ribbons after 5 h annaeal-ing. The coexistence of sufficient fine Fe3B with Fe2B andg1-FePt grains was the main reason to result in the

ARTICLE IN PRESSC.W. Chang et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2593–2595 2595

improvement of remanence and magnetic energy productfor the (Fe0.675Pt0.325)84B16 ribbons. The magnetically softFe2B and Fe3B phases were not only detected in TMAscans but also directly evidenced by HRTEM analysis.

This paper was supported by National Science Council,Taiwan under grant no. NSC-94-2112-M-194-001.

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