3862 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

A Comparative Study on the Microstructure andMagnetic Properties of Melt-Spun RE2Fe14B=�-Fe

and RE2Fe14B=Fe3B (RE = Nd; Pr) NanocompositesZ. Chen1, D. N. Brown1, Bao Min Ma2, P. Campbell1, Y. Q. Wu3, and M. J. Kramer3

Magnequench Technology Center, Singapore 117525, SingaporeMagnetics Hong Kong, Kowloon, Hong Kong

Ames Laboratory, Iowa State University, Ames, IA 50011 USA

Abstract—The microstructure and magnetic properties of melt-spunRE2Fe14B -Fe andRE2Fe14B Fe3B nanocomposite pow-ders were studied using Pr and Nd versions ofRE9Fe86B5(RE2Fe14B -Fe) andRE9Fe79B12(RE2Fe14B Fe3B) alloys. It wasfound that the RE9Fe86B5 exhibited a finer and more uniform microstructure with an average grain size of 10 nm, whereas a rel-atively coarse microstructure (grain size up to 100 nm) was observed in the RE9Fe79B12. It was also found that the RE9Fe86B5

yielded higher r and ( )max, and a more square demagnetization loop, although the ci was slightly lower. The superior mag-netic properties observed in the RE9Fe86B5 can be attributed to the more favorable intrinsic magnetic properties of -Fe (higher s

and lower ) and the much finer microstructure, which produces a stronger intergrain exchange coupling between the RE2Fe14Band -Fe phases. The results indicate that RE2Fe14B -Fe nanocomposites are more desirable than RE2Fe14B Fe3B for futuredevelopment of RE–Fe–B nanocomposite magnets.

Index Terms—Magnetic property, melt spinning, microstructure, nanocomposite magnet, RE–Fe–B.

I. INTRODUCTION

NANOCOMPOSITE RE–Fe–B magnets have attractedmuch attention in recent years due to their enhanced mag-

netic performance and potential application in electric motorand sensor devices [1]. Such materials consist of a fine mixtureof magnetically hard and soft phases, either -[2], [3] or [4], [5]. However, to the best ofour knowledge, there has not been a direct comparison madebetween these two ternary alloy nanocomposite materials.Such a comparison is essential for the further developmentof RE–Fe–B nanocomposite magnets. Thus, the purpose ofthis paper is to make such a direct comparison of ternary

- and nanocomposite mag-nets.

II. EXPERIMENTAL PROCEDURES

The compositions of , , ,and were used in this study. The position of thesecompositions on the RE–Fe–B phase diagram meant that eachhad equivalent volume fraction (about 23%) of soft phase

- . The alloy ingots were produced by arc meltingNd, Pr, Fe, and Fe–B constituents under an Ar atmosphere. Thearc-melted ingots were then broken into pieces andmelt spun under argon to produce ribbons. Each compositionwas melt spun with wheel speeds between 14 and 25 m/s sothat the transition from under optimal to over quench could beobserved. Ribbon thickness ranged from 80 to 45 . Part ofthe ribbon was crushed and sieved to a size of ( 40mesh). Crystallization annealing of overquenched samples was

Digital Object Identifier 10.1109/TMAG.2005.854950

carried out at 650–750 for 5–10 min. However, powdersamples in the most optimally quenched condition yieldedthe highest magnetic performance, and so optimally quenchedpowder/ribbon samples were selected for characterization.

The phases present in each material were studied using aPhilips X-ray diffractometer (XRD) and a Perkin-Elmer thermo-gravimetric analyzer (TGA). A Philips CM30 transmission elec-tron microscope (TEM) was used to examine the microstructure.Demagnetization curves of powder samples were measured witha Lakeshore vibrating sample magnetometer (VSM). Sampleswere pulse magnetized with a peak field of 4.5 T before themeasurement. A demagnetization factor of 0.21 was used to cor-rect the shape effect on the demagnetization curve. Room-tem-perature hysteresis curves of the Pr-based ribbon samples wereprepared using a Quantum superconducting quantum interfer-ence device (SQUID) under a maximum applied field of 5 T.No demagnetization factor was applied on SQUID data sincethe ribbon was sufficiently long. The parameter was intro-duced to evaluate demagnetization loop squareness. By defini-tion, , and an value of100% would represent perfect squareness.

III. RESULTS AND DISCUSSIONS

The best magnetic properties for both andwere achieved from samples in the optimally

quenched as-spun condition. Table I summarizes the magneticproperties from melt-spun andpowder in their optimally quenched condition. It can be seenthat the optimal wheel speed is 21 m/s for and 15m/s for . This confirms that higher boron levelsimprove the glass formability of rapidly solidified alloys. Formagnetic properties, has higher andand a more square demagnetization loop for both Nd-based and

0018-9464/$20.00 © 2005 IEEE

CHEN et al.: MELT-SPUN RE Fe B -Fe AND RE Fe B Fe B RE Nd Pr NANOCOMPOSITES 3863

TABLE IOPTIMAL WHEEL SPEED AND MAGNETIC PROPERTIES OF Pr Fe B

AND Pr Fe B MELT-SPUN POWDER MEASURED IN VSM.S IS

DENOTED AS THE SQUARENESS OF THE DEMAGNETIZATION CURVE

AND IS DEFINED AS 4� (BH) =B

Fig. 1. Room-temperature hysteresis loops of Pr Fe B and Pr Fe Bmelt-spun ribbon.

Pr-based compositions. However, the is slightly lower inboth - samples. Fig. 1 compares the hysteresisloops of and ribbon samples.

Since both Nd- and Pr-based compositions exhibited similartrends in magnetic properties, the microstructural study wasfocused on the Pr-based samples in the optimally quenchedcondition. Figs. 2 and 3 compare the XRD and TGA profilesof and , respectively. It can be clearlyseen that the consists of and - phasesand the consists of and phases.From the TGA data, a small fraction of - is also observed in

. The TEM images of andin Fig. 4 illustrate the profound difference in grain size be-tween the two structures. The material exhibits amuch finer and more uniform microstructure (average grainsize 10 nm) than the material, which exhibitsgrains as large as 100 nm. This contrast indicates a differ-ence in solidification and crystallization behavior between the

- and systems. The rolesof - and during thermal processing appear quitedifferent, with the offering less grain growth control than

- .The superior magnetic performance of -

type nanocomposites can be explained by the difference

Fig. 2. XRD spectra of Pr Fe B and Pr Fe B melt-spun powder.

Fig. 3. TGA curve of Pr Fe B and Pr Fe R melt-spun powder.

in the intrinsic magnetic properties of the soft phases,together with microstructural refinement. It is known[5] that the - has a higher saturation magnetiza-tion and a lower magnetocrys-talline anisotropy constant than

, which explainswhy the - nanocomposite system has higher

and lower than the equivalent . Inaddition, the much finer microstructure in wouldlead to a stronger intergrain exchange coupling between

- than in , which, in turn,would lead to a higher and a more square demagnetizationcurve. To further compare the degree of exchange coupling, a

plot analysis was conducted, and the results are illustratedin Fig. 5. (For more information on plot analysis, pleasesee [6].) It can be seen that a positive is present in both

3864 IEEE TRANSACTIONS ON MAGNETICS, VOL. 41, NO. 10, OCTOBER 2005

Fig. 4. TEM images of Pr Fe R and Pr Fe B melt-spun ribbon.

and powders, indicating the existenceof intergrain exchange coupling in both structures. The higherpeak in the indicates a stronger exchange couplingbetween and - than that between and

.In summary, our study has demonstrated that

samples exhibit finer and more uniform microstructures, higherand , and more square demagnetization curves

than their equivalent material. The higher perfor-mance of - nanocomposites can be attributedto the more favorable intrinsic magnetic properties of the

- component (higher and lower ) and a much finermicrostructure. The results indicate that - is amore desirable system for future developments of high-perfor-mance nanocomposite magnets.

Fig. 5. �M plots of Pr Fe B and Pr Fe B melt-spun powder.

ACKNOWLEDGMENT

The work at Ames Laboratory was supported by the UnitedStates Department of Energy (USDOE), Office of Science (OS),Office of Basic Energy Sciences (BES), and the Technology Re-search Division, Office of Advanced Scientific Computing Re-search, through Iowa State University under Contract W-7405-ENG-82.

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[3] G. Mendoza-Suarez and H. A. Davies, “The coercivities of nanophasemelt-spun PrFeB alloys,” J. Alloys Comp., vol. 281, no. 1, pp. 17–22,1998.

[4] R. Coehoorn, D. B. De Mooij, and C. De Waard, “Melt-spun permanentmagnet materials containing Fe B as the main phase,” J. Magn. Magn.Mater., vol. 80, no. 1, pp. 101–104, 1989.

[5] E. F. Kneller and R. Hawig, “The exchange-spring magnet: A new ma-terial principle for permanent magnets,” IEEE Trans. Magn., vol. 27, no.4, pp. 3588–3600, Jul. 1991.

[6] Z. Chen, H. Okumura, G. C. Hadjipanayis, and Q. Chen, “Enhance-ment of magnetic properties of nanocomposite Pr Fe B=�-Fe mag-nets by small substitution of Dy for Pr,” J. Appl. Phys., vol. 89, no. 4,pp. 2299–2303, 2001.

Manuscript received February 3, 2005.