Improved thermal stability of hard magnetic properties in rapidly solidified RE–TM–B alloys

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Ramanujan School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

H.A. Davies Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, United Kingdom (Received 20 March 2008; accepted 26 June 2008)

Rapidly solidified nanocrystalline RE–TM–B (RE ⳱ Nd, Pr, Dy, TM ⳱ Fe, Co) alloys with enhanced hard magnetic properties were synthesized by melt spinning. The composition- and microstructure-dependent elevated temperature magnetic properties were investigated. The temperature coefficients of remanence (␣) and coercivity (␤) were determined. The effects of Pr substituting Nd, Co substituting Fe, Dy substituting RE, and grain size on the Curie temperature and thermal stability were studied. Co or Dy substitutions were found to have a significant beneficial effect on the thermal stability. Reducing grain size could also improve elevated temperature behavior. Maximum energy product (BH)max > 100 kJ/m3 could be obtained in compositionally optimized nanophase alloys at temperature of 473 K. Extremely low coefficients of ␣ and ␤ were realized in exchange coupled nanocomposite alloys. Bonded nanocomposite magnets with ␣ ⳱ −0.052%/K and ␤ ⳱ −0.0365%/K for 300–400 K were also successfully fabricated.

I. INTRODUCTION

Nanocrystalline hard magnetic Nd–Fe–B-based alloys have been extensively investigated since the last decade.1,2 The enhanced remanence and energy product of nanocrystalline Nd–Fe–B alloys result from the increased exchange coupling between magnetically hard grains and, for nanocomposite alloys, between magnetically hard and magnetically soft grains.3 However, in spite of their excellent room temperature (RT) properties, Nd–Fe–B magnets have had few high-temperature applications due to their relatively low Curie temperature (TC) and poor thermal stability. The relevant temperature coefficients of ternary Nd–Fe–B magnets are 3.0–3.6 times greater than those of SmCo magnets so that the use of Nd–Fe–B alloys is restricted to low temperatures, typically less than 100 °C.4 Moreover, for nanophase magnets, coercivity (jHC) tends to be reduced by exchange

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Address all correspondence to this author. e-mail: [email protected], [email protected] DOI: 10.1557/JMR.2008.0337 J. Mater. Res., Vol. 23, No. 10, Oct 2008

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coupling, and for Nd2Fe14B/Fe nanocomposites in particular, jHC can be rather low ( 130 kJ/m3 and (BH)max >100 kJ/m3 can be obtained at temperatures of 400 and 473 K, respectively. The relationships between (BH)max, Jr, and jHC at 473 K for the examined RE–TM–B (RE ⳱ Nd, Pr, Dy; TM ⳱ Fe, Co) alloys are shown in Fig. 8. These relationships are similar to those which we obtain at RT for a broad range of RE–TM–B compositions.19 Jr decreases approximately linearly with increasing jHC, whereas (BH)max has a maximum value for all compositions. The maximum value of (BH)max for T ⳱ 473 K is 100– 110 kJ/m3. To achieve this maximum value, jHC values of ∼250–450 kA/m must be maintained at 473 K. T