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α-Fe-bonded magnets
Journal of Alloys and Compounds 269 (1998) 284–287
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Magnetic properties of nanocomposite Nd 2 (Fe,Co,M) 14 B / a-Fe-bonded magnets J. Jakubowicz, M. Jurczyk* Institute of Materials Science and Engineering, Poznan´ University of Technology, M. Sklodowska-Curie 5 Sq., 60 -965 Poznan´ , Poland Received 16 December 1997
Abstract Nanocomposite Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe (M5Al-Cr, V, Cr, Ni, Zr or Mo) powders, with a volume fraction of magnetically soft a-Fe phase of 10 and 37.5%, have been prepared by a high-energy ball-milling (HEBM) and annealing. Enhanced remanent magnetic polarizations up to 1.15 T were obtained. It has been found that addition of a small amount of Al-Cr, Cr, Zr or Mo to Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe can improve the intrinsic coercivity and the hysteresis squareness of the Nd 2 (Fe,Co) 14 B / a-Fe-based nanocomposite materials. The improvements may be related to refined grain sizes realized by these additives. Bonded magnets of Jr and 21 to 1.10 T and 760 kA m 21 have been produced from Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe, with a 10 J Hc ranging from 0.81 T and 840 kA m and 37.5% volume fraction of soft a-Fe phase, respectively. 1998 Elsevier Science S.A. Keywords: Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe; Remanence-enhanced materials; Intrinsic coercivity; Bonded magnets
1. Introduction In recent years, there has been much interest in composite materials of magnetically hard and soft phases, which are candidates for high-performance permanent magnets [1–4]. In composite permanent magnets soft magnetic grains, of high saturation polarization, enhance the low remanence of the hard magnetic phase by a form of exchange coupling [1]. Modelling of hysteresis curves of isotropic nanocrystalline permanent magnet powders by Kneller and Hawig [1], Schrefl et al. [2], and Skomski [5], have shown that the coercivity and enhancement of the remanent magnetic polarization are highly dependent on the grain size of the soft magnetic phase. In order to achieve a significant enhancement of the remanent magnetic polarization, and to preserve a high intrinsic coercivity in isotropic nanocrystalline Nd 2 Fe 14 B-based magnets, a mean grain size of less than 20 nm is required [1,2]. Micromagnetic calculations by Schrefl et al. [2] on cellular structures in two and three dimensions have shown that the size of the soft grains should ideally be about twice the domain wall width (dw ) of the hard magnetic phase. Skomski obtained a maximum energy product (BH) max of up to 1 MJ m 23 for a multilayer composed of alternating layers of oriented hard Sm 2 Fe 17 N 3 skeleton phase and a *Corresponding author. Fax: 146 61 8313276. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 98 )00145-5
soft Fe 65 Co 35 phase [5], but so far there has been no practical realization of such a structure. Magnetic materials with enhanced remanent magnetic polarization were first reported in a single-phase system by McCallum et al. [6] for melt-spun NdFeB alloys containing Si or Al. Coehoorn et al. [7] described a new type of Nd-poor Nd 3.8 Fe 77 B 19.2 ribbons, where the hard phase is Nd 2 Fe 14 B and the soft phase is mainly Fe 3 B. Similarly, remanence enhancements has been reported in a variety of nanostructured Fe-rich rare-earth magnet alloys, prepared by recrystallization of either mechanically alloyed [8,9] or of high-energy ball-milled materials [4]. Gong et al. [8] investigated two-phase nanocomposite magnets in which the minor Nd 2 Fe 14 B hard phase is coupled to an a-Fe or Fe 70 Co 30 soft phase. They reported an increase in remanence and a decrease in intrinsic coercivity for material containing an increasing weight percentage of the Fe 70 Co 30 soft phase. The additions of Zr has been shown to be advantageous in improving the energy product and squareness of the hysteresis loop in two-phase nanocomposite (Nd,Dy) 2 (Fe,Co,Zr) 14 B / a-Fe materials [4]. Optimum magnetic properties are obtained for Nd 2 (Fe,Co,Zr) 14 B / aFe materials with a volume fraction of a magnetically soft phase of 37–40%. So far, experiments have given a remanent magnetic polarization of 1.14 T, and an intrinsic coercivity of 504 kA m 21 for Nd 12.6 Fe 69.3 Co 11.6 Zr 0.5 B 6 / a-Fe, with a 37.5% volume fraction of a-Fe [4]. Energy
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285
products (BH) max of 220 kJ m 23 are achieved in these materials. In this work, the effect of M (M5Al-Cr, V, Cr, Ni, Zr or Mo) substitutions for iron on the magnetic properties in nanocomposite two-phase Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe materials, with an excess of a-Fe of 10 and 37.5%, has been investigated using X-ray and magnetometric techniques. All materials have been prepared by high-energy ballmilling (HEBM) and then annealing.
2. Materials preparation and structure Alloy lumps of Nd 12.6 Fe 69.82x Co 11.6 M x B 6 (M5Zr 0.5 , Mo 0.5 , Cr 2 , Al 1 Cr 2 , V5 , Ni 5 ) were prepared by arc melting stoichiometric amounts of the constituent elements (purity 99.9 at.% or better) in an argon atmosphere. The remanence-enhanced powders were prepared by HEBM procedures, which have been described in detail elsewhere [4,10]. The powders were typically milled in a high-energy planetary ball mill (Fritsch P5) at a rotation speed of 120 rpm in an argon (99.998%) atmosphere for 48 h, followed by annealing at a temperature in the range 630–7708C for 30 min under high-purity argon to form the tetragonal Nd 2 Fe 14 B phase. The powders were examined by XRD analysis, using Co Ka radiation, at the conclusion of milling, prior to annealing and at various stages during annealing. The mean grain size of the powders, at various stages of the experiment, was determined from X-ray line broadening using the Scherrer formula [11]. Fig. 1 shows the XRD patterns of HEBM Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders with a 10% volume fraction of a-Fe, milled and annealed, as a representative alloy example. After 48 h milling the alloy had decomposed into an amorphous phase and nanocrystal-
Fig. 2. The grain size of high-energy ball-milled (a) Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe, and (b) Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders with a volume fraction of magnetically soft a-Fe phase of 10%, without annealing and after heat treatment at different temperatures (lines are provided as a guide to the eye).
line a-Fe (Fig. 1a). The following annealing in high-purity argon, at 7008C for 30 min, leads to the formation of the tetragonal Nd 2 Fe 14 B-type crystal structure (Fig. 1b) which coexists with the a-Fe phase, resulting in a two-phase nanocomposite material. The grain size prior to annealing and as a function of the annealing temperature for Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe and Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders with a volume fraction of magnetically soft a-Fe phase of 10%, is shown in Fig. 2. At the conclusion of milling, the mean grain size of the crystalline component in Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe was of the order of 10 nm, and after annealing at 6308C for 30 min it had increased to about 32 nm. XRD spectra of the Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe revealed a similar pattern of behaviour as it decomposed into an amorphous phase and nanocrystalline a-Fe, though the mean grain size was a little larger, being of the order of 18 nm. Annealing at 6308C for 30 min also increases the grain size of the a-Fe phase to around 40 nm (see Fig. 2).
3. Magnetic properties
Fig. 1. XRD spectra of Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders with a volume fraction of magnetically soft a-Fe phase of 10%: (a) high-energy ball-milled for 48 h, (b) heat treated at 7008C for 30 min.
Samples for the measurements of magnetic properties were made by mixing the powders with epoxy resin in suitable moulds. All data have been normalized with respect to 100% density. The magnetic hysteresis curves were measured at room temperature using a vibrating sample magnetometer with external fields up to 2.1 MA m 21 . Honda extrapolations (J vs 1 /H ) were utilized to determine the zero field saturation magnetic polarization Js . The results obtained for the Nd 12.6 Fe 69.82x Co 11.6 M x B 6 /
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Table 1 Saturation magnetic polarization Js , remanent magnetic polarization Jr , reduced remanent magnetic polarization a, and intrinsic coercivity J Hc , at 295 K of HEBM and annealed Nd 2 Fe 14 B-type powders with an excess of a-Fe Hc (kA m 21 )
Material composition
Volume fraction of magnetically soft a-Fe phase (%)
Js (T)
Jr (T)
a (Jr /Js )
J
Nd 12.6 Fe 81.4 B 6 / a-Fe Nd 12.6 Fe 81.4 B 6 / a-Fe a Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe a Nd 12.6 Fe 69.3 Co 11.6 Zr 0.5 B 6 / a-Fe Nd 12.6 Fe 69.3 Co 11.6 Zr 0.5 B 6 / a-Fe a Nd 12.6 Fe 69.3 Co 11.6 Mo 0.5 B 6 / a-Fe Nd 12.6 Fe 69.3 Co 11.6 Mo 0.5 B 6 / a-Fe Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe Nd 12.6 Fe 64.8 Co 11.6 V5 B 6 / a-Fe Nd 12.6 Fe 64.8 Co 11.6 Ni 5 B 6 / a-Fe
10 37.5 10 37.5 10 37.5 10 37.5 10 37.5 10 37.5 10 10
1.14 1.46 1.19 1.62 1.16 1.62 1.17 1.62 1.12 1.55 1.11 1.50 1.09 1.15
0.62 0.63 0.50 0.55 0.86 1.14 0.85 1.15 0.81 1.10 0.85 1.08 0.81 0.80
0.54 0.43 0.42 0.34 0.74 0.70 0.73 0.71 0.72 0.71 0.76 0.72 0.74 0.70
460 160 322 208 662 504 487 4.35 840 760 638 586 415 423
a
Data were taken from Refs. [4,10].
a-Fe (M5Zr 0.5 , Mo 0.5 , Cr 2 , Al 1 Cr 2 , V5 , Ni 5 ) powders, with a 10 and 37.5% volume fraction of a-Fe, examined in the current study, are presented in Table 1 and plotted in Figs. 3 and 4. All the powders are magnetically isotropic. The intrinsic coercivity J Hc and the reduced remanent magnetic polarization a of Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / aFe powder with a volume fraction of magnetically soft a-Fe phase of 10%, as a function of annealing temperature for 30 min anneal, are shown in Fig. 3. A maximum intrinsic coercivity of 638 kA m 21 is obtained at an annealing temperature of 7008C. J Hc is lower in powders annealed at both lower and higher temperatures, as the grain size of the soft a-Fe phase is either too small or too large for optimum exchange coupling [1]. The reduced
remanent magnetic polarization decreases from 0.78 to 0.66 as the annealing temperature increases. Generally, the grain growth which occurs at the higher temperatures leads to deterioration in the magnetic properties. The principal advantages of Co addition to Nd 2 Fe 14 B / a-Fe, as we discussed recently, are increases in the Curie temperature of the hard phase, and a possible increase in the saturation polarization arising from the higher saturation of Fe(Co) compared to that of pure a-Fe [4]. Fig. 4 shows demagnetization curves of the Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe, Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe and Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders, with a volume fraction of a-Fe of 10%, after HEBM and optimal annealing treatment. The Cr 2 - and Al 1 Cr 2 -free
Fig. 3. The variation of the intrinsic coercivity J Hc and reduced remanent magnetic polarization a 5Jr /Js with annealing temperature for the Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powder with a volume fraction of magnetically soft a-Fe phase of 10% (lines are provided as a guide to the eye).
Fig. 4. Demagnetization curves of (a) Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe, (b) Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe and (c) Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders, after HEBM and optimal annealing treatment with a volume fraction of a-Fe of 10% (all data have been normalized with respect to 100% density).
J. Jakubowicz, M. Jurczyk / Journal of Alloys and Compounds 269 (1998) 284 – 287
Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe powder has a lower remanent magnetic polarization and lower intrinsic coercivity. With a small amount of Al-Cr, Cr, and other metals, such as Zr or Mo (not presented in Fig. 4), added to the cobalt substituted Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe, Jr and J Hc are significantly improved (see Table 1). The improvement may be related to refined grain sizes realized by these additives. Bonded magnets of Jr and J Hc ranging from 0.81 T and 840 kA m 21 to 1.10 T and 760 kA m 21 have been produced from Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe, with a 10 and 37.5% volume fraction of soft a-Fe phase, respectively. Partial replacement of Fe by Co and M additives in Nd 12.6 Fe 81.4 B 6 / a-Fe can also improve the temperature coefficients of their remanence a and intrinsic coercivity b in the bonded magnets studied. A better understanding of the role that cobalt and M plays in these materials will be the subject of further study. The Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe powder, with 10% a-Fe by volume, showed the highest intrinsic coercivity, J Hc , of 840 kA m 21 , which compares with |800 kA m 21 reported earlier by Hirosawa and Kanekiyo [12] for melt-spun and annealed Nd 4.5 Fe 57 Cr 20 B 18.5 ribbons.
4. Conclusions Remanence-enhanced Nd 12.6 Fe 69.82x Co 11.6 M x B 6 (M5 Zr 0.5 , Mo 0.5 , Cr 2 , Al 1 Cr 2 , V5 , Ni 5 ) powders, with a volume fraction of a-Fe of 10 and 37.5%, have been produced. It has been found that addition of a small amount of Al-Cr, Cr, Zr or Mo to Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe can improve the intrinsic coercivity and the hysteresis squareness of the Nd 2 (Fe,Co) 14 B / a-Fe-based nanocomposite
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materials. The Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe powders, with 37.5% by volume of a-Fe, where M5Zr 0.5 , Mo 0.5 , Cr 2 , or Al 1 Cr 2 , would seem to offer most promise as bonded magnets, because they have an intrinsic remanence of order of 1.1 T and an acceptable intrinsic coercivity.
Acknowledgements This research was supported by the INCO-Copernicus project Nr IC15 CT 96-0758 of the European Commission, Brussels.
References [1] E.F. Kneller, R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. ¨ [2] T. Schrefl, R. Fischer, J. Fidler, H. Kronmuller, J. Appl. Phys. 76 (1994) 7053. [3] J.M.D. Coey, K. O’Donnell, J. Appl. Phys. 81 (1997) 4810. [4] M. Jurczyk, S.J. Collocott, J.B. Dunlop, P.B. Gwan, J. Phys. D: Appl. Phys. 29 (1996) 2284. [5] R. Skomski, J. Appl. Phys. 76 (1994) 7059. [6] R.W. McCallum, A.M. Kadin, G.B. Clemente, J.E. Keem, J. Appl. Phys. 61 (1987) 3577. [7] R. Coehoorn, D.B. de Mooij, J.P.W.B. Duchateau, K.H.J. Buschow, J. Phys. Paris C8 (1988) 669–670. [8] W. Gong, G.C. Hadjipanayis, R.F. Krause, J. Appl. Phys. 75 (1994) 6649. [9] J. Ding, P.G. McCormick, R. Street, J. Magn. Magn. Mater. 124 (1993) L1. [10] M. Jurczyk, J. Alloys Comp. 235 (1996) 232. [11] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, London, 1978, p. 102. [12] S. Hirosawa, H. Kanekiyo, Mater. Sci. Engin. Vols. A217, A218 (1996) 367.
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Magnetic properties of nanocomposite Nd 2 (Fe,Co,M) 14 B / a-Fe-bonded magnets J. Jakubowicz, M. Jurczyk* Institute of Materials Science and Engineering, Poznan´ University of Technology, M. Sklodowska-Curie 5 Sq., 60 -965 Poznan´ , Poland Received 16 December 1997
Abstract Nanocomposite Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe (M5Al-Cr, V, Cr, Ni, Zr or Mo) powders, with a volume fraction of magnetically soft a-Fe phase of 10 and 37.5%, have been prepared by a high-energy ball-milling (HEBM) and annealing. Enhanced remanent magnetic polarizations up to 1.15 T were obtained. It has been found that addition of a small amount of Al-Cr, Cr, Zr or Mo to Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe can improve the intrinsic coercivity and the hysteresis squareness of the Nd 2 (Fe,Co) 14 B / a-Fe-based nanocomposite materials. The improvements may be related to refined grain sizes realized by these additives. Bonded magnets of Jr and 21 to 1.10 T and 760 kA m 21 have been produced from Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe, with a 10 J Hc ranging from 0.81 T and 840 kA m and 37.5% volume fraction of soft a-Fe phase, respectively. 1998 Elsevier Science S.A. Keywords: Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe; Remanence-enhanced materials; Intrinsic coercivity; Bonded magnets
1. Introduction In recent years, there has been much interest in composite materials of magnetically hard and soft phases, which are candidates for high-performance permanent magnets [1–4]. In composite permanent magnets soft magnetic grains, of high saturation polarization, enhance the low remanence of the hard magnetic phase by a form of exchange coupling [1]. Modelling of hysteresis curves of isotropic nanocrystalline permanent magnet powders by Kneller and Hawig [1], Schrefl et al. [2], and Skomski [5], have shown that the coercivity and enhancement of the remanent magnetic polarization are highly dependent on the grain size of the soft magnetic phase. In order to achieve a significant enhancement of the remanent magnetic polarization, and to preserve a high intrinsic coercivity in isotropic nanocrystalline Nd 2 Fe 14 B-based magnets, a mean grain size of less than 20 nm is required [1,2]. Micromagnetic calculations by Schrefl et al. [2] on cellular structures in two and three dimensions have shown that the size of the soft grains should ideally be about twice the domain wall width (dw ) of the hard magnetic phase. Skomski obtained a maximum energy product (BH) max of up to 1 MJ m 23 for a multilayer composed of alternating layers of oriented hard Sm 2 Fe 17 N 3 skeleton phase and a *Corresponding author. Fax: 146 61 8313276. 0925-8388 / 98 / $19.00 1998 Elsevier Science S.A. All rights reserved. PII S0925-8388( 98 )00145-5
soft Fe 65 Co 35 phase [5], but so far there has been no practical realization of such a structure. Magnetic materials with enhanced remanent magnetic polarization were first reported in a single-phase system by McCallum et al. [6] for melt-spun NdFeB alloys containing Si or Al. Coehoorn et al. [7] described a new type of Nd-poor Nd 3.8 Fe 77 B 19.2 ribbons, where the hard phase is Nd 2 Fe 14 B and the soft phase is mainly Fe 3 B. Similarly, remanence enhancements has been reported in a variety of nanostructured Fe-rich rare-earth magnet alloys, prepared by recrystallization of either mechanically alloyed [8,9] or of high-energy ball-milled materials [4]. Gong et al. [8] investigated two-phase nanocomposite magnets in which the minor Nd 2 Fe 14 B hard phase is coupled to an a-Fe or Fe 70 Co 30 soft phase. They reported an increase in remanence and a decrease in intrinsic coercivity for material containing an increasing weight percentage of the Fe 70 Co 30 soft phase. The additions of Zr has been shown to be advantageous in improving the energy product and squareness of the hysteresis loop in two-phase nanocomposite (Nd,Dy) 2 (Fe,Co,Zr) 14 B / a-Fe materials [4]. Optimum magnetic properties are obtained for Nd 2 (Fe,Co,Zr) 14 B / aFe materials with a volume fraction of a magnetically soft phase of 37–40%. So far, experiments have given a remanent magnetic polarization of 1.14 T, and an intrinsic coercivity of 504 kA m 21 for Nd 12.6 Fe 69.3 Co 11.6 Zr 0.5 B 6 / a-Fe, with a 37.5% volume fraction of a-Fe [4]. Energy
J. Jakubowicz, M. Jurczyk / Journal of Alloys and Compounds 269 (1998) 284 – 287
285
products (BH) max of 220 kJ m 23 are achieved in these materials. In this work, the effect of M (M5Al-Cr, V, Cr, Ni, Zr or Mo) substitutions for iron on the magnetic properties in nanocomposite two-phase Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe materials, with an excess of a-Fe of 10 and 37.5%, has been investigated using X-ray and magnetometric techniques. All materials have been prepared by high-energy ballmilling (HEBM) and then annealing.
2. Materials preparation and structure Alloy lumps of Nd 12.6 Fe 69.82x Co 11.6 M x B 6 (M5Zr 0.5 , Mo 0.5 , Cr 2 , Al 1 Cr 2 , V5 , Ni 5 ) were prepared by arc melting stoichiometric amounts of the constituent elements (purity 99.9 at.% or better) in an argon atmosphere. The remanence-enhanced powders were prepared by HEBM procedures, which have been described in detail elsewhere [4,10]. The powders were typically milled in a high-energy planetary ball mill (Fritsch P5) at a rotation speed of 120 rpm in an argon (99.998%) atmosphere for 48 h, followed by annealing at a temperature in the range 630–7708C for 30 min under high-purity argon to form the tetragonal Nd 2 Fe 14 B phase. The powders were examined by XRD analysis, using Co Ka radiation, at the conclusion of milling, prior to annealing and at various stages during annealing. The mean grain size of the powders, at various stages of the experiment, was determined from X-ray line broadening using the Scherrer formula [11]. Fig. 1 shows the XRD patterns of HEBM Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders with a 10% volume fraction of a-Fe, milled and annealed, as a representative alloy example. After 48 h milling the alloy had decomposed into an amorphous phase and nanocrystal-
Fig. 2. The grain size of high-energy ball-milled (a) Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe, and (b) Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders with a volume fraction of magnetically soft a-Fe phase of 10%, without annealing and after heat treatment at different temperatures (lines are provided as a guide to the eye).
line a-Fe (Fig. 1a). The following annealing in high-purity argon, at 7008C for 30 min, leads to the formation of the tetragonal Nd 2 Fe 14 B-type crystal structure (Fig. 1b) which coexists with the a-Fe phase, resulting in a two-phase nanocomposite material. The grain size prior to annealing and as a function of the annealing temperature for Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe and Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders with a volume fraction of magnetically soft a-Fe phase of 10%, is shown in Fig. 2. At the conclusion of milling, the mean grain size of the crystalline component in Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe was of the order of 10 nm, and after annealing at 6308C for 30 min it had increased to about 32 nm. XRD spectra of the Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe revealed a similar pattern of behaviour as it decomposed into an amorphous phase and nanocrystalline a-Fe, though the mean grain size was a little larger, being of the order of 18 nm. Annealing at 6308C for 30 min also increases the grain size of the a-Fe phase to around 40 nm (see Fig. 2).
3. Magnetic properties
Fig. 1. XRD spectra of Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders with a volume fraction of magnetically soft a-Fe phase of 10%: (a) high-energy ball-milled for 48 h, (b) heat treated at 7008C for 30 min.
Samples for the measurements of magnetic properties were made by mixing the powders with epoxy resin in suitable moulds. All data have been normalized with respect to 100% density. The magnetic hysteresis curves were measured at room temperature using a vibrating sample magnetometer with external fields up to 2.1 MA m 21 . Honda extrapolations (J vs 1 /H ) were utilized to determine the zero field saturation magnetic polarization Js . The results obtained for the Nd 12.6 Fe 69.82x Co 11.6 M x B 6 /
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Table 1 Saturation magnetic polarization Js , remanent magnetic polarization Jr , reduced remanent magnetic polarization a, and intrinsic coercivity J Hc , at 295 K of HEBM and annealed Nd 2 Fe 14 B-type powders with an excess of a-Fe Hc (kA m 21 )
Material composition
Volume fraction of magnetically soft a-Fe phase (%)
Js (T)
Jr (T)
a (Jr /Js )
J
Nd 12.6 Fe 81.4 B 6 / a-Fe Nd 12.6 Fe 81.4 B 6 / a-Fe a Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe a Nd 12.6 Fe 69.3 Co 11.6 Zr 0.5 B 6 / a-Fe Nd 12.6 Fe 69.3 Co 11.6 Zr 0.5 B 6 / a-Fe a Nd 12.6 Fe 69.3 Co 11.6 Mo 0.5 B 6 / a-Fe Nd 12.6 Fe 69.3 Co 11.6 Mo 0.5 B 6 / a-Fe Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe Nd 12.6 Fe 64.8 Co 11.6 V5 B 6 / a-Fe Nd 12.6 Fe 64.8 Co 11.6 Ni 5 B 6 / a-Fe
10 37.5 10 37.5 10 37.5 10 37.5 10 37.5 10 37.5 10 10
1.14 1.46 1.19 1.62 1.16 1.62 1.17 1.62 1.12 1.55 1.11 1.50 1.09 1.15
0.62 0.63 0.50 0.55 0.86 1.14 0.85 1.15 0.81 1.10 0.85 1.08 0.81 0.80
0.54 0.43 0.42 0.34 0.74 0.70 0.73 0.71 0.72 0.71 0.76 0.72 0.74 0.70
460 160 322 208 662 504 487 4.35 840 760 638 586 415 423
a
Data were taken from Refs. [4,10].
a-Fe (M5Zr 0.5 , Mo 0.5 , Cr 2 , Al 1 Cr 2 , V5 , Ni 5 ) powders, with a 10 and 37.5% volume fraction of a-Fe, examined in the current study, are presented in Table 1 and plotted in Figs. 3 and 4. All the powders are magnetically isotropic. The intrinsic coercivity J Hc and the reduced remanent magnetic polarization a of Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / aFe powder with a volume fraction of magnetically soft a-Fe phase of 10%, as a function of annealing temperature for 30 min anneal, are shown in Fig. 3. A maximum intrinsic coercivity of 638 kA m 21 is obtained at an annealing temperature of 7008C. J Hc is lower in powders annealed at both lower and higher temperatures, as the grain size of the soft a-Fe phase is either too small or too large for optimum exchange coupling [1]. The reduced
remanent magnetic polarization decreases from 0.78 to 0.66 as the annealing temperature increases. Generally, the grain growth which occurs at the higher temperatures leads to deterioration in the magnetic properties. The principal advantages of Co addition to Nd 2 Fe 14 B / a-Fe, as we discussed recently, are increases in the Curie temperature of the hard phase, and a possible increase in the saturation polarization arising from the higher saturation of Fe(Co) compared to that of pure a-Fe [4]. Fig. 4 shows demagnetization curves of the Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe, Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe and Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders, with a volume fraction of a-Fe of 10%, after HEBM and optimal annealing treatment. The Cr 2 - and Al 1 Cr 2 -free
Fig. 3. The variation of the intrinsic coercivity J Hc and reduced remanent magnetic polarization a 5Jr /Js with annealing temperature for the Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powder with a volume fraction of magnetically soft a-Fe phase of 10% (lines are provided as a guide to the eye).
Fig. 4. Demagnetization curves of (a) Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe, (b) Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe and (c) Nd 12.6 Fe 66.8 Co 11.6 Al 1 Cr 2 B 6 / a-Fe powders, after HEBM and optimal annealing treatment with a volume fraction of a-Fe of 10% (all data have been normalized with respect to 100% density).
J. Jakubowicz, M. Jurczyk / Journal of Alloys and Compounds 269 (1998) 284 – 287
Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe powder has a lower remanent magnetic polarization and lower intrinsic coercivity. With a small amount of Al-Cr, Cr, and other metals, such as Zr or Mo (not presented in Fig. 4), added to the cobalt substituted Nd 12.6 Fe 69.8 Co 11.6 B 6 / a-Fe, Jr and J Hc are significantly improved (see Table 1). The improvement may be related to refined grain sizes realized by these additives. Bonded magnets of Jr and J Hc ranging from 0.81 T and 840 kA m 21 to 1.10 T and 760 kA m 21 have been produced from Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe, with a 10 and 37.5% volume fraction of soft a-Fe phase, respectively. Partial replacement of Fe by Co and M additives in Nd 12.6 Fe 81.4 B 6 / a-Fe can also improve the temperature coefficients of their remanence a and intrinsic coercivity b in the bonded magnets studied. A better understanding of the role that cobalt and M plays in these materials will be the subject of further study. The Nd 12.6 Fe 67.8 Co 11.6 Cr 2 B 6 / a-Fe powder, with 10% a-Fe by volume, showed the highest intrinsic coercivity, J Hc , of 840 kA m 21 , which compares with |800 kA m 21 reported earlier by Hirosawa and Kanekiyo [12] for melt-spun and annealed Nd 4.5 Fe 57 Cr 20 B 18.5 ribbons.
4. Conclusions Remanence-enhanced Nd 12.6 Fe 69.82x Co 11.6 M x B 6 (M5 Zr 0.5 , Mo 0.5 , Cr 2 , Al 1 Cr 2 , V5 , Ni 5 ) powders, with a volume fraction of a-Fe of 10 and 37.5%, have been produced. It has been found that addition of a small amount of Al-Cr, Cr, Zr or Mo to Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe can improve the intrinsic coercivity and the hysteresis squareness of the Nd 2 (Fe,Co) 14 B / a-Fe-based nanocomposite
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materials. The Nd 12.6 Fe 69.82x Co 11.6 M x B 6 / a-Fe powders, with 37.5% by volume of a-Fe, where M5Zr 0.5 , Mo 0.5 , Cr 2 , or Al 1 Cr 2 , would seem to offer most promise as bonded magnets, because they have an intrinsic remanence of order of 1.1 T and an acceptable intrinsic coercivity.
Acknowledgements This research was supported by the INCO-Copernicus project Nr IC15 CT 96-0758 of the European Commission, Brussels.
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