The Influence of Zirconium Addition and Process Parameters On

Journal of Magnetism and Magnetic Materials 238 (2002) 56–64

The inﬂuence of zirconium addition and process parameters on the magnetic properties of Pr-Fe-B sintered magnets

R.N. Faria Instituto de Pesquisas Energeticas! e Nucleares IPEN-CNEN, CEP 05422-970, Sao* Paulo–SP, Brazil

Received 19 June 2001; received in revised form 23 August 2001

Abstract

Sintered magnets based on the compositions Pr16Fe76B8 and Pr16Fe75.5B8Zr0.5 were produced using the hydrogen decrepitation process. Sintered magnets prepared under speciﬁc processing conditions from the zirconium-free alloy exhibited excellent remanence (1.22 T), intrinsic coercivity (1.22 T) and energy product (278 kJm3). The squareness factor of magnets prepared from the Pr16Fe75.5B8Zr0.5 alloy was improved considerably (0.96). This investigation also shows the remarkable inﬂuence of zirconium addition on the intrinsic coercivity of these permanent magnets. r 2002 Elsevier Science B.V. All rights reserved.

Keywords: PrFeB magnets; Zirconium addition; Hydrogen decrepitation

1. Introduction the addition of Zr [4]. None of these investigations have reported about the influence of zirconium Recent work has shown that addition of addition on the magnetic properties of PrFeB zirconium to Pr-based magnets, produced via the sintered magnets. In this investigation, sintered hydrogenation disproportionation, desorption, permanent magnets with compositions of and recombination process was an effective way Pr16Fe76B8 and Pr16Fe75.5B8Zr0.5 were prepared to developremanence and coercivity in these via the hydrogen decrepitation (HD) process. The materials [1]. Addition of Zr to nanocrystalline influence of zirconium addition, sintering tem- PrFeB-based magnets led to a dramatic decrease in perature and cooling rate from the sintering size of the hard magnetic grains (o10 nm), with temperature on the magnetic properties of these remarkable influence on the magnetic properties of HD magnets has been investigated. single phase magnets [2]. Addition of Zr to A feature of melt-spun Nd–Fe–B alloy and sintered NdFeB permanent magnets reduced mechanically alloyed Sm–Fe–N alloy is the inverse coercivity from 0.9 to B0.6 T [3]. Conversely, correlation between their remanence and coerciv- intrinsic coercivity of mechanically milled PrFeB ity [5]. Similar magnetic behavior has been powders was enhanced from B1.6 to 1.8 T with observed in various strontium hexaferrites [6]. An empirical correlation between remanence and E-mail address: [email protected], [email protected] coercivity for Pr-based magnets that exhibited (R.N. Faria). optimum magnetic properties has been reported

[7]. In this study, magnetic properties of the where /cos yS is the degree of easy-axis align- Pr16Fe76B8 and Pr16Fe75.5B8Zr0.5 HD sintered ment of the Pr2Fe14B matrix phase (f). It is 0.5 for magnets have been compared to those in which an isotropic magnet and increases to 1 for a the inverse correlation between remanence and perfectly aligned magnet (good practical magnets coercivity of PrFeB-type magnets were observed should have cos y around 0.92–0.95 [12]). The (only for Br and m0 iHcX1 T). The maximum volume fraction (0pf p1) of the Pr2Fe14B matrix observed magnetic properties and the calculated phase in an ideal magnet is unity (to achieve high properties of these magnets have been compared. densification, practical magnets are produced with Various Pr–Fe–B-type magnets, prepared by the higher amounts of rare-earth and the volume standard powder metallurgy (PM) route, the HD fraction is maintained around 0.82–0.85). The process, hot pressing (HP), hot rolling (HR) and packing factor or relative density (P=magnet die upsetting (DU), have also been included for density/theoretical density) for an ideal permanent comparison. magnet is also unity (in practical magnets produced by powder metallurgy it can reach 0.96 [12]). Is is spontaneous polarization of the Pr2Fe14B 2. Experimental matrix phase (and is dependent on the temperature coefficient b). Two commercial alloys in the as-cast state were The spontaneous magnetization (Ms) is given by studied. To produce Pr-based HD magnets [8,9], the ratio of the magnetic moment and the volume. B35 g of the cast alloy ingot was placed in a Pr2Fe14B has a tetragonal crystal structure with 10 stainless steel hydrogenation vessel. This was then lattice constants a ¼ 8:81 10 m and c ¼ 10 2 28 3 evacuated to backing-pump pressure. Hydrogen 12:27 10 m [13] (V ¼ a c ¼ 9:52 10 m ). was introduced to a pressure of 1 bar, and this The magnetization in units of Bohr magnetons 24 1 resulted in decrepitation of the bulk material. The (mB ¼ 9:27 10 JT ) per formula unit at room vessel was then evacuated to backing-pump temperature (300 K) is 31.9 mB/f.u. [13]. Four pressure for 30–40 min, the material transferred Pr2Fe14B formula units in a cell [14] with these to a roller ball mill under a protective nitrogen dimensions gives a total magnetic moment of 21 atmosphere and milled for 20 h using cyclohexane 127.6 mB (4 31.9 mB=1.183 10 ). There- as the milling medium. The resultant fine powder fore, the spontaneous magnetization of Pr2Fe14B 5 1 21 28 was then dried for 1 h and transferred to a small is 12.42 10 Am (1.183 10 /9.52 10 ). cylindrical rubber tube under nitrogen atmo- The spontaneous polarization is then 1.56 T 7 1 sphere. The fine powder was aligned by pulsing (Is ¼ Ms m0; m0 ¼ 4p 10 TmA ). Reported three times in a 4.5 T magnetic field, pressed spontaneous polarization for the Pr2Fe14B matrix isostatically at 1000 kg cm2, vacuum sintered for phase is 1.56 T [15] and 1.575 T (at 293 K) [16]. The 1 h at 10601C followed by slow cooling (SC) in the differences in these values can be attributed to the furnace (B3.51C min1) or fast cooling (FC) temperature coefficient of Is: The highest rema- outside the furnace. For comparison, some sam- nence of a PrFeB permanent magnet (ideal) ples were sintered at 1100oC for 1 h and rapidly can be considered to be around 1.58 T cooled to room temperature. (/cos yS ¼ 1; f ¼ 1 and P ¼ 1). Magnetic characterization of the sintered The molecular mass of Pr2Fe14B is 1074.47 g PrFeB(Zr) magnets was carried out using a (2 140.9+14 55.847+10.811) and four formu- 28 3 permeameter. Measurements were carried out la units in a cell with a volume of 9.52 10 m after saturation in a pulsed field of 4.5 T. In order resulted in theoretical density of approximately 3 23 28 to compare with the experimental results, the 7.5 g cm (4 1074.47/6.02 10 /9.52 10 = 3 theoretical remanence of the HD magnets was 7.499 Mg m ). Room temperature X-ray density 3 estimated from the expression [10,11] for Pr2Fe14B is 7.53 g cm [17]. Previously calculated and observed densities [14] for Pr2Fe14B were Br ¼ hicos y fPIs; ð1Þ 7.513 and 7.51 g cm3, respectively. The calculated 58 R.N. Faria / Journal of Magnetism and Magnetic Materials 238 (2002) 56–64

density for Nd Fe B has a slightly higher value ) 2 14 3 (7.6 g cm3) [18]. 0.03) (g cm 7 r ( 3. Results and discussion

Table 1 shows the magnetic properties of the 5) 7

Pr16Fe76B8 and Pr16Fe75.5B8Zr0.5 HD magnets )( 3 sintered at 10601C/11001C and fast or slow cooled max to room temperature. The remanence (1.22 T) of (BH) (kJ m the magnet sintered at 11001C and fast cooled was higher than that of the magnet sintered at 10601C and slowly cooled (1.18 T). Conversely, the intrinsic coercivity of the former (1.22 T) was lower than that of the latter (1.37 T). In both cases, the 0.02) 7 density (r) of the HD magnets was close to the SF (ratio) ( calculated value and the packing factor, around 0.97–0.98. This high packing density is due probably to the HD process that produces a (T) c H hydrogen atmosphere during sintering [19]. Similar 0.02) b 0 7 m behavior with respect to remanence and intrinsic ( coercivity has been previously reported for Pr16Fe76B8 HD sintered magnets produced with powders milled for different times [9]. It has also (T) been reported that in the case of Sm–Co magnets, c H 0.02) i 0 the increase in grain size, due to increase in 7 ( m sintering temperature, led to higher remanence values even when the density was unaffected [20,21]. Zirconium increases significantly the

squareness factor (SF) at the expense of intrinsic (T) 0.02) r 7 B coercivity. SC produces samples with higher ( intrinsic coercivity, compared to rapid cooling. HD sintered magnets 0.5

Small changes in remanence of these magnets can Zr 8 be observed but intrinsic coercivity changed B dramatically with Zr addition (and change in 75.5 Fe cooling rate). 16 The magnetic properties of various permanent magnets [22–30] and the HD sintered magnets and Pr 8 B

studied here are compared in Table 2. The C-FCC-FCC-SCC-FCC-SC 1.22 1.20 1.18 1.16 1.12 1.22 0.67 1.37 1.21 1.02 1.06 0.66 1.10 0.93 0.86 0.82 0.96 0.80 0.68 0.61 278 266 254 253 227 7.39 7.33 7.35 7.34 7.35 76 1 1 1 1 1

Pr16Fe76B8 HD magnet sintered at 11001C for 1 h Fe 16 1100 1060 1060 1060 1060 showed very good energy product (278 kJ m3) temperature–cooling rate alongside a sintered permanent magnet prepared from a totally desorbed (TD) powder (286 kJ m3)

[9], but the TD magnet showed a much lower 0.5 0.5 Zr Zr 8 8

intrinsic coercivity. Thus, the Pr Fe B HD 8 8 8

16 76 8 B B B B B

sintered permanent magnet exhibited excellent 76 75.5 76 76 75.5 overall magnetic properties. The slightly higher Fe Fe Fe Fe Fe 16 16 16 16 16 Table 1 Magnetic properties of Pr Pr Pr Pr Pr Alloy Sintering magnetic properties of the Pr15Fe79B6 magnet [23] Pr R.N. Faria / Journal of Magnetism and Magnetic Materials 238 (2002) 56–64 59

Table 2 Magnetic properties of various Pr-based magnets [19–27]

3 2 3 Alloy Proc. route Br (T) m0 iHc (T) (BH)max (kJ m ) Br =4m0 (kJ m ) Refs. Magnet type I (Heat-treated)

Pr16Fe76B8 TD 1.22 1.06 286 296 [9] Pr16Fe76B8 HD 1.22 1.22 278 296 This work Pr16Fe76B8 HD 1.20 1.31 268 286 [22] Pr16Fe76B8 HD 1.18 1.37 254 277 This work Pr16Fe76B8 PD 1.18 1.52 280 277 [9] Pr16Fe76B8 HD 1.15 1.49 255 263 [9] Pr16Fe76B8 HD 1.12 1.67 245 250 [9] Pr16Fe76B8 HD 1.12 1.53 245 250 [9] Pr16Fe75.5B8Zr0.5 HD 1.12 1.02 227 250 This work Pr16Fe76B8 HD 1.09 1.75 232 236 [9] Pr16.9Fe79.1B4 HD 1.06 2.03 201 223 [8] Pr16Fe76B8 (9 h, undermilled) HD 1.04 1.44 208 215 [9] Pr15Fe62.5Co16Al1B5.5 PM 1.27 0.96 302 321 [23] Pr17Fe76.5B5Cu1.5 HP 1.26 1.00 288 316 [24] Pr14Dy1Fe62.5Co16Al1B5.5 PM 1.23 1.15 275 301 [25] Pr14Tb1Fe62.5Co16Al1B5.5 PM 1.18 1.38 258 277 [25] Pr13Dy2Fe62.5Co16Al1B5.5 PM 1.15 1.43 256 263 [25] Pr13Tb2Fe62.5Co16Al1B5.5 PM 1.13 1.52 248 254 [25] Pr12Dy3Fe62.5Co16Al1B5.5 PM 1.12 1.71 243 249 [25] Pr17Fe76.5B5Cu1.5 HR 1.10 1.62 226 241 [26] Pr20.5Fe73.8B3.7Cu2 HD 1.07 1.98 198 228 [8] Pr11Dy4Fe62.5Co16Al1B5.5 PM 1.04 2.20 209 215 [25] Pr19Fe74.5B5Cu1.5 HP 0.99 1.10 191 195 [27]

Magnet type II (As-sintered)

Pr16Fe75.5B8Zr0.5 HD 1.20 0.67 266 286 This work Pr16Fe76B8 TD 1.20 0.99 253 286 [9] Pr16Fe76B8 PM 1.19 1.10 F 282 [28] Pr16Fe76B8 HD 1.17 1.23 255 272 [22] Pr16Fe76B8 HD 1.16 1.21 253 268 This work Pr16Fe76B8 HD 1.14 1.39 248 258 [9] Pr16Fe76B8 PD 1.13 1.45 247 254 [9] Pr16Fe76B8 HD 1.11 1.43 229 245 [9] Pr16Fe76B8 HD 1.10 1.32 228 241 [9] Pr16Fe76B8 HD 1.09 1.49 234 236 [9] Pr16Fe76B8 (9 h, undermilled) HD 1.01 1.36 190 203 [9]

Magnet type III

Pr15Fe79B6 PM 1.29 1.24 314 331 [23] Pr13.75Fe80.25B6 DU 1.24 1.57 289 306 [29] Pr14Tb1Fe79B6 PM 1.23 1.66 267 301 [30]

can be attributed to the lower rare earth and boron rare earth content, although some scatter can be content in this material. seen. The high remanence value of the hot pressed Fig. 1 shows the maximum observed remanence magnet Pr17Fe76.5B5Cu1.5 can be attributed to the as a function of rare earth content for various ‘‘squeezing out’’ of the praseodymium-rich mate- PrFeB-type permanent magnets. In general, re- rial during the HP operation. The saturation manences tend to be lower for magnets with higher polarization of an alloy can be calculated as the 60 R.N. Faria / Journal of Magnetism and Magnetic Materials 238 (2002) 56–64

Fig. 1. Maximum remanence obtained for various Pr-based permanent magnets as a function of rare-earth content (Please refer to Table 2 to identify the alloy). product of the volume fraction (f ) of the matrix where the magnetocrystalline anisotropy field phase present in the alloy and the spontaneous (m0 HA) for the matrix phase is 9.15 T 1 polarization of the RE2Fe14B matrix phase (fIs) (7.28 MA m ) and c and N are the microstructur- [31]. The reported volume fraction of the matrix al factors for the sintered magnet [16]. In a well- phase in Nd16Fe76B8 alloy is 0.82 and in aligned Pr15Fe77B8 sintered magnet (degree of Nd15Fe77B8 sintered magnet is 0.85. Assuming alignment=0.92) the reported microstructural the same volume fraction of f in Pr-based magnets parameters c and N are equal to 0.38 and 1.1, (and alloys), and that a stoichiometric magnet respectively. According to Hirosawa and Tsubo- (Pr11.8Fe82.3B5.9 or Pr2Fe14B) has a matrix phase kawa [16] the degree of alignment of the c-axis volume fraction of 0.98 (upper limit that can be affects both parameters, c and N: Improved align- achieved in practice [32]), a plot can be drawn to ment yields a smaller intrinsic coercivity through roughly estimate remanence as a function of the decrease in c; which surpasses the effect of reduc- alloy volume fraction. Fig. 1 also shows the tion in N: The post-sintering heat treatment makes calculated remanences for good permanent mag- N smaller than in the as-sintered state and c does nets (assuming: /cos ySfPIs ¼ 0:92 f 0:98 not change with this treatment. In the Pr16Fe76B8 1:58 ¼ 1:424 f ) with volume fractions of 0.98 HD sintered magnets studied here, slow cooling (hypothetical), 0.85 and 0.82. The remanence of can be considered as the post-sintering heat a magnet with 16 at% of praseodymium should be treatment and it produces improved isolation of around 1.168 T. This agrees well with the average the matrix phase (yielding better intrinsic coerciv- value (1.176 T) obtained for Pr16Fe76B8 HD ity). Hence, in the present magnets, the c factor sintered permanent magnets studied here. For a should remain constant and N in the slowly cooled Pr15Fe77B8 (f ¼ 0:85) magnet, Br would be 1.21 T. magnets should be smaller than in the fast cooled The intrinsic coercivity (m0 iHc) of sintered magnets. Substituting Eq. (1) in (2): Pr Fe B permanent magnets at room tempera- 15 77 8 m H ¼ cm H ðNB =hicos y fPÞ: ð3Þ ture can be described by [16] 0 i c 0 A r The slow and fast cooled Pr16Fe76B8 HD sintered m0 iHc ¼ cm0HA NIs; ð2Þ magnets studied here have a relative density of R.N. Faria / Journal of Magnetism and Magnetic Materials 238 (2002) 56–64 61

98% (P ¼ 0:98) and a volume fraction of around in the as-sintered condition (type II) are not fully 82% (f ¼ 0:82). Assuming a degree of c-axis developed to permit a good curve fit and type III alignment and c factor identical to those reported magnets shift to higher Br values. Zirconium for the Pr15Fe77B8 sintered magnets (/cos yS ¼ addition has so much influence in these Pr-based 0:92 and c ¼ 0:38), the parameter N can be HD magnets that no curve fitting has been possible determined for the Pr16Fe76B8 HD magnets ðN ¼ for these samples. A data point for a Pr16Fe76B8 ð3:48 m0 iHcÞ=1:353 BrÞ: The remanence and sintered magnet (reported in Ref. [9]) lies far away coercivity of the fast cooled HD Pr16Fe76B8 from the trend shown by most of the other data. magnets are 1.16 T and 1.21 T (10601C), respec- The reason for this outlying point is the short tively, and N ¼ 1:45: For the slowly cooled HD milling time (9 h) used for processing this HD magnets, N ¼ 1:32 since Br ¼ 1:18 T and magnet (undermilling yields magnets with large m0 iHc ¼ 1:37 T, confirming the expected reduction polycrystal grains). Also included in this graph is in this parameter with post-sintering heat treat- the calculated value of coercivity (1.74 T) using ment. Since no significant change in remanence Eq. (3) for a hypothetical Pr15Fe77B8 magnet with Zr addition has been observed in these HD with Br ¼ 1:21 T (employing microstructural magnets, a significant change in microstructural parameters c ¼ 0:38 and N ¼ 1:1 [16]; / S factors probably occurs, in view of the observed cos y ¼ 0:92; f ¼ 0:85; P ¼ 0:98 and m0 iHc ¼ dramatic decrease in coercivity. This is surprising 3:4821:435Br). The calculated value of coercivity as Zr-addition is known to increase the anisotropy for another hypothetical Pr15Fe77B8 permanent field of PrFeB compounds [33] and according to magnet with an inferior degree of easy axis Eq. (2), an increase in iHc: alignment (0.86) and a Br ¼ 1:13 T (c ¼ 0:39 Fig. 2 shows calculated and experimental values and N ¼ 1:11 [16]; f ¼ 0:85; P ¼ 0:98 and of remanence and intrinsic coercivity of various m0 iHc ¼ 3:5721:549Br) is about 1.82 T. Pr-based permanent magnets. There is a decreas- Calculated values of the theoretical upper limit 2 ing monotonic behavior for heat-treated magnets (for bHcX0:5Br) of the energy product (Br =4m0) (type I), independent of composition, additions have been included in Table 2. Fig. 3 shows the and processing route. Coercivities of the magnets experimental values of energy product and iHc for

Fig. 2. Calculated and experimental values of remanence and coercivity of various Pr-based magnets (1: Pr16Fe75.5B8Zr0.5 sintered at 10601C/SC, 2: Pr16Fe76B8 sintered at 10601C/SC and 3: Pr16Fe76B8 sintered at 11001C/FC). 62 R.N. Faria / Journal of Magnetism and Magnetic Materials 238 (2002) 56–64

Fig. 3. Energy product and intrinsic coercivity of various Pr-based permanent magnets (1: Pr16Fe75.5B8Zr0.5 /10601C/SC, o 2: Pr16Fe76B8/1060 C/FC, 3: Pr16Fe76B8 /11001C/FC and 4: Pr16Fe76B8 /10601C/SC).

Fig. 4. Calculated and experimental values of energy product and remanence of various Pr-based permanent magnets.

2 the Pr-based magnets. Fig. 4 shows Br =4m0; between these parameters and, as expected, the (BH)max and Br for the Pr-based magnets. The theoretical energy product is always higher than 3 calculated energy product (291 kJ m ) for the the measured (BH)max. Practical Pr-based magnets Pr15Fe77B8 magnet (with a remanence of 1.21 T can be improved by increasing the volume fraction and a calculated intrinsic coercivity of 1.76 T) has and degree of alignment to 0.95 and if the relative also been included. There is good agreement density is decreased to only 0.95 (by decreasing the R.N. Faria / Journal of Magnetism and Magnetic Materials 238 (2002) 56–64 63 amount of RE), the remanence could reach 1.355 T References and the energy product 365 kJ m3. It has been possible to sinter high-energy magnets with Nd [1] R.N. Faria, D.N. Brown, I.R. Harris, J. Alloys Com- content as low as 13 at% [34]. The degree of pounds 296 (2000) 219. [2] D. Goll, H. Kronmuller, Fifteenth International Work- alignment in Pr16Fe76B8 HD magnets, determined shop on Rare-Earth Magnets and their Applications, by X-ray diffraction (pole figure), varies from 88% Vol. 1, Dresden, Germany, 1998, p. 189. to 97% (according to milling time) [35]. [3] M. Leonowicz, J. Magn. Magn. Mater. 83 (1990) 211. [4] D.G. Lee, D.Y. Kim, W.Y. Jeung, I.K. Kang, IEEE Trans. Nd16Fe76B8 HD sintered magnets showed much less tolerance to sintering at temperatures higher Magn. Mag. 28 (5) (1992) 2148. o [5] J.M.D. Coey, J. Magn. Magn. Mater. 140–144 (1995) than 1050 C and a dramatic decrease in the intrinsic 1041. coercivity [36]. The excellent magnetic properties of [6] R.A. McCurrie, Ferromagnetic Materials Structure and o the Pr16Fe76B8 HD magnets sintered at 1100 Care Properties, Academic Press, New York, 1994, p. 238. in agreement with data reported recently which [7] N.B. Lima, M.M. Serna, H.O. Santos, R.N. Faria, showed that Pr- and Nd-based magnets have a A.J. Williams, I.R. Harris, Proceedings of the Eighth International Symposium on Magnetic Anisotropy and different grain growth mechanism. Pr-based mag- Coercivity in RE Transition Metal Alloys. Birmingham, nets withstand high temperature annealing without 1994, p. 109. grain growth [21]. Evidence of significantly re- [8] R.N. Faria, J.S. Abell, I.R. Harris, J. Alloys Compounds duced crystal alignment, in the case of annealed 177 (1991) 311. Nd Fe B magnets, has also been found. [9] R.N. Faria, A.J. Williams, J.S. Abell, I.R. Harris: 16 76 8 Fourteenth International Workshopon RE Magnets, S. Paulo, 1996, p. 570. [10] R.A. McCurrie, J. Appl. Phys. 52 (12) (1981) 7344. 4. Conclusions [11] C.W. Searle, V. Davis, R.D. Hutchens, J. Appl. Phys. 53 (3) (1982) 2395. [12] Y. Kaneko, K. Tokuhara, N. Iahigaki, Vacuum 47 (6–8) In this investigation Pr16Fe76B8 HD magnet 1 (1996) 907. (sintered at 1100 C for 1 h) with excellent [13] S. Hirosawa, Y. Matsuura, H. Yamamoto, S. Fujimura, properties (Br ¼ i Hc ¼ 1:22 T, SF=0.82, BHmax= M. Sagawa, J. Appl. Phys. 59 (3) (1986) 873. 3 3 278 kJ m and r ¼ 7:4gcm ) has been pro- [14] T. Jinghua, H. Yiying, L. Jingkui, Sci. Sin. (Series A) XXX duced. In rapidly cooled specimens sintered at (6) (1987) 607. 1060oC, zirconium increased the SF (from 0.68 to [15] K.H.J. Buschow, Handbook of Magnetic Materials, Vol. 10, North-Holland, Amsterdam, 1977, p. 506. 0.96) and remanence (from 1.16 to 1.20 T), at the [16] S. Hirosawa, Y. Tsubokawa, J. Magn. Magn. Mater. 84 expense of intrinsic coercivity. Sintering at higher (1990) 309. o temperature (1100 C) had the same effect on SF [17] M. Jurczyk, O.D. Chistjakov, J. Magn. Magn. Mater. 78 (increased from 0.68 to 0.82) without affecting (1989) 279. intrinsic coercivity. The magnetic properties of the [18] J.J. Croat, J.F. Herbst, R.W. Lee, F.E. Pinkerton, J. Appl. Phys. 55 (6) (1984) 2078. Zr-free magnets only were in good agreement with [19] I.R. Harris, P.J. McGuiness, Eleventh International the inverse correlation between remanence and Workshop on RE Magnetism and their Applications, coercivity of Pr-based magnets. Good agreement Pittisburgh, 1990, p. 29. between calculated and observed magnetic proper- [20] M.F. Campos, F.J.G. Landgraf, R. Machado, D. Rodri- ties has been achieved in PrFeB-type permanent gues, S.A. Romero, A.C. Neiva, F.P. Missell, J. Alloys Compounds 267 (1998) 257. magnets. Further studies are being carried out to [21] F.G. Jones, M. Doser, T. Nezu, IEEE Trans. Magn. Mag. optimize the amount of zirconium added to the 13 (1977) 1320. alloy and the sintering temperature. [22] M.R. Corfield, A.J. Williams, I.R. Harris, J. Alloys Compounds 296 (2000) 138. [23] S.Y. Jiang, H.Y. Chen, S.F. Cheng, E.B. Boltich, S.G. Sankar, D.E. Laughin, W.E. Wallace, J. Appl. Phys. Acknowledgements 64 (10) (1988) 5510. [24] T. Shimoda, K. Akioka, O. Kobayashi, T. Yamagami, Many thanks are due to FAPESP and IPEN- T. Ohki, M. Miyagawa, T. Yuri, IEEE Trans. Magn. Mag. CNEN for supporting this investigation. 25 (1989) 4099. 64 R.N. Faria / Journal of Magnetism and Magnetic Materials 238 (2002) 56–64

[25] S.Y. Jiang, J.X. Yan, B.M. Ma, S.G. Sankar, [31] W. Fernengel, J. Magn. Magn. Mater. 83 (1990) 201. W.E. Wallace, Tenth International Workshopon Rare- [32] Kees de Kort, Fourteenth International Workshopon Earth Magnets and their Applications, Kyoto, Japan, Rare-Earth Magnets and their Applications, Sao* Paulo, 1989, p. 457. 1996, p. 47. [26] T. Shimoda, K. Akioka, O. Kobayashi, T. Yamagami, [33] M. Jurczyk, W.E. Wallace, J. Magn. Magn. Mater. 59 A. Arai, Eleventh International Workshopon Rare-Earth (1986) L182. Magnets and their Applications, Pittsburgh, PA, 1990, p. 17. [34] D.W. Scott, B.M. Ma, Y.L. Liang, C.O. Bounds, J. Appl. [27] Z. Chen, Z. Shi, L. Wang, H. Fu, J. Appl. Phys. 71 (6) Phys. 79 (8) (1996) 4830. (1992) 2799. [35] R.N. Faria, A.R. Castro, N.B. Lima, J. Magn. Magn. [28] Z. Shouzeng, L. Lin, Z. Lidong, H. Qin, J. Magn. Magn. Mater. (2001) accepted for publication. Mater. 54–57 (1986) 521. [36] P.J. McGuiness, E. Devlin, I.R. Harris, E. Rozendaal, [29] F.E. Pinkerton, C.D. Fuerst, J. Appl. Phys. 69 (8) (1991) J. Ormerod, J. Mat. Sci. 24 (1989) 2541. 5817. [30] S.Y. Jiang, J.X. Yan, F.S. Li, C.L. Yang, Tenth Interna- tional Workshopon Rare-Earth Magnets and their Applications, Kyoto, Japan, 1989, p. 409.