Structural and magnetic characteristics of the Cfmm2Fe14-xCoxB system (Cfmm = Ce-free mischmetal)

Journal of Magnetism and Magnetic Materials 103 (1992) 245-249 North-Holland

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Structural and magnetic characteristics of the Cfmm 2Fe14_xfOx B system (Cfmm = Ce-free mischmetal) L.Y. Zhang, Meiqing H u a n g and W.E. Wallace The Advanced Materials Corporation, 100 N. Bellefield Avenue, Pittsburgh, PA 15213, USA Received 19 August 1991

lntermetallics of Cfmm2Fel4_xCOxB systems, where Cfmm stands for Ce-free mischmetal, were synthesized. The structural and magnetic properties were studied by means of bulk magnetometry for the entire composition range (x = 0-14). All materials have a tetragonal crystal structure with decreasing lattice parameters upon Co substitution. The Curie temperature, Tc, rises with x. At room temperature, saturation magnetization, Ms, shows a maximum at x = 2.0, while the anisotropy field, H A, and the theoretical energy product, (BH)max, exhibit weak maxima at around x = 4 and then gradually decrease for higher Co concentrations. X-ray diffraction patterns for the sample studied with (a) zero and (b) 20 kOe applied field show that magnetization lies along the c-axis at room temperature.

1. Introduction

Nd2Fel4B and alloys based on Nd2Fe14B are now recognized as very important high energy permanent magnet materials [1-3]. Extensive studies have been carried out on Nd2Fet4B-based alloys in which Fe has been partially or totally replaced by Co and/or Nd has been partially or totally replaced by another rare earth. Significant alterations in magnetic properties, e.g., Curie temperature, anisotropy field, spin-reorientation temperature, occur as a result of these replacements [4-9]. One of the important current objectives with Nd2FelnB magnets is reduction of cost. This can be accomplished by replacing Nd with mischmetal (mm) [10,11], which is much less costly than Nd. However, there is a major objection to this replacement in that mm has Ce as its major ingredient. Ce in mm2F%aB is quadripositive, giving rise, perhaps by electron transfer, to a reduced Fe moment and a lowered saturation magnetization for mm2Fe14B. It is possible to obtain cerium-free mischmetal (Cfmm), which, while more costly than mm, is substantially cheaper (a factor of 2 or 3) than Nd. The present

study is devoted to the study of (Cfmm)2Fe14 B alloys as potential permanent magnet materials and also those systems in which Fe has been partially or totally replaced by Co. Structural and magnetic features of these polycomponent systems are presented in this paper.

2. Experimental details

The composition of the Ce-free mischmetal used for this work is shown in table 1. The samples studied with Cfmm2F%a_xCoxB (x = 0-14) were prepared from 99.9% (or better) purity starting materials by means of induction melting in a water-cooled copper boat under purified argon atmosphere. As-cast ingots were wrapped in a Ta-foil, sealed into evacuated quartz tubes filled with 1/3 of atmosphere of argon and annealed at 1000 °C for one week. They were Table 1 Mischmetal used Element (at%)

Ce none

La 63.9

Pr 8.5

Nd 27.2

Ca 0.33

Fe 0.39

Mg <0.01

246

L. Y Zhang et a l. / Properties of Cfmm 2 Fe z4 ~Cox B

then rapidly cooled to room temperature. X-ray diffraction and thermomagnetic analysis (TMA) were employed to ensure the homogeneity of the samples. X-ray diffraction analysis was performed at room temperature on randomly oriented powders using a Rigaku diffractometer with CrK~ radiation. Powders aligned in a field of 20 kOe were also examined by X-ray diffraction in order to establish their easy direction of magnetization. Lattice parameter refinement was done by a computer program based on Cohen's method. Magnetic measurements were made by standard methods in fields up to 20 kOe. The measurements at 298 and 77 K were carried out using a P A R vibrating Sample magnetometer. Saturation magnetizations, M s, were obtained from magnetization isotherms using Honda plots (M versus 1 / H ) . Anisotropy fields, H A, were determined at 298 K by measuring the M versus H curves for the easy and hard directions for powder samples (size _< 37 Ixm) aligned in wax. H A values were deduced from the extrapolated intersection of the curves. All other measurements were made by the Faraday technique. Thermomagnetic analysis was performed at low external field ( = 0.5 to 3 kOe) in the temperature range 298 to = 1100 K for a rough chunk of the sample. The Curie temperatures, T~, were obtained by plotting M2 versus T and extrapolating the steep part of the curve to M2 = 0.

Table 2 Crystal structure data for Cfmm2Fe14 ~CoxB (Cfmm = Cefree mischmetal) Compounds

Lattice parameter a (~,)

V ~) (~3)

p b) (g/cm.~)

952.072 945.971 937.823 928.611 918.081 905.799 892.833 873.924

7.49 7.58 7.69 7.81 7.94 8.10 8.26 8.48

c (~,)

Cfmm2Fe14B 8.808 Cfmm2Fet2Co2B 8.789 Cfmm2Fel0Co4B 8.773 C f m m 2 FesCo6B 8.742 Cfmm2Fe6CosB 8.716 Cfmm2Fe4COll)B 8.695 CfmmEFe2Cot2B 8.675 Cfmm2Co14B 8.66(/

12.272 12.246 12.185 12.151 12.085 11.981 11.864 l 1.653

") Unit cell volume. b) Calculated density. cobalt atoms in these systems possess a smaller size than iron atoms. 3.2. T h e C u r i e t e m p e r a t u r e

The magnetic ordering temperatures for the Cfmm2Fe]4 xCOxB alloys examined in this study are observed to be between 567 and 975 K. The composition dependence of the Curie tempera-

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3. Results and discussion Structural and magnetic properties measured for the intermetallics under investigation in this work are presented in tables 2 and 3 and plotted in figs. 1-5.

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The X-ray diffraction measurements showed that all of the compounds exhibit the tetragonal Nd2Fe~4B-type structure. With increasing Co concentration, the lattice parameters, a and c, and, in turn, the unit cell volume, V, decrease monotonically, while the calculated density increases (table 2 and fig. 1). This indicates that

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Fig. 1. Crystal structure parameters as a function of cobalt content in Cfmm2Fe14 ~CoxBsystems.

L.Y. Zhang et aL / Properties of Cfmm 2Fe l4 _ xCo x B

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interaction [3,12]. Tc of R2COl4B is much higher than that of RzFe14B , indicating that the exchange interaction between Co-Co and R - C o in R2Co14B is much stronger than the corresponding interactions in R2Fe~aB compounds. Buschow et al. found [12] that the J 3 d - 3 d and J R - 3 d e x c h a n g e s are about 3-fold and 2-fold stronger in R2Co14 B than those in R2FelaB systems, respectively. In addition, Herbst and Yelon found by neutron scattering studies [13] that in R 2 Fela_xCOxB intermetallics Co and Fe exhibit different site-occupying preferences. These preferences may also affect 3d-3d and R-3d exchange interactions [14,15], and hence Tc.

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Fig. 2. The concentration dependence of saturation magnetization, M s, and the Curie temperature, Tc, in the Cfmm 2 Fe 14-xCOx B systems.

ture is shown in fig. 2 and table 3. Substitution of Fe by Co produces a sharp increase in Tc at low Co concentration (x < 6) followed by a much smaller change at higher Co content. The behavior is very similar to that observed for R 2 Fe14_xCoxB systems in which R is a single rare earth. The Tc behavior in the Cfmm2Ft4_xCOxB systems signifies a net increase of positive exchange interactions as Co replaces Fe. It is accepted that in R2T14B systems (T = Fe or Co), Tc is primarily determined by 3d-3d exchange interaction. There is a small contribution due to the R-3d exchange

3.3. Magnetizations The composition dependence of the saturation magnetization of Cfmm 2Fe 14-xCOxB intermetallics at 298 and 77 K is displayed in table 3 and fig. 2. As Co substitutes for Fe, the saturation magnetization shows a slight increase and reaches a maximum at around x = 2, and then monotonically decreases with increasing Co concentration. It is well established that in R2(Fe, Co)14B compounds the Fe and Co sublattices exhibit ferromagnetic coupling. For the Ce-free mischmetal used in this work, La is non-magnetic. The other main constituents are Pr and Nd, which are believed to be ferromagnetically coupled with the (Fe, Co)-3d sublattice [3,12]. The measured data in this study for Cfmm2Fe14_xCOxB are in accord with this type of magnetic coupling. Cobalt has a lower magnetic moment than iron. When

Table 3 Magnetic data for CfmmzFe14_xCoxB (Cfmm = Ce-free mischmetal) Compounds

C f m m 2 Fe 14B C f m m z Fe t 2Co z B C f m m z Fe 10Co4 B Cfmm2FesCo6B Cfmm z Fe6C°s B Cfmm2Fe4Col0B Cfmm 2 Fe 2Co 12B C f m m 2Co 14 B

T¢

M s (/x B / f . u . )

H A (kOe)

(BH)ma x a)

(K)

298 K

77 K

298 K

(MGOe)

567 692 798 868 909 943 971 975

26.9 28.2 28.0 27.6 25.6 23.7 21.7 19.2

30.0 31.5 29.7 28.9 27.9 25.1 22.7 19.6

52 45 48 55 41 32 28 25

42 48 48 48 42 37 32 26

a) Theoretical energy product at 298 K.

L.Y Zhang et al. / Properties of Cfmm2Fe H ~Co~B

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iron is replaced by Co, the net 3d magnetization and, in turn, the total magnetization will be progressively decreased. This is the expected behavior if Fe and Co possess composition-independent moments. However, the dependence of saturation magnetization on the composition in the Cfmm 2 Fe 14 -xCOxB systems clearly indicates that the Fe and Co moments are not invariant with composition. The dependence on composition can be explained in terms of a rigid band model and the boron acting as a donor of electrons [5,15,16]. Finally, it should be noticed that Cfmm 2 Fe]a_xCo~B with x < 6 possesses favorable values of the theoretical energy product, (BH) . . . . which are shown in table 3 and fig. 3. Cfmm 2 Fe12Co2B and Cfmm2Fe~0Co4B seem to be good candidates for high energy magnet fabrication since they exhibit promising behavior both in Tc and (BH)ma x values.

H curve and the composition dependence of H A are plotted in figs. 4 and 5. The bulk anisotropy of R2T14B compounds (T = Fe or Co) arises from two contributions the R sublattice and the 3d sublattice [3,12]. The Fe and Co sublattice favors an axial and planar anisotropy, respectively. La is non-magnetic and makes no contribution to the anisotropy, whereas both Pr and Nd contribute to the uniaxial direction of magnetization. It is a remarkable feature that the composition dependence of H A values exhibits a similar trend with that of M S, showing a maximum at about x = 4 for 298 K (see fig. 5). This observation may imply that the magnetization of the 3d sublattice has an important effect on the magnetocrystalline

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3.4. Anisotropy field X-ray diffraction studies for randomly oriented and magnetically aligned fine powder (see fig. 3) revealed that the Cfmm2Fet4_xCoxB alloys are uniaxial. The measured anisotropy field, H A, at 298 K is listed in table 2. A representative M vs.

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L.Y. Zhang et al. / Properties of Cfmm 2Eel4 -

anisotropy in the Cfmm2Fe14xCOxB intermetallics, probably via the exchange field which is directly proportional to the magnitude of the 3d-magnetization [17]. Additionally, the composition dependence of H A observed may involve preferential substitution of Co for Fe in the 3d sublattice [4] as well as the rare earth elements in the R sublattice [18].

x COx

B

249

and Cfmm2FelzC02 B are suggested to be promising as permanent magnet candidates. To date their potential as candidates for permanent magnet materials has not been realized.

References

3.5. Cfmm-Fe-B magnet A permanent magnet having the composition of Cfmm18Fe74B 8 was fabricated by means of conventional powder metallurgical techniques. Its coercivity, iHc, was observed to be only = 400 Oe, although its remanence, Br, is 7.3 kG. This observation implies that the processing may be important, and some doping may be necessary to realize the potential of this material.

4. Conclusions

CfmmeFe~4_xCOxB (Cfmm=Ce-ffee mischmetal) intermetallics, with x varying from 0 to 14, have been investigated. All materials crystallize in the tetragonal Nd2Fe14B-type crystal structure, with decreasing lattice parameters upon Co substitution for Fe. The Curie temperature, To, rises with Co content from 567 to 975 K. All compounds show the easy direction of magnetization along the c-axis at room temperature. The saturation magnetization, M s, exhibits a slight maximum at x = 2, while the anisotropy field, HA, and the theoretical energy product, (BH)m~x, exhibit a maximum at x = 4 and then monotonically decrease for higher Co concentrations, respectively. Based on the results of Curie temperature and the theoretical energy product, Cfmm2Fel0CO4 B

[1] M. Sagawa, C. Fujimura, N. Togawa and Y. Matssuura, J. Appl. Phys. 53 (1984) 2083. [2] J.J. Croat, J.F. Herbst, R.W. Lee and F.E. Pinkerton, Appl. Phys. Lett. 44 (1984) 148. [3] W.E. Wallace, Prog. Solid State Chem. 16 (1986) 127. [4] M.Q. Huang, E.B. Boltich, W.E. Wallace and E. Oswald, J. Magn. Magn. Mater. 60 (1986) 270. [5] Y. Matsuura, S. Hirosawa, H. Yamamoto, S. Fujimura and M. Sagawa, Appl. Phys. Lett. 46 (1983) 308. [6] Ying-Cheang Yang, Wen-Wang Ho, Hai-Ying Chen, Jin Wang and Jian Len, J. Appl. Phys. 57 (1985) 4118. [7] H. Fujii, W.E. Wallace and E.B. Boltich, J. Magn. Magn. Mater. 61 (1986) 251. [8] F. Pourarian, S.G. Sankar, A.T. Pedziwiatr, E.B. Boltich and W.E. Wallace, Mater. Res. Soc. Symp. Proc. 96 (1987) 103. [9] E.B. Boltich, A. T. Pedziwiatr and W.E. Wallace, ibid., 208. [10] B.M. Ma and C.J. Willman, ibid., 307. [11] Y. Yamasaki, H. Soeda, M. Yanagida, K. Mohri, N. Teshima, O. Kohmoto, T. Yoneyama and N. Yamaguchi, IEEE Trans. Magn. MAG-22 (1986) 763. [12] K.H.J. Buschow, D.B. de Mooij, S. Sinnema, R.J. Radwafiski and J.J.M. Franse, J. Magn. Magn. Mater. 51 (1985) 211. [13] J.F. Herbst and W.B. Yelon, J. Appl. Phys. 60 (1986) 4224. [14] H.M. van Noort and K.H.J. Buschow, J. Less-Common Met. 113 (1985) L9. [15] A.T. Pedziwiatr, S.Y. Jiang and W.E. Wallace, J. Magn. Magn. Mater. 62 (1986) 29. [16] K.H.J. Buschow, Rep. Prog. Phys. 40 (1977) 117o. [17] E.B. Boltich and W.E. Wallace, J. Less-Common Met. 126 (1986) 35. [18] K.H.J. Buschow, Mater. Res. Soc. Symp. Proc. 96 (1987) 1.