Spin reorientations in the Cfmm2Fe14−xCoxB systems

Journal of Magnetism and Magnetic Materials ! !1 (1992) 75-78 North-Holland

Spin reorientations in the Cfmm2Fe14_xCoxB systems * L.Y. Zhang, E.B. Boltich a n d W . E . W a l l a c e Carnegie Mellon Research h~stitute, Carnegie Melhm Unicersity, Pittsbur 5 PA 15213, USA Received 11 November 1991

The magnetic phase diagram for the :ystem Cfmm2Fet4_,Co,B, where Cfmm stands for Ce-free mischmetal, was determined. These systems were found to pt~ssess Tsal'S lower than their Nd counterparts. This is directly attributable to the fact that Ce-free mischmetal contains both La, which tends to dilute the anisotropy of the Nd, and Pr, which exhibits axial anisotropy. Those systems rich in Co (x > lid were four~d to exhibit two spin reorientations, a type 1 spin reorientation at low temperature, characteristic t)f the Nd in the Ce-free mischmetal, and a type 2 spin reorientation at higher temperatures, resulting from the competing anisotropies of the rare earth and transition metal sublattices.

I. Introduction Ternary intermetallics based o n R2"I-14B (R = rare earth atom, T = Fe and Co) possess interesting and technologically important m~gnetic properties [1-3]. One of the most interesting fundamental characteristics of these compounds is the fact that some exhibit spin reoriemations. For example, it has been observed that Nd2Fe~4B possesses a cone-to-axis spin rcoric~atation at = 135 K, while Pr2Fet4B does not [4,5]. The nature of the spin reorientations observed in R zTI4B systems has been studied by several groups [6-11]. Among the various R2Tt4B systems studied, two distinct types of spin reorientations have been identified. The first type (type 1) is that which occurs in NdzFe~zB and Ho2Fev~B. These spin reorientations are a characteristic of "'ae rare earth ion alone, and occur in those sy,,tevas in which the directional preference of the rare earth ion is temperature dependent. The second type of spin reorientation (type 2~ is observed in such

Correspondence to: Prof. W.E. Wallace, Carnegie Mellon Research Inst., Carnegie Mellon University, Pittsburgh. PA 15213, USA. * This work was supported by a ,'ontract with the US Army Research Office, Research Tri~ ngle, NC, USA.

systems as Er2Fel4B and Pr2Col4B. This type of spin reorientation is the result of competing anisotropies between the rare earth and the transition metal. Hence, systems in which the rare earth and the transition metal favor different directions, are likely candidates for spin reorientations. From the applied point of view, the spin reorientations are important because they produce various types of M versus T behavior and determine the temperature rangc of uniaxial anisotropy, which is an essential characteristic ot a permanent magnet material. Considerable efforts have been made to improve the intrinsic characteristics of Nd2Fe~4I?; type compounds. Various substituents have been introduced to both R-atom sites and Fe-atorn sites [4,5,12,13] to enhance the Curie temperature, Tc, and anisotropy field, H A. Of course, these substitutions can result in dramatic changes in the spin-reorientation temperatuces. A good candidate for lo~"-cost R - F c - B - t y p e permanent magnets is Cfmm 2 l-¢j~_ ~ . o , B, whece Cfmm stands for Ce-free mischmet~l, one of the most abundant and least costly forms of the 1arc earths. While much more economical than their Nd counterparts, these materials also exhibit very good magnetic properties [14]. However, in order to accurately assess the potential of these systems

0304-8853 /92 /$0:~.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

76

L.Y. Zhang et aL / .~pin reorientations in the Cfmm 2 Fe t4 - xCox B systems

Table 1 Ccmposition of mischmetal used (at%)

3. Results and discussion

Ce

La

Pr

Nd

Ca

Fe

Mg

none

63.9

8.5

27.2

0.03

0.39

< 0.01

,.s permanent magnet materials, an understandmg of their fundamental magnetic properties is essential. Therefore, an investigation of spin reorientations in Cfmm2Fe14_xCoxB systems was undertaken for the entire composition range (x = 0-]L4). The chemical makeup of the Ce-free mischmetal used in the present work is given in table 1. The introduction of a large amount of Co into the 3d sublattice in CfmmzFe~4_xCoxB resulted in not only the appearance of a type 2 spin reorientation, TsR2, at high temperature, but also a change in the type 1 spin-reorientation transition temperature (TsR 1).

2. Experimental The sample were prepared by induction melting stoichiometric proportions of the constituent elements (99.9% purity or better) in a watercooled copper boat under flowing ultra-high purity argon. The as-cast ingots were annealed at 1000°C for two weeks and then quenched to room temperature. X-ray powder diffraction analysis was performed at room temperature using a Rigaku diffractometer with Cr K~ radiation. Thermomagnetic analysis (TMA) was also employed to characterize the sample and ascertair., that they were single phase. TMA was performed by measuring magnetization as a function of temperature in the tempera. ture range 4.2--1100 K in a low external field ( = 1.0 kOe) on a rough chunk sample, using the. ]Faraday method. To effectively detect the spin.. ~:eorientation transition, the rough chunk sample was exposed to tl'e largest possible field at 4.2 K, the field was then reduced to = 1.0 kOe and the temperature was scanned [8]. The spin reorientations were identified as either spike or step-likv irregularities in the M versus T curves, de "lading on the natt, re of spin reorientation.

X-ray diffraction and TMA ana)ysis showed tha t all of the samples studied were ',tingle phase, and posssessed the NdzFe~4B-type tetragonal structure. The Curie temperatures and spin-reorientation transition temperatures of the various sys~:ems studied are presented in table 2. The low ~emperature spin reorientation, TSR~, is a transformation with rising temperature from a conical t~ an axial (along the c-axis) spin arrangement. The high temperature spin reorientation, TSR2, is attributable to a change in the direction of magnetization from axial to planar (in the a-b plane) [15]. As shown in fig. 1, Cfmm2Fej4B exhibits only the low temperature (type 1) spin reorientation, while Cfmm2Co14B displays both low (type 1) and high (type 2) temperature spin reorientations. A drop in both TSR! and TSRz with increasing Co content was observed, as shown in table 2. Compared with NdzFe14B (TsR ! = 135 K) and NdzCo14B (TsR I --- 34 K), Cfmm2Fel4B and Cfmm2Co14B possess lower spin-reorientation temperatures, as listed in table 2. That this is true for the type 1 spin reorientation is understood as follows. An ongoing analysis of the origin of spin reorientations in Nd2FelaB-type compounds [6] has indicated that the intrinsic directi~mal preference of the Nd magnetization :s determined by a complex interplay between the crystalline electric

.r~

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i

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I F----

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400

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800

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T(K) Fig. 1. Magnetization versus temperatare curves for Cfmm, Fe t4 B and Cfmm 2Col~ B, showing spin-reorientation transitions.

L.Y. Zhang et. aL / Spitz reorientations in the C f m m 2 Fez4 _ , C o , B s>'stems Table 2 T h e Curie and spin-reorientation Cfmm 2 Fe 14 - , C o x B systems

temperatures

Composition [xl

T, [K]

TSR 1 [K]

TSR2 [KI

0 2 4 6 8 10 12 14

567 692 798 868 909 943 971 975

110 102 95 86 78 70 50 26

732 435 390

for

the

field and the rare earth-transition metal exchange interaction. As far as the rare earth characteristics are concerned, it has been determined that, in the Nd2Fet4B structure, Nd favors an axial structure at temperatures above about 135 K and a conical one at lower temperatures. (The actual transition temperature is determined by the strength of the rare earth-transition metal exchange interaction and, therefore, the magnetization of the transition metal sublattice.) Unlike Nd, Pr has been found to possess an axial preference at all temperatures, in this structure. Based on this model, the observation that TSR~ is lower in the Cfmm-containing systems than in their Nd counterparts can be explained by the fact that not only are the cone-seeking (at low temperatures) Nd 3 + ions diluted in the Ce-free mischmetal (27.2 at% Nd), they are substituted, to a significant extent, by axis-seeking Pr 3+ (8.5 at% Pr). Hence, the axial state persists to significantly lower temperatures. The Tsm decrease upon Co substitution for Fe in the Cfmm2Fe14_xCOx B compounds may be (at least partly) attributed to the decrease in the exchange field. Boltich and Wallace [16] have proposed that the type 1 spin-reorientation temperature is directly correlated with the exchange f i e l d , Hexch, which is, to a first approximation, directly proportional to the magnitude of the 3d-sublattice magnetization. When the transition metal sublattice contains cobalt which possesses a lower magnetic moment than Fe, the o,verall magnetization of the transition metal sublattice is decreased, relative to the Fe compound. There-

77

fore, as Co is substituted for Fe, the exchange field, H,~xch, erxperienced by the Nd 3+ ion becomes weakened and, in turn, TsR I shifts to lower temperatures. For Fe-rich compounds, the average decrease in Tsm upon Co substitution is about 5 K per Co atom. However, the decrease in Tsm is higher in very Co-rich systems, suggesting that more than a reduction of the exchange field is involved in this composition range [17]. For alloys with x > 10, a second (type 2) spin reorientation was observed (TsR2), as shown in fig. 2. As mentioned earlier, this type of spin reorientation is a consequence of competing anisotropies of the transition metal (Fe and Co) and rare earth sublattices. Here the anisotropy of the rare earth sublattice is attributable to Nd and Pr, both of which are axis seeking in the high temperature regime. The transition metal sublattice, on the other hand, contains both Fe and Co, which favor axial and planar orientations, respectively [1]. Therefore, the most direct effect of Co substitution is to change the intrinsic directional preference of the transition metal sublattice from axial to planar, at sufficiently high Co content. This w~,s revealed in our earlier study of YeFe14_x(exB and GdzFel4_~Co~B (Y and Gd are isotropi:) [4]. As discussed previously, when the rare earth and transition metal sublattices exhibit differer,t directional preferences, the rare earth sublatfice generally dominates at low temperatures and the transition metal sublattice dominates ~t higher ones. Since, in the present sysi

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~

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,~ ~

TSR~ ~

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0

I

200

I

I

400

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I

600

I

i

I

800

1000

T(K)

Fig. 2 Magnetization versus t e m p e r a t u r e curves for C f m m 2 F e l 4 _ x C o ~ B (x = 10, 12, 14) systems, showing spin-re3rientation transitions above room temperature.

78

L. ~: Zhang et al. / Spin reorientations in the Cfmm2Fei4_ xCox B systems 1ooo

disordered o"~ //

~plonor

8o0

600

400

/iio,

zool c 0

!

that, in contrast with TSRI, the decrease in TSR2 is smaller in the more Co-rich systems. The magnetic phase diagram obtained for the Cfmm2Fe 14-xC°xB system is shown in fig. 3. From this figure, it is obvious that there is a large

region of uniaxial anisotropy, making certain compositions of these systems potential candidates as permanent magnet materials.

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References

Composition (x)

Fig. 3. Magnetic phase diagram of the Cfmm,Fet4_xCo~B systems.

terns, both of the anisotropic rare earths (Pr and Nd) are axis seeking in the temperature range of the type 2 transition, those compounds sufficiently rich in Co for the net transition n:etai anisotropy to favor the plane (x >_ 10) will e~,hibit a type 2 transition from the axis to the plane. In addition to this direct effect on the net transition metal anisotropy, Co substitution also modifies the strength of the exchange field acting on the rare earths and, hence, affects the strength of the rare earth anisotropy. This latter effect is much more subtle and its significance is currently under investigation. All of the samples under investigation exhibit an axial direction of magnetization at temperatures below = 400 K. This implies that, below this temperature, the anisotropy of Nd and Pr is strong enough to override the transition metal anisotropy, even for compounds with high Co concentration. It is to be noted that, in the Cfmm2Fet4_~CoxB systems, Nd and Pr are diluted by a large amount of non-magnetic La. This is why, for a given cGmposition, the Cfmm-containing system shows lower TSR2 than the purc Nd and Pr based compounds [4,5]. On the average, the decrease in TSR2 upon Co substitution is about 85 K per Co atom. It is interesting to note

[1] W.E. Wallace, Prog. Solid State Chem. 16 (1986) 127. [2] 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. [3] J.F. Herbst, J.J. Croat, F.E. Pinkerton and W.B. Yelon, Phys. Rev. B 29 (1984) 4176. [4] M.Q. Huang, E.B. Boltich, W.E. Wallace and E. Oswald, J. Magn. Magn. Mater. 60 (1986) 270. [5] A.T. Pedziwiatr, S.Y. Jiang and W.E. Wallace, J. Magn. Magn. Mater. 62 (1986) 29. [6] E.B. Boltich and W.E. Wallace, Solid State Commun. (1985) 523. [7] R.L. Davis, R.K. Day and J.B. Dunlop, Solid State Commun. 56 (1985) 181. [8] E.B. Boltich, A.T. Pedziwiatr and W.E. Wallace, J. Magn. Magn. Mater. 66 (1987) 317. [9] M. Yamada, Y. Yamaguchi, H. Kato, H. Yamamoto, Y. Nakagawa and S. Hirosawa, Solid State Commun. 56 (1985) 333. [10] A.T. Pedziwiatr and W.E. Wallace, J. Magn. Magn. Mater. 65 (1987} 139. [11] J.M. Cadogan, J.P. Gavigan, D. Givord and H.S. Li, J. Phys. F 18 (1988) 779. [12] Ying-chang Yang, Wen-wang Ho, Hai-ying Chen, Yin Wang and Jian Lan, J. Appl. Phys. 57 (1985) 4118. [13] F. Pourarian, S.G. Sankar, A.T. Pedziwiatr, E.B. Boltich and W.E. Wallace, Materials Research Soc. Syrup. Proc. 96 (1987) 103. [14] L.Y. Zhang, M.Q. Huang, W.E. Wallace, J. Magn. Magn. Mater. 103 (1992) 245. [15] E.B. Boltich, A.T. Pedziwiatr and W.E. Wallace, in: Proc. MRS Conf., eds. S.G. Sankar, J.F. tlerbst and N.C. Koon (Materials Research Society, Pitt. burgh, PA, 1987) p. 119. [16] E.B. Boltich and W.E. Wallace, J. Less-Common Met. 125 (1986) 35. [17] F. Bolzoni, J.M.D. Coey, J. Gavigan, D. Givord, O. Moze, L. Pareti and T. Viadieu, J. Magn. Magn. Mater. 65 (1987) 123.