Spin reorientations in R2Fe14-xCoxB systems (R = Pr, Nd and Er)

Journal of Magnetism and Magnetic Materials 65 (1987) 139-144 North-Holland, A m s t e r d a m

139

S P I N R E O R I E N T A T I O N S IN R2Fel4_ xCoxB S Y S T E M S (R = Pr, Nd A N D Er) *

A.T. P E D Z I W I A T R ** and W.E. WALLACE MEMS Department and Magnetics Technology Center, Carnegie-Mellon University, Pittsburgh, PA 15213, USA Received 29 May 1986; in revised form 18 August 1986

The effect of cobalt substitution on spin reorientation p h e n o m e n a in R2Fel4_xCoxB systems (R = Pr, Nd, Er) was studied by means of bulk magnetometry in the temperature range 4.2-1100 K. It was established that in the Nd-based system the introduction of cobalt resulted not only in a shift of the low temperature spin reorientation (cone to axis) but also triggered, for x/> 10, an appearance of a second spin reorientation (axis to plane) at high temperature. In the Pr-based system, for x/> 9.5, a high temperature spin reorientation (axis to plane) was also observed. Its dependence on Co content was determined. For the Er-based system, an increase of the spin reorientation temperature (plane to axis) was observed as more cobalt was introduced into that system. It was also established that the tetragonal single-phase materials in this system exist only up to x = 5. Temperature-composition diagrams are presented, indicating types of spin arrangements observed in the investigated systems.

1. Introduction

Ternary intermetallics based on RaFe14B (R = rare earth atom, Y and Th) [1-8] exhibit a variety of unusual magnetic features, including spin reorientations in some of these compounds. Apart from scientific interest, the spin reorientations are also important because they determine temperature regions of uniaxial anisotropies which are critical for technical applications of R2Fe14B as a basis for permanent magnet manufacturing (mainly for R = Nd and Pr). The nature of spin reorientation phenomena in R2Fe14B ( R = N d , Er, Tm) was recently studied by several groups [8-16]. It was shown that NdaFe14B undergoes a spin reorientation at = 135 K from conical to axial spin arrangement. In the EraFe14B compound the magnetocrystalline anisotropy changes from planar to axial at about 325 K, marking the change of the easy direction of magnetization from [100] to [001]. Non-collinear spin structure in the (001) plane was postulated [13] for Er2FexgB. * The work was supported by a contract with the A r m y Research Office. ** O n leave from Institute of Physics, Jagiellonian University, 30°059 Cracow, Poland.

The same type of spin reorientation as in Er 2 Fea4B was observed for Tm2Fel4B at the temperature of -~ 310 K. Pr2Fe14B does not exhibit any change of spin arrangement in the temperature range 4.2-565 K. Various substituents introduced either to Ratom sites or to Fe-atom sites in NdEFe14B-type crystal structure affect the spin reorientation temperature, TSR. The most often studied substitution has been the introduction of cobalt in place of iron. This has been investigated for R = Nd, Y, Gd and Pr, [18-27], but little attention has been paid to the change of TSR resulting from such substitution. Here we report our studies of TSR in Co-substituted 2 : 1 4 : 1 systems for R = Nd, Pr and Er. Introduction of a large amount of cobalt into the 3d element sublattice in the Nd2Fel4_xCox B system resulted not only in the change of the low temperature TSR but also in an appearance of a second spin reorientation (at high temperature) which is different in nature than that observed at low temperatures. In the case of the Pr-based system, a spin reorientation at high temperature was also discovered for compounds with high Co content. These spin reorientations are of the axisto-plane type. The appearance of axis-to-plane

0304-8853/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

140

A.T. Pedziwiatr, W.E. Wallace / Spin reorientations in R_, Fe 14 - ~Cox B systems

reorientation in Pr- and Nd-based systems is a new phenomenon observed, to our knowledge, for the first time.

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2. Experimental ¢:

The samples were prepared by melting stoichiometric proportions of the constituent elements (99.9 wt% purity or better) in a water-cooled copper boat by induction heating under flowing ultra-high purity argon. As-cast ingots were annealed at 900 ° C for two weeks and then rapidly cooled to room temperature. X-ray diffraction analysis was performed at room temperature on powdered samples with the use of a Rigaku diffractometer and Cr-K~ radiation. Randomly oriented and aligned (in fields up to 20 kOe) samples were used in order to draw conclusios as to their anisotropy at room temperature. Thermomagnetic analysis (TMA) and optical metallographical microscopy were also employed to check samples and ascertain that they were single phase. T M A was performed by measuring magnetization vs. temperature, M vs. T, dependencies at low external magnetic fields in the temperature range 295-1100 K with the use of the Faraday-type magnetic balance. The Curie temperatures, T~, were also determined from these measurements. X-ray, T M A and optical microscopy revealed that single-phase materials, exhibiting Nd2Fel4Btype crystal structure, were formed for the entire composition range (x = 0-14) in the case of Prand Nd-based systems. For Er-based systems only samples with x ~< 5 turned out to be single phase. Single-phase materials were studied for spin reorientation appearance in the temperature region 4.2 K up to their Tc. For temperature scans between 4.2 and 295 K a Faraday unit with 20 kOe field capability was used. For temperature scans between 295 K and the Tc of the studied material, another Faraday unit was used - one with field capability of 5 kOe. The experimental procedure applied was as follows: rough chunks of the studied material were exposed to as large a field as possible at the lowest temperature (4.2 K for low temperature scans; 295 K for high temperature scans). The field was then reduced to about

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Fig. 1. Magnetization vs. temperature curves recorded for end members of R2Fel4_~,CoxB systems. Curves were obtained at H~xt = 0.5 kOe, using an experimental procedure described in the text. 0.5 kOe and the M vs. T curve was recorded. The reason for such procedure was to align rough particles in such a way that their resultant magnetization would point in the direction of the external field. This orientation of rough particles would be maintained during the temperature scan. This method has been found very effective in searching for spin reorientation in polycrystalline materials. Depending on the type of spin reorientation, the M vs. T curves exhibited irregularities (spikes or steps), as shown in fig. 1.

3. Results and discussion Fig. 1 illustrates typical shapes of M vs. T curves observed for end members of the investi-

,4. T. Pedziwiatr, W.E. Wallace / Spin reorientations in R , Fe14 ~CoxB systems

141

-

gated systems. T h e spin r e o r i e n t a t i o n temperatures derived from M vs. T curves a n d the Curie temperatures for all investigated c o m p o u n d s are s u m m a r i z e d in tables 1 a n d 2. T¢ data are i n c l u d e d for the purpose of completeness a n d also to show how the T¢ compares to TSR. The dependencies of spin r e o r i e n t a t i o n temperatures o n cobalt c o n t e n t in R2Fe14_xCoxB systems are plotted in fig. 2. With the use of X-ray p a t t e r n s of oriented powders o b t a i n e d at r o o m temperature a n d also according to theoretical considerations [28] based o n a two-dimensional, two-crystaUite grain model, it was possible to distinguish the spin reorientations a n d attribute them to magnetocrystalline a n i s o t r o p y changes (cone to axis, axis to p l a n e or p l a n e to axis). F o r each investigated system (discussed below) a d i a g r a m of spin a r r a n g e m e n t types observed in that system was constructed. These diagrams are shown in figs. 3 - 5 .

Table 2 The Curie and spin reorientation temperatures in R2Fea4_x CoxB systems (R = Pr and Er)

3.1. T h e N d 2 F e 1 4 _ x C o x B s y s t e m

smaller when the 3d sublattice becomes " d i l u t e d " b y cobalt, which has a lower magnetic m o m e n t t h a n iron. It is k n o w n that Hexch is directly prop o r t i o n a l to the m a g n i t u d e of the 3d-magnetization. As shown in ref. [29], changes in TsR' can be

T h e spin r e o r i e n t a t i o n observed at low temperatures, TsR1, is a t r a n s f o r m a t i o n with rising temperature from a conical spin a r r a n g e m e n t to a n axial one (along the c-axis). It is observed in the entire range of cobalt c o n c e n t r a t i o n a n d decreases m o n o t o n i c a l l y (fig. 1) as more cobalt is i n t r o d u c e d i n t o the system. A gradual drop in TsR' c a n be explained b y the fact that the exchange field, Hexch, experienced by the N d 3+ ion becomes Table 1 The Curie and spin reorientation temperatures in the Nd2Fela_,CQ~B system Composition

T~ (K)

TSR' (K)

x= 0 2a) 4 a) 6 a) 8.5 9.5 10 10.5 11 13 14

589 727 820 880 943 964 973 980 987 999 1004

134 125 112 105 103 100 98 96 93 69 34

") Data taken from ref. [25]. T¢ and TSR experimental errors: _+3 K.

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556 631 747 831 863 893 920 942 950 958 967 979 986

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142

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related to changes in Hexch: the lower the Hexch , the lower the TSR,. A n appearance of a second spin reorientation at temperature TSR2 for alloys with x >/10 is a consequence of competing anisotropies of 3d- and Nd-sublattices. For a given composition (x >~ 10), the decreased exchange interaction becomes comparable to the crystal field interaction and, as disordered

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Fig. 5. Temperature-composition diagram of spin configuration types observed in the Er 2Fex4_ xCoxB system. temperature increases, a spin reorientation occurs, leading to a more energetically favorable spin arrangement. It is a transition from axial to planar spin configuration as temperature is increased. The value of TSR2 decreases when Co content increases because with more cobalt in the alloy t h e transition metal anisotropy has enhanced planar tendency and can be manifested over a larger temperature region (further away from Tc, as shown in table 1). A n average decrease in TSR 2 is about 83 K per one Co atom substituted into the 3d sublattice. Fig. 3 shows a t e m p e r a t u r e - c o m postion diagram of different spin arrangement types observed in the NdEFe14_xCoxB system. The diagram indicates that an axial anisotropy range is prevailing.

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xCoxB system

F o r the Pr-based system, in contrast to the N d - b a s e d system, no spin reorientation at low temperatures was observed. This is a result of differences in crystal field interactions between Pr- and N d - b a s e d c o m p o u n d s [9]. Only spin reorientations at high temperatures were observed for cobalt concentrations x >/9.5 (table 2; fig. 2). These are transitions between axial and planar

A.T. Pedziwiatr, W.E. Wallace / Spin reorientations in R_, Fe 14 - ~Co~.B systems

anisotropy (see fig. 4). They are of the same nature as those observed at TSR2 for the Nd-based system. The same arguments as those used for the Nd-based system can explain the occurrence of this phenomenon. The decrease of TsR with Co content in the Pr system is not as pronounced as in the case of the Nd system (only 38 K per one substituted atom as opposed to 83 K for the Nd system). It is also worth emphasizing that the high temperature spin reorientation appears for slightly lower Co concentration in the case of the Pr system (critical composition is 9.0 < x ~< 9.5 as opposed to 9.5 < x ~< 10.0 in the case of the Nd system), indicating that a lower amount of Co in the 3d sublattice is needed to bring the 3d sublattice anisotropy down to the level where the high temperature spin reorientation occurs. The temperature-composition regions of the observed spin orientations in the Pr-based system are shown in fig. 4. It is evident that the axial anisotropy range is even broader than in the Nd-based system. 3.3. The Er2Fe14 _ xCoxB system

This is a ferrimagnetic system in which iron can be successfully replaced by cobalt in the tetragonal lattice only up to x = 5. The spin reorientation observed for Er2Fe14B is of a different nature than that observed for the Pr- and Nd-based systems at high temperatures. It is a transition, as temperature is increased, from planar to axial anisotropy. The TsR is shifted towards higher temperatures (fig. 2; table 2) by cobalt addition. Fig. 5 indicates the temperature-composition regions of planar and axial anisotropies observed in this system. Unlike Pr and Nd, the Er ion is characterized by a positive Stevens coefficient (preferring planar anisotropy). Also, the cobalt atom seeks planar anisotropy in intermetallic compounds; therefore, it is not surprising that the TsR increases as more cobalt is introduced into the crystal lattice. Simply, planar anisotropy dominates over a larger temperature region when Co substitutes for iron.

4. Conclusions The application of bulk magnetometry for polycrystalline samples proves to be an effective

143

way of determining spin reorientations in R2Fe14_xCOxB systems. The substitution of cobalt for iron in the Nd-based system results not only in a monotonic decrease of the low temperature spin reorientation (cone to axis) (TsR ' = 134 K for x = 0 and 34 K" for x = 14), but also induces an appearance of a second spin reorientation (axis to plane) for high (x >1 10) Co concentrations. The second spin reorientation appears at high temperatures (TsR 2 = 878 K for x = 10). Tsv,2 gradually decreases to reach a value of 546 K for Nd2Co14B. For the Pr-based system a high temperature spin reorientation (axis to plane) is also observed for cobalt content x >/9.5. The value of Tsa in this system decreases from 836 K for x = 9.5 to 664 K for Pr2Co14B. The temperature-composition region of axial anisotropy is larger in the Pr-based system than in the Nd-based system. An increase of spin reorientation temperature (plane to axis) is observed for the Er-based system with an increase of Co content. However, the amount of cobalt which an substitute for iron in the tetragonal lattice is only x ~< 5 in this system.

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A.T. Pedziwiatr, W.E. Wallace / Spin reorientations in R 2FeI4 xCo~B systems

[14] N.C. Koon, M. Abe, E. Callen, B.N. Das, S.H. Liou, A. Martinez and R. Segnan, J. Magn. Magn. Mat. 54-57 (1986) 593. [15] N.C. Koon, B.N. Das and C.M. Williams, ibid. (1986) 523. [16] H. Hiroyoshi, N. Saito, G. Kido, Y. Nakagawa, S. Hirosawa and M. Sagawa, ibid. (1986) 583. [17] S.G. Sankar and K.S.V.L. Narasimhan, ibid. (1986) 530. [18] Y. Matsuura, S. Hirosawa, H. Yarnamoto, S. Fujimura and M. Sagawa, Appl. Phys. Lett. 46 (1985) 308. [19] K.H.J. Buschow, D.B. de Mooij, S. Sinnema, R.J. Radwanski and J.J.M. Franse, J. Magn. Magn. Mat. 51 (1985) 211. [20] Ying-Chang Yang, Wen-Wang Ho, Hai-ying Chen, Yin Wang and .Jian Lan, J. Appl. Phys. 57 (1985) 4118. [21] E. Burzo, L. Stanciu and W.E. Wallace, J. Less-Common Metals 111 (1985) 83.

_

[22] H.M. van Noort and K.H.J. Buschow, ibid. 113 (1985) L9. [23] Z. Maocai, M. Deqing, Y. Xiuling and L. Shigiang, in: Proe. of the 8th Intern. Workshop on Rare Earth Magnets and Their Applications, Dayton, Ohio (6-8 May, 1985) ed. K.J. Strnat, Univ. of Dayton, p. 541. [24] C.D. Fuerst, Y.F. Herbst and E.A. Alson, J. Magn. Magn. Mat. 54-57 (1986) 567. [25] M.Q. Huang, E.B. Boltich, W.E. Wallace and E. Oswald, J. Magn. Magn. Mat. 60 (1986) 270. [26] A.T. Pedziwiatr, E. Burzo and W.E. Wallace, to be published. [27] A.T. Pedziwiatr, S.Y. Jiang and W.E. Wallace, to be published. [28] E.B. Boltich, A.T. Pedziwiatr and W.E. Wallace, to be published. [29] E.B. Boltich and W.E. Wallace, J. Less-Common Metals, to appear.