Journal of Magnetism and Magnetic Materials 66 (1987) 63-68 63 North-Holland, Amsterdam

STRUCTURE AND MAGNETISM OF THE R2Fe14_xCoxB F E R R I M A G N E T I C S Y S T E M S (R -- Dy AND Er) *

A.T. P E D Z I W I A T R ** and W.E. W A L L A C E

MEMS Department and Magnetics Technology Center, Carnegie-Mellon University, Pittsburgh, PA 15213, USA

Received 4 August 1986; in revised form 22 September 1986

R2Fe14_xCq~B ferrimagnetic systems (R = Dy and Er) have been synthesized and studied by X-ray and magnetometry methods to determine the lattice parameters, Curie temperatures, saturation magnetizations, anisotropy fields and spin-re- orientation temperatures. It has been established that the single phase materials, exhibiting a tetragonal crystal structure, can be formed in the Dy-based system only for x ~< 8, while in the Er-based system only for x ~< 5. An average increase of the Curie temperature of 58 K per one substituted Fe atom by Co is observed for both systems (x ~< 5). A characteristic maximum in composition dependence of the saturation magnetizations at 295 K is found for x ~ 2, although this is less pronounced than in light rare earth-based Co-substituted systems. The composition dependencies of the anisotropy fields for the Dy-based system at 77 and 295 K show distinct maxima for low Co concentrations and a sharp decrease for higher Co content. The spin-reorientation temperature for the Er-based system is shifted towards higher temperatures by the Co substitution. A brief comparison between ferro- and ferrimagnetic R 2 Fel4- xCc~ B systems is included.

1. Introduction

Some of the te rnary in termeta l l ic c o m p o u n d s of the formula R2Fe14B (R = rare earth) recent ly have become a basis for deve loping the highest energy p roduc t p e r m a n e n t magnets known [1-3]. A signif icant d r a w b a c k of these mater ia l s is their re la t ively low Curie temperature . This has been subs tan t ia l ly e l imina ted by a pa r t i a l r ep lacement of i ron by coba l t in their crysta l s tructure. This observa t ion has induced an interest in fundamen- tal studies of R2Fela_xCOxB systems with the pu r pose of ga in ing a be t te r unde r s t and ing of the or igin of the high an i so t ropy in some of these systems, preferent ia l occupa t ion of sublat t ices, magne t ic in te rac t ions be tween subla t t ices and the in t e rp lay be tween the crystal and exchange fields.

The effect of coba l t subs t i tu t ion has been s tud ied in RzFe14B systems for R = Y, Nd , Pr

* This work was supported by a contract with The Lawrence Livermore National Laboratory.

** On leave from Institute of Physics, Jagiellonian University, 30-059 Cracow, Poland.

and G d [4-16]. R2Co14B c o m p o u n d s have also been s tudied [9-1,13]. Genera l ly , the coba l t sub- s t i tu t ion for i ron in the above systems results in a decrease of la t t ice cons tants and thus in changes of in te ra tomic d is tances which are crucial for the exchange interact ions. The Curie t empera tu re in- creases for abou t 50 K per one subs t i tu ted Fe a tom by Co in the region of low Co concen t ra t ion ( x ~< 5). T h e s a t u r a t i o n m a g n e t i z a t i o n of R2Fe14 xCoxB c o m p o u n d s increases sl ightly to a m a x i m u m a round x - - 2 . 0 and af terwards de- creases with larger coba l t content . A n i s o t r o p y fields decrease mono ton ica l ly as coba l t replaces i ron in the N d - b a s e d system, while in the case of the Pr -based system they show a d rama t i c rise for x > 10 [14]. F o r x >/10 in the N d - b a s e d system and for x >t 9.5 in the Pr-based system high tem- pe ra tu re spin reor ien ta t ions (axis- to-plane) are in- duced by a p lane-seeking coba l t which results in the fact that two spin reor ien ta t ions are observed in the case of the N d - b a s e d system for large Co

con ten t [15]. The Dy2Fex4B and Er2Fe14B c o m p o u n d s form

easi ly but, as r epor ted in ref. [9], DY2Co14B and

0304-8853 /87 /$03 .50 © Elsevier Science Publ ishers B.V. ( N o r t h - H o l l a n d Physics Publ ishing Divis ion)

64 A.T. Pedziwiatr, W.E. Wallace / Structure of R 2 Fe l 4 - ~Cox B ferrimagnetic systems

Er2Co14B cannot be synthesized using the same procedure as for the Fe-based counterparts. It was a purpose of this study to investigate the range of cobalt substitution in these systems. AdditiOnally, the studies of Co substitution on magnetic proper- ties of R2Fe14B phases have been conducted so far mainly for light rare earths. In the case of heavy rare earth-based systems (HRES) the mode of coupling between 3d moments and 4f moments is antiferromagnetic as opposed to ferromagnetic observed for light rare earth-based systems (LRES). Dy- and Er-based systems were chosen for study because these ions have the Stevens factor of opposite sign, which affords an oppor- tunity of comparing their anisotropy behavior. Also, the Dy-based system may have some techni- cal importance in permanent magnet manufactur- ing, as Dy is known to enhance the anisotropy field in 2 : 14 : 1 materials.

2. Experimental

For the preparation of the samples we used 99.9 wt% (or better) purity starting materials which were alloyed by means of induction heating in a purified argon atmosphere. As-cast ingots were wrapped in Ta-foil, sealed into quartz tubes filled with argon and annealed at 900°C for two weeks. X-ray diffraction, thermomagnetic analysis (TMA) and metallographical microscopy were employed to analyze materials to establish that they were single phase. X-ray diffraction analysis was per- formed at room temperature on randomly ori- ented powdered samples with the use of a Rigaku diffractometer and C r - K a radiation. Lattice parameters refinement was conducted by a com- puter procedure using Cohen's least square method. For several samples, aligned powders were also studied by X-ray diffraction in order to draw conclusions as to their anisotropy. TMA was per- formed by recording magnetization vs tempera- ture curves at low external magnetic fields in the temperature range 295-1100 K, with the use of a Faraday-type magnetic balance. The Curie tem- peratures, T~, and the spin-reorientation tempera- tures, TSR, were also determined from these mea- surements. For low temperature measurements

(4.2-295 K) a Faraday-type magnetic balance was applied. The saturation magnetizations were ob- tained from magnetization isotherms using Honda plots. The anisotropy fields, H A, were determined at 295 and 77 K for powders ( < 37 ~m) aligned in wax by recording the magnetization vs. field curves in the easy and hard directions. H A was taken as an extrapolated intersection of these curves. A vibrating sample magnetometer with external fields up to 20 kOe was used for H A measurements.

3. Results and discussion

The structural, thermomagnetic and metallo- graphical analyses revealed that single-phase materials having the characteristic tetragonal crystal structure form in the Dy-based system only up to x = 8, while for the Er-based system they form only up to x = 5. For samples with x > 8 and > 5, respectively, X-ray patterns showed mainly the sequence and intensity of peaks typical of the hexagonal structure found in 2 : 17 systems. For the nontetragonal samples TMA showed one and in some cases two magnetic phases. Similar observations concerning .structure were made by Buschow et al. [9] when attempting to synthesize R2Co14B compounds for R = Ce, Dy, Ho, Er, Trn, Lu. The alloys with high cobalt concentration were identified recently as a mixture of three phases: 2: 17, 1 : 4 : 1 and a solid solution of transition metals [16].

Single-phase materials show a Nd2Fe14B-type crystal structure [17-19] with lattice parameters decreasing as cobalt replaces iron (see fig. 1). The decrease is linear (within the limit of experimental error) with a constant c/a ratio of 1.368 for the Er-based system and 1.371 for the Dy-based sys- tem. The decrease of lattice parameters most probably occurs because cobalt atoms are smaller in size than iron atoms.

The composition dependencies of the Curie temperatures for the systems investigated are shown in fig. 2 and summarized in table 1. Sub- stituting Fe by Co produces an average increase in T c of 58 K per one substituted Co atom (x ~< 5) in both systems. This rate is slightly ( = 5 K / a t o m ) higher than that observed in Nd- and Pr-based

A. T. Pedziwiatr, 14/. E. Wallace / Structure of R 2 Fe l 4 -- x C o x B ferrimagnetic systems 65

12 .0T_ ' , , ,

oc~ II 9 " | o.

°

~ 8 . 9

/ o Dy #_ 8.8 Er !

._u ,q

8.7

i I I I I 0 2 4 6 8 I0

C o m p o s i t i o n ( x )

Fig. 1. Lattice parameters as functions of cobalt content in R 2 Fe 14- xC°~ B systems.

systems for the same composition range [15]. This difference may be due to smaller interatomic dis- tances in the case of HRES as compared to those

I 0 0 0

8 0 0

6 0 0 , I.-

400

i r ; i /°/°

• Er, TSR

"Is R ,.,.&,.,,,,,.A . . . . . ~ J

2 0 0 i I I I 0 2 4 6 8 I0

C o m p o s i t i o n ( x )

Fig. 2. The composition dependence of the Curie temperature, To, and spin-reorientation temperature, TsR, in R 2 Fe14_xCO~ B

systems.

for LRES, and thus to enhanced 3d-3d exchange interaction. It is also plausible that the relative importance of R - 3 d exchange is larger in the case of HRES due to larger magnetic moments of heavy rare earths. Similar rates of T~ increase observed in LRES and in HRES indicate that a similar mechanism of T c increase is involved in both groups of systems. As proven by MSssbauer spectroscopy for the Nd-based system [7] and advocated in refs. [15] and [20] for R = Pr and Nd systems, the decrease of interatomic distances and a preferential substitution of Co into 16 k 2 and 8 J2 crystal sites (these are involved in negative exchange interactions) can account for the strengthening of an overall exchange interaction and thus for an increase of T c.

The composition dependencies of the satura- tion magnetization, M s, at 295 and 77 K are plotted in fig. 3 and shown in table 1. At 77 K, for both systems, there is a broad maximum in M s observed for x = 2 followed by a subsequent de- crease, whereas at 295 K the M s drops monotoni- cally as Co substitutes for Fe. This behavior is similar to that observed for LRES and can be explained in the same way [4,12,14]. The presence of a maximum in M s is pronounced and char- acteristic not only of Fe -Co binary alloys, but

GO

=L

t-

O

0

t-

O

=o

0 O3

• 7 7 K ~ D y 0 295KJ

• 77K "~ Er S z~ 2 9 5 K

15 6 , ,~<o- - .~o~ ~ & O.

, 0

I I I I 0 2 4 6 8 I0

Composition ( x )

Fig. 3. The saturation magnetization dependence on cobalt content in R2Fe14 xC0~B systems at 295 and 77 K.

'66 A. T. Pedziwiatr, W.E. Wallace / Structure o f R 2 Fe ~ 4 - xC°x B ferrirnagnetic systems

Table 1 Magnetic data for RzFet4_xCoxB systems (R = Dy and Er)

Composition rc a) rsR a) Ms b) (~tB/f.U.) (~3d) c) (~B/atom) HA d) (kOe)

(K) (K) 295 K 77 K 77 K 295 K 77 K

R = Dy

x = 0 592 - 14.1 12.1 2.15 158 192 2 725 - 14.8 12.1 2.15 168 198 4 820 - 14.7 11.2 2.09 180 170 6 916 - 13.5 10.0 ZOO 156 153 8 976 - 12.0 8.5 L90 128 135

R = Er

x = 0 554 330 19.6 14.7 2.12 planar planar 2 698 349 20.2 14.4 2.10 planar planar 4 803 373 19.8 13.2 2.01 planar planar 5 843 390 19.5 12.5 1.76 planar planar

a) Tc and TsR experimental error: + 3 K. b) Ms error: _+0.2 t~B/f.u. c) (~3d) error: +0.02. d) HA values are too large to be measured accurately with our present technique. They can be treated only as approximate data.

also R - F e - C o ternaries. The magnet ic p roper t ies of F e l _ x C o ~ al loys were recent ly discussed in ref. [21].

The magnet ic s t ructure of Dy2Fe14B was s tudied by neut ron di f f rac t ion [22]. I t was con- f i rmed that all magnet ic momen t s lie in the c-di- rec t ion with D y and Fe momen t s being coupled ant ipara l le l . The values of the i ron momen t s dif- fer, depend ing on c rys ta l lographic site. The aver- age D y m o m e n t at 77 K was found to be 9.05btB. By assuming that the above value is cons tan t th roughou t the ent i re series of DyzFe14_xCO~B and a col l inear an t i fe r romagne t ic coupl ing takes place, we ca lcu la ted a change of the mean mag- net ic m o m e n t of the t rans i t ion meta l at 77 K. The ob ta ined values decrease mono ton ica l ly f rom 2.15/~ B for x = 2 to 1.9~t B for x = 8 ( table 1). The subs t i tu t ion of coba l t which has a magnet ic mo- men t lower than that of i ron clear ly con t r ibu tes to the decrease of the 3d subla t t ice magnet iza t ion .

By compar ing the M~ value at 77 K for ErzFe14 B (14.7/~B/f.u.) with its coun te rpa r t for Y2Fe14 B (30.4/~a/f .u.) [13], one can deduce the Er moment . Such p rocedure yields the Er m o m e n t at 77 to be 7.85t~B. This p rocedure assumes, however, a col l inear a r rangement for all Er and F e spins

which, in reali ty, is not the case as they are p r o b a b l y only co -p lana r [23]. The above proce- dure provides only a p ro jec t ion of an average Er m o m e n t on the d i rec t ion of F e moments . Very recent neu t ron d i f f rac t ion measu remen t s for Er2Fe14 B at 77 K [24] yield an average Er m o m e n t to be 7.23/~ B. This value is ob ta ined using also a s tr ict ly col l inear mode l which seems to be suffi- c ient to fit the observed d i f f rac t ion pat tern . None - theless, by assuming that the p ro jec t ion of the Er m o m e n t on the d i rec t ion of F e momen t s remains cons tan t th roughou t the Er2Fe14_ xCOxB series, one can es t imate the mean 3d m o m e n t for each com- posi t ion. The calcula t ions yield the mean 3d mo- men t to be 2.12~t B for x = 0 and 1.96~t B for x = 5 (see table 1). These are typical values for 3d m o m e n t s in R2Fe14B-based systems [14,25]. In reali ty, the p ro jec t ion of the Er m o m e n t is p rob - ab ly changing smooth ly with Co concen t ra t ion which requires cor rec t ion to the net Er m o m e n t for different Co contents , bu t that cor rec t ion is very small , as it depends on the cosine of a small can t ing angle.

Al l inves t iga ted Dy-based c o m p o u n d s show an axial an i so t ropy with no ind ica t ion of its change in the t empera tu re range 4.2 K - up to their T c.

A. T. Pedziwiatr, W.E. Wallace / Structure of R 2 Fe l 4 _ xCox B ferrimagnetic systems 6 7

This was established by M vs. T measurements at low external fields in the above temperature range. Anisotropy fields determined for Dy-based com- pounds at 295 and 77 K are plotted in fig. 4 and given in table 1. At both temperatures H A shows initial increase and after reaching a maximum there is a gradual decrease observed as more cobalt is introduced into the system. It is worth noting that the above-described composition dependence of H A for the Dy system displays exactly the opposite trends as those observed in H A vs. com- position for the Pr-based system [14]. The reason for this is not clear. The initial increase of H A in the Dy-based system (for low Co content) may be associated with anisotropic exchange or with a small increase of M s observed in this composition region. It may also be the result of a change in crystal field induced by a change in electric charge brought on by cobalt introduction. The observed H A values are smaller at 295 than those at 77 K, which is the result of decreasing anisotropy of the Dy 3+ ion with rising temperature. The same rea- soning can explain the fact that the maximum of the H A vs. composition curve is shifted towards lower Co concentration at lower temperature (see fig. 4). At 77 K the Dy 3÷ anisotropy is large enough (compared to its anisotropy at 295 K) to dominate the 3d sublattice anisotropy, even for compounds with higher Fe content.

Er-based compounds show planar bulk ani-

i i f v

o. ~" 150 o • rrK O v, o 2 9 5 K 0

<~

I 0 0 J I I I 0 2 4 6 8

Composition ( x )

Fig. 4. The dependence of anisotropy fields on cobalt content in Dy2Fe]4 ~CoxB systems at 77 and 295 K.

sotropy up to their spin reorientation temperature, TsR, above which they become axial. The TSR is plotted in fig. 2 and shown in table I as a function of cobalt content. Unlike the Dy ion, the Er ion is characterized by a positive Stevens factor (prefer- ring planar anisotropy). Also, the cobalt a tom seeks planar anisotropy in intermetallic com- pounds; therefore, an increase in the temperature range of planar anisotropy is expected as more cobalt is introduced into the system. This is ex- perimentally observed, TsR being shifted toward high temperature by cobalt substitution.

4. Conclusions

Single-phase materials having a tetragonal crys ta l s t ruc ture can be fo rmed in the Dy2Fe14_xCoxB system only for x ~< 8 and for the Er2Fela_xCoxB system only for x ~< 5. This is in contrast with the LRES (Nd, Pr, Gd, Y) which crystallize in tetragonal form for the entire com- position range. Such difference is, most probably, the result of the lanthanide contraction.

Substituting Fe by Co (x ~< 5) produces an in- crease in Curie temperature of 58 K per one substituted Fe a tom by Co in Dy- and Er-based systems. It is only 5 K / a t o m larger than the rate of T c increase observed for LRES. This suggests a similar mode of preferential substitution within the 3d sublattice for both groups of systems.

A characteristic maximum in the composition dependence of the saturation magnetization ob- served for x = 2 in LRES is also observed for HRES, though less pronounced.

The mean 3d magnetic moments show similar values for LRES and HRES.

The dependencies of anisotropy fields on cobalt concentration do not show systematic similarities between LRES and HRES. They are distinctly different for each rare earth system, reflecting fundamental differences in crystal field between compounds of different rare earths.

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68 A. 7". Pedziwiatr, W.E. Wallace / Structure of R e Fel 4 - xCo:~ B ferrirnagnetic systems

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