Journal of Magnetism and Magnetic Materials 80 (1989) 159-164 159 North-Holland, Amsterdam

MAGNETIC ANISOTROPY OF THE Y(Co 1_ xFex)3 PSEUDOBINARY COMPOUNDS

Nguyen Minh HONG 1, J.J.M. FRANSE

Natuurkundig Laboratorium der Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands

and

Nguyen Phu THUY

Cryogenic Laboratory, Faculty of Physics, University of Hanoi, Vietnam

Received 3 January 1989; in revised form 21 March 1989

The anisotropy energy of the Y(Col_xFex) 3 compounds has been determined between 4.2 and 250 K. The concentration dependence of the anisotropy energy behaves anomalously in this pseudo-binary series. Upon increasing the iron content, the anisotropy energy increases up to x = 0.25, then decreases, changes sign around x = 0.45 and reaches a flat minimum at the iron-rich side. This anomaly can satisfactorily be explained in terms of the individual site anisotropy model by considering the different occupancy factors of iron and cobalt for the 3d sites.

1. Introduction

A large part of the study of intermetallics be- tween rare-earth (R) and 3d-transition-metal (T) elements is dealing with their magnetocrystalline anisotropy that is related to the possibility of permanent magnet applications. The RT 3 com- pounds, however, are not very favourable for that purpose and are less investigated.

The anisotropy of the R - T compounds is gen- erally treated as a sum of two independent contri- butions, one from the R sublattice and another from the 3d sublattice. The former contribution is satisfactorily described in terms of the crystal-field model; the latter, however, is more complicated. Developing the ideas of individual site anisotro- pies [1], Thuy and Franse [2] were able to describe the anisotropy behaviour of the Y2(Col_xFex)17 pseudobinary compounds. Application of this model to the R(Co, Fe)5 [3], R(Co, Fe)4B [4] and R2(Co, Fe)I~B compounds [5] also leads to a rea- sonable description for the 3d-anisotropy data and stimulates the interest in other R - T intermetallics, among them the Y(Co, Fe)3 compounds.

I On leave from Cryogenic Laboratory, University of Hanoi.

The RT 3 compounds (with T = Co or Fe) crys- tallize in the PuNi 3 type of structure which can be deduced from the CaCu 5 as follows:

2RT 5 + R - T ~ 3(RT3).

The R atoms occupy two unequivalent crystallo- graphic sites and the 3d atoms lie at the 3b, 6c and 18h sites according to the Wyckoff notations [6]. Studies of the magnetic properties indicate that the cobalt sublattice anisotropy is axial [7], oppo- site to the planar behaviour of the iron sublattice [81.

In order to understand the 3d contribution to the overall anisotropy in the RT 3 compounds, the study of the present paper was performed on the pseudo-binary Y(Col_xFex) 3 series. In the next section, the experimental details are presented and in the third section the results are discussed.

2. Experimental

The Y(Co l_xFex)3 compoUnds with x = 0, 0.25, 0.5, 0.75, 0.9 and 1.0 were prepared by arc-melting the constituents in an argon atmosphere. The sam- ples were subsequently subjected to an X-ray anal-

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160 N.M. Hong et al. / Magnetic anisotropy of Y(Co I ~ Fe~)~

Table 1 Structural and magnetic properties of the Y(Co 1 ~Fe~)3 compounds

x Lattice parameters ~ • Tc

a (nm) c (nm) ( / z ~ / 3 d - at) (K)

E~ (4.2 K) (K/ f .u . )

0 0.5023 2.4417 0.53 301 4.06 0.25 0.5045 2.4466 1.44 590 7.86 0.50 0.5079 2.4499 1.75 632 - 1.35 0.75 0.5100 2.4579 1.82 618 - 4.67

1.71 6.07 0.90 1.0 0.5122 2.4606 1.67 530 - 4.01

ysis. Except for YFe 3, a sample in which a small amount of the 6 :23 phase was found, all the samples are single phase and of the PuNi 3-type of structure. An attempt to grow single crystals of YCo 3 and YFe 3 was also made using a three-arc Czochralski apparatus [9]. For YCo 3, a small piece of single-crystalline material (25.5 mg) was ob- tained by starting with an ingot of the Y3Cov composition. For YFe 3, the attempts were not successful; in all seeds the 6 : 23 phase turned out to be present.

Values for the Curie temperature were deduced from low-field (0.1 T) magnetization measure- ments. The results are consistent with existing data [10] and are collected in table 1.

Magnetization measurements at various tem- peratures, ranging from 4.2 to 250 K, have been carried out in fields up to 6 T on the piece of single-crystalline material and on free powders in order to determine the spontaneous magnetization and on cubic-shaped aligned-powder samples to determine the magnetic anisotropy. The con- centration dependences of the magnetic moment and the Curie temperature are shown in fig. 1.

As already shown in the case of Lu(Col_x- Fex)4B , Y(COl_xFex)aB [4] and Y2(Cot_xFex)a4B [5], the demagnetizing factors along different axes of the cubic-shaped oriented-powder samples de- pend on the direction of the freezing field. Values for the demagnetizing factor along the freezing direction were determined directly from the slope of the magnetization curves and found to be about 0.2. The corresponding values for the perpendicu- lar directions, therefore, were estimated to be around 0.4 for the investigated series.

The sign of the first anisotropy constant, K1, was determined by an X-ray diffractogram on the aligned-powder sample. We emphasize that the sign of the first anisotropy constant can also be recognized by inspecting the magnetization mea- surements on aligned-powder samples [5]. In case of samples with axial anisotropy, a computation program based on a Gaussian distribution of c-axes of grains around the alignment direction, was used. In case of samples with planar ani- sotropy, the c-axes of the crystallites tend to ori- ent themselves in the direction perpendicular to the alignment field. An uniform distribution of c-axes in the plane perpendicular to the alignment direction was therefore assumed. The anisotropy energy can next be determined either by consider-

I 125 1oo

E

75

50 /

/

t 800

6OO

t 000 ° 25 i ' x

0 - I I 0.25 0.5 0.75 10

Fig. 1. The concentration dependence of magnetization and Curie temperature of Y(Co I xFex)3 compound. The dotted line represents the calculated values for T c according to the expression: ZAFe_Ve = k B T c / S ( S + 1) with S the value for the pseudospin as derived from the saturation magnetization; full

line is a guide to the eye for the magnetization.

N.M. Hong et aL/ Magnetic anisotropy of Y(Co~ _ xFex)3

ing the area limited by the easy-direction and the hard-direction magnetization curves or by fitting the hard-direction magnetization curve [11,12]. The values obtained by the former method, in fact, were used as starting values for calculations by the latter method. The results are shown in fig. 2. For all samples, the anisotropy constant K 2 is negligi- ble compared to K 1. Hereafter, we will denote Ea (defined as K] + K2) for the anisotropy energy. For the YCo 3 sample, the hard-direction magneti- zation curves resulted in a width of the Gaussian distribution function of 12.5 °.

The anisotropy-energy data are collected in table 1 together with the values for lattice parame- ters and the magnetic moment of the considered compounds.

i .0

_ 2 0

- - 1.0

UJ

- 1.0

- 2.0

x ~ 0 0.5

3 . R e s u l t s a n d d i s c u s s i o n s

The magnetic moment of the pseudobinary Rn(Co, Fe),, compounds exhibits "S la te r - Pauling" behaviour similar to that observed in binary Co-Fe alloys. As can be seen in fig. 1, Y(Co, Fe)3 is no exception. The same tendency has been observed by Arif et al. [10] and Gubbens

I 2.0

o t

No 1.0

I..tJ

- 1.0

- 2.0

I I I I I 100 200

0.

Fig. 2. The temperature dependence of the anisotropy energy of the Y(Co 1 _xFex)3 compounds.

161

Fig. 3. The concentration dependence of the anisotropy energy of the Y(Co] _xFex)3 compounds; the solid line represents eq. (1); the inset shows the occupancy factors used in the calcula-

tions (details are given in the text).

et al. [13] in the hyperfine field of the iron ions in Y(Co, Fe)3 , Dy(Co, Fe)3 and Er(Co, Fe)3.

In contrast to R(Co, Fe)5 and R2(Co , Fe)17, the Curie temperature of Y(Co, Fe)3 shows a max- imum around x = 0.5. It follows the concentration dependence of the magnetic moment indicating that a mean-field approximation might be applica- ble. The Curie temperature, calculated with a value for the exchange parameter, ZZ3d_3d, of 15.1 × 10-21j (z is the number of interacting neighbour- ing atoms) is given by the dotted line in fig. 1. The good agreement with the experimental data for x > 0.5 shows that this exchange parameter is little changed in that composition range. For lower x values, however, deviations are found.

At 4.2 K, the anisotropy energy varies anoma- lously upon substituting cobalt for iron. As shown in fig. 3, there is a maximum in EA(X ) around x = 0.25 and a change in sign around x = 0.25.

A similar complex change of Ea(x ) has also been observed in Y(Co, Fe)s , Y2(Co, Fe)a 7 and Y2(Co, Fe)14B and has been described in the model of individual site anisotropy (see refs. [2-4] and references therein). For the investigated com- pounds, the anisotropy energy Ea(x ) per formula unit can be expressed as follows:

1 3 E , ( x ) = Ea(O ) + ~ E nff~,FeAE~, (1)

t = l

162 N.M. Hong et al. / Magnetic anisotropy of Y(Co I ~ Fe ~)3

Table 2 Individual site anisotropy parameters AE~,, E~,co al and E'a,F~ of the Y(Col ~Fe,)? compounds (see text). ( In units of K / 3 d - i o n )

Site T / T c = 0.0 T/T~=O.1

Ea,(. o Ea.Fe Ea.re A E~ i ~ A E~ , a) , , b~ Ea.co Ea.Fe

3b 30.89 - 12.82 18.07 29.70 12.07 17.63 37.4 6c 40.55 6.18 - 34.37 - 38.35 5.95 - 32.40 - 31.4

181, 3.14 1.37 4.51 2.89 1.27 4.16 -

~) Taken from ref. [16]. b) Deduced using eq. (3) (see text).

where

AE~' ' ' (2) = Ea , Fe -- Ea,co

is the difference between the anisotropy-energy contributions of iron and cobalt ions at the site i; n , = 3, 6 and 18 for the b, c and h sites, respec- tively; f,,ve is the occupancy factor of iron ions at

the site i. Experimental values for fi,Fe can be determined

from MiSssbauer or neutron-diffraction experi- ments on pseudo-binary R(Co, Fe) 3 compounds. Such information, unfortunately, is not available. The preference of an a tom for a specific crystallo- graphic site is mainly determined by the ionic radii and by the volume of the site. For transition metal atoms of the 1 : 5-type of structure, Eleman and Buschow [14] found that the preference for the site 2c decreases going from nickel to cobalt to iron. Following these authors, we assume that the preferential occupation of iron atoms in R(Ni, Fe)3 is similar to that in R(Co, Fe) 3. The values for AE~ can then be deduced from eq. (1) using our experimental results for E~(x) and using the val- ues for f,,w obtained from neutron-diffraction measurements on Er(Nil_xFex)3 by Tharp et al.

[151. Because of the large scatter in the reported

values for the occupancy factors, f,,ve, in Er (Nil_xFex)3, we were not able to derive a con- sistent set of AE / values that fits the observed E~(x) behaviour. We considered, therefore, these occupancy factors as a guide to deduce a new set of f,.w data which is shown in the insert of fig. 3. This insert shows a clear tendency of the iron atoms to occupy preferentially the site 3b. With these data a satisfying fit has been obtained for

E a ( x ) a t 4.2 K (see fig. 3). The following parame- ter values result from this fit:

AE 2h = 30.89 K/3d- ion ,

AE,~ c = - 40.55 K / 3 d - i o m

AK~ ah = 3.14 K/3d- ion ,

The deviation between the calculated curve and the experimental result in fig. 3 is relatively large for YFe 3. Because it is just this compound in which the presence of a second phase has been found in the X-ray analysis, no attempts were made to reduce this deviation for YFe 1 at the expense of larger deviations for the other (single- phase) compounds.

The contributions of the individual sites to the anisotropy of YCo 3 have been calculated by Kgkol et al. [16] within a crystalline electric field ap- proach. Results of these calculations are quoted in table 2. The values for Ea.Fe c a n now be deduced using eq. (2) and the results are also collected in

table 2. Although the reliability of the deduced h E ;

values depends strongly on the accuracy of the occupancy factors, it is clear that the values for Ea,co' and Ea,Fe' at both the 3b and 6c sites, are opposite in sign. These contributions are the main contributions to the resulting overall anisotropy. Moreover, the sign of A E 6c coincides with that of AEa 2c in R(Co, Fe) 5 [31 and with that of AE~ 8f in R2(Co, Fe)17 [2]. This result is quite reasonable because these three sites have similar nearest neighbour surroundings. The opposite anisotropy contributions of cobalt and iron ions at the same site and their difference in magnitude (those for iron are usually larger than those for cobalt) are to

N.M. Hong et al. / Magnetic anisotropy of Y(Co I _ xFex)3 163

~0

20

-20

- . 3b

o

- o

18h

i T r - - - ~ 0.2 0./.

( T / Tc) -~ l~ .

~ o

6c • ~ "

~ e ~

/

/

I I I I

Fig. 4. The temperature dependence of the difference in the anisotropy energy of iron and cobalt ions at different sites in

Y(Col _xFex)3; AE / i i = Ea, Fe -- Ea,co.

be expected as has been verified for various types of compounds [2-5] and have tentatively been explained as a consequence of the opposite signs of the Stevens factors of the different electronic configurations of cobalt and iron ions in the con- sidered compounds [17]. The 18h-site of Y(Co, Fe)3 does not follow these systematics neither does the 9d-site of Y2(Co, Fe)]7 [2,3]. These two sites are closely related as they can be derived directly from the site 3g in the CaCu5-type of structure. The origin of this discrepancy has still to be solved. Nevertheless, the 18h-sites provide only a minor contribution to the overall ani- sotropy of Y(Co, Fe)3 and do not lead to the same signs for the anisotropy energy of the border compounds YCo 3 and YFe 3 as is observed in the Y2(Co, Fe)a7 system for Y2Co17 and Y2Fe]6 [2,3].

The above-observed anomaly in Ea(x ) exists also at elevated temperature. The same procedure has, therefore, been performed to analyse the Ea(x) curves at the reduced temperatures (T /Tc) up to 0.4. We present in fig. 4 the results for AE~' as a function of temperature. We note that the contribution from the 18h-sites decreases faster than those from the other two sites and becomes very small around T/T~ = 0.4. Using the calcu- lated values for E,'co(T)'[16], values for E~,F~(T )

can be deduced. The results for Ei, Fe at T I T c = 0.1 are also reported in table 2.

Values for E~,Fe , on the other hand, can also be deduced from 57Fe M~issbauer experiments on ErFe 3 [13] by using the step-wise changes in the hyperfine fields at the spin-reorientation tempera- ture, TSR. Denoting the step in the hyperfine field at the site i by AB i and the average step for all sites by AB we can write at the spin-reorientation temperature TSR ( = 50 K):

Ea/,Fe A B i

Ea (YFe3) - 3AB" (3)

The values for E~,F~ (50 K) deduced in this way correspond to a T / T c value of about 0.1. These values are also collected in table 2 and turn out to be in fair agreement with those deduced by means of the occupancy factors. It should be noted here once more that all conclusions are tentative due to the fact that the calculated overall anisotropy of YCo 3 in ref. [16] is found to be equal to 5.7 x 105 J / m 3 at 0 K, whereas the experimentally de- termined value is equal to 9.45 x 1135 J / m 3 at 4.2 K.

In conclusion, we stress that the complex com- position dependence of the anisotropy energy of the Y(COl_xFex) 3 compounds can satisfactorily be explained in terms of the model of individual site anisotropy. Neutron-diffraction experiments on R(Co, Fe)3 compounds would be helpful to verify the tentative results for the occupancy fac- tors of cobalt and iron atoms on the different crystallographic sites.

Acknowledgements

The authors wish to express their thanks to Dr. F.F. Bekker and Dr. T.D. Hien for their kind support and continuous interest in this work, To Dr. A.A. Mehovsk.¢¢ and Ji song-quan for their tireless efforts to grow single crystals, to H. Schlatter and A. Riemersma for their help in the sample preparation, and to R. Verhoef for assis- tance at the experiments.

The authors are indebted to Prof. Dr. K.H.J. Buschow for his suggestion to employ the f,,Fe values of R(Ni, Fe)3.

164 N.M. Hong et aL / Magnetic anisotropy of Y(Co I x Fex).~

The c o m p u t a t i o n p r o g r a m s for the a n i s o t r o p y

c o n s t a n t s were k i n d l y p r o v i d e d b y Dr . Z. K ~ k o l f r o m the U n i v e r s i t y of M i n i n g a n d M e t a l l u r g y

(Cracow, Po land) . Th i s work was s u p p o r t e d b y the N e t h e r l a n d s

U n i v e r s i t y F o u n d a t i o n for I n t e r n a t i o n a l Co o p e r -

a t i on ( N U F F I C ) a n d b y the E u r o p e a n C o m m i s -

s ion in the scope of the C E A M p r o g r a m .

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