On the magnetic properties of Y(FexCo1−x)2 compounds

Solid State Communications, Vol. 25, pp. 525—529, 1978. Pergamon Press. Printed in Great Britain

ON THE MAGNETIC PROPERTIES OF

Y(Fe~Coi_~)

2 CO)IPOUNDS

E .Burzo National Center of

Physics,

P.O.Box

52o6, Bucharest-Romania

(Received 21~ October by E.F.Bertaut) The composition dependence of the mean magnetic moment of cobalt atoms in Y(Fe~Col_~)2 compounds is analysed in the local environment model. Cobalt has a magnetic moment of 1.56 iig if there are least two Fe atoms as nearest neighbours.The maximum in the composition dependence of the transition metal moments is due to the magnetic contributions of iron atoms only. The thermal variation of reciprocal susceptibility obeys a Curie—Weiss behaviour, in addition to the Pauli paramagnetic term. Finally, the influence of the variable magnetic inter — actions on the transition metal moments is discussed.

In the last years considerable at— tention has been given to the analysis of magnetic behaviour of 3d transition metals in rare—earth or yttrium com — pounds. In these systems, as shown by the experimental data, the Fe or Co 13 both band and localized atoms present The study of pseudobinary behaviour. Y(FexCol_~)2 compounds may supply useful information on this matter . Yttrium has no magnetic moment and thus the magnetic behaviour of Y(FexCol_x) 2 is given by the Fe and Co contributions. YCo2 presents and en — hanced Pauli paramagnetism with the ~us— cepti~ility x —• 2o.lo~ emu/formula unit.~ While the cobalt moment is nil in YC02, the iron atoms carry a magnetic contribution in YFe2 of l.41~ 1IB.S the influence of the variable magnetic It is therefore of interest to analyse interactions on the Fe or Co atoms, by substituting cobalt to iron. The compounds crystallize in the MgCu2 structure type. Yttrium atoms oc— cupy ~3m sites, while the 3d atoms are distributed in ~m positions. Because of this high symmetry, simple models may be used to describe the magnetic behaviour. The previous data regarding the composition deDendence of the saturation magnetization ~8 are interpreted on the basis of non localized moments associated with the transition metal ions. By substitution of cobalt, the moment should increase at a rate corresponding to the addition of one electron per atom, since these electrons are assumed to fill only the majority spin band. When the majority spin band is filled, the moment decreases as result of fil— ling the minority spin band?. It is our purpose to show that the magnetic behaviour of cobalt in Y(FexCol_~)2 is different from that of

iron. The magnetic interactions invol — ving the cobalt atoms are essentially of short—range and consequently the corn— position dependence of these moments can be analysed in the local environ — ment model. The interactions involving range. In atoms addition we present the the iron however are also of re— long sults of magnetic measurements above the Curie temperatures. The sample were prepared by melting the constituents in an are furnace.

a ~

-

2~ —

per formuLa unit

E 0

1

-

• per

iron atom

c __________

10 Fig.l

08 —

0.6

04

0.0

The composition dependence of the saturation magnetization and the magnetic contributions of iron atoms.

The compounds were thermally treated at 950°C for then days, and then fast cooled. By quenching the samples, we cx— pect that the Fe or Co atoms be randomly distributed in ~m sites. The X— rays analyses show the presence of one phase only. The lattice parameters do not much 525

526

ON THE MAGNETIC PROPERTIES OF Y (Fe Co

1

differ from those previously reported 7. We present in Fig.l the composition dependence tion per formula of the unit saturation for Y(FexCoi_x) magnetiza— 2 compounds. As previously demonstrated 10,12 the MFe,iron 57 magnetization hyperfine isfield pro-values, H~=ll45 portional to theMFC Fe 12 where Hn is given in kGs and MFe in tiB. The magnetic con— tributions of the iron atoms, determined by the Mossbauer effect 9 are presented in Fig.l.

2

2 0

~

o -~

~

9

‘N •

\

opresent data

1 E

~ref [7] ,ref [8]

0 O

Fig.2

\ \

~ ~

\ \, \ I

0.2

0.4

_____

0.6

08

1,0 (1-x)

The mean magnetic moment of the cobalt atoms in Y(FexCol_x)2 con— pounds. By solid line we plotted the theoretical prediction accor— ding to the relation (]~).

—

The composition dependence of the magnetic moments of cobalt atoms is plotted in Fig.2, together with, values obtained from previous measurements?,8. The cobalt moment varies little in the composition range x > o.5. A collapse of the cobalt moment at a composition x~ o.5 is observed.The behaviour shown in Fig.2 may be considered using the local environment model 13 The model has been used to describe the composi — tion dependence of the cobalt moment in Gd(COxNIlx) 3~lS. 2 114 and latter on in other We suppose that the cobalt has a rare—earth transition metal atoms systems magnetic moment MC 0c nearest neighbours (NN)areiron 0, if there at atoms least being nil for n < nc. In case of random distribution of Fe and Co in ‘~m sites, the mean magnetic moment per cobalt atom of the alloy is given by: M V p ) (1) Co Co n~n ~ X where

~

COMPOUNDS

given cobalt atom has n Fe 6h1 NN environment 61 xnl(l_x) P~(x) = (6—n)!n!

Vol. 25, No. 8 atoms

in

(2)

results The is best obtained fit with for the fl~2 experimental and MC 6 ~‘B It is to be noted that the va0=l.5 lues obtained from three independent mea— surements agree rather well.In the compo— sition range o.2o.6, in the li— mit of the experimental errors, a Curie— Weiss law is observed (X,~O). In Figs.3 and 14 we plotted in solia lines the calculated curves according to(3), using the parameters x~ C and e~given in Figs.5 and 6. The composition dependence of the Pauli paramagnetic term is presented in Fig.5. By solid line we plotted the va— lues calculated from statistical weight of the cobalt atoms which have n<2 iron atoms in NN environment. A reasonable agreement with the experimental data is obtained. The Curie constants and also the paramagnetic Curie temperatures, vary moments —Fig. 6. mean It is transition to be notedmetal that similarly as the the Curie temperatures are sensibil We than cannotthose distinguish the reported?. paramag— smaller previously metic contributions of cobalt from those of iron. The estimation of the mean ratio, r, between the number of magnetic carriers deduced from paramagnetic data and from saturation measurements has a similar variation with Tc, as previously reported2. The ratio r increases when

P 0(x)

is the probability

that a

the Curie temperature decrease, illus—

Vol. 25, No. 8

ON ThE MAGNETIC PROPERTIES OFY(FexCOi~)

2COMPOUNDS

527

x=0.1 0 3 0 3

E 300

x=O.2

-

0

100

200 Fig.3

3~

—

400

500

600

700

8~XJ 900 1000 Temperature (K)

1100

The thermal variation of reciprocal susceptibility for the com — pounds with x = o.1 , o.2, o.6 and o.9.

C

3 0 3

500

600

Fig.4

700

—

800

900

1000 1100 Temperature (K)

The thermal variation of reciprocal susceptibility for the corn — pounds with x 0.14, o.8 and 1.0.

trating an increase of the itinerancy degree of the transition metal moments, Substituting iron by cobalt the cx— change interactions in the system vary. Among the factors which contribute to 14 and we the distances between these variations mention the number them1? or the modification of the tran— of NR atomsl sition metal moment by partial filling

of their 3d bands, as result of in— creasing the electron concentration of the system. Starting from the band structure of cobalt and iron in YT2(T = Fe or Co) corn— of electron when an replacing pounds hi,5, concentration we do not expect increase Fe by Co. It seems that in the case of Y(FexCo 1..~)2 compotnds the number of NN

528

ON

THE MAGNETIC PROPERTIES OF Y

atoms and the distances between them are the dominant parameters for the exchange interactions,

0

—

a

~ 2

-

9 3 E

-

600~ aE

/

E

/ /

C

/

/ 0

C

4OO~

(Fe Co ) COMPOUNDS x 1-x 2

Vol. 25, No. 8

magnetic environment. Even for a very low content ( x = 0.022) iron has a magnetic moment as shown by M.E. measurements 19, The magnetic interactions between the iron atoms are both of short and long— ranges 2,15, The sensitivity of the transition metal moments to the magnetic interac tions or electronic concentration are typical for band behaviour. On the other hand, the local environment effects or the Curie—Weiss behaviour imply locali— haviour of Fe Therefore or Co may the be analysed zed features. magnetic only be— in models which take into account both20 features, as proposed by Friedel et al or Stearns 21,

I

a C o

~

I

a

0

°~ -200~

2

1 o

o~

0

0 0

I

I

0.2

0.4

—

E clj

I

0.6

0.8

1.0

0

x—~. Fig.5

~20-

~10-

The composition dependence of the Pauli paramagnetic term.

a 0~

The variation of the exchange inter— actions is reflected in the modification of the transition metal moment 18. The critical magnitude of the interactions leading to the appearance of cobalt mo— ment, corresponds to those atoms which have at least two iron atoms as Nfl. In case of the iron atoms, their magnetic contributions are not so sensitive to the

a ~

0 0

I 0.1

I 0.2

I

0.3

0.4

I 0.5 X

Fig.6

—

The composition dependence Curie netic

of the

constants, C, and paramag— Curie temperatures e P’

REFERENCES

1. 2. 3.

14. 5. 6. 7.

8. 9. lo. 11. 12. 13. 114. 15. 16. 17.

BURZO, E., Proceed.of Tenth Rare—Earth Research Conference, Carefree, Arizona, 1973, p.96. BURZO, E., Phys.Rev. Bl6 (in press) BUSCROW, K.H.J. et al., Physica 91 B+C, 261 (1977). LEMAIHE, R., Cobalt, 33, 2ol (1966). GIVORD, D., GIVORD, F., and LEMAIRE, R., J.Physique, 32, Cl—668 (1971). BURZO,E., Zeit.angew.Phys. 32, 127 (1971). ABEL,A. and CRAIG,R.S., J.Less.Common Metals, 16, 77 (1968). PIERCY, A.R. and TAYLOR, K.N.R., J.Phys. Cl, 1112 (1968). STEINER,W. and ORTBAUER, H., Phys.Stat.Sol. (a), 26, 1451 (19714). LUIJPEN, M.G. et al., Physica 86—88 B+C , 1141 (1977). GUIMARAES, A.P., and BUNBURY— D.St., J.Phys. P3, 885 (1973). MORARIU, M., BURZO , E., and BARB, D., Phys.Stat.Sol. (b), 62, K55 (19714). MORARIU, M., BURZO, E., and BARB, D., J’.Phys. ~3 , C6—6l5 (1976). JACCARINO, V. and WALKER, L.R., Phys.Rev.Letters, ~, 258 (1963). BURZO, E., LAZAR, D.P., and CIORASCU,M., Phys.Stat.Sol.(b) ~, Kl145 (19714). BENSUS, M.F. et al., Physica 86—88 B+C, 85 (1977). COLLINS, M.F., and FORSYTH, J.B., Phil.Mag. 8 , 14o1 (1963). NEEL, L., Ann.Phys. (Paris), 8, 237 (1937).

Vol. 25, No. 8

18. 19. 2o. 21.

ON THE MAGNETIC PROPERTIES OF I (Fe Co ) COMPOUNDS x 1-x2

BURZO, E., Solid State Commun., 20, 569 (1976). CORSON, M.R., et al., Hyperfime Interactions (in press). F’RIEDEL, J., LEMAN, ~., and OLSZEWSKI, D., J.Appl.Phys. 323, 325 (1961). STEARNS, M.B., Phys.Rev., B8, 14383 (1973).

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