DETERMINATION OF THE NUMBER OF MAGNETIC COBALT ATOMS IN DILUTE AuCo AND AuCoFe ALLOYS FROM NUCLEAR SPECIFIC HEAT MEASUREMENTS

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DETERMINATION OF THE NUMBER OF MAGNETIC COBALT ATOMS IN DILUTE AuCo AND AuCoFe ALLOYS FROM NUCLEAR SPECIFIC

HEAT MEASUREMENTS

P. Costa-Ribeiro, J. Souletie, D. Thoulouze, R. Tournier

To cite this version:

P. Costa-Ribeiro, J. Souletie, D. Thoulouze, R. Tournier. DETERMINATION OF THE NUMBER OF MAGNETIC COBALT ATOMS IN DILUTE AuCo AND AuCoFe ALLOYS FROM NUCLEAR SPECIFIC HEAT MEASUREMENTS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-753-C1- 754. �10.1051/jphyscol:19711261�. �jpa-00214092�

JOURNAL DE PHYSIQUE Colloque C 1, supplbment au no 2-3, Tome 32, Fkvrier-Mars 1971, page C 1

-

753

DETERMINATION OF THE NUMBER

OF MAGNETIC COBALT ATOMS IN DILUTE AuCo AND AuCoFe ALLOYS FROM NUCLEAR

SPECIFIC

HEAT MEASUREMENTS

P. COSTA- RIBEIRO, J. SOULETIE, D. THOULOUZE and R. TOURNIER Centre de Recherches sur les Tres Basses Temp6ratures

Cedex 166, 38, Grenoble-Gare, France

RBsumB. - Nous avons mesurk la chaleur spkcifique d'alliages diluks &Co et AsCoFe jusqu'a 20 m OK. Le terme hyperfin permet de compter le nombre d'atomes de Co magnktiques qui, dans l'alliage &Co, se trouvent &tre ceux appar- tenant a des groupes d'au moins trois atomes de Co. En prksence d'atomes de fer, un grand nombre d'atomes de Co grou- pks en paires ou isolks, auparavant non magnktiques, le deviennent par effet #interactions.

Abstract. - We present specific heat measurements of dilute A s o and AuCoFe alloys down to 20 m OK. The hyper- fine term allows to count the magnetic Co atoms which are foundto be those included in groups of at least 3 Co atoms, in the AuCo alloys. On introducing Fe atoms, a large number of previously non magnetic pairs or isolated Co atoms becomemagnetic, due to interaction effects;

The appearance (or disappearance) of localized moments on impurities in a metallic host depends strongly upon interaction effects [l] : such local envi- ronment effects have been used to explain experimental data not only on transition metal hosts [2], but also on noble metal matrices [3].

The case of impurities of Co in Au, non magnetic at low concentrations while presenting the characte- ristics of a localized moment at higher concentrations, can be considered in that context. The density of states pd(EF) on each Co atom will be dependent on the local surroundings ; thus each principal type of configuration will have its own characteristic tempe- rature TK.

We have measured the specific heat of dilute AuCo and AuCoFe alloys down to 20 m ^OK. The h y p x n e t e r m o f the specific heat, mainly produced by the effective field of the ordered magnetic Co atoms on their own nuclei, allows us to count them.

The samples were prepared by a method [4] which ensures reproducible and efficient quenching. The nominal concentrations c are 0.75, 1 .O, 1.50, 2, 3 and 4 at. % Co for the AuCo alloys and 1.5 at. % Co and 0.1, 0.3, 1 and 3 at.% Fe for the AuCoFe alloys.

The specific heat was measuredwith an apparatus previously described 151. For the experimental points of the AuCoFe alloys, the temperature drifts were extrapolated with a computer [6]. A correction of the magnetic temperature scale by

+

3.3 m OK was esti- mated necessary from the evaluation of a spurious T-3 term in the low temperature specific heat of all our samples. The AuCo results previously reported [4] have been corrected in this way (the new values are about 10 % higher than the old ones).

Figures l a and 2a show the experimental points in a CT2 versus T 3 diagram, for T < 0.1 OK. The solid lines represent the best fits obtained with the law C = AT-'

+

BT, where A is the hyperfine term and B the magnetic ordering plus electronic term.

For the AuCo alloys (Fig. la), A varies approxima- tely as thecube of the Co concentration : for the

FIG. la. - CT2 versus T3 for 4 different concentrations of Co in Au. The solid lines show the best fits obtained with the law CT2 = A

+

BT3, where higher temperature results have

also been used.

b) The hyperfine coefficient A as a function of the concentra- tion X3 of CO atoms included in groups of at least three Co

atoms.

0.75, 1, 1.5, 2, 3 and 4 at. %, we find respectively 8, 11, 18,38, 122 and 260 erg OK mole-'. This suggests that a magnetic moment can be associated with the groups of 3 atoms, assuming the moment is the same for all the magnetic Co atoms. More precisely, in figure l b we see that A is a linear function of the concentration X3 of Co atoms which belong to groups of 3 or more Co atoms. Assuming the alloy is per- fectly disordered, we have

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711261

C 1

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⁷⁵⁴ P. COSTA- RIBEIRO, J. SOULETIE, D. THOULOUZE AND R. TOURNIER

FIG. 2a. - CT2 versus T3 for 4 different concentrations of Fe in a &Co 1.5 at. % matrix. The solid lines show the best fits obtained with the law CT2 = A

+

BT3 where higher

temperature results have also been used.

b) The hyperfine coefficient A as a function of the Fe concen- tration.

A&!.L&c0,.5 Fi? deduce TK 2: 50 OK for the pairs and 700 OK for the

isolated impurities.

CT* (crg.~/mok) Till now, we have neglected the contribution of the

matrix and of the non magnetic Co atoms. The hyper-

3 0 0 - 3% fine magnetic term of the matrix induced by the magnetic Co atoms is proportional to c3 and may give rise to a reduction of 1.5 % of H,,,. The quadru-

From the slope of this straight line which is propor- tional to H;, we obtain He,, = 200 kOe (+ lo), in good agreement with He, = 225 kOe for bulk Co [7]

and 180 kOe for Co in Pt [8].

Then the interaction effects between the Co in groups of at least 3 atoms are sufficient for these Co to be magnetic at temperatures higher than 20 m OK.

An analysis of the B term [4] which we measured below the ordering temperature of the magnetic atoms and other results [9] at higher temperature allows to

200 -

for corresponding AuFe alloys [lo]. We obtain values of 33, 67, 142 a n d 2 4 8 erg0K mole-' for 0.1, 0.3, 1 and 3 at. % Fe respectively. For 3 % Fe, the magnetic hyperfine term due to Co is increased by a factor 14 showing that many pairs and isolated atoms now give a contribution.

This large contribution cannot be quantitatively explained by an argument as restrictive as the first neighbour effect previously considered. Since T K for the pairs is lower by a factor 10 than that for the isolated impurities, we should expect long distance effects to be more effective on the pairs. The curvature which is observed in the curve A(c,,) would then be attributed to the saturation of pairs (the only contri- bution of all the pairs would yield A ⁼ 85 erg OK mole-' with N2 = 12 c2(1 - c)18) and the final slope to the isolated impurities. Further experiments are in progress to test this assertion quantitatively.

polar term induced by all the Co atoms is proportional

/ ^a

^@ to c and is probably relatively small since it does not

//

References

0.3% appear in our results. The contribution induced by the

100 - magnetic Co groups on the non magnetic pairs and

0.1% isolated atoms is expected to be proportional to c5 and c4 respectively. We estimate this correction to be less

(KI)

^{than 15 %} from the results on the AuCoFe that we

0 -

10-4 1 0 - ~ now present.

We have measured the specific heat of AuCoFe

A @rg ~ / m d e ) alloys to follow the effect on the non

magnetic Co atoms of a magnetic impurity, Fe, which itself gives a known contribution [lo]. The addition of

@

0.1 to 3 at. % Fe in a sample containing 1.5 at. % Co

100

0

^{causes as may a be large} seen increase in figure of 2a. the In hyperfine figure 2b the specific hyperfine heat,

0 ck6t%) contribution of these alloys is given as a function of

I 2 3 the concentration of Fe after subtraction of the values

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. -

^{, ,,}

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