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Submitted on 1 Jan 1971

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MAGNETIC HYPERFINE FIELDS AT 61Ni NUCLEI IN Ni-Pd ALLOYS

F. Obenshain, W. Glaeser, G. Czjzek, J. Tansil

To cite this version:

F. Obenshain, W. Glaeser, G. Czjzek, J. Tansil. MAGNETIC HYPERFINE FIELDS AT 61Ni NUCLEI IN Ni-Pd ALLOYS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-783-C1-784.

�10.1051/jphyscol:19711274�. �jpa-00214106�

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JOURNAL DE PHYSIQUE

Colloque C 1, supplbment au no 2-3, Tome 32, Fe'vrier-Mars 1971, page C 1 - 783

MAGNETIC HYPERFINE FIELDS AT 61Ni NUCLEI IN Ni-Pd ALLOYS (*)

F. E. OBENSHAIN, W. A. GLAESER (*), G. CZJZEK (**) and J. E. TANSIL (***)

Oak Ridge National Laboratory, Oak Ridge, Tennessee, U. S . A.

R6sum6. - Nous avons obtenu des spectres de l'effet Mossbauer du 6lNi dans des alliages de Ni-Pd, sur tout le domaine des concentrations. La temperature de la source et de I'absorbeur etait de 4,2 OK. Les spectres montrent, a toutes les concentrations de Pd, une structure magnttique hyperfine. La moyenne de la valeur absolue du champ magnk- tique mesure, < I Haf I >, dkcroit de 76,l kOe, pour le nickel pur, a un minimum de 31 kOe, pour le Ni-Pd (45 at. %), et augmente jusqu'a 173 kOe pour le Ni-Pd (90 at. %). Les alliages sont ferromagnktiques jusqu'au Ni-Pd (98 at. -%).

Le signe de Ham est nkgatd dans le nickel pur et positif pour le Ni-Pd (90 at. %). Un model bask sur les suppositions que : (a) le champ magnktique au site de tous les noyaux de 61Ni est distermink d'aprks la distribution des atomes, de nickel et de palladium aux sites voisins, et ( 6 ) Ies atomes de palladium ont une forte contribution positive a ce champ donne un accord qualitatif avec nos observations.

Abstract. - We have obtained nuclear gamma resonance absorption spectra of 61Ni in Ni-Pd alloys over the entire concentration range. Both source and absorber were held at 4.2

OK.

The spectra show at every Pd concentration a distri- bution of magnetic hyperke fields. The measured average absolute value of the hyperfine field < < H~hn I > decreases from 76.1 kOe at pure nickel to a minimum of 31 kOe at Ni-Pd (45 at. %) and then increases to 173 kOe at Ni-Pd (90 at.

%). The alloys are ferromagnetic up to Ni-Pd (98 %). The sign of Hap in pure nickel is negative and at Ni-Pd (90 %) it is

positive.

A model based on the assumptions : (a) the hyperfine field at any 6lNi nucleus is determined by the distri- bution of nickel and palladium atoms on neighboring lattice sites, and ( b ) the palladium atoms give a strong positive contribution to the field, gives qualitative agreement with our observations.

We have taken nuclear gamma resonance spectra

1 000 ORNL-DWG 70-1307

of 6 1 ~ i in Ni-Pd alloys in the concentration range 5

to 97 atomic percent Pd. The source was 6 1 ~ o in a

o 995

nonmagnetic matrix 64Ni-V (14 at. %). The 61Co

o 990

is produced by the reaction 64Ni(p, U)~'CO with a

proton energy of 22 MeV. Both source and absorber

z - V) 0 985

were immersed in liquid helium.

0980

The spectra showed a partly resolved magnetic

Z

g

0 975

splitting for all concentrations. We have fitted each

0 970

of these spectra with 12 Lorentz lines, assuming purely

magnetic splitting and equal width for all lines. As an

o 965

example, the spectrum for Ni-Pd (85 at. %) is shown in

o 960

figure la. Since all spectra were symmetric to within

I 000

statistics, the average quadrupole interaction was

o 995

taken to be zero. The dependence of the absolute

o 990

value of the effective magnetic field at 6 1 ~ i on concen-

tration is shown in figure 2. This concentration depen-

g V) - 0 985

dence bears no simple relationship [I] to the average 2

o 980

moment of the alloys, or with the measured magnetic

Z 0 975

moments of nickel atoms 121, pNi. An interpretation

of our results can be given if we consider the fact the

0 970

splitting of the Mossbauer spectra is determined by

o 965

1 H,, 1, irrespective of the sign of H,,. Also, there is

o 960

considerable broadening of the individual lines for all

-6 -5 -4 -3 -2 VELOCITY (mm/s) -1 0 4 2 3 4 5 6

palladium concentrations. If we assume this broade- ning to be caused by a distribution of magnetic fields

for a given concentration, and hence to be nonuni- FIG. 1. - Mossbauer absorption spectrum of 6lNi in Ni-Pd (85 at. %) at 4.2

OK

(a) Fitted with

12

lines of equal width assu- form over the spectrum, we obtain a significantly ming a unique magnetic hyperhe field at nickel nuclei. (b) Fit- better fit to our spectra, as demonstrated in figure lb. ted with the assumption of a distribution of magnetic hyperfine The average field < Hhf > at 61Ni is negative [3] fields.

(*)

Research sponsored by the U. S. Atomic Energy Com- in pure nickel, changes sign near 50 at. % Pd and mission under contract with the Union Carbide Corporation. becomes positive for higher concentrations of palla-

(**)

Present address

:

Kernforschungszentrum Karlsruhe, dium [ I ] . ~h~ sign of the hyperfine field, determined by Germany.

(***)

Oak Ridge Graduate Fellow from the University of applying an external magnetic field of -50 kOe, was

Tennessee. found to be positive at Ni-Pd (90 at. %). The fact that

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711274

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F. E. OBENSHAIN, W. A. GLAESER, G. CZJZEK AND JOHN E. TANSIL

ORNL-DWG 70-10485

TABLE I

Magnetic fields (") and second moments of the magne- tic field distribution for several palladium concentra- tions.

~d Conc. < I

Hhf

1 >

M2(l

H ~ f I)

at. % kOe (k0e)'

-

- -

0 76.1 (0.4) 0

5 72.8 (0.2) 24 (3)

10 68.5 (0.2)

25 52.8 (0.5) 45 (3)

120 (12)

40 34.2 (0.3) 198 (13)

45 31.0 (0.7)

50 31.6 (0.6) 82 (7)

60 38.3 (1.1) 49 (7)

251 (19)

70 68.4 (1.0) 478 (32)

85 148.6 (0.6) 565 (26)

90 173.0 (0.9) 320 (28)

95 156.5 (0.6) 265 (16)

97 44.5 (2.2)

-

FIG.

2.

- Average magnetic hyperfine field at 61Ni nuclei in Ni-Pd alloys for several palladium concentrations. Circles are measured values. The solid line is the function <I

Hhf

I>

calculated with the values A

= 10.2

kOe/p~, B

=

16.4 koe/,U~, C

=

- 140 k o e / p ~ .

the experimental data do not cross zero (Fig. 2) is caused by the distribution of fields, since

In Table I we list the value < I H,, I > and the second moment M2 (I H,, I) of the distribution of [ H,, 1,

obtained by fitting the spectra with these assumptions.

The qualitative features of the dependence of

< I Hhf I > on palladium concentration may be pro- duced if we assume that H, at a 6 1 ~ i nucleus is the algebraic sum of three terms :

C) Standard deviations are given in parentheses.

1. a term due to core polarization and the orbital field which is proportional to the magnetic moment of the nickel 3 d-electrons, Ha

=

A . pNi,

2. a term which describes the influence of palladium neighbors H,

=

n.B.,uNi, where n is the number of palladium neighbors in the first coordination shell, 3. a contribution from the bulk conduction-elee- tron polarization, H,

=

C.F, where A, B, and C are concentration independent parameters and is the average magnetic moment of the alloy.

A calculation of < 1 H,, I > with this model agrees qualitatively with the measured values (Fig. 2 conti- nuous curve). Also, the distribution of ( H,, I in these alloys, derived from line broadening, is reproduced by this model if we assume that the magnetic moment of the 3 d-electrons on any nickel atom depends on the local palladium concentration.

References

[I] ERICH (U.), GORING (J.), HUFNER (S.) and KANKE-

LEIT

(E.), Phys. Letters, 1970, 31A, 492.

[2] CABLE ( J . W.) and CHILD (H. R.), Phys. Rev. B, 1970, 1, 3809.

[3] WEGENER (H. H. F.) and OBENSHAIN (F. E.), 1961,

163, 17.

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