MAGNETIC PROPERTIES OF VERY DILUTE Pd-Fe ALLOYS AT VERY LOW TEMPERATURE

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MAGNETIC PROPERTIES OF VERY DILUTE Pd-Fe ALLOYS AT VERY LOW TEMPERATURE

G. Chouteau, R. Tournier

To cite this version:

G. Chouteau, R. Tournier. MAGNETIC PROPERTIES OF VERY DILUTE Pd-Fe ALLOYS AT VERY LOW TEMPERATURE. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-1002-C1-1004.

�10.1051/jphyscol:19711357�. �jpa-00214393�

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

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MAGNETIC PROPERTIES OF VERY DILUTE Pd-Fe ALLOYS AT VERY LOW TEMPERATURE

G. CHOUTEAU, R. TOURNIER

C. R. T. B. T., C. N. R. S., Cedex 166, 38, Grenoble-Gare, France

Rksum6. - L'aimantation B saturation d'alliages trks dilues de Pd-Fe proportionnelle A la concentration ( c 5 0,2 % at.) conduit B un moment moyen de 10p~lat. Fe, tandis que la constante de Curie, egalement proportionnelle

a

c, dkduite des lois de Curie-Weiss entre 1 et 4 OK donne un moment effectif de 13 PB. En dessous de 0,l %, concentration critique correspondant a une distance moyenne entre impuretes de 15-20

A

le ferromagnktisme disparaft rapidement. I1 est possible que I'interaction entre deux impuretes distantes de plus de 15

A

soit antiferromagnktique.

Abstract. - The saturation magnetization of very dilute Pd-Fe alloys is proportional to the concentration ( c

5

0.2 at. %) and leads to an average moment of 10 pB/Fe atom. The Curie constant deduced from Curie-Weiss law (1 K < T < 4.2 K) 1s also proportional to c and gives an effective moment of 13 PB. The ferromagnetic state begins to disappear below 2: 0.1 %.

This critical concentration corresponds to an average distance of 15-20

A

beyond which two impurity atoms may interact antiferromagnetically.

It is well known that a small amount of iron in Palladium sets up ferromagnetic ordering and giant moments [I], [2]. Neutron scattering experiments 131,

141

show that the polarization range on the Palladium atoms extends up to 10

A

and gives an explanation for the existence of ferromagnetism at low concentrations c of iron. The saturation magnetization has been mostly measured between 0.1

%

and 15

%,

in the region where it is not proportional to the concentration [I], [2]. The paramagnetic behaviour has been studied by Mossbauer effect [5] and magnetization measurements [6]. The magnetization as a function of the magnetic field was compared with the Brillouin func- tion. The gJ values [6] obtained vary with the concen- tration (12

<

^gJ

<

20). The Curie temperatures were determined using Arrott's criterion [7], or the charac- teristic temperature of the resistivity anomaly [8], [9]

or the specific heat anomaly [lo], [ll]. The values of T, depend on the criterion chosen, but when they are plotted on the same diagram versus c, it is clear that their law variation is the same. It has been suggested that the Curie temperature should become proportional to the second power of the concentration for low concentrations [9].

In the present work, all the samples (1 5 specimens) were melted in a vacuum furnace, in alumina crucibles.

The iron concentration was checked by an activation technique. The variation of the magnetization with the field (0

<

^h^,(35 kOe) and the temperature (0.05

<

T

<

^4.2K and 15

<

^T

<

²⁰K) is studied.

The magnetization at all temperatures is measured by an extraction method. From the measurements, the saturation magnetization, the Curie constant and the Curie temperature for very low concentrations (0 < c < 0.17

%)

are deduced.

The magnetization due to the iron atoms is easily saturated at 0.05 K in low fields (Fig. 1). The mean value of the saturation magnetization per iron atom is 10 Bohr magnetons (Fig. 2). The paramagnetic magnetization of Palladium atoms is proportional to the external field (Fig. 1). This susceptibility remains constant and equal to that of pure Palladium metal for c 5 0.3

%.

For c > 0.3

%

we observe a decrease

FIG. 1. - Magnetization curves for various concentrations.

1.25 K, 0 0.05 K.

when c increases in agreement with previous results 1121. Then, in the region of low concentrations, where additive effects are observed (Fig. 2) for the induced polarization on the matrix, under the influence of an increasing concentration, the paramagnetic susceptibility does not depend on the concentration (Fig. 1).

When saturation effects are observed on the induced polarization, the susceptibility decreases slightly. This indicates that the susceptibility of the matrix, and thus the enhancement factor, decreases when the local field created by the iron atoms on the Palladium increases.

The initial susceptibility is always measured in suffi- ciently low fields, where the magnetization is proportional to the external field (h < 200 Oe). The addi-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711357

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MAGNETIC PROPERTLES OF VERY DILUTE Pd-Fe ALLOYS AT VERY LOW TEMPERATURE C 1

-

¹⁰⁰³

Fe conc. [at "/I

FIG. ^2.- Saturation magnetization and Curie constant of Pd Fe alloys. 0 our measurements. x Crangle's.

-

Mc Dougald's.

a critical range beyond which two impurities atoms are not necessarily ferromagnetically coupled. This critical range is approximately equal to 15-20

Hi

since co

-

^{l/z with}^z

^--

1 000. For very low concentrations one should observe the antiferromagnetic state of dilute alloys. This is clearly shown by specific heat data, but because of statistical fluctuations of concentration many little ferromagnetic regions will exist in the alloy and then the antiferromagnetic state will be more difficult to observe by susceptibility measurements which are ma~nly sensitive to the presence of these regions with strongly superparamagnetic behaviour. However, at very low temperatures, the temperature TN at which the curves 1/x(T) deviate from Curie-Weiss law is proportional to the concentration (Fig. 4) T,

--

2 600 XC, this can be compared to the

tional part due to iron atoms follows a Curie-Weiss law between 1 and 4.2 K, for the lowest concentrations (c

<

0.03

%).

The Curie constants deduced are proportional to the concentration and correspond to an effective moment of 13 bohr magnetons per iron atom. The paramagnetic Curie temperature 8 seems to be proportional to the second power of the concentration for the lowest concentrations (c < 0.03

%)

0 ¹^.5.8 x lo6 c2 (Fig. 3). Ferromagnetic Curie tem-

FIG. 4. - Reciprocal initial susceptibility v. s. temperature below 1 K. The straight lines represent the high temperature

Curie-Weiss law.

1 I F? conc. [a) 0/.J

I

0 0.5 1.0 1.5

FIG. 3. - Ferromagnetic Curie temperature (To) and parama- gnetic Curie temperature (8). 0 our measurements.

.

Crangle's.

@ Mc Dougald's. We have only represented the Te determined by Arrott's criterion.

peratures determined using Arrott's criterion (if they have a physical sense) are also proportional to c2, T,

--

^1.06^xlo6 c2 for 0.02

%

^<^c< 0.1

%.

For

c > 0.1

%

the curve T, versus c can be analysed as a straight line which meets the concentration axis at a critical concentration co (Fig. 3), below which the ferromagnetic state disappears rapidly because T, va- ries like c2. These properties are similar t o those of AuFe and CrFe - alloys. They imply the existence of

results of CuMn and AuFe alloys. The magnetization curve of a3.006 9

at?^

alloy at 0.05 K varies less r a ~ i d l y than the Brillouin function for J = 5. This may be the indication of the existence of antiferromagnetic interactions.

To conclude, we have shown that the saturation magnetic moment per iron atom in PdFe alloys is constant and equal to 10 pg for c < 0.2

%.

The effective moment deduced from the Curie constant has an average value of 13 p ~ . The ferromagnetic state begins to disappear below a critical concentration correspond- ing to an average distance between impurity atoms of 15-20

A.

It is possible that impurity atoms at a distance larger than 15-20

A

interact antiferromagnetically with an RKY type interaction. A study at concentrations lower than 70 ppm will be necessary to confirm this assumption.

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C 1

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¹⁰⁰⁴ ^G. ^CHOUTEAU,^R.^TOURNIER

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