MÖSSBAUER STUDY OF DILUTE FeX ALLOYS WITH X THE NON-TRANSITION ELEMENTS Al, Si, Ga, Ge, As, Sn AND Sb

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MÖSSBAUER STUDY OF DILUTE FeX ALLOYS

WITH X THE NON-TRANSITION ELEMENTS Al, Si,

Ga, Ge, As, Sn AND Sb

K. Maring, A. Algra, F. Stubbe, F. van der Woude

To cite this version:

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MOSSBAUER

STUDY OF DILUTE

FeX

ALLOYS WITH

X

THE

NON-TRANSITION ELEMENTS Al,

z,

Ga, Ge,

As,

Sn

AND Sb

K'. W. MARING, A. J. ALGRA, F. STUBBE and F. VAN DER WOUDE Solid .State Physics Laboratory, Materials Science

Center, University of Groningen, Groningen, The Netherlands

R6ume. - Par des experiences Mossbauer dependantes de la temperature les champs magneti- ques hyperfins sur des positions differentes de fer sont determines dans les alliages Fe - 1 alo Ge, Fe-3 alo Ge, Fe- 3,9 alo Sn, Fe- 5 alo Ga, Fe- 5,l alo Sb, Fe - 3 a10 Sb and Fe - 1 alo Sb. Les differences des champs hyperfins r6duits sont petites indiquant que les effets itinerants sont d'une importance majeure. Les temperatures de Curie sont determinees.

I1 parait qu'il y a une correspondance globale entre Tc et H(0,O) de ces alliages comparee au fer pur, indiquant que cette variance de Tc est contr616e aussi par des interactions non locales.

Abstract.

-

By temperature dependent Mossbauer experiments magnetic hyperfine fields

H((m, n) T) at different iron sites are determined in the alloys Fe - 1 alo Ge, Fe - 3 a10 Ge, Fe

- 3.9 a10 Sn, Fe

-

5 alo Ga, Fe

-

5.1 a10 Sb, Fe - 3 alo Sb and Fe - 1 alo Sb. The differences of the reduced hyperfine fields are small indicating that itinerant effects are of major importance. The Curie temperatures are determined. There appears to be a rough correspondence between Tc and H(0,O) of these alloys compared with pure iron, indicating that variation of TC is also governed by non local interactions.

1. Introduction. - In search for the origin of ferromagnetism in iron, not only pure iron itself but also and to a larger extent solid solutions, alloys and compounds with iron as an important constituent have been investigated by a variety of experimental methods and techniques such as Mossbauer effect, neutron diffraction, magnetization measurements, resistivity and specific heat measurements. Because of the favorable properties of iron with respect to the Mossbauer effect e. g. large hyperfine interactions relative to the linewidth, high recoilless fraction and the long life-time, this technique is a very suitable microscopic technique in this field of interest.

The non-transition elements Al, Si, Ga, Ge, As, Sn and Sb are assumed to be non-magnetic when dissolved in iron because of their electronic structure. Neutron diffraction measurements indeed do show no well localised magnetic moment at the impurity atom [I]. Some years ago these impurities were thought to act like pure magnetic dilutents, but more recent measurements suggest that this is not the case [2,3,4]. Moreover these data show a rather peculiar systematics along the rows of the periodic system for the impurity elements. The bulk magnetization measurements of Aldred show that A1 and Si behave as simple magnetic dilutents in agreement with the change of - 2.2 pB per guest atom

A1 or Si. For Ga, Ge and As a change of

-

1.4 p~

per guest atom is found, while this value for Sn and Sb impurities is

-

1 pB per atom. The deviation from simple magnetic dilution is also reflected in the Curie temperature of the corresponding dilute iron base alloys.

To investigate the effects of these non-transition impurities on the electronic and magnetic properties of dilute iron-base alloys as a function of impurity concentration and temperature and to check whether and in which way the systematics of the bulkmagne- tization measurements are reflected in the Mossbauer data, temperature dependent Mossbauer measurement were performed.

2, Experimental methods.

-

The alloys were pre- pared by melting together the pure elements (99.999

%)

in the desired composition in a high frequency furnace in hydrogen atmosphere or in vacuum, The alloys were checked with X-ray diffraction. From weight losses upon alloying deviations from the nominal composi- tions could be established, but in most cases the actual composition appeared to be the same as the nominal one.

Dependent on the kind and the concentration of the impurity the alloys may become too brittle to be rolled. In that'case the alloys were powdered by filing and

sieved to a maximum grain size of 20 p. The samples were quenched in hydrogen atmosphere in water from 800 O C to room temperature.

The spectra were recorded with a springless constant acceleration Mossbauer spectrometer and a 25 mCi source of Co5' in Cr matrix.

3. Experimental results and discussion.

-

The spectra obtained by temperature dependent Mijssbauer effect study on Fe - 3.9 a/o Sn, Fe - 5.1 a/o Sb, Fe

- 3 a/o Sb, Fe - 1 a/o Sb, Fe

-

5 a/o Ra, Fe - 3 a/o Ge

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C6-394 K. W. MARING, A. J. ALGRA, F. STUBBE AND F. VAN DER WOUDE and Fe

-

1 a10 Ge can be analysed in two different

ways. The alloys mentioned above have the b. c. c.- structure and assuming a random distribution of the impurity atoms, which can be achieved by a proper heat treatment, the probabilities of the different iron sites are given by a binominal distribution. In our analysis we included the first two shells, so H((m, n) T) will denote the magnetic hyperfine filed at an iron nucleus with m impurities in the first shell and n impurities in the second shell at a temperature T. Many authors have assumed additivity in their analysis, stating

and also

I.S.(m, n) = I.S.(O, 0)

+

m AI. S. (1,O)

+

n AI.S.(O, 1) where AH(1,O) denotes the change in magnetic hyperfine field with respect to H(0, 0) by the addition of one impurity in the first shell and AH(0, 1) the change by the addition of one impurity in the second shell. The additivity is generally assumed to be valid for the small impurity concentrations (< 5 alo). It is our impression however, that this validity depends rather strongly on the kind of impurity elements. Except for the analysis of the Fe

-

1 a10 Ge, Fe

-

3 a10 Ge, Fe

-

1 a10 Sb spectra, the extra constraint of additivity has not been used.

In this way we obtained the magnetic hyperfine fields H((m, n) T) at the different iron sites up to the Curie temperature. In figure 1 we plotted

H((m, n) T )

-

H((0, 0) T ) T

h((m, n) 7) = versus -

.

H((m, n) 0) H((O,O) 0 ) T c

A general trend of h((1,O) T) and h((0, 1) T) from liquid nitrogen temperature up to 1 000 K can be seen. Schurer, Maring Van der Woude and Sawatzky have pointed out how to relate the behaviour of h with the different, more or less, extreme models to explain ferromagnetism in iron 15-12]. The result of h((1, 0) T) and h((0, 1) T) being almost zero indicates that in these alloys the itinerant aspects are dominant in contrast with the local effects which are small. Quadrupole effects are small and neglected.

The sequence of the satellite peaks in the Mossbauer spectra is not always the same for the different alloys. At room temperature for Fe

-

5.1 a10 on 3 a/olSb

1

H(0, 1)

I

>

I

H(0,O)

I

>

I

H(1, 0)

I

applies whereas for Fe

-

3 a10 Ge both

and

yield a rather good curve fitting, though the- first method gives a better x2-value. For the other alloys the

Fr(;. 1. -The difference of the redueed magnetic hyperline fields at different iron sites

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result has been

I

H(0, 0)

I

>

I

H(0, 1)

1 >

1

H(1,O)

1.

Whether the differences in H(0, 1) and H(l, 0) have to be sought in different iron moments, which could be reflected in magnetization measurements, or in an oscillatory behaviour of the conduction electron polari- zation is not yet clear.

Another important parameter is the Curie temperature of these alloys. We measured these Curie temperatures by a thermal scanning procedure. Avoiding diffi- culties with the calibration of the thermocouple, we simultaneously measured the Tc of a pure iron sample and a FeX sample, yielding reliable values for the Curie temperature of the alloys with respect to pure iron. The results are given in figure 2.

Imp. Conc. I%)

-

FIG. 2. -The change of the Curie temperature of the various dilute iron-base alloys, with respect to pure iron, as a function of the impurity concentration. The experimental values have been determined by a thermal scanning procedure.

ATc =

~g~

- T:' ; FeSb + ; FeGe 0 ; FeSn ; FeAl _-x ;

FeAs _-

n.

~ h e G l i d line r2ers to pur~dilution.

The most striking feature in this result is the maximum in the FeSb and the FeGe curves 1131. Moreover the trends oBgure 2 are reflected in the H(0,O) of the different alloys at high temperatures (800 K-1 000 K) (Table I).

In the Fe

-

5.1 a/o Sb, Fe - 3 a10 Sb and Fe - 1 a10 Ge

alloys a Curie temperature higher than for pure iron is associated with magnetic hyperfine fields H(0,O) at iron sites without impurities in the first two shells at elevated temperatures, which are larger than corresponding hyperfine fields in pure iron. For Fe

-

3.9 a10 Sn and Fe

- 5 a/o Ga a Curie temperature lower than for

pure iron is associated with values of H(0, 0) which are smaller than the hyperfine fields in pure iron. Finally for Fe

-

3 a10 Ge and Fe

-

1 a10 Sb this correlation is not present.

The fact, that a rough correspondence between the behaviour of Tc and H(0, 0) exists, emphasizes the importance of the non-local interactions because H(0, 0) is determined mainly by non-local effects. Through a modified Zener-Vonsovskii model the change in Curie temperature dTc/dc can be related to the h((m, n) T) curves [5-121. Using this model a value dTc/dc =

-

1.3 K/at

%

has been calculated for non- magnetic impurities in iron, in agreement with the results of FeSn, FeGa and FeA1. The discrepancy of the value withthoseobtainedfor FeGe, FeSb and FeAs indicates the necessity of c h o Z g a different s z o f parameters for these alloys to account for their different behaviour of T, as a function of concentration. This may be found in a reduction of the s-d mixing [14] caused by some impurities like Ge and Sb. The s-d mixing is usually thought not to be important in iron in contrast to nickel [15]. However, for effects of the order of 1 percent or lower it should be considered. Since this would not strongly affect the magnetic moments this explanation also indicates why no correlation between changes in average magnetic moments and changes in Tc can be found.

Acknowledgements.

-

This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (Organization for Fundamental Research on Matter - F. 0. M.) and has been made possible by financial support from the Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek (Netherlands Organization for the Advancement of Pure Research - Z. W. 0.).

H((0,O)T)

EX

AH =

[

]

-

Y

G

]

at diffeeent temperaturesfor the various dilute iron-base ~~110);s. H((O,O) 0)

The experimental error in AH is 0.002

Temp. Fe

-

5 a10 Ga Fe

-

1 a10 Ge Fe

-

3 a10 Ge Fe

-

3.9 ajo Sn Fe

-

1 a10 Sb Fe

-

3 a10 Sb Fe

-

5.1 a10 Sb

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C6-396 K. W. MARING, A. J. ALGRA, F. STUBBE AND F. VAN DER WOUDE References

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