MÖSSBAUER LINE BROADENING IN HEXAGONAL ε-PHASE IRON ALLOYS

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MÖSSBAUER LINE BROADENING IN

HEXAGONAL ϵ-PHASE IRON ALLOYS

J. Williams, D. Pearson

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, .supple'ment au no 12, Tome 37, De'cembre 1976, page C6-401

MOSSBAUER LINE BROADENING IN HEXAGONAL

8-PHASE

IRON ALLOYS

J. M. WILLIAMS and D. I. C. PEARSON

Department of Physics, University of Sheffield, Sheffield S3 7RH, England

ResumB. - Pour essayer de comprendre les proprietes magnetiques de 1'6tat hexagonal de haute pression du fer (l'ktat E), les &tats equivalents des alliages du fer avec le ruthenium et l'osmium

ont etk Btudies au moyen de I'effet Mossbauer dans 57Fe. L'ktat hexagonal est stable dans ces alliages sans l'application des hautes pressions de l'exterieur. Les spectres Mossbauer, qui, a hautes tempb ratures, peuvent 6tre tous caracterises par une interaction quadrupble seule, s'elargissent a de tr6s basses temperatures. Cet Blargissement peut 6tre partiellement decrit comme des effets d'ipaississe-

ment, mais dans le cas des alliages de Ru, il serait aussi en concordance avec un ordre magnetique de champ hyperfine

-

13 kG.

Abstract. - In an attempt to understand the magnetic properties of the high pressure hexagonal phase of iron (&-phase), the equivalent phases of iron with ruthenium or osmium have been studied by means of the Mossbauer effect in 57Fe. The hexagonal phase is stable in these alloys without the application of large external pressures. The Mossbauer spectra, which at high temperatures, may all be characterized by a quadrupole interaction alone become broadened at very low tempe- ratures. This broadening may partly be described by thickness effects, but in the case of the Ru alloys would also be consistent with the existence of a magnetic ordering with a hyperfine field

-

13 kG.

1. Introduction.

-

Considerable interest has recently been shown in the magnetic properties of the hexagonal high pressure phase of pure metallic iron (E-Fe). Due to the high pressures (P > 130 kbar) required t o stabilize this phase direct experimental studies have proved difficult. It is well known that the b. c. c.-phase (a-Fe) orders ferromagnetically at high temperatures and the f. c. c.-phase (y-Fe) orders anti- ferromagnetically at

--

80 K. Recently however high pressure Mossbauer experiments have been succesfully carried out at temperatures down to 2.2 K [I, 21, but show little evidence of any magnetic ordering in the hexagonal phase.

Iron may be stabilized in the hexagonal &-phase at normal pressures by the addition of ruthenium or osmium and these alloys thus provide an alternative method of studying this phase. Results obtained from an early study of these alloys by Ohno [3], when extrapolated to pure &-iron, suggested an antiferromagnetic coupling with an internal field of 16 kG and a NCel temperature around 100 K. A more recent theoretical paper by Fletcher and Addis [4] however reports on a band theory calculation made for &-phase iron in which the Stoner criterion and a similar one for the occurrence of antiferromagnetism were applied and it was found that neither magnetic state would be predicted because of the low value for the density of states at the Fermi energy. These calculations also failed to predict any

antiferromagnetic coupling in the s-Fe alloys with Ru or 0s.

In this work we report a careful Mossbauer study of the &-phase alloys with Ru and 0 s using the "Fe y-resonance. Measurements have been carried out for various Ru and 0 s concentrations and at temperatures down to liquid helium.

2. Sample preparation.

-

Alloys of iron with up to 30

%

ruthenium or osmium were prepared by melting the relevant amounts of pure constituents in an argon arc furnace. They were then annealed at 1 400 OC for 9 hours and quenched into water. The hexagonal structures were then checked by x-ray analysis. Apart from the 15

%

Ru sample which showed a small second phase contribution, the x-ray patterns of all the alloys indicated no detectable phase other than the h. c. p. e-phase.

3. Experimental method and results.

-

Mossbauer absorption spectra were obtained for the various hexagonal alloys at temperatures ranging from 5 K

to 680 K. All the alloy spectra exhibit some broadening at low temperatures and typical spectra, measured at room temperature and at 5 K, are shown for a range of alloys in figures 1-4. With the exception of the 15

%

Ru alloy the high temperature data may be satisfactorily fitted assuming a small quadrupole interaction. The

26

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C6-402 J. M. WILLIAMS AND D. I. C. PEARSON I I . RELATIVE VELOCITY l m m 1 s - 1 0 I RELATlVE VELOCITY ( mm / s) (bl

1

I I 0 1 R E L A T I V E M L O C I T Y i m m 1 s )

FIG. 1. - 57Fe Mossbauer absorption spectra for Feo.7Ruo.3 at (a) 298 K and (b) 5 K, relative to S7Co in Rh source at 298 K.

room temperature fitted parameters are listed in table I. The 15

%

Ru spectrum (Fig. 2) shows a small contamination from another phase consistent with that expected from the x-ray analysis, the two weak satellite lines being, probably, the inner lines of a small 6 -line u-phase contribution. The values for the isomer shift and quadrupole splittings when extrapolat-

ed to zero Ru or 0 s concentrations are aIso listed in table I and it is seen that they are in very good agreement with the zero pressure extrapolated values obtained from the pure &-iron results, i. e.

I. S. (relative to a-Fe) =

-

0.14

mm

s- l ;

obtained by Williamson et al. [I]. These results thus

Host A F e ~ .7 RuO .3 F e ~ . 8 5 Ru0.15 Fe1,o Ruo e-Fe Ref. [l] Fe0.7 Os0.3 Fee.,, Oso.15 Fe100 0 % Temp. 300 K 300 K 300 K 300 K 300 K 300 K 300 K 1 I 0 I RELATIVE VELOCITY (mm 15

Flo. 2. - 57Fe Mossbauer absorption spectra for Fe0.85Ru0.15 at (a) 298 K and (b) 5 K, relative to 57Co in Rh source at 298 K.

confirm the similarity of these hexagonal alloys to &-iron and justify their choice in this study.

The broadened low temperature spectra cannot, in general, be fitted satisfactorily with a quadrupole doublet alone, so other causes of broadening have to be considered. Firstly we consider thickness broadening due to saturation effects in the absorbers. For small value of the effective thickness t = nf, go (0

<

t

<

5) the broadening is given approximately by

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increased value of fa. In the absence of any measured values we shall assume that its value is approximately

fa

(5 K) = 0.7 in accordance with the value measured at 4.2 K for pure iron 151. Substituting the equivalent value of t into equation (1) gives a value of

for the broadened value at 5 K. The low temperature 0 s alloy spectra may be fitted in terms of a broadened quadrupole doublet and the broadening could be accounted for by thickness effects given the large f values assumed. The low temperature Ru alloy spectra cannot, however, be satisfactorily fitted as quadrupole doublets. The line shapes are different and the broadening is much larger than that which could be accounted for by increased thickness effects. [Apart from the incorrect line shapes, values of fa

>

1 would be required.]

We are thus forced to examine other sources of broadening in the low temperature Ru alloy spectra and the obvious choice is magnetic broadening. The interpretation however is complicated by the fact that any magnetic hypedine interaction present would be comparable in magnitude to the quadrupole interaction which is already known to exist from the high temperature data. One has also of course to allow for any temperature dependance of this quadrupole coupling in the interpretation of low temperature data. It is

I I I - 1 0 I RELATIVE VELOCITY (mm I s ) I

-

1 0 1 RELATIVE VELOCITY ( m m l s )

known from measurements of electric field gradients at Fe in h. c. p. transition metals [6] that their values change only by about, at maximum, 10

%

between 4.2 K and 300 K. We may thus be justified in assuming a low temperature quadrupole coupling of about 10

%

greater than the measured room temperature value.

The interpretation of Mossbauer spectra of "Fe nuclei subjected to a combined magnetic dipole and electric quadrupole interaction is a complicated pro- blem and in general a unique solution cannot be obtained [7]. The situation is further complicated in our case by the lack of resolution in the spectra resulting from the compariability of the hyperfine splittings and the line widths. In order to obtain some estimate of the magnitude of these interactions we have used the method of analysis suggested in 171. Thus the low temperature spectra, for the Ru alloys, were fitted with a least squares fitting proceedure to the sum of eight Lorentzian lines whose positions are constrained to be consistent with the expected energy level scheme so that only five position parameters are involved. [See Fig. 1 of ref. 171.1 For non-saturated powder spectra the number of intensity parameters may also be reduced to five. Each spectral component was given the same line width (fixed at a suitable value to include thickness broadening) and in this way one may calculate H from the ground state splitting, in addition to the quadrupole splitting (2 8 ) and the isomer shift. In

I

- I

4

1

RELA1 lVE VELOCITY ( m m l s )

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C6-404 J. M. WILLIAMS AND D. I. C. PEARSON

general, by comparing the line positions and relative intensities with those calculated by Kundig [S], the range of possible y-values and associated values of the spherical angle 0 are estimated. Preliminary fits to the

5 K 30 %-Ru alloy spectrum, shown in figure lb, which we observe to be symmetrical in shape, indicate a set of parameters consistent with an axially symmetric field gradient (y = 0) accompanied by an internal magnetic field H

-

13 kG orientated at 8

--

50° to the symmetry axis. The parameters, when compared with the Kundig plots, were adjusted in a self-consistent way until the best fit was obtained.

This fitting proceedure will be improved and similar

fits made of spectra measured at increasing temperatures. Full results will be published at a later date. 4. Discussion. - All the spectra show some broadening at low temperatures, but in the 0 s alloys this broadening can mainly be accounted for by an increase in the effective thickness. However, for the Ru alloys, thickness effects only partially explain the observed broadening and a preliminary analysis indicates the possibility of magnetic ordering with a corresponding hyperfine field H

--

13 kG orientated at about 500 to

the axis of symmetry. The study is continuing and further results will be published later.

References

[I] WILLIAMSON, D. L., BUKSHPAN, S. and INGALLS, R., Phys. [5] WILLIAMS, J. M. and BROOKS, J. S., Nucl. Instrum. Meth.

Rev. B 6 (1972) 4194-4206. . . 128 (1975) 363-372. . .

[2] KONIG, K., WORTMANN, G. and ~ L V I U ~ , G . M., Proceedings _[6]_WORTMANN,_G._{and WILLIAMSON,}_D._{L., Hyperfine Interuc-}

of the International Conference on Mossbauer Spec- _tion_{1 (1975) 167-176.} troscopy, Cracow, Poland 1975, Vol. 1, 189.

[3] OHNO, H., J. Phys. Soc. Japan 31 (1971) 92-101. [7] VAN DONGEN TORMAN, J., JAGANNATHAN, R. and T ~ o o s -

[4] FLETCHER, G. C. and ADDIS, R. P., J. Phys. F : Metal TER, J. M., Hyperfne Interactions 1 (1975) 135-144.