CORRELATION OF ISOMER SHIFT AND HYPERFINE FIELD IN AMORPHOUS (Fe0.2Co0.8) 80P17Al3

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CORRELATION OF ISOMER SHIFT AND

HYPERFINE FIELD IN AMORPHOUS (Fe0.2Co0.8) 80P17Al3

C. Chien, H. Chen

To cite this version:

C. Chien, H. Chen. CORRELATION OF ISOMER SHIFT AND HYPERFINE FIELD IN AMOR- PHOUS (Fe0.2Co0.8) 80P17Al3. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-118-C2-119.

�10.1051/jphyscol:1979240�. �jpa-00218638�

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JOURNAL DE PHYSIQUE Colloque C2, supplkment au n o 3, Tome 40, mars 1979, page (22-118

CORRELATION OF ISOMER

SHIFT AND HYPERFINE FIELD IN

AMORPHOUS ( ~ e ~ . 2 ~ 0 0 , 8)80~1 7 ~ 1 3 + C.L. Chien and H.S. Chen X

Department o f Physics, Ths Johns Hopkins University, Baltimore, MaqZand 21 218, U. S. A.

% ~ e Z l Laboratories, Murray H i l l , New Jsrsey 07974, U. S. A.

Rdsum5.- Le compos6 magndtique amorphe (Fe,, 2C00 8)80P17A13 prdsente un spectre hyperfin magndtique trSs assymdtrique. On a montrd que cette as;ymdt;ie est le rdsultat de la distribution P la fois des ddplacements isomeriques et des champs hyperfins, et de la corrdlation lindaire I.S.=(I.S.),,+b(H-H,).

On a ddtermind la valeur de b = -(2,5+0,5) 10-~mm/s-k0e. Le spectre assymdtrique ne peut pas Ctre ex- pliqud par une interaction quadrupole rdsiduelle.

Abstract.- Amorphous magnetic (Fe,, 2C00 8)80P17A13 exhibits a highly asymmetric magnetic hyperfine spectrum. This asymmetry is shown to be'the result of distributions of both isomer shifts and hyperfine fields, and a linear correlation of I.S. = (I.S.),,+b(H-no). The value of b = -(2.510.5)10-3mm/s- kOe has been found. The asymmetric spectrum cannot be explained with a residual quadrupole interaction.

1. Introduction.- In amorphous magnetic solids there are a very large number of inequivalent atomic sites.

The resultant magnetic hyperfine spectrum usuallyhas broad line widths primarily due to a distribution of hyperf ine fields {P (H) ) / 1 /

.

The effective quadrupde interaction is expected to be negligible because of the spatial averaging of its directional dependence 121. However, it is sometimes observed that the intensities and the apparent line widths of the 5 7 ~ e spectrum are slightly asymmetric about the centroid;

e.g. in amorphous Fee,, B,,, /3,4/, and (FexNil-J Pl,, B6 151. Although a residual quadrupole interaction has been suggested as a possible cause 141, we show in the present case that it is more likely to be the result of a distribution of isomer shifts (p(1.S.)) and more importantly, a direct correlation of P(H) and P(1.S.).

2. Results and discussion.- Amorphous (Fe,,

.

2 C ~ o . 8)

PI7Al3 has been chosen for its narrow P(H), high value of TC(665 K) and low Fe concentration so that the spectral lines are not significantly affected by the thick-absorber effect. The magnetic hyperfine spectrum at 4.2 K, as shown in figure 1, exhibits a pronounced-asymmetry;

s.

the intensities and the apparent line widths of the spectral lines on the right-hand side are significantly different from the corresponding ones on the left-hand side. The posi- tions of the six lines are very symmetric about centroid with a negligible effe,ctive quadrupole interaction { (e2q~)eff 2 0.05 rn/~). This argues against the possibility of residual quadrupole interaction being responsible for the large asymmetry in the spectrum.

+work supported by NSF Grant No. DMR77-09598.

Fig. 1 : ~Essbauer spectrum of amorphous

!Fe,.,Co,. ,),,P,,Al, at 4.2 K(top). The solid curve 1s the result of a linear correlation of P(H) and P(1.S.) shown in the bottom. The scale for I.S. is relative to that of a-Fe at 4.2 K.

From the random atomic arrangement in amorphous solids, it is reasonable to expect a distribution of isomer shifts (p(1.S.)) as well as a distribution of hyperfine fields {P(H)}. However if these two distributions are independent of each other it is easy to show that, regardless of the shapes of the distributions, the resultant spectl-um would be a symmetric pattern. Thus the asymmetry in the spectrum suggests a correlation between P(1.S.) and P(H). Without an a priori knowledge of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979240

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the functional dependence of isomer shift on H, one can expand the isomer shift as a power series I.S. = (I.S.), + b (H-H,) +

....

⁽¹⁾

where (I.S.), and H, are the most probable isomer shift and hyperfine field respectively. The quadra- dic and higher terms are not used in the present case since the linear term appears to be adequate.

In order to estimate the value of b, some theoretical spectra have been calculated as shown in figure 2. For these spectra the P(H) has been assumed to be a Lorentzian about H,. The Fe atoms with hyperfine field H are assumed to have an isomer shift described by equation (I), and the values of b =

(0, - 1

,

-5 and

-

¹⁰⁾10-~mm/s-k0e have been used. It

is clear that the asymmetry in the spectrum depends critical on the value of b. Furthermore, the features exhibited by the spectra with b between -0.001 and -0.005 mm/s-k0e resemble remarkably well to those of the experimental spectrum shown in figure I.

with the P(H) shown in the bottom of figure 1. Becau- se of the assumption of a linear relation as described by equation (I), the P(1.S.) also has the same shape. The peak values of Ho = 315

*

⁵^{kOe and}

(I.S.), = 0.08

+

0.02 mm/s (relative to a-Fe at 4.2K) and the proportionality constant of

b =

-

^(2.5^f^0.5) ^-1s-k0e ⁽²⁾

have been determined. The negative sign of b indica- tes that the Fe atoms which have smaller isomer shifts have larger hyperfine fields. This conclusion is consistent with the results obtained from a number of series of amorphous magnetic systems /5,7/.

In conclusion, we have shown that the asymmetry in the magnetic hyperfine spectrum of amorphous

(Fe,. 2C00. 7A1 can be well accounted for by the existence of distribution of isomer shifts and its correlation with a distribution of hyperfine fields, without invoking a residual quadrupole interaction.

References

/I/ Sharon, T.E. and Tsuei, C.C., Phys. Rev.

(1972) 1047.

/ 2 / See e.g. Wertheim, G., "Mb'ssbauer Effect : Prin- ciples and Applications", (Academic Press, New York) 1964.

/3/ Chien, C.L., Phys. .Rev. B A , (1978) 1003.

/4/ Vincze, I., Solid State Commun.

5

( 1 978) 689.

/5/ Chien, C.L., Musser, D.P., Luborsky, F.E.,Becker, J.J. and Walter, J.L., Solid State Commun.

(1977) 231.

161 Window, B., J. Phys.

2

(1971) 401.

/7/ Chien, C.L., Musser, D., Luborsky, E. and Walter, J.L., this Conference.

Fig. 2 : Calculated spectra due to a correlation of P(H) and P(1.S.) with a relation of I.S. = (I.S.), + b (H-H,). The P(H) is taken to be a Lorentzian about H, = 315 kOe with a HWHM = 25 kOe. Values of b =

(0, -1, -5 and -10) 10-~mm/s-k0e have been used.

We have fitted the spectrum of amorphous (Fe,. 2Coo. ,) aoP, ,A1, by including both P(H) and P(1.S.) in the fitting routine with the two distributions related by equation (1). The Fourier series method developed by Window 161 has been used to expand the two distributions eash in 15 terms of a cosine series. No effective quadrupole interaction has been included. The best-fit results, as shown by the solid curve in figure 1 , have been obtained