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MÖSSBAUER EXPERIMENTS ON DILUTE IRON-BASED ALLOYS WITH NON-TRANSITION
ELEMENTS
L. Cser, I. Vincze
To cite this version:
L. Cser, I. Vincze. MÖSSBAUER EXPERIMENTS ON DILUTE IRON-BASED ALLOYS WITH NON-TRANSITION ELEMENTS. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-787-C1-789.
�10.1051/jphyscol:19711276�. �jpa-00214108�
JOURNAL DE PHYSIQUE Colloque C 1, supplkment au no 2-3, Tome 32, Hvrier-Mars 1971, page C 1 - 787
MO s SBAUER EXPERIMENT s ON DILUTE IRON-BASED ALLOY s
WITH NON-TRANSITION ELEMENTS
L. CSER and I. VINCZE
Central Research Institute for Physics, Budapest, Hungary
Rksum6. - La distribution de champ hyperfin et le dkplacement isomerique causk par une impuretb unique dans le fer ont kt6 examinks dans le cas des contaminations de Al, Si, Ga et Ge. Nous avons essay6 de mettre au point les causes du desaccord entre les donnQs obtenues par l'effet de Mossbauer et celles observkes par divers techniques de RNM. La diffkrence entre les d6placements des champs hyperfins et les changements dans le dkplacement isomkrique dans Fe-A1 et Fe-Si sont discut6s en termes du modkle de l'effet d'6cran de charge. Dans le cas des impuretks de Ga et Ge les change- ments des champs hyperfins et des dkplacements isomkriques sont expresskment differents de ceux observks pour A1 et Si.
Abstract. - The hyperfine field distribution and the isomer shift caused by a single impurity in iron was investigated in the case of Al, Si, Ga and Ge. An attempt was made to clear up the disagreement between the data obtained by Mossbauer effect and those observed by various NMR techniques. The difference between the shifts of the hyperfine fields and the changes of the isomer shifts in Fe-A1 and Fe-Si are discussed in terms of the charge screening model.
In the case of Ga and Ge impurities the changes of the hyperfine fields and the isomer shifts are markedly different from those observed for Al and Si.
I. Introduction. - In the alloyed iron matrix the magnetic moment and the charge distribution of iron are perturbed in the environment of the impurity atoms. The measure of the perturbation can be assessed by different experimental techniques (ME, SE and cw NMR) by which one observed changes of the hyperfine field at the iron atoms surrounding the impurities. These changes reflect the perturbation of the spin density. In addition the Mossbauer effect shows the changes in the s-electron charge density
as it can be inferred from the measured isomer shift.
The nature of the perturbation can be well studied in the case of alloys containing A1 or Si impurity, since for these elements no perturbation of the d-band has been observed. A systematical discrepancy exists, however between the ME [I] and SE [2, 31 data on the magnitude of the hyperfine field shifts at the second and third neighbours of the impurity atom (Table I).
a) A1 impurity Coordination
spheres of the
neighbours 100 A H/H, (*) 100 A H/Ho (**) 100 A H/Ho (* * *) A i/(mm/s)
- - - - -
1. - 7.0 + 0.2 - 7.0 , 0.2 - 7.1 + 0.1 0.017 0.002
2. - 3.7 f 0.3 - 1.7 + 0.2 1.5 + 0.2 - 0.001 +_ 0.005
3. 1.3 + 0.2 0.9 + 0.3 - 2.1 + 0.1 - 0.009 + 0.002
4. - 0.1 f 0.1 0.5 +_ 0.3 (****) 0.007 + 0.003
5. - 0.4 f 0.2
b) Si impurity Coordination
spheres of the
neighbours 100 A H/Ho (*) 100 A H/Ho (**) 100 A H/Ho (***) A i/(mm/s)
- - - - -
1. - 8.0 4 0.3 - 8.0 3- 0.2 - 8.1 + 0.1 0.041 + q.002
2. - 3.8 +_ 0.7 - 1.8 3- 0.4 1.9 + 0.3 - 0.007 + 0.005
3. 1.9 4 0.5 1.3 + 0.3 - 2.2 & 0.2 - 0.002 +_ 0.002
4. 0.0 4 0.7 0.6 + 0.3 (****) 0.001 + 0.003
(*I ')[ I ME.
(**) [2] SE.
(***) Present ME.
(****) The hyperfine fields and isomer shifts at the 4th neighbours were estimated from the average values of these shifts.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711276
C 1 - 788 L. CSER AND I. VINCZE In the present paver some possible causes of this AH(r) . , z discrepancy are coniidered in -the case of A1 and Si 1
rcos (2 KF r) - sin (2 KF r) (2 KFrll.
impurities inquiring in addition, into the possible r
nature of t h e he measurements on 2,,d ---
samples containing Ga and Ge impurities show an
appreciable perturbation of the d-band of iron, too. Ai(r) x - [cos (2 K~.r)/(2 KF r) + sin (2 K ~ . r ) ] l / r ~ 11. Experimental. - The Mossbauer spectra were
obtained generally at room temperature. As a rule powder samples of 22 y and 56 p were used. Only one of the specimens was FeAl (6 at. % Al) plate rolled to 40 y thickness.
The spectra were decomposed by the least means squared method assuming that the hyperfine field shifts and the changes of the isomer shift to be additive and the impurity atoms distributed at random.
To give an account of the possible causes of the discrepancy between the ME and SE data of the following effects were investigated.
a) Different experimental temperatures were used in the reported measurements (ME at room temperature, SE at liquid helium temperature'). The present spectra measured at liquid nitrogen temperature did n o t differ from those taken at room temperature.
b) The tickness of the sample causes a deviation of the Mossbauer line shape from a Lorentzian curve.
For this reason an exact calculation of the absorption curve was made [4]. Though the difference between the ME and SE data could be reduced somewhat by this way, the discrepancy remained still appreciable.
c) An ordering of the impurity atoms might also influence the parameters of the Mossbauer pattern.
However, the agreement between the data obtained at different concentrations precludes an ordering effect.
d) The best agreement between the ME and SE data was found by assuming that the satellite, attributed by [2] to the second neighbours of the impurities, is the contribution of the third neighbours and vice versa.
The results are listed in Table I.
In Table I1 the values of dH/dc, dildc and the per- turbations of the first neighbour of Al, Ga, Si or Ge impurity are listed (where H and i are the hyperfine field and isomer shift, respectively ,and c is the impu- rity concentration).
111. Discussion. - Two models are available to evaluate the perturbation of conduction electrons by impurity atoms.
a) In terms of the RKKY model [5] the perturbation is interpreted in first approximation as the spin density oscillation of the conduction electrons around the impurity, while in the second order approximation [6]
also a charge density oscillation arises. The depen- dence of these oscillations on the distance from the impurity site gives
where AH(r) and Ai ( r ) are the changes of the hyperfine field and the isomer shift, caused by the impurity, respectively, and KF is the Fermi vector. The amplitude of the oscillations is determined by the strength of the s-d exchange interaction.
b) In terms of the charge screening model [7, 8, 91 the excess charge of the impurity is screened by the oscillating conduction electron density polarized by the s-d exchange. The difference between the numbers of spin-up and spin-down electrons leads to a spin density oscillation. If the s-d exchange energy is negli- gible relative to the perturbation potential of the impurity, the screening in the two sub-bands is similar, and the distance dependence of the oscillations has the form
AH(r) z sin (2 KF r + q)/(2 KF r)' (2) Ai(r) z [cos (2 KF r + q)] . l/r3
Here cp and oscillation amplitude are determined by the potential due to the impurity.
Calculating with the reported values of KF, cp and the amplitude (e. g. [I, 9, 101) it was found that the measured hyperfine field and isomer shift (spin density and charge density) oscillations cannot be satisfactorily described by either of the above models.
The RKKY model does not yield any explanation of the difference between the oscillation amplitudes of the hyperfine fields and isomer shifts observed for A1 and Si. In the charge screening model the difference between the A1 and Si potential results in a larger oscillation amplitude and phase shift difference than observed.
The difference between the observed data and the values calculated with the use of the above models might be due to the asymptotic caracter of the function by which the distance dependence of the perturbations is approximated. The use of preasymptotic forms could perhaps improve the description [l 11.
A much more important error seems to be due to the approximation, that the energy of the s-d exchange is taken to be negligible as compared to the perturba- tion potential of the impurity. If this approximation does not hold, the wave functions of the screening electrons in the two sub-bands can be considerably different and then the distance dependence of the oscillations is inconsistent with the description (2).
As apparent from Table I1 in spite of the similar excess charges of A1 or Ga, and of Si or Ge, there is
a great difference between the magnitudes and distri- Acknowledgments. - Authors are indebted to Prof.
butions of the perturbations caused by the different L. Psi1 for his interest in this work, to Dr C . Hargitai impurities. This can be attributed to the fact that the and Dr G . Griiner for stimulating and informative d-electrons of Ga and Ge appreciably interact with discussions, to Dr I. Gladkih for the decomposition .the d-band of iron and consequently the above models programs, to Dr G. Konczos for preparing the samples applying to conduction electron perturbation cannot and to Dr D. L. Nagy for participation in the mea-
be used for Ga and Ge. surements.
References
[I] STEARNS (M. B.), Phys. Rev., 1966, 147, 439. [6] EVERTS (H. U.) and GANGULY (B. N.), Phys. Rev.
121 RUBINSTEIN (M.), STAUSS (G. H.) and STEARNS (M. B.), 1968, 174, 594.
J. Appl. Phys., 1966, 375, 1334. [7] DANIEL (E.), Hyperfine Interactions, Academic [3] MURPHY (J. J.), BUDNICK (J. I.) and SKALSKI (S.), Press N. Y. 1967.
J. Appl. Phys., 1968, 39, 1239 ; BUDNICK (J. I.), [8] CAROLI (R.) and BLANDIN (A.), J. Phys. Chem. Solids, Colloqu. Ampere XV, North-Holland, Amster- 1966, 27, 503.
dam, 1969. [9] GRUNER (G.) Solid State Comm., 1969, 7 , 1421.
[4] SHIRLEY (D. A.) and KAPLAN (M.), Phys. Rev., 1961, [lo] KOHN (W.) and V o s ~ o ( S . H.), Phys. Rev., 1960,
123. 816. 119. 912. 7 -
151 RUDERMAN(M. A.) and KITTEL (C.), Phys. Rev., 1954, [Ill ALFRED (L. C. R.) and VAN OSTENBURG (D. O.), 96, 99 ; KASUYA (T.), Progr. Th. Phys., 1956, Phys. Letters, 1967, 26A, 27.
16,45 ; YOSIDA (K.), Phys. Rev., 1957,106,893.
