Solid State Communications, Vol. 17, pp. 639—641, 1975 Pergamon Press. Printed in Great Britain
MöSSBAUER MEASUREMENTS IN IRON BASE ALLOYS WITH Be, Cu AND Au IMPURITIES I. Vlncze
Central Research Institute for Physics, Budapest, Hungary and
A.T. Aidred
Argonne National Laboratory, Argonne, IL 60439, U.S.A.
(Received8May 1975 byA. Zawadowskl)
An initial determination of the hyperfine field and isomer shift at iron sites in dilute hon-base alloys containing Be, Cu and Au impurities is presented.
The data on the charge and spin perturbation are compared with those ob- tained earlier for non-transition metal solutes, and it is concluded that the effect of Be is similar to other nontransition metal solutes, whereas the ef.
fects of Cu and Au are similar to those of transition metal solutes.
THE MAGNETIC perturbation caused by nonmagnetic measurements have been published on iron-base alloys impurities in iron is thought to be simple, because of with nontransition elements (see references in refer- the absence of a magnetic moment localized at the ence 4), the effect of the above-mentioned impurities impurity site. Although the electronic structure of has not been studied, except for a preliminary report5 these impurities is relatively uncomplicated, i.e., non- on ~Au.
existent or fully occupied d-shells, simple dilution of
the magnetization (dji/dc=—2 .2MB/atom) occurs The present study may make an important con- only for Be,’ Al and Si.2 In the case of other nonmag- tribution to our understanding of the charge-screening netic impurities in iron, greater or smaller deviations mechanism because the solutes Be, Al and Si have dif- from simple dilution are found. In addition to average ferent outer electron configurations (252, 3s2p and magnetization and diffuse neutron-scattering measure- 3s2p2, respectively), whereas they each produce simple ments, Mossbauer investigations can give useful infor. dilution of the average magnetization when added to mation about the nature of the perturbation due to iron as impurities. From this point of view, it is inter- these nonmagnetic impurities. esting to compare the effect of Be with that of Cu as
an impurity because Cu also causes a magnetic pertur- We present the results of our MOssbauer measure- bation very near simple dilution (dji/dc= —2.OJiB/CU
ments on dilute FeBe, FeCu and ~Au alloys. The atom) despite its filled 3d shell. Although Au impurities alloys prepared for earlier magnetization measure- have a filled 5d shell, they do not behave as simple ments1’3 were used. These samples have been re- diluents (dji/dc=— 1.1MB/Au atom).3 The only ex- examined metallographically and no precipation was planation for this is an increase in the neighboring observed, which is in agreement with the present iron moments.
Mossbauer results. Although many MOssbauer
___________ The experimental apparatus and the computer
~ supported in part by the U.S. Energy Research technique used for the evaluation of the Mossbauer and Development Adminstratlon. spectra have been described previously.4 The only
639
640 IRON BASE ALLOYS WITH Be, Cu AND Au IMPURITIES Vol. 17, No.5 Table 1. Change ofthe hyperfinefield z~H1and isomer shift ~i1 at thejirst iron
neighbours ofa Be impurity; the total change of the hyperfine field dH/dc and of the isomer shift d I S/dc are also given
Concentration ~ dH/dc d IS/dc
(at.%) (kOe) (mm/sec) (kOe) (mm/sec)
0.74 — 17.6(0.4) 0.005(4) — 165(10) 0.04(4)
6.52 — 18.7(0.3) 0.01 1(2) — 170(10) 0.00(4)
10.69 — 20.0(0.3) 0.009(2) — 180(5) 0.00(3)
FeBe: The Mossbauer spectra are similar to those for other nontransition element solutes in iron,4 that
f~Cu is, they have observable satellites,andonly the effects of first neighbor impurities could be reliably deter-
R[kOe] ~~f5(10)~J~ ~ mined. The data are given in Table I.
C FeCuand FeAu:No satellites were observed; even the line broadening is quite small. The values of the
0 average hyperfine field and isomer shift obtained from
o 1 2 3 0 2 ~ the fit of a single six-line pattern are shown in Figs. 1
c~%Cu cit%Cu and 2.
FIG. 1. Average hyperfme field and isomer shift at iron We will compare these results with those4 obtained
atoms in FeCu as a function of Cu concentration. . . .
— on dilute iron alloys contaming other nontransition
metal impurities. This comparison is especially inter-
FeAu esting for the effects of Be, AlandSi impurities. As it is well known, AlandSi solutes cause a somewhat
~ different perturbation4 of the hyperfme field and
isomer shift at the iron sites. The changes at the first iron neighbors of the impurity are =—22.6 and
3~ ,,/I ~~9~lOe- ~2 083(5)~ —26.3 kOe;respectively. Two explanations are given for the dif-i~i1= 0.023(3) and 0.040(3) mm/sec
~ 11
, 0 _________________ ference: (1) According to the first model,6 this iso 2 4 6 0 2 4 6 caused by an overlap contribution, i.e., Al enlarges
at%Au whereas Si contracts the iron lattice; the relative changes
FIG. 2. Average hyperfme fieldandisomer shift at iron in the lattice parameter are7 1/a(cia/dc)= + 0.050 atoms in FeAu as a function of Au concentration. and—0.028, respectively. (2) The difference in charge
screening, because Si has a larger excess charge than Al, can explain8 the observed tendencies in both the change in procedure is that now we have used a perturbations of isomer shift and hyperfine field. Thus, 50 mCi57Co in Pd source for the measurements (pre. the effect of Be as an impurity, whichhasan outer pared by The Radiochemical centre, Amersham, electron number one less than that of Al but contracts England). The spectra taken on powder samples(grain the iron lattice two times more strongly than Si [1/a size < 50 Mm) contained 500—1000 x iO~counts (cia/dc)= — 0.052J~is of crucial importance as regards per channel,andthe depth of the outerpeakswas the twomodels mentioned.
-~40.-60 x iO~counts per channel in each 1024-
channel spectrum, which was collected during a I to According to our measurements, Be impurities
2-day run, cause a decrease of~.H1=—(18.7±0.3) kOe and an
increase of&1 =(0.009 ±0.002) mm/sec in the hyper.
The results may be summarized as follows: fine field and isomer shift of the first neighbor iron
Vol. 17, No.5 IRON BASE ALLOYS WITH Be, CuANDAu IMPURITIES 641 atoms, respectively. These values clearly demonstrate in iron. However, a relatively large differenceexists the absence ofanycorrelation with the overlap, which between the measured value of dR/dc= —(55±10) kO therefore should be quite small, and, at the same time, andthe value of— 145 kOe calculated on the basisof they are in good agreement with the expectation based equation (12) in reference 10. The value of dR/dc deter on the charge-screening model. According to the model, mined here is in a very good agreementwiththat~-’
the contribution from the charge screening are expected obtained at 77 K onhighlysupersaturated (metastable) to be proportional to i~Z,the excess charge, exactly Fe-rich bcc ~Cu thin filmalloys produced by simul- what was found. On the other hand, the existence of taneous vapor deposition of FeandCu onto substrates.
a relativelyimportantcharge-screening contribution Moreover, the fact that the isomer shift increases and to the iron hyperfme field distribution around the the hyperfme field decreases as Cu is added to iron is nonmagnetic impurity calls into question the identi- opposite to the effect of the transition—metal impurities fication6 of this hyperfine field distribution as a simple
RKKY oscillation in the conduction electron band. The effect of Au as a solute is rather surprising.
This is the first case known to us in which dR/dcand Despite the almost identical average manetization dNdc have different signs. The increase in the average decrease due to BeandCu impurities, Cu causes a sig- iron hyperfine field and the absence ofanysateffites nificantly smaller decrease in both the hyperfme fields (even thelinebroadening is quite small, 10% at at the first iron neighbors and the average iron hyper- 6 at.% Au) together suggest a long-range perturbation fme field. The absence ofthe smallfield satellite, which of the neighboring iron atoms, which is in keeping is observed for all non-transition metal impurities, with the relatively large change in the isomershift.
shows that the Cu solutes behave more like transition The calculated value of dH/dc is— 10 kOe,andthe metal impurities. Thisis supported by the correlation agreement with the measured value (90 ±10) kOe is that exists between the isomer shift of Fe in copper similarto that obtained for other impurities in the 5d andthe isomershiftchange caused by nearest Cu series.
neighbors at iron sites, as extrapolated from the total change of the isomer shift dTh/dc. A similar correlation
9 .. Acknowledgement—Weare pleased to acknowledge
hasbeen found for almost all transition metal solutes the very stimulating discussions with Dr. l.A.Campbell.
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