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MÖSSBAUER STUDIES OF FeCu THIN FILMS
W. Keune, J. Lauer, D. Williamson
To cite this version:
W. Keune, J. Lauer, D. Williamson. MÖSSBAUER STUDIES OF FeCu THIN FILMS. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-473-C6-476. �10.1051/jphyscol:1974698�. �jpa-00215855�
JOURNAL DE PHYSIQUE Colloque C6, suppliment au no 12, Tome 35, DCcembre 1974, page C6-473
MOSSBAUER STUDIES OF FeCu THIN FILMS
W. KEUNE, J. LAUER and D. L. WILLIAMSON (*)
Fachbereich Angewandte Physik, Universitat des Saarlandes, 66 Saarbriicken, W. Germany
R6sum6. - Des alliages mktastables FeCu ont etk obtenus par condensation simultanke de vapeurs de Fe et de Cu sur des substrats a 295 et 77 K. Les phases suivantes ont BtC identifiks par spectroscopie Mossbauer et microscopie Blectronique : 0-40 at % Cu, alliage CC ; 40-65 at % Cu, melange d'alliages CC et CFC ; 65-100 at % Cu, alliage CFC. Pour les alliages CC on a determine la diminution du champ hyperfin (hf) a 77 et 295 K par plus proche et second voisin, respectivement.
Une tempkrature de Curie trks basse T,
-
600 K a pu &re estimBe pour une concentration c de cuivre aussi faible que 3 %. Pour des valeurs plus elevkes de c, hf reduit moyen ne suit pas la relation 1-0,42 c donnk par Stearns. Cet Ccart peut s'expliquer par une agrkgation des atomes de cuivre, et le parametre moyen d'ordre a courte distance Z est estime a+
0,7. Les alliages CFC rkvklent un ordre magnetique 4,2 K, probablement dfi des agrkgats de fer dans la matrice de cuivre.Abstract. - Metastable FeCu alloys have been produced by simultaneous vapor deposition of Fe and Cu onto substrates at 295 and 77 K. The following phases have been identified by Mossbauer spectroscopy and electron microscopy : 0-40 at % Cu, bcc alloy ; 40-65 at % Cu, mixture of bcc and fcc alloys ; 65-100 at % Cu, fcc alloy. For the bcc alloys the decrease in the hyperfine field (h. f.) at 77 and 295 K per nearest and next-nearest neighbor, respectively, has been determined. A very low Curie temperature Tc
-
600 K could be estimated for such a small Cu concentration c as 3 %.For larger c the reduced average h. f. does not follow the relation 1-0.42 c given by Stearns. This deviation can be explained by clustering of Cu atoms, and the average short-range order parameter 5 is estimated to be
+
0.7. FCC alloys show magnetic order at 4.2 K, p-obably due to Fe clusters in the Cu matrix.This article reports results of Mossbauer studies on highly supersaturated (metastable) Fe rich FeCu thin film alloys, and to a lesser extent of Cu rich alloys, produced by simultaneous vapor deposition of Fe and Cu onto substrates at 295 and 77 K [I]. Emphasis has been given to the investigation of Fe rich alloys for which line structure in the magnetic hyperfine spectrum is observed due to local changes of the Fe57 hyperfine field (h. f.) near solute (Cu) atoms, and for which the average h. f. varies with changing solute concentration. Stearns [2, 31 has given a theoretical description for the decrease of the average hyperfine field with increasing solute concentration for dilute FeSi and FeAl alloys. This decrease is attributed to the loss o f t h e 4s-like conduction electron hyperfine field contribution of the missing Fe atom, and impor- tant information about the origin of the hyperfine field in the pure a-Fe matrix could be obtained [4].
Criteria for the choice of such non-transition solute atoms which are best to use in order to study the pure iron matrix itself are : (i) the impurity should not develop a magnetic moment, (ii) the localized Fe moment as well as the spin-density distribution of itinerant 3d electrons should not change upon alloying,
(*) Present address : Department of Physics, University of North Carolina, Chapel Hill, N. C. 27514, USA.
and (iii) the volume overlap contribution to the h. f.
at the solute atom, Hv, has to be small in order that it does not interfere with the spin density of the iron.
It has been suggested [4] that Cu might be a suitable solute atome in this sense, since the atomic radii of Cu and Fe are of similar magnitude, and requirement (iii) thus might be fulfilled. Furthermore, the localized magnetic moment of Cu impurities in iron is zero [I], though some ambiguity exists at low temperature.
Our present investigation shows, however, that Cu impurities affect the magnetic properties of a-Fe in a very different way than do Si or A1 impurities, as is revealed by a strong temperature dependence of the average h. f.
Polycrystalline films with thicknesses of
-
2 000were deposited from two independently controllabje electron guns on A1 or mylar substrates held at 295 or 77 K in an oilfree vacuum system of
-
loe6 torrduring deposition. For unenriched samples natural Fe of 99.996
%
purity and 99.999 % pure Cu were used, and in order to increase their effective resonance absorber thickness they had to be taken out of the vacuum system, cut to pieces and sandwiched. Some samples were prepared by evaporation of 86%
enriched Fe57 from a special furnace [5] instead of using the e-gun ; these samples were deposited onto 77 K substrates and Mossbauer measurements were per-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974698
C6-474 W. KEUNE, 3. LAUER AND D. L. WILLIAMSON
formed in situ. The evaporation rate of up to 50 A/s of each component was measured with a calibrated quartz crystal thickness and rate monitor and was usually constant during a deposition-run within a few percent.
The film composition was calculated from the ratio of the respective evaporation rates of Cu and Fe, and the error in composition was estimated from the largest deviation in the rate observed during sample preparation. An independent determination of the film composition has been made by mieroprobe analysis.
The structural analysis was done by 100 kV transmission electron diffraction on samples which were evaporated (simultaneously with samples for Mossbauer studies) on NaCl and floated off in water.
From electron micrographs grain diameters between 100-500
A
were found. Alloys with up to 40 at%
Cu have bcc structure, and alloys with more than 65 at%
Cu are fcc, while in the 40-65 at
%
Cu transition region mixtures of both phases exist. In some fcc films few y-Fe precipitates could be observed several weeks after film deposition.Some typical Mossbauer spectra of ferromagnetic bcc alloys are shown in figure 1 for different Cu concen- trations c. In all spectra a partially resolved satellite hyperfine field can be observed in addition to the main pattern. The line intensity ratio of nearly 3 : 4 : 1 in each hyperfine spectrum indicates that the magne- tization lies in the film plane. We assume that the observed satellites correspond to the effects of the Cu impurity on Fe neighbor atoms in the first and second neighbor shell around the impurity, as has been done by Wertheim et al. [6].
Data at 77 and 295 K of the low-concentration sample (c = 0.031) could be analyzed by a computer least-squares fit with four partial six-line spectra.
Their relative line intensities agreed within error limits with the probabilities of occurence of (0,O)-, (1,O)-, (0,l)-, and (2,O)-Fe atoms, assuming random impurity distribution in this alloy. By using the expression H(n, m) = H,, (1
+
Ah, n+
Ah, m) (1+
kc) for the h. f. of an Fe atom with n nearest and m next-nearest neighbor impurity atoms [6] we obtained the relative h. f. change Ah, and Ah, per nearest and next-nearest Cu atom, respectively, as well as the constant k. The values for Cu are :and
Ahl = - 0.05(+ 0.01), Ah2 =
-
0.02(+ 0.01), k = - 0.39(+ 0.005) at 295 K.The isomer shift change is about
+
0.05 mm/s per nearest neighbor Cu atom, and corresponds to a decrease of the s-electron density at the Fe nucleus upon alloying.Figure 2 shows the measured reduced average hyperfine field
hHf
as a function of Cu concentrationVelocity IrnmlsJ
FIG. 1. - Mossbauer spectra of E C u thin films with : a) 3.1 at % Cu, deposited and measured at 295 K, b) 10.6 at % Cu, deposited and measured at 77 K in situ, c) 26.4 at % Cu, deposited and measured at 77 K in situ. (Co57 in Cu source).
at 77 and 295 K, together with the theoretical 1-0.42 c dependence which is expected for impurity atoms acting as simple magnetic holes in the iron matrix (as Si and A1 do) if saturation effects are neglected [2].
The average h. f. at 77 K for c = 0.031 lies near this line, since Ah,, Ah, and k for Cu at low temperature are not very different from corresponding values for Si or A1 [3, 61.
XH,
for c = 0.031 at 295 K, however, is very low and falls even on the line I-c (simple dilution) for the c-dependence at 295 K of the reduced average moment per atom [I], ]ti/pFe. We conclude from this low h. f. value that the Curie temperature Tc of the 3.1%
Cu sample is appreciably lower than for pure iron. Assuming for the temperature depen- dence ofh,,
a BrilIouin function with S = $ and saturation at 77 K, a value Tc-
600 K can be esti-OF FeCu THIN FILMS C6-475
\ \ Steorns 1 - O L 2 c
0.6 ', Kneller
1 - c 0.6
0
0 01 0 2 03 0.L 05 06 0 7 0.8 09 1 0 at. Cu concentration c
-
FIG. 2. - Reduced average hyperfine field
hzn = HEI~(c, T ) I H ~ ( T )
of E C u thin films as a function of at. Cu concentration c. Full circles : T = 77 K, open circles : T = 295 K. Dashed-dotted line : 1-0.42~ (Stearns, ref. [2]), dashed line : reduced average magnetic moment per atom c / p ~ e = 1 - c at room temperature
(Kneller, ref. [I]).
" I I I I
0 10 2 0 3 0 LO 50
C u concentrotion [ a t . % I
-
FIG. 3. - Estimated Curie temperature Te (K) as function of at % Cu concentration in E C u thin films. Full circles : present data, triangles : from magnetization measurement (ref. [I]).
mated (I) for the 3.1
%
Cu alloy, compared to Tc = 1 043 K of pure Fe (Fig. 3).The strong decrease in Tc for such a dilute FeCu alloy means that the effective exchange coupling strength of the localized 3d moments of iron has been significantly changed by a small percentage of Cu solute atoms. This is in contrast to dilute FeSi or FeAl alloys for which T, decreases only some
-
percent relative to T, of pure Fe [7]. The decrease of Tc in the case of FeSi is related to a small decrease in the average exchange coupling by a simple dilution effect [8]. The large drop in To for the dilute FeCu -
(1) Footnote : above N 400K 3 C u alloys segregate into nearly pure cl-Fe and nearly pure Cu.
alloy, however, must have a different reason. Recently it has been shown that ferromagnetism in a-iron is caused by the indirect coupling of localized Femoments through some (- 5
%)
itinerant 3d electrons [2, 4, 81.Since the localized Fe moment does not change upon alloying with Cu at room temperature [I] it is suggested that the spin density distribution of the itinerant 3d elec- trons is disturbed by the 3d electrons of the Cu impu- rity, thus lowering the exchange coupling strength which is of long-range nature.
One can observe in figure 1 that even for such high Cu concentrations as 26.4 % the overall shape of the spectra are not very different from the one for 3.1
%
Cu. One would expect from work on other concentrated alloys [9, 101 that with increasing c more satellite lines appear and that the outer lines finally become very broad due to the large probability of occurence of a large number of neighbor configu- rations. The peculiar property of the FeCu thin film alloys is revealed also in figure 2, where the average h. f. change for c 5 0.03 is much smaller than for c 2 0.03. Thus the slope ofXH,
(c) for cs
0.2 is only about - 0.09 and - 0.26 at 77 and 295 K, respectively, compared with initial slopes of about - 0.42 and - 1.0, respectively. Increasing the Cu concentration from more than-
4%
Cu has also no appreciable effect on the Curie temperature which decreases only slightly compared to the initial change (Fig. 3).We interprete these anomalities with the sudden onset of atomic clustering during film deposition for c 7 3-4
%
Cu. Deposition at 77 K gives not a suffi- ciently low surface diffusion constant in order to prevent cluster formation, since Mossbauer spectra of films (with the same c) which were deposited and measured in situ at 77 K and of films deposited at 295 K and subsequently measured at 77 K show no difference.The value for the average hyperfine field (at low temperature) reflects the local impurity concentra- tion in the first and second neighbor shell of an Fe atom. Since the 3.1
%
Cu sample is statistically ordered, and since its reduced average hyperfine fieldZ,,
at 77 K is equal tohHf
of the 10.6 % Cu alloy at 77 K, one has the possibility to obtain an estimate for the degree of local clustering in the 10.6%
Cu sample. If we assume proportionality between the average hyperfine field and the average magnetic moment a relation similar to the one given by Beck [l 11 can be defined between the average local Cu concen- tration y within the first two neighbor shells of an Fe atom and the nominal alloy concentration c :- a is an average short-range order parameter for the first and second neighbor shell of an Fe atom. With y = 0.031 and c = 0.106 one obtains Z =
+
0.71 for the 10.6%
Cu film. An estimate of E for theC6-476 W. KEUNE, J. LAUER AND D. L. WILLIAMSON
1 I I I
I 1 I I
- 8 -6 - L -2 0 2 L 6 8
Velocity [ rnrn 1 s ]
FIG. 4. - Mossbauer spectra of aCuFe thin film (84.4 at % Cu) at 300, 77 and 4.2 K (C057 in Pd source).
39
%
Cu alloy can be obtained by assuming thatAH,
would follow approximately the relation 1-0.42 c at 77 K if no clustering had occurred. The result is+
0.74 for the 39%
Cu film. Our result shows that for Cu concentrations c y 4%
sudden atomic cluster formation occurs to such a high degree that the local Cu concentration in the first and second neighbor shells of Fe atoms is only about4
of the average alloy concentration c.Typical spectra of an fcc Cu-rich alloy with 84.4 at % Cu are shown in figure 4. The sample was deposited at 77 K, subsequently warmed up to room temperature and measured at 300, 77 and 4.2 K. FCC alloys are paramagnetic at room temperature, in agreement with ref. [I]. In all cases measured so far two lines could be fitted to room-temperature spectra, with one narrow line at - 0.09
+
0.01 mm/s, and a second broader line which shifted from+
0.2 to+
0.7 mm/s with decreasing Cu content (relative to a-Fe). At lower temperature magnetic ordering occurs, the order being complete at 4.2 K with a h. f. of roughly 270 kOe.The ordering temperature decreases with increasing Cu concentration and is above 77 K for 84.4
%
Cu, and at about 25 K for 88.7%
Cu. The spectra in figure 4 are fully reversible with respect to temperature.Based on the described results for bcc alloys it is assumed that cluster formation occured in fcc S F e films, too. The spectra in figure 4 then demonstrate the magnetic ordering in Fe clusters in the Cu matrix.
It is worthwhile to mention that the peak at
- 0.09 mm/s at 300 K has the same isomer shift as y-Fe precipitates in Cu [12, 131 at room tempera- ture. We can exclude the presence of y-Fe in the fcc films, however, since y-Fe is antiferromagnetic at 4.2 K with a small (-- 25 kOe) hyperfine field, leading to a broadened single line [12, 131 ; no such line has been detected in the 4.2 K spectrum, however (Fig. 4).
This work was supported by the Deutsche Fors- chungsgemeinschaft.
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