Hyperfine Interactions 126 (2000) 171–174 171
Hyperfine field at grain boundary atoms in iron nanostructures
J. Balogha, L. Bujdos´oa, D. Kapt´asa, T. Kem´enya, I. Vinczea,b, S. Szab´oc and D.L. Bekec
aResearch Institute for Solid State Physics and Optics, P.O. Box 49, H-1525 Budapest, Hungary
bGeneral Physics Department, E¨otv¨os University, Budapest, Hungary
cDepartment of Solid State Physics, Lajos Kossuth University, Debrecen, Hungary
Iron nanocrystallites of ball-milled iron powder, partially crystallized melt quenched amorphous alloys and polycrystalline multilayers were studied. The change in the hyperfine field at iron atoms due to grain boundaries does not exceed the experimental linewidth.
1. Introduction
It is widely assumed that nanocrystalline materials possess a special grain- boundary structure, which is very different from that of the usual polycrystalline grain boundaries. The Gleiter model [1] depicted nanocrystalline substances as essentially perfect fine grains with wide grain boundaries of significantly reduced density. On the contrary, recent high resolution transmission electron microscopy [2] and X-ray fine structure measurements [3] indicate that the nanocrystalline grain boundaries are very similar to the usual polycrystalline grain boundaries. Significant structural disorder ex- tends no further than the planes immediately adjacent to the boundary plane [2] and the average coordination numbers of nanocrystalline materials agree with the bulk value within the experimental error [3]. These results are in contradiction to some previous interpretations of M¨ossbauer data on different nanocrystalline Fe (n-Fe) samples.
The first M¨ossbauer study of n-Fe was made on a sample prepared by consolida- tion of small clusters [4] and the results were described by two hyperfine components:
a sharp sextet corresponding to atoms inside the crystallites with a nearly perfect order with the parameters of pure bcc-Fe and a broad sextet which was assigned to atoms in the strongly distorted grain boundary region. There were large uncertainties in the ratio of the two components and in the hyperfine parameters of the broad component [5].
Later on n-Fe particles were prepared by chemical [6], cluster beam [7] or gas [8] de- position and mechanical milling [9,10], but these studies gave quite different results.
The inconsistency of the data [11] was explained by preparation-specific structural differences. However, different hyperfine fields were determined by different groups for the most extensively studied mechanically milled samples, as well.
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2. Results and discussion
The n-Fe samples were prepared by three different methods:
(i) ball-milling of Fe powder,
(ii) partial crystallization of Fe–B–Zr–Cu amorphous ribbons, and (iii) vacuum evaporation of Fe–B and Fe–Ag multilayers.
(i) The grain size after different milling times were analyzed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). According to the TEM results the expected grain-boundary fraction approaches 10% after 160 h of milling and 25%
after 470 h of milling if a 1 nm grain-boundary thickness is supposed. The linewidth of the six-line pattern observed in the M¨ossbauer spectra increased by about 20%
after 160 h of milling, but 0.3% Cr content was also detected by EDX and X-ray fluorescent analysis. After 470 h milling the Cr contamination reached 2.1 at% and the M¨ossbauer spectrum also shows satellites [10] characteristic of Cr impurities. The spectral fractions were in fairly good agreement with the measured impurity levels.
The results show that the width of the grain-boundary is well below 1 nm and the hyperfine field of the grain-boundary atoms is higher than those in the vicinity of Cr impurities.
(ii) Nanocrystallized (nc) Fe–Zr–B–Cu samples were prepared from amorphous rib- bons by heat treatment at a temperature above the first crystallization stage to form bcc nanocrystallites with an average diameter of 5–20 nm which are embedded in a residual amorphous matrix. The room temperature M¨ossbauer spectra of these samples contain two well resolved sextets (these will be referred as the main and the satellite components) superimposed on the hyperfine field distribution of a residual amorphous phase. The first sextet with the larger splitting has hyperfine parameters similar to α-Fe, but the origin of the second sextet is disputed [12,13]. The observation [12]
that the hyperfine field of the satellite and the main component follows the same tem- perature dependence – even in case of the nc Fe80Zr7B12Cu1 sample where the Curie temperature is at least 100 K lower than the bcc-Fe value – strongly supports the notion that these two components belong to the same phase, i.e., bcc-Fe containing a few at% Zr and B impurities. The spectral component of atoms at the interface of the bcc grains and the amorphous matrix cannot be distinguished from this impurity contribution.
(iii) In order to separate the grain-boundary contribution of n-Fe samples polycrys- talline multilayers consisting of insoluble elements are regarded the most relevant. The grain size is influenced by the columnar growth and is roughly proportional to the layer thickness [14]. There are not only bcc–bcc grain boundaries but other (bcc-amorphous in case of Fe–B and bcc–fcc in case of Fe–Ag) interfaces in the sample, a system- atic study is able to distinguish them. The results on Fe–B multilayers could be well
J. Balogh et al. / Hyperfine field at grain boundary atoms 173
Figure 1. M¨ossbauer spectrum of Fe–Ag multilayer measured at 12 K. The spectrum was fitted by two sextets (full and dashed lines) belonging to bcc-Fe and Fe–Ag interface, respectively, and a doublet
(dotted lines) due to impurities in the Al substrate.
explained by the formation of an amorphous interfacial alloy [15] and no broadening of the pure bcc-Fe lines occurred in samples with Fe layer thickness in the range of 3.5–10 nm. It was only in the case of samples with nominal Fe layer thickness of about 2.5 nm that broadened bcc-Fe lines (0.36±0.02 mm/s) could be observed.
Spectrum of a Fe–Ag multilayer measured at 12 K is shown in figure 1. It was fitted by two sextets with 35.5 and 33.8 T hyperfine fields. The higher field component can be assigned to the Fe–Ag interface [16]. Both components show a slight broadening (Γ=0.37 mm/s), but no other well resolved component is observed.
3. Conclusion
The results on polycrystalline multilayers show that the perturbation which can be expected in the hyperfine field due to changes of the iron coordination numbers and distances in bcc grain-boundaries is close to the resolution of M¨ossbauer spectroscopy.
This is in line with high resolution TEM and EXAFS results indicating no anomalous grain-boundary structure in nanocrystalline materials. The appearance of well resolved components with a smaller hyperfine field in ball-milled Fe and in nanosize iron clusters of Fe–Zr–B–Cu composite materials is explained by specific impurities in the bcc phase.
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Acknowledgements
This work was supported by the Hungarian Research Fund OTKA T020624 and F020092.
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