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Chemical and hyperfine field analysis of sputtered iron
Journal of Magnetism and Magnetic Materials 40 (1983) 219-223 North-Holland Publishing Company
219
CHEMICAL AND HYPERFINE FIELD ANALYSIS OF SPUTTERED IRON S. B J A R M A N and R. W ~ , P P L I N G Institute of Physics, University of Uppsala, P.O. Box 530, S.751 21 Uppsala, Sweden
Received 23 February 1983
The previously reported amorphous structure of sputtered iron is shown to be stabilized by carbon impurities as deduced from high energy ion beam scattering and M6ssbauer spectroscopy. The minimum amount of carbon needed has not been determined but amorphous samples were obtained for carbon contents of 6% or even less. Spin wave excitation behaviour up to room temperature is revealed. A possible relation between s-electron densities and magnetic hyperfine fields at the iron nuclei, as indicated by the M6ssbauer spectra, is discussed.
1. I n t r o d u c t i o n
In the field of magnetism there has been a large interest in investigations of a m o r p h o u s magnetic materials during the last ten years. To a considerable extent the investigations have focussed on the metallic glass systems, i.e., a m o r p h o u s systems of transition metals and some glass forming metalloids [1]. Of particular interest for theoretical m o d elling and understanding of this kind of magnetism are the pure a m o r p h o u s 3d metals [2,3]. Resistivity measurements on such materials have shown that the conductivity is m u c h lower in the a m o r p h o u s than in the corresponding crystalline phases [4]. These results together with those of Hall effect studies [5] indicate that the electrons are considerably more localized in the a m o r p h o u s than in the crystalline phase. Whether this applies to d-electrons as well as to s-electrons is still an o p e n question. It is also interesting to notice that the magnetic m o m e n t per volume seems to be considerably suppressed in the a m o r p h o u s phase
a
-6
~
3
6
Veloc~ t7 (mm/8)
b
[51. Some preliminary results on sputtered, amorp h o u s iron films that were stable well above r o o m temperature have been reported earlier [6]. The films had been examined b y Mbssbauer spectrosc o p y which showed the typical distribution of hyperfine fields characteristic of a m o r p h o u s mag-
-3
-6
-3
~
Velocity
3
6
(totals)
Fig. 1. Mrssbauer conversion electron spectra of a sputtered iron film before (a) and after (b) crystallization. The solid lines are theoretical fits as discussed in the text. The a-iron lines have been excluded in (b) for clarity.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
220
S. Bjarman, R. W~ippling / Chemical and hyperfine field analysis of sputtered iron
netic materials. Also, electron diffraction had been used revealing the typical diffuse pattern of an amorphous structure. Amorphous iron made by vapour deposition at liquid helium temperature under different experimental conditions, on the other hand, crystallized already at low temperatures [7,8]. It was thus natural to seek an explanation of the stabilization of the amorphous structure in terms of impurities present. In this paper we report on chemical analysis utilizing both the Rutherford backscattering technique and phase analysis using Mrssbauer spectra of crystallized samples. The temperature dependence of the field distribution is discussed as well as a possible relation between the isomer shift and the magnetic hyperfine field.
2. Sample preparation All samples were prepered using a similar apparatus and the same collecting geometry as described in ref. [9]. The sputtering conditions were as follows. Krypton ions of 10-12 keV energy were focused on a pure iron ingot enriched to 25% in 57Fe. The collecting foils were placed at a distance of 2 - 4 cm from the iron target. Using a beam intensity of 0.35 mA a deposition rate of approximately 0.4 .A/s was achieved. The substrates were Al-foils of different thickness, very thin gold films or simply a glass slide. Self-supporting iron films were made by using the floating off technique discribed in ref. [9] where in our case CsI was used as water soluble salt instead of NaC1. T h e initial pressure in the chamber was (1-10) x 10 - 6 Torr rising "to ( 5 - 5 0 ) × 10-6 Torr during sputtering. Other features as e.g. the kinetic energy distribution of the sputtered atoms can be found in refs. [9,10]. The typical energy is 1-10 eV per incoming iron atom. The iron films produced as described above were usually 1 0 0 - 2 0 0 }xg/cm2 a n d the thickest film formed was about 1 m g / c m 2 thick.
3. Experimental details In the chemical analysis high energy ion scattering [11] and Mrssbauer spectroscopy was ulitized. The a-particles for the scattering experiments were obtained from the Tandem accelerator in Uppsala and the equipment used was similar to the one described in ref. [12]. The Mrssbauer measurements were made using a conventional constant acceleration spectrometer allowing for simultaneous calibration using two 57C0 Rh sources.
4. Chemical analysis From the studies of Litterst et al. [13] it is clear that the presence of oxygen has a stabilizing effect on the amorphous phase and since the presence of oxygen during the sputtering process cannot be completely avoided, we first set out to determine a possible oxygen contamination. By utilizing the a - 1 6 0 r e s o n a n c e at 3.045 MeV, it is possible to perform a depth selective determination of the relative oxygen content [12]. A scanning through the self-supporting iron films is conveniently made by varying the a-energy above 3.045 MeV, thus obtaining resonance at different depths. The absolute values can be obtained by recording a spectrum at e.g. 3 MeV and use known cross-sectional data [14]. The results of these studies clearly showed that the oxygen was confined to the film surfaces and that no heavier impurities were present. The only lighter element that can be present in any appreciable amount is carbon and a rather good quantitative determination of possible carbon contamination can be made using alpha particles at 4 MeV energy. At this energy and down to 3.3 MeV there is a rather high and constant cross section, which makes it possible to calculate the carbon content by simply measuring the peak areas. Figs. 2 and 3 show back-scattering spectra of two different iron-on-gold films recorded at 4 MeV. Spectra of both sides of the films, i.e. a) and b), are shown in these figures. The first film (fig. 2) was made under the same sputtering conditions as the films reported earlier
S. Bjarman, R. Wllppfing / Chemical and hyperfine field analysis of sputtered iron Fe
0
Au
a
221
Fe
Au
.,/;'"":":.. [:.:....::.j.." '.~',~.~.
-....". t~
C
,~
0
~_..i~ /! b
t/)..
u. "
L
Fe
Au
(_3"
:.
<__
co .~ I
"
... Fe
Au
•%-..-. "70 C B. 4
0 1.2
•
¢~" . U•
2.8
3.5
Aipha energy (Me V) Fig. 2. Alpha particle back scattering spectra of both sides (a)
and (b) of an iron film sputtered on gold under similar conditions as those reported in [6]. The gold peak (Au) is cut off so as to show the full intensity of the carbon peak (C) compared to the iron peak (Fe). Other peaks originate from surface oxygen and remaining CsI on the gold surface. An enlarged carbon peak is inserted in each diagram.
[6]. This film also proved to be amorphous. In the sputtering of the second film (fig. 3) extraordinary precautions were taken in order to prevent carbon contamination. The sputtering chamber was thoroughly cleaned, a liguid nitrogen cold trap was installed and the previously used target holder of graphite was exchanged against an iron holder. In spite of all precautions there is evidently some carbon left as can be seen in fig. 3. The structure of the peaks, however, shows t h a t the carbon is confined to the surfaces and the F e - A u interface. This was also evident f r o m a M6ssbauer spectrum of the same film, which showed the usual narrow six-line spectrum of crystalline bee-iron with no visible impurities at all. The conclusion is thus that the amorphous structure is stabilized by carbon impurities. The
..
-O
.
~
?..fl
":'::
"~':":'~"~"
l~ fl, 4
j 1.2
~.
." 2. fl
2.8
9.6
Alpha energy (geV) Fig. 3. Same of fig. 2 for a more recently made film as discussed in the text.
minimum amount of carbon needed to stabilize the amorphous structure has, however, to he determined in further studies. By using the backscattering spectrum of fig. 2 and subtracting the surface carbon peaks, we deduce a carbon content for this film of approximately 10%. We have also used M6ssbauer spectra of previously amorphous samples that were crystallised by a heat treatment at 250°C. By analysing the spectrum in terms of bee-iron and different precipitated iron carbides, as shown in fig. lb, the carbon content can readily be determined. The spectrum was fitted using the known hyperfine parameters of a-iron and of the two Fe3C phases, i.e., the ferromagnetic and paramagnetic ones [15]. This analysis results in a 6% carbon content. 5. F i e l d d i s t r i b u t i o n a n a l y s i s
Fig. l a shows a room temperature spectrum of the 6% carbon sample discussed above. The spec-
222
s. Bjarman, R. W@pling / Chemical and hyperfine field analysis of sputtered iron
trum was recorded immediately after the sputtering, but no changes were seen in a spectrum of the same sample a year later (no precautions were taken when storing the sample). The spectrum clearly shows the typical distribution of hyperfine fields of an amorphous magnetic material. Fig. 4a shows the fitted field distribution as obtained by a model-independent method described elsewhere [2]. As can be been, almost the entire distribution lies well below the a-iron field value of 33 T. The average field obtained was 20 T. The isomer shift (IS) was fitted using two different models. Firstly a constant IS was assumed resulting in IS = 0.15 m m / s . Secondly a linear relation between IS and the magnetic hyperfine field (B) was assumed giving IS = (0.08 + 0.012 x B) m m / s (B in tesla). The second model gave a considerably better fit and was found to be a way to explain the small asymmetry in the spectra. The results of the second model indicates
•co
• •
•
•
oil
.
lfl
5
15
2fl
Hyper?.field
25
•.• •
38
that the s-electron density at the iron nucleus decreases as the hyperfine field increases. In a naive picture it thus seems that the hyperfine field increases as the local atomic arrangement deviates from close packing. It should be emphasized that the density of amorphous iron is not believed to deviate much from that of bcc-iron since the bcc structure is not very close packed [3]. The relative intensities (Hi) of the sublines of the individual sextets were treated in the following manner HI,6 = 3H3,5,
H2, 5 = a.
The first relation reflects the assumption of a thin sample. The parameter a describes the spin anisotropy where a = 0 corresponds to all spins parallel to the g a m m a ray direction and a = 4 corresponds to all spins perpendicular to that direction [2]. The parameter a was fixed to different values in different fittings. A best fit was obtained for a = 3, i.e., showing a pronounced in plane alingment of the spins contrary to the results of alloys more concentrated in carbon [16]. Fig. 4b shows the relative deviation of the average field from the saturation value as a function of T 3/2. T h e linear behaviour clearly demonstrates the existence of spin waves. The fitted straight lines gives a slope of 3.1 × 10 -5 K -3/2, which is almost an order of magnitude larger than for bcc-iron [17]. This results reflects the fact that the density of low energy spin waves is high in an amorphous ferromagnet.
B (Tesla)
b
Acknowledgements
The authors would like to thank L. Einarsson and J. Kjellberg for performing the sputtering. Thanks also to G. Possnert and L.O. Norlin for introducing us to the backscattering technique and helping us to do the chemical analysis. Financial support was obtained through the Swedish Natural Science Research Council (NFR).
~15 1B
1
2
3
4
5
6
References
Fig. 4. (a) The magnetic hyperhne field distribution at room temperature obtained from the spectrum in fig. la. (b) The T 3/2 dependence of the magnetic hyperfine field. The solid line is a least squares fit as discussed in the text.
[1] C.C. Tsuei, Amorphous Magnetism, eds. H.O. Hopper and A.M. de Graaf (Plenum Press, New York, London, 1973) p. 299.
S. Bjarman, R. W@pling / Chemicaland hyperfinefield analysis of sputtered iron [2] S. Bjarman, Report UUIP-1046 (Institute of Physics, University of Uppsala, April 1981). [3] G.S. Cargill III, Solid State Physics, vol. 30, eds. H. Ehrenreich, F. Seitz and D. Turnbull (Academic Press, New York, 1975) p. 227. [4] J.G. Wright, IEEE Trans. Magn. MAG-12 (1976) 95. [5] S.J. Raeburn and R.V. Aldrldge, J. Phys. F 8 (1978) 1917. [6] S. Bjarman, R. Kamal and R. Wiippling, J. Magn. Magn. Mat. 15-18 (1980) 1389. [7] T. Ichikawa, Phys. Stat. Sol. (a) 19 (1973) 707. [8] P.K. Leung and J.G. Wright., Phil. Mag. 30 (1974) 995. [9] G. Sletten and P. Knudsen, Nucl. Instr. and Meth. 102 (1972) 459. [10] O. Almen and G. Bruce, Nucl. Instr. and Meth. 11 (1961) 257.
223
[11] A.E. Morgan and H.W. Werner, Phys. Seripta 18 (1978) 451. [12] L.O. Norlin, B. Orre, G. Possnert and K. Johansson, Phys. Scripta 17 (1978) 439. [13] F.J. Litterst, A. Ogrodnik and G.M. Kalvius, Hyperfine Interactions 4 (1978) 879. [14] R.A. Jarls, Nuclear Cross Section Data for Surface Analysis, vol. 2 (Report. Dep. of Phys., Univ. of Manchester, Dec. 1979). [15] N.N. Greenwood and T.C. Gibb, M6ssbauer Spectroscopy (Chapman and Hall, London, 1971) p. 318. [16] N. Kazama, N. Heiman and R.L. White, J. Appl. Phys. 49 (1978) 1706. [17] C.L. Chien and R. Hasegawa, Hyperfine Interactions 4 (1978) 866.
219
CHEMICAL AND HYPERFINE FIELD ANALYSIS OF SPUTTERED IRON S. B J A R M A N and R. W ~ , P P L I N G Institute of Physics, University of Uppsala, P.O. Box 530, S.751 21 Uppsala, Sweden
Received 23 February 1983
The previously reported amorphous structure of sputtered iron is shown to be stabilized by carbon impurities as deduced from high energy ion beam scattering and M6ssbauer spectroscopy. The minimum amount of carbon needed has not been determined but amorphous samples were obtained for carbon contents of 6% or even less. Spin wave excitation behaviour up to room temperature is revealed. A possible relation between s-electron densities and magnetic hyperfine fields at the iron nuclei, as indicated by the M6ssbauer spectra, is discussed.
1. I n t r o d u c t i o n
In the field of magnetism there has been a large interest in investigations of a m o r p h o u s magnetic materials during the last ten years. To a considerable extent the investigations have focussed on the metallic glass systems, i.e., a m o r p h o u s systems of transition metals and some glass forming metalloids [1]. Of particular interest for theoretical m o d elling and understanding of this kind of magnetism are the pure a m o r p h o u s 3d metals [2,3]. Resistivity measurements on such materials have shown that the conductivity is m u c h lower in the a m o r p h o u s than in the corresponding crystalline phases [4]. These results together with those of Hall effect studies [5] indicate that the electrons are considerably more localized in the a m o r p h o u s than in the crystalline phase. Whether this applies to d-electrons as well as to s-electrons is still an o p e n question. It is also interesting to notice that the magnetic m o m e n t per volume seems to be considerably suppressed in the a m o r p h o u s phase
a
-6
~
3
6
Veloc~ t7 (mm/8)
b
[51. Some preliminary results on sputtered, amorp h o u s iron films that were stable well above r o o m temperature have been reported earlier [6]. The films had been examined b y Mbssbauer spectrosc o p y which showed the typical distribution of hyperfine fields characteristic of a m o r p h o u s mag-
-3
-6
-3
~
Velocity
3
6
(totals)
Fig. 1. Mrssbauer conversion electron spectra of a sputtered iron film before (a) and after (b) crystallization. The solid lines are theoretical fits as discussed in the text. The a-iron lines have been excluded in (b) for clarity.
0 3 0 4 - 8 8 5 3 / 8 3 / 0 0 0 0 - 0 0 0 0 / $ 0 3 . 0 0 © 1983 N o r t h - H o l l a n d
220
S. Bjarman, R. W~ippling / Chemical and hyperfine field analysis of sputtered iron
netic materials. Also, electron diffraction had been used revealing the typical diffuse pattern of an amorphous structure. Amorphous iron made by vapour deposition at liquid helium temperature under different experimental conditions, on the other hand, crystallized already at low temperatures [7,8]. It was thus natural to seek an explanation of the stabilization of the amorphous structure in terms of impurities present. In this paper we report on chemical analysis utilizing both the Rutherford backscattering technique and phase analysis using Mrssbauer spectra of crystallized samples. The temperature dependence of the field distribution is discussed as well as a possible relation between the isomer shift and the magnetic hyperfine field.
2. Sample preparation All samples were prepered using a similar apparatus and the same collecting geometry as described in ref. [9]. The sputtering conditions were as follows. Krypton ions of 10-12 keV energy were focused on a pure iron ingot enriched to 25% in 57Fe. The collecting foils were placed at a distance of 2 - 4 cm from the iron target. Using a beam intensity of 0.35 mA a deposition rate of approximately 0.4 .A/s was achieved. The substrates were Al-foils of different thickness, very thin gold films or simply a glass slide. Self-supporting iron films were made by using the floating off technique discribed in ref. [9] where in our case CsI was used as water soluble salt instead of NaC1. T h e initial pressure in the chamber was (1-10) x 10 - 6 Torr rising "to ( 5 - 5 0 ) × 10-6 Torr during sputtering. Other features as e.g. the kinetic energy distribution of the sputtered atoms can be found in refs. [9,10]. The typical energy is 1-10 eV per incoming iron atom. The iron films produced as described above were usually 1 0 0 - 2 0 0 }xg/cm2 a n d the thickest film formed was about 1 m g / c m 2 thick.
3. Experimental details In the chemical analysis high energy ion scattering [11] and Mrssbauer spectroscopy was ulitized. The a-particles for the scattering experiments were obtained from the Tandem accelerator in Uppsala and the equipment used was similar to the one described in ref. [12]. The Mrssbauer measurements were made using a conventional constant acceleration spectrometer allowing for simultaneous calibration using two 57C0 Rh sources.
4. Chemical analysis From the studies of Litterst et al. [13] it is clear that the presence of oxygen has a stabilizing effect on the amorphous phase and since the presence of oxygen during the sputtering process cannot be completely avoided, we first set out to determine a possible oxygen contamination. By utilizing the a - 1 6 0 r e s o n a n c e at 3.045 MeV, it is possible to perform a depth selective determination of the relative oxygen content [12]. A scanning through the self-supporting iron films is conveniently made by varying the a-energy above 3.045 MeV, thus obtaining resonance at different depths. The absolute values can be obtained by recording a spectrum at e.g. 3 MeV and use known cross-sectional data [14]. The results of these studies clearly showed that the oxygen was confined to the film surfaces and that no heavier impurities were present. The only lighter element that can be present in any appreciable amount is carbon and a rather good quantitative determination of possible carbon contamination can be made using alpha particles at 4 MeV energy. At this energy and down to 3.3 MeV there is a rather high and constant cross section, which makes it possible to calculate the carbon content by simply measuring the peak areas. Figs. 2 and 3 show back-scattering spectra of two different iron-on-gold films recorded at 4 MeV. Spectra of both sides of the films, i.e. a) and b), are shown in these figures. The first film (fig. 2) was made under the same sputtering conditions as the films reported earlier
S. Bjarman, R. Wllppfing / Chemical and hyperfine field analysis of sputtered iron Fe
0
Au
a
221
Fe
Au
.,/;'"":":.. [:.:....::.j.." '.~',~.~.
-....". t~
C
,~
0
~_..i~ /! b
t/)..
u. "
L
Fe
Au
(_3"
:.
<__
co .~ I
"
... Fe
Au
•%-..-. "70 C B. 4
0 1.2
•
¢~" . U•
2.8
3.5
Aipha energy (Me V) Fig. 2. Alpha particle back scattering spectra of both sides (a)
and (b) of an iron film sputtered on gold under similar conditions as those reported in [6]. The gold peak (Au) is cut off so as to show the full intensity of the carbon peak (C) compared to the iron peak (Fe). Other peaks originate from surface oxygen and remaining CsI on the gold surface. An enlarged carbon peak is inserted in each diagram.
[6]. This film also proved to be amorphous. In the sputtering of the second film (fig. 3) extraordinary precautions were taken in order to prevent carbon contamination. The sputtering chamber was thoroughly cleaned, a liguid nitrogen cold trap was installed and the previously used target holder of graphite was exchanged against an iron holder. In spite of all precautions there is evidently some carbon left as can be seen in fig. 3. The structure of the peaks, however, shows t h a t the carbon is confined to the surfaces and the F e - A u interface. This was also evident f r o m a M6ssbauer spectrum of the same film, which showed the usual narrow six-line spectrum of crystalline bee-iron with no visible impurities at all. The conclusion is thus that the amorphous structure is stabilized by carbon impurities. The
..
-O
.
~
?..fl
":'::
"~':":'~"~"
l~ fl, 4
j 1.2
~.
." 2. fl
2.8
9.6
Alpha energy (geV) Fig. 3. Same of fig. 2 for a more recently made film as discussed in the text.
minimum amount of carbon needed to stabilize the amorphous structure has, however, to he determined in further studies. By using the backscattering spectrum of fig. 2 and subtracting the surface carbon peaks, we deduce a carbon content for this film of approximately 10%. We have also used M6ssbauer spectra of previously amorphous samples that were crystallised by a heat treatment at 250°C. By analysing the spectrum in terms of bee-iron and different precipitated iron carbides, as shown in fig. lb, the carbon content can readily be determined. The spectrum was fitted using the known hyperfine parameters of a-iron and of the two Fe3C phases, i.e., the ferromagnetic and paramagnetic ones [15]. This analysis results in a 6% carbon content. 5. F i e l d d i s t r i b u t i o n a n a l y s i s
Fig. l a shows a room temperature spectrum of the 6% carbon sample discussed above. The spec-
222
s. Bjarman, R. W@pling / Chemical and hyperfine field analysis of sputtered iron
trum was recorded immediately after the sputtering, but no changes were seen in a spectrum of the same sample a year later (no precautions were taken when storing the sample). The spectrum clearly shows the typical distribution of hyperfine fields of an amorphous magnetic material. Fig. 4a shows the fitted field distribution as obtained by a model-independent method described elsewhere [2]. As can be been, almost the entire distribution lies well below the a-iron field value of 33 T. The average field obtained was 20 T. The isomer shift (IS) was fitted using two different models. Firstly a constant IS was assumed resulting in IS = 0.15 m m / s . Secondly a linear relation between IS and the magnetic hyperfine field (B) was assumed giving IS = (0.08 + 0.012 x B) m m / s (B in tesla). The second model gave a considerably better fit and was found to be a way to explain the small asymmetry in the spectra. The results of the second model indicates
•co
• •
•
•
oil
.
lfl
5
15
2fl
Hyper?.field
25
•.• •
38
that the s-electron density at the iron nucleus decreases as the hyperfine field increases. In a naive picture it thus seems that the hyperfine field increases as the local atomic arrangement deviates from close packing. It should be emphasized that the density of amorphous iron is not believed to deviate much from that of bcc-iron since the bcc structure is not very close packed [3]. The relative intensities (Hi) of the sublines of the individual sextets were treated in the following manner HI,6 = 3H3,5,
H2, 5 = a.
The first relation reflects the assumption of a thin sample. The parameter a describes the spin anisotropy where a = 0 corresponds to all spins parallel to the g a m m a ray direction and a = 4 corresponds to all spins perpendicular to that direction [2]. The parameter a was fixed to different values in different fittings. A best fit was obtained for a = 3, i.e., showing a pronounced in plane alingment of the spins contrary to the results of alloys more concentrated in carbon [16]. Fig. 4b shows the relative deviation of the average field from the saturation value as a function of T 3/2. T h e linear behaviour clearly demonstrates the existence of spin waves. The fitted straight lines gives a slope of 3.1 × 10 -5 K -3/2, which is almost an order of magnitude larger than for bcc-iron [17]. This results reflects the fact that the density of low energy spin waves is high in an amorphous ferromagnet.
B (Tesla)
b
Acknowledgements
The authors would like to thank L. Einarsson and J. Kjellberg for performing the sputtering. Thanks also to G. Possnert and L.O. Norlin for introducing us to the backscattering technique and helping us to do the chemical analysis. Financial support was obtained through the Swedish Natural Science Research Council (NFR).
~15 1B
1
2
3
4
5
6
References
Fig. 4. (a) The magnetic hyperhne field distribution at room temperature obtained from the spectrum in fig. la. (b) The T 3/2 dependence of the magnetic hyperfine field. The solid line is a least squares fit as discussed in the text.
[1] C.C. Tsuei, Amorphous Magnetism, eds. H.O. Hopper and A.M. de Graaf (Plenum Press, New York, London, 1973) p. 299.
S. Bjarman, R. W@pling / Chemicaland hyperfinefield analysis of sputtered iron [2] S. Bjarman, Report UUIP-1046 (Institute of Physics, University of Uppsala, April 1981). [3] G.S. Cargill III, Solid State Physics, vol. 30, eds. H. Ehrenreich, F. Seitz and D. Turnbull (Academic Press, New York, 1975) p. 227. [4] J.G. Wright, IEEE Trans. Magn. MAG-12 (1976) 95. [5] S.J. Raeburn and R.V. Aldrldge, J. Phys. F 8 (1978) 1917. [6] S. Bjarman, R. Kamal and R. Wiippling, J. Magn. Magn. Mat. 15-18 (1980) 1389. [7] T. Ichikawa, Phys. Stat. Sol. (a) 19 (1973) 707. [8] P.K. Leung and J.G. Wright., Phil. Mag. 30 (1974) 995. [9] G. Sletten and P. Knudsen, Nucl. Instr. and Meth. 102 (1972) 459. [10] O. Almen and G. Bruce, Nucl. Instr. and Meth. 11 (1961) 257.
223
[11] A.E. Morgan and H.W. Werner, Phys. Seripta 18 (1978) 451. [12] L.O. Norlin, B. Orre, G. Possnert and K. Johansson, Phys. Scripta 17 (1978) 439. [13] F.J. Litterst, A. Ogrodnik and G.M. Kalvius, Hyperfine Interactions 4 (1978) 879. [14] R.A. Jarls, Nuclear Cross Section Data for Surface Analysis, vol. 2 (Report. Dep. of Phys., Univ. of Manchester, Dec. 1979). [15] N.N. Greenwood and T.C. Gibb, M6ssbauer Spectroscopy (Chapman and Hall, London, 1971) p. 318. [16] N. Kazama, N. Heiman and R.L. White, J. Appl. Phys. 49 (1978) 1706. [17] C.L. Chien and R. Hasegawa, Hyperfine Interactions 4 (1978) 866.