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Antiferromagnetism of FCC Fe thin films
Journal of Magnetism and Magnetic Materials 6 (1977) 192-195 © North-Holland Publishing Company
ANTIFERROMAGNETISM OF FCC Fe THIN FILMS W. KEUNE *, R. HALBAUER, U. GONSER, J. LAUER * and D.L. WILLIAMSON ** Fachbereich 12.1- Angewandte Physik, Universitiit des Saarlandes, D-6600 Saarbrf~cken, W. Germany
Received 5 April 1977
Thin iron films (-18 A thick and 90% enriched in STFe) were prepared on (001) Cu single crystal substrates. By using M~ssbauer spectroscopy it was found that antiferromagnetic ordering begins at around 80 K ± 10 K with averagehyperfine fields of about 14-20 kOe at 4.2 K. Additional proof for the existence of antiferromagnetism has been obtained by measurements in a longitudinal external magnetic field. The apparent discrepancy in the literature of ferro- and antiferromagnetic ordering in fcc Fe films is discussed.
It haS been well established by neutron diffraction [1-3] and M6ssbauer effect experiments [ 4 - 9 ] that fcc (?-) Fe orders antiferromagnetically at low temperature. In these studies the 3,-Fe state was stabilized either in the form of coherent ? - F e precipitates in a Cu matrix [ 1 , 2 , 6 - 9 ] or by alloying (stainless steel) [3,5]. Surprisingly, fcc single crystal Fe thin films (~30 A thick) grown by electrodeposition on (110) bulk Cu single crystal substrates have been found by ferromagnetic resonance studies to be ferromagnetic at room temperature [10]. Recently, ferromagnetic order at room temperature and below in fcc Fe films epitaxially grown on (111) Cu substrates by vapor deposition has been inferred from magnetization measurements [11 ]. In view of the contrasting results for thin films of 3,-Fe [10,11] compared to those of bulk ? - F e [1-9], the M6ssbauer effect, being a microscopic method, appears to be a useful additional technique to determine the magnetic state of the s7 Fe atom in a thin film. This technique should be particularly useful since M6ssbauer spectral parameters of the various Fe modifications are well established, and since information
about the type of magnetic ordering (ferro- or antiferromagnetic) easily can be obtained from a M6ssbauer spectrum by application of a magnetic field. In the present study, we have investigated " 1 8 A thick Fe films vacuum-deposited on (001) Cu thin film substrates by the MSssbauer effect. Thin film samples were prepared by vapor condensation of a base layer of 2000 A single-crystalline Cu onto a cleaved and polished NaC1 crystal which was maintained at 300°C during the entire specimen preparation process [12]. Subsequently, in order to obtain a sufficiently large effective MSssbauer absorber thickness, four layers of " 1 8 A Fe (90% enriched in s7 Fe) separated by 1000 A Cu, were successively deposited and finally covered by 2000 A Cu for protection. Vapor deposition was performed in an oil-free UHV system with a base pressure of 10 -~° torr and 1 0 - 7 - 1 0 -8 torr during deposition. Evaporation rates were ~ 3 - 4 A/s for Cu and "/2-3 A/s for STFe. The resulting multilayer film was stripped off carefully from the NaC1 in water. A 100 keV transmission electron diffraction from thinned edges of the sample yielded only (somewhat broadened) fcc diffraction spots and indicated epitaxial growth of (001) Cu planes parallel to the film surface. Our observation suggests that the 18 A deposits are pseudomorphic with the Cu layers and have a lattice constant scarcely distinguishable from that of the Cu films [13].
* Present address: Laboratorium fiir Angewandte Physik, Gesamthochschule, D-4100 Duisburg, Germany. ** Present address: Department of Physics, Colorado School of Mines, Golden, Colorado 80401, USA. 192
W. Keune et aL / A ntiferromagnetism o f fcc Fe thin films
1.00-
i........./
09,
0.92
~
K
1'0°• ~-
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77K
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'
d -:.:,:].: v..',,~..~......]..:~13 K "" ~';': :':" 49 kOe -~ 6 i Velocity [ram/s]
Fig. 1. M6ssbauer spectra of ~18 A thick Fe on (001) Cu at, (a) 295 K; (b) 77 K; (c) 4.2 K; (d) 13 K and with an applied longitudinal field of 49 kOe. Source: 57Co in Rh.
Clear evidence for fcc Fe (paramagnetic at 295 K) can be obtained from the room temperature MSssbauer spectrum (fig. la) for which data analysis by a leastsquares computer fit with two independent lorentzian lines (dashed curves) yielded a strong absorption line with an isomer shift (IS) of (-0.084 + 0.003) mm/s (relative to ot-Fe at room temperature), and a weaker satellite line with an IS of (+0.24 +--0.01) mm/s. Within error limits the IS of the main line is identical to that of (paramagnetic) 3,-Fe precipitates in Cu at room temperature [4,6-9] (-0.088 + 0.003 mm/s) which indicates that the majority of the Fe atoms in our sample condensed in the 3,-phase. The measured full width at half maximum of the main line was 0.31 mm/s which is only slightly broader than the natural line width due to the effect of finite absorber thickness. At lower temperature (examples are given in figs. lb and 1c for 77 and 4.2 K, respectively) a drastic broadening of the thin film ~ - F e line can be seen. This observation is analogous to the case of'),-Fe precipitates
193
in Cu below their N6el temperature where a reduction in temperature leads to increasing line broadening due to an increasing degree of antiferromagnetic ordering [4,6-9]. The antiferromagnetic state is indicated by a line broadening only, since the magnetic hyperfine field saturation value (24 kOe for large precipitates) is of the order of the natural line width, and thus the full STFe six-line Zeeman pattern cannot be resolved [4,6-9]. The similarities in the low-temperature spectra of -),-Fe precipitates and our thin fcc Fe films suggest that antiferromagnetic order occurred also for the latter. From the measured line width in fig. lc the magnitude of the hyperfine field H o is estimated to be 14-20 kOe for the fcc film at 4.2 K. A direct proof for antiferromagnetism has been obtained by application of a longitudinal magnetic field HA of 49 kOe (parallel to the "),-raydirection) while the sample was held at 13 K. The measured spectrum (shown in fig. ld) is very similar to that of antiferromagnetic l ' - F e precipitates at low temperature in an external field [7,8]. It is characterized by very broad outer lines due to a distribution of magnetic fields at the s7 Fe nucleus which is typically observed for an antiferromagnet with random spin orientation in an applied field [14]. The separation of the outer lines at maximum resonance is equivalent to a most probable effective field at the 57Fe nucleus of(52 + 2) kOe. Ferromagnetic behavior of the film can be excluded positively from this measurement since it is well known that in such a case (the magnetic moments being aligned parallel to the external field) a magnetic hyperfine pattern would be expected with sharp lines ("43.3 mm/s width in this case) and without the two Am = 0 lines. Furthermore, the outer peak distance should correspond to an effective field H etf of either H A - Ho or HA + Ho (i.e. about 30 kOe or about 70 kOe), depending on whether the internal field Ho (taking Ho ~ 20 kOe) is negative or positive, respectively. However, none of these consequences of ferromagnetism was observed. Paramagnetism, too, can be discarded by similar arguments concerning line width and the Am = 0 line intensity. The paramagnetic-antiferromagnetic transition temperature has been determined by measuring the 3,-Fe line width as a function of temperature. The results (fig. 2) which are entirely reversible with temperature, indicate a slight increase in line width in going from room temperature to about 100 K due to
194
W. Keune et al. / Antiferromagnetism of fcc Fe thin films
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c•
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-°-
-
--° .
.
.
.
.
°_
_
03
100
200 Tempero|ure IK}
300
Fig. 2. ~-Fe line width as a function of temperature. an increase of the effective absorber thickness (being proportional to the Debye-Waller factor). The drastic line broadening due to antiferromagnetic ordering begins at about 80 K -+ 10 K. The observed transition temperature is not as sharp and is higher than the N6el temperature of 67 K measured for large 7 - F e percipitates [2,4,7-9]. Recent work on fcc Fe alloys has shown [15] that the N6el temperature of pure 7 - F e could be higher than 67 K (between 67 and 90 K), because large 7 - F e precipitates may contain about 1-3% Cu impurities [16,17] which could lower the N6el temperature to 67 K. Thus, the higher transition temperature of the fcc Fe films might indicate a very low Cu impurity content, i.e. little or no interdiffusion of Cu into the Fe layer during film deposition. The "smearing" of the transition could be due to: (i) varying local strains in the film which cause a distribution of N~el temperatures, since TN of fcc Fe (stainless steel) is known to depend quite sensitively on pressure [18]; or (ii) a broad distribution of island sizes in the ~18.8, thick Fe films, since it is known from M6ssbauer studies of 7 - F e precipitates that the transition temperature strongly drops with decreasing particle size [7,9] possibly due to superparamagnetic relaxation. In summary, our M6ssbauer investigation of " 1 8 A thick fcc Fe films vacuum deposited on (001) Cu substrates gives clear evidence for antiferromagnetism at low temperature and paramagnetism at 295 K. This result is not necessarily in contradiction with ferromagnetism observed at room temperature in fcc Fe films on (110) and (111) Cu substrates [10,11]. For example, these results could be seen as demonstrating a critical dependence of exchange energy on lattice
spacing, which might be slightly different for the various Cu substrate orientations. The apparent discrepancy of ferro- and antiferromagnetic ordering of fcc Fe films might also be explained by considering the spin structure. By using neutron diffraction of coherent 7 - F e precipitates in copper [1 ] and stainless steel [3] it was concluded that the atomic magnetic moments are oriented close to the [001 ] direction and that the spins in alternating ferromagnetic sheets of (001) planes are antiferromagnetically coupled to each other. Thus, the spins of (001) fcc films can align themselves within the film plane. In contrast, the spins of (111) films with the same magnetic ordering along [001 ] would form large angles with the film plane which might be energetically unfavorable, particularly for the surface spins. In a (001) film one can construct an antiferromagnetic [001] spin arrangement oriented within the film plane; however, the situation is more complex than that of (001) or (I 11) films because of the following: (001) and (111) surfaces are uniform in that the atoms on surfaces have a unique number of nearest neighbor surface atoms (eight in the case of (001) surfaces and nine in the case of (111) surfaces) while on (01 I) surfaces, atomic positions with seven and eleven nearest neighbors occur. Thus, it is suggested that the accommodation and alignment of the spins with regard to the film orientation could be a determining factor in the magnetic ordering of fcc Fe films. Specifically, (001) films maintain the bulk an tiferromagnetic ordering while (011) and (111 ) films change to a ferromagnetic ordering.
Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft.
References [1] S.C. Abrahams, L. Guttman and J.S. Kasper, Phys. Rev. 127 (1962) 2052. [2] G.J. Johanson, M.B. McGirr and D.A. Wheeler, Phys. Rev. B1 (1970) 3208. [3] Y. Ishikawa, Y. Endoh amd T. Takimoto, J. Phys. Chem. Solids 31 (1970) 1225. [4] U. Gonser, C.J. Meechan, A.H. Muir and H. Wiedersich, J. Appl. Phys. 34 (1963) 2373.
I¢. Keune et al. / Antiferromagnetism of fcc Fe thin films [5] L.D. Flansburg and N. Hershkowitz, J. Appl. Phys. 41 (1970) 4082 and references quoted therein. [6] B. Window, Phil. Mag. 26 (1972) 681. [7] D.L. Williamson, W. Keune and U. Gonser, Proc. Intern. Conf. on Magnetism, Vol. 1 (2) (Publ. House Nauka, Moscow, 1974). p. 246. [8] S.J. Campbell and P.E. Clark, J. Phys. F: Metal Phys. 4 (1974) 1073. [9] D.L. WiUiamsonand W. Keune, Proc. Intern. Conf. on M6ssbauer Spectroscopy, Vol. 1, A.Z. Hrynkiewicz and J.A. Sawicki, eds. (Akademia G6rniczo-Hutnicza, Cracow, Poland, 1975) p. 133. [10] J.G. Wright, Phil. Mag. 24 (1971) 217. [11] U. Gradmann, W. KIlmmerle and P. TiUmanns, Thin Solid Films 34 (1976) 249.
195
[12] L.O. Brockway and R.B. Marcus, J. Appl. Phys. 34 (1963) 921. [13] W.A. Jesser and J.W. Matthews, Phil. Mag. 15 (1967) 1097. [14] G.K. Wertheim, D.N.E. Buchanan and J.H. Wernick, Solid State Commun. 8 (1970) 2173. [15] H.H. Ettwig and W. Pepperhoff, Arch. Eisenhiittenw. 46 (1975) 667. [16] K.E. Easterling and H.M. Miekk-Oja, Acta Met. 15 (1967) 1133. [17] L.J. Swartzendruber and L.H. Bennett, Magnetism and Magnetic Materials, AlP Conf. Proc., No. 5, H.C. Wolfe, ed. (AIP, New York, 1972) p. 408. [18] D.R. Rhiger, R. Ingalls and Chun-Mai Liu, Solid State Commun. 18 (1976) 681.
ANTIFERROMAGNETISM OF FCC Fe THIN FILMS W. KEUNE *, R. HALBAUER, U. GONSER, J. LAUER * and D.L. WILLIAMSON ** Fachbereich 12.1- Angewandte Physik, Universitiit des Saarlandes, D-6600 Saarbrf~cken, W. Germany
Received 5 April 1977
Thin iron films (-18 A thick and 90% enriched in STFe) were prepared on (001) Cu single crystal substrates. By using M~ssbauer spectroscopy it was found that antiferromagnetic ordering begins at around 80 K ± 10 K with averagehyperfine fields of about 14-20 kOe at 4.2 K. Additional proof for the existence of antiferromagnetism has been obtained by measurements in a longitudinal external magnetic field. The apparent discrepancy in the literature of ferro- and antiferromagnetic ordering in fcc Fe films is discussed.
It haS been well established by neutron diffraction [1-3] and M6ssbauer effect experiments [ 4 - 9 ] that fcc (?-) Fe orders antiferromagnetically at low temperature. In these studies the 3,-Fe state was stabilized either in the form of coherent ? - F e precipitates in a Cu matrix [ 1 , 2 , 6 - 9 ] or by alloying (stainless steel) [3,5]. Surprisingly, fcc single crystal Fe thin films (~30 A thick) grown by electrodeposition on (110) bulk Cu single crystal substrates have been found by ferromagnetic resonance studies to be ferromagnetic at room temperature [10]. Recently, ferromagnetic order at room temperature and below in fcc Fe films epitaxially grown on (111) Cu substrates by vapor deposition has been inferred from magnetization measurements [11 ]. In view of the contrasting results for thin films of 3,-Fe [10,11] compared to those of bulk ? - F e [1-9], the M6ssbauer effect, being a microscopic method, appears to be a useful additional technique to determine the magnetic state of the s7 Fe atom in a thin film. This technique should be particularly useful since M6ssbauer spectral parameters of the various Fe modifications are well established, and since information
about the type of magnetic ordering (ferro- or antiferromagnetic) easily can be obtained from a M6ssbauer spectrum by application of a magnetic field. In the present study, we have investigated " 1 8 A thick Fe films vacuum-deposited on (001) Cu thin film substrates by the MSssbauer effect. Thin film samples were prepared by vapor condensation of a base layer of 2000 A single-crystalline Cu onto a cleaved and polished NaC1 crystal which was maintained at 300°C during the entire specimen preparation process [12]. Subsequently, in order to obtain a sufficiently large effective MSssbauer absorber thickness, four layers of " 1 8 A Fe (90% enriched in s7 Fe) separated by 1000 A Cu, were successively deposited and finally covered by 2000 A Cu for protection. Vapor deposition was performed in an oil-free UHV system with a base pressure of 10 -~° torr and 1 0 - 7 - 1 0 -8 torr during deposition. Evaporation rates were ~ 3 - 4 A/s for Cu and "/2-3 A/s for STFe. The resulting multilayer film was stripped off carefully from the NaC1 in water. A 100 keV transmission electron diffraction from thinned edges of the sample yielded only (somewhat broadened) fcc diffraction spots and indicated epitaxial growth of (001) Cu planes parallel to the film surface. Our observation suggests that the 18 A deposits are pseudomorphic with the Cu layers and have a lattice constant scarcely distinguishable from that of the Cu films [13].
* Present address: Laboratorium fiir Angewandte Physik, Gesamthochschule, D-4100 Duisburg, Germany. ** Present address: Department of Physics, Colorado School of Mines, Golden, Colorado 80401, USA. 192
W. Keune et aL / A ntiferromagnetism o f fcc Fe thin films
1.00-
i........./
09,
0.92
~
K
1'0°• ~-
.c o 0.96 -
77K
E 0.92e
100-
=
:;. . . . . . .
..... :~
-~ 0.980
/
oc 0.961.0O- ..:'...
:,..,!.:':.
•" ":....:..~...
0.99
.~::: .".
..:.:
0.98 -2
'
d -:.:,:].: v..',,~..~......]..:~13 K "" ~';': :':" 49 kOe -~ 6 i Velocity [ram/s]
Fig. 1. M6ssbauer spectra of ~18 A thick Fe on (001) Cu at, (a) 295 K; (b) 77 K; (c) 4.2 K; (d) 13 K and with an applied longitudinal field of 49 kOe. Source: 57Co in Rh.
Clear evidence for fcc Fe (paramagnetic at 295 K) can be obtained from the room temperature MSssbauer spectrum (fig. la) for which data analysis by a leastsquares computer fit with two independent lorentzian lines (dashed curves) yielded a strong absorption line with an isomer shift (IS) of (-0.084 + 0.003) mm/s (relative to ot-Fe at room temperature), and a weaker satellite line with an IS of (+0.24 +--0.01) mm/s. Within error limits the IS of the main line is identical to that of (paramagnetic) 3,-Fe precipitates in Cu at room temperature [4,6-9] (-0.088 + 0.003 mm/s) which indicates that the majority of the Fe atoms in our sample condensed in the 3,-phase. The measured full width at half maximum of the main line was 0.31 mm/s which is only slightly broader than the natural line width due to the effect of finite absorber thickness. At lower temperature (examples are given in figs. lb and 1c for 77 and 4.2 K, respectively) a drastic broadening of the thin film ~ - F e line can be seen. This observation is analogous to the case of'),-Fe precipitates
193
in Cu below their N6el temperature where a reduction in temperature leads to increasing line broadening due to an increasing degree of antiferromagnetic ordering [4,6-9]. The antiferromagnetic state is indicated by a line broadening only, since the magnetic hyperfine field saturation value (24 kOe for large precipitates) is of the order of the natural line width, and thus the full STFe six-line Zeeman pattern cannot be resolved [4,6-9]. The similarities in the low-temperature spectra of -),-Fe precipitates and our thin fcc Fe films suggest that antiferromagnetic order occurred also for the latter. From the measured line width in fig. lc the magnitude of the hyperfine field H o is estimated to be 14-20 kOe for the fcc film at 4.2 K. A direct proof for antiferromagnetism has been obtained by application of a longitudinal magnetic field HA of 49 kOe (parallel to the "),-raydirection) while the sample was held at 13 K. The measured spectrum (shown in fig. ld) is very similar to that of antiferromagnetic l ' - F e precipitates at low temperature in an external field [7,8]. It is characterized by very broad outer lines due to a distribution of magnetic fields at the s7 Fe nucleus which is typically observed for an antiferromagnet with random spin orientation in an applied field [14]. The separation of the outer lines at maximum resonance is equivalent to a most probable effective field at the 57Fe nucleus of(52 + 2) kOe. Ferromagnetic behavior of the film can be excluded positively from this measurement since it is well known that in such a case (the magnetic moments being aligned parallel to the external field) a magnetic hyperfine pattern would be expected with sharp lines ("43.3 mm/s width in this case) and without the two Am = 0 lines. Furthermore, the outer peak distance should correspond to an effective field H etf of either H A - Ho or HA + Ho (i.e. about 30 kOe or about 70 kOe), depending on whether the internal field Ho (taking Ho ~ 20 kOe) is negative or positive, respectively. However, none of these consequences of ferromagnetism was observed. Paramagnetism, too, can be discarded by similar arguments concerning line width and the Am = 0 line intensity. The paramagnetic-antiferromagnetic transition temperature has been determined by measuring the 3,-Fe line width as a function of temperature. The results (fig. 2) which are entirely reversible with temperature, indicate a slight increase in line width in going from room temperature to about 100 K due to
194
W. Keune et al. / Antiferromagnetism of fcc Fe thin films
0.g
E "{ _Eo.7 }
-~.
x:
c•
(1.5
-4
-'~-
--
-°-
-
--° .
.
.
.
.
°_
_
03
100
200 Tempero|ure IK}
300
Fig. 2. ~-Fe line width as a function of temperature. an increase of the effective absorber thickness (being proportional to the Debye-Waller factor). The drastic line broadening due to antiferromagnetic ordering begins at about 80 K -+ 10 K. The observed transition temperature is not as sharp and is higher than the N6el temperature of 67 K measured for large 7 - F e percipitates [2,4,7-9]. Recent work on fcc Fe alloys has shown [15] that the N6el temperature of pure 7 - F e could be higher than 67 K (between 67 and 90 K), because large 7 - F e precipitates may contain about 1-3% Cu impurities [16,17] which could lower the N6el temperature to 67 K. Thus, the higher transition temperature of the fcc Fe films might indicate a very low Cu impurity content, i.e. little or no interdiffusion of Cu into the Fe layer during film deposition. The "smearing" of the transition could be due to: (i) varying local strains in the film which cause a distribution of N~el temperatures, since TN of fcc Fe (stainless steel) is known to depend quite sensitively on pressure [18]; or (ii) a broad distribution of island sizes in the ~18.8, thick Fe films, since it is known from M6ssbauer studies of 7 - F e precipitates that the transition temperature strongly drops with decreasing particle size [7,9] possibly due to superparamagnetic relaxation. In summary, our M6ssbauer investigation of " 1 8 A thick fcc Fe films vacuum deposited on (001) Cu substrates gives clear evidence for antiferromagnetism at low temperature and paramagnetism at 295 K. This result is not necessarily in contradiction with ferromagnetism observed at room temperature in fcc Fe films on (110) and (111) Cu substrates [10,11]. For example, these results could be seen as demonstrating a critical dependence of exchange energy on lattice
spacing, which might be slightly different for the various Cu substrate orientations. The apparent discrepancy of ferro- and antiferromagnetic ordering of fcc Fe films might also be explained by considering the spin structure. By using neutron diffraction of coherent 7 - F e precipitates in copper [1 ] and stainless steel [3] it was concluded that the atomic magnetic moments are oriented close to the [001 ] direction and that the spins in alternating ferromagnetic sheets of (001) planes are antiferromagnetically coupled to each other. Thus, the spins of (001) fcc films can align themselves within the film plane. In contrast, the spins of (111) films with the same magnetic ordering along [001 ] would form large angles with the film plane which might be energetically unfavorable, particularly for the surface spins. In a (001) film one can construct an antiferromagnetic [001] spin arrangement oriented within the film plane; however, the situation is more complex than that of (001) or (I 11) films because of the following: (001) and (111) surfaces are uniform in that the atoms on surfaces have a unique number of nearest neighbor surface atoms (eight in the case of (001) surfaces and nine in the case of (111) surfaces) while on (01 I) surfaces, atomic positions with seven and eleven nearest neighbors occur. Thus, it is suggested that the accommodation and alignment of the spins with regard to the film orientation could be a determining factor in the magnetic ordering of fcc Fe films. Specifically, (001) films maintain the bulk an tiferromagnetic ordering while (011) and (111 ) films change to a ferromagnetic ordering.
Acknowledgement This work was supported by the Deutsche Forschungsgemeinschaft.
References [1] S.C. Abrahams, L. Guttman and J.S. Kasper, Phys. Rev. 127 (1962) 2052. [2] G.J. Johanson, M.B. McGirr and D.A. Wheeler, Phys. Rev. B1 (1970) 3208. [3] Y. Ishikawa, Y. Endoh amd T. Takimoto, J. Phys. Chem. Solids 31 (1970) 1225. [4] U. Gonser, C.J. Meechan, A.H. Muir and H. Wiedersich, J. Appl. Phys. 34 (1963) 2373.
I¢. Keune et al. / Antiferromagnetism of fcc Fe thin films [5] L.D. Flansburg and N. Hershkowitz, J. Appl. Phys. 41 (1970) 4082 and references quoted therein. [6] B. Window, Phil. Mag. 26 (1972) 681. [7] D.L. Williamson, W. Keune and U. Gonser, Proc. Intern. Conf. on Magnetism, Vol. 1 (2) (Publ. House Nauka, Moscow, 1974). p. 246. [8] S.J. Campbell and P.E. Clark, J. Phys. F: Metal Phys. 4 (1974) 1073. [9] D.L. WiUiamsonand W. Keune, Proc. Intern. Conf. on M6ssbauer Spectroscopy, Vol. 1, A.Z. Hrynkiewicz and J.A. Sawicki, eds. (Akademia G6rniczo-Hutnicza, Cracow, Poland, 1975) p. 133. [10] J.G. Wright, Phil. Mag. 24 (1971) 217. [11] U. Gradmann, W. KIlmmerle and P. TiUmanns, Thin Solid Films 34 (1976) 249.
195
[12] L.O. Brockway and R.B. Marcus, J. Appl. Phys. 34 (1963) 921. [13] W.A. Jesser and J.W. Matthews, Phil. Mag. 15 (1967) 1097. [14] G.K. Wertheim, D.N.E. Buchanan and J.H. Wernick, Solid State Commun. 8 (1970) 2173. [15] H.H. Ettwig and W. Pepperhoff, Arch. Eisenhiittenw. 46 (1975) 667. [16] K.E. Easterling and H.M. Miekk-Oja, Acta Met. 15 (1967) 1133. [17] L.J. Swartzendruber and L.H. Bennett, Magnetism and Magnetic Materials, AlP Conf. Proc., No. 5, H.C. Wolfe, ed. (AIP, New York, 1972) p. 408. [18] D.R. Rhiger, R. Ingalls and Chun-Mai Liu, Solid State Commun. 18 (1976) 681.