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Submitted on 1 Jan 1979
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DEPTH PROFILING OF THE MAGNETIC
HYPERFINE FIELD IN ULTRATHIN FILMS OF Fe USING MÖSSBAUER SPECTROSCOPY
A. Owens, C. Chien, J. Walker
To cite this version:
A. Owens, C. Chien, J. Walker. DEPTH PROFILING OF THE MAGNETIC HYPERFINE FIELD
IN ULTRATHIN FILMS OF Fe USING MÖSSBAUER SPECTROSCOPY. Journal de Physique Col-
loques, 1979, 40 (C2), pp.C2-74-C2-75. �10.1051/jphyscol:1979225�. �jpa-00218623�
JOURNAL DE PHYSIQUE Colloque C 2 , supplkment au n o 3, Tome 40, mars 1979, page (22-74
DEPTH P R O F I L I N G OF THE MAGNETIC HYPERFINE F I E L D I N U L T R A T H I N F I L M S OF Fe U S I N G MOSSBAUER SPECTROSCOPY +
A.H. Owens, C.L. Chien and J.C. Walker
The Johns Hopkins University, Baltimore, Mary land 21218, U.S.A.
RQsum6.- Nous utilisons la spectroscopie Msssbauer pour ddterminer les champs hy erfins 1 des pro- fondeurs diffsrentes dans des couches minces de Fe. Les couches se composent de 96Fe avec une pe- tite couche de 5 7 ~ e situde B des profondeurs diff6rentes dans les couches principales.
Abstract.- Mzssbauer spectroscopy has been used to determine the hypergine fields and isomer shifts at different depths in a series of epitaxial Fe films all of about 60 A thickness. The films were made of isotopically pure 5 6 ~ e with a thin probe layer of 5 7 ~ e placed at different depths.
1. Introduction.- Over a number of years we have been studying ultrathin epitaxial films of iron using MGssbauer spectroscopy in order to investigate the effects of reduced dimensionality on the proper- ties of magnetic systems. These effects have shown up as changes in the magnetic hyperfine field and Curie temperatures as a function of film thickness /I/. In this system, the magnetic hyperfine field is proportional to the magnetization. Hyperfine field measurements as a function of temperature and film thickness serve to provide information on the behavior of the magnetization as a function of these two variables. In our earlier work we observed a decrease of average hyperfine field with decreasing film thickness for temperatures above about 50 K, but an increase of average hyperfine field with de- creasing film thickness for lower temperatures.
Along with this behavior we observed an increase in line width and an increase in isomer shift for smal- ler film thicknesses. These observations indicated to us that much of this unusual magnetic behavior might be associated with the film surface, and that
it would be useful to determine the magnetic hyper- fine field as a function of depth in the film. This was made possible by growing a series of films, all
2 and 18
1
and get quite food statistics in the spec- tra.2. Thin Film Preparation.- The iron films are grown by vapor deposition onto single crystal (111) Ag substrates which had been produced just prior to the Fe deposition by epitaxial growth of Ag onto air- cleaved iron-free synthetic mica surfaces in an ul- trahigh vacuum system. Maximum pressures during de- position were 8x10-' torr. From previous studies of this epitaxial system it is known that iron grows on the silver with the (110) plane parallel to the (111) Ag surface. The interdiffusion of Fe in Ag should be negligible, and flat growth of the Fe films is expec- ted with complete surface coverage for Fe thicknesses of 50
fi.
An overcoat of about 20001
of Ag is added over the iron to prevent oxidation. These expecta- tions were verified by layer-by-layer Auger electron spectroscopy following argon iron sputtering of the completed films.The film thicknesses was determined to an accu- racy of 10% using an in-situ quartz crystal oscilla- tor during deposition and X-ray fluorescence whenthe film was removed from the vacuum system for analysis.
3. Results.- Figure 1 shows a typical spectrum taken at 4.2 K.
of about 66
1
thickness, using for most of the filml.0000
isotopically pure 5 6 ~ e which shows no Mijssbauer ef-
fect. During deposition a thin layer of " ~ e was ,9989 place at varying depths in the films to serve as a 9976 probe of local magnetic hyperfine field when the
,9962
~zssbauer spectrum was taken. Very high count rate
techniques developed in this laboratory /2/ made it ,9942
possible to use 5 7 ~ e layers with thicknesses between -8 -6 -4 -2 0 2 4 6 8 VELOCITY(rnm/s)
+ Work supported by NSF Grant DMR 77-07-127 Fig. 1 : A typical spectrum taken at 4.2 K 18 of 5 7 ~ e on the lower surface of 62 i( total thickness.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979225
Very high count rate techniques were used for a 62 film with an 18
%
layer of 5 7 ~ e at the lower surface. The average magnetic hyperfine field is 344 kG. Figure 2 shows the magnetic hyperfine fields for the group of films all of about 20 atomic layers in thickness, with the probe " ~ e layers at diffe- rent depths.NUMBER OF LAYERS
Fjg. 2 : Magnetic hyperfine fields for the group of fllms of -20 layers at 4 . 2 K.
The vertical error bars indicate uncertainties in determination of the average hyperfine field, while the horizontal bars, indicate the thickness of the 5 7 ~ e layer used as a probe. The solid line is not a fit to the data but is only drawn as a guide It can be noted that the center of the film shows a hyper- fine field of 341 kG which is approximately equal to the bulk value at 4.2 K, but near the surface the hyperfine field increases by about 8 kG. The isomer shift measurements shown in figure 3 show the same trend (the isomer shifts showed no qualitative chan- ge as a function of temperature).
I I 298K
327 . '
4 8 12 16 2 0
NUMBER
OF LAYERS
Fjg. 3 : Isomer shift variation for the group of fllms of -20 layers at 298 K.
Finally, the hyperfine fields at 298 K are shown in figure 4 . It can be seen here that the hyperfine field near the surface is now than the value at the center of the film by about 5 kG. These results are similar to those seen for whole films of 5 7 ~ e /I/, but it is now clear that departures from bulk values of the magnetic hyperfine field are associa- ted with the surface. The isomer shift indicates a
decrease in s-electron density at the nucleus of Pe atoms near the surface which may imply a decrease in the conduction electron contribution to the magnetic hyperfine field. Since the overall field is negative and the conduction electron contribution positive
/ 3 / , this might explain the low temperature increase
in hyperfine field near the surface.
NUMBER
OF
LAYERSFig. 4 : Magnetic hyperfine fields for the group of films of -20 layers at 298 K.
At higher temperatures all molecular field calcula- tions indicate a decrease in magnetization near the surface as observed / 4 / . Clearly more study is nee- ded of this complex problem, but Mgssbauer spectros- copy seems to offer a unique means of probing the magnetization distribution across the thickness of ultrathin ferromagnetic films.
References
/ 1 / Duncan, S., Owens, A., Semper, R. and Walker, 3 . Hyperfine Interactions ( 1 9 7 8 ) 386.
/ 2 / Semper, R., Guarnieri, C., and Walker, J., Nucl.
Instrum. Methods
2
( 1 9 7 5 ) 4 4 7 ./ 3 / Watson, R., and Freeman, A., Phys. Rev.
123
( 1 9 6 1 ) 2027.
/ 4 / Corciovei, A . , Vananu, D., Rev. Roum. Phys.
2
( 1 9 7 0 ) 4 7 3 .
