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Optical and magnetooptical study of Fe-Al2O3 granular thin films
Journal
of Magnetism
OPTICAL
and Magnetic
Materials
54-57
AND MAGNETOOPTICAL
F. D’OkAZIO,
J.L. DORMANN
1309
(1986) 1309-1310
STUDY
OF Fe-Al *03 GRANULAR
*, D. FIORANI
THIN
FILMS
** and F. LUCARI
The optical absorption, magnetic circular dichroism and Faraday rotation on four samples of Fe-Al :O, granular film\ have been measured in the near infrared. One of these shows large FR values. Many optical transitions are observed and they are connected with Fe’+ and Fe’+ 10”s present inside the films.
1. Introduction Fe-Al,O,
granular
Table 1 thin
films
are composite
materi-
of small iron particles (50 A < $J < 80 A) dispersed in an amorphous Al,O, insulating matrix. Their magnetic properties have been extensively studied [1,2] and have found to be strongly dependent on the volume of the metallic particles and their concentration. The dynamic properties, investigated by AC susceptibility and Mbssbauer measurements, show some similarities with respect to the spin-glass behaviour. while significant differences are observed in the static ones [2]. als consisting
2. Experiments and results Four samples were deposited by cosputtering Fe and Al,O, as thin films (thickness about 0.5 pm) onto a silica substrate. X-ray photoemission spectroscopy and Massbauer spectra indicate that at the surface the iron is in a fully oxidized 3 + state, while the “bulk” is constituted by metallic iron and iron aluminate. The Fe’+ aluminate, which indicates an interaction between the metal particles and the alumina matrix, shows increasing percentages with decreasing particle size [3]. The bulk weight percentages were determined by EMB analysis and the particle size was measured by electron microscopy (table 1). On these samples we made measurements with the apparatus described in ref. [4] improved and connected on-line with a microcomputer. The magnetic field used in the measurements of magnetic circular dichroism and Faraday effect is static and its value is approximately 2.5 kOe. We devoted our attention principally to the sample S16 which has the highest iron content and which shows the highest blocking temperature of the magnetization III. In fig. 1 the optical absorption of this sample is reported. The measurements were made at room temperature in the range of wavelength 0.6-2.7 pm. The figure shows that this sample absorbs light strongly in the visible region, where our apparatus reaches its limit, * Laboratoire de Magnetisme, CNRS, 1 Place A. Briand, 92190 Meudon Bellevue. France. ** ITSE, CNR. CP 10, 00016 Monterotondo
0304-8853/86/$03.50
0 Elsevier
Stazione,
Science
Italy.
Publishers
Percentages in weight of iron and mean particles different samples
diameter
Sample
0(A)
Fe”
FeLi
Fe”
s12 s13 s17 Slh
50 60 70 80
33 36 37 38
7 8 10 14
10 12 13 14
for
while in the near infrared the absorption decreases rapidly as the wavelength increases. However. above this strong decrease it is possible to identify four small enhancements of the curve placed around 1.0. 1.2. 1.8 and 2.2 km indicative of the presence of optical transitions of the atoms composing the film. Fig. 2 shows the results of MCD measurements carried out at room and liquid nitrogen temperatures. The results are affected by a strong noise but is appears that the absolute differences in the absorption originated by the magnetic field are very small because the samples are very thin. However, we note that the signal intensity nearly doubles with the decrease in temperature and that at both temperatures the sample S16 has values a little higher than the other samples especially in the interval of wavelength I-1.6 pm. In addition it is evident that the tails of the stronger transitions are nearly exhausted over 1.8 Km. The strong magnetic dichroism signal indicates that the transitions. present in this wavelength range, originate from magnetic ions. Fig. 3 shows the results of the measurements of Faraday rotation (FR) at room and liquid nitrogen temperatures. Despite the small thickness of the samples the measurements do not show a particularly high noise and many interesting experimental conclusions can be obtained. The increase observed on lowering the temperature is almost the same as in the MCD measurements. The differences among the samples here are more evident especially looking at the results for Sl6. This sample reaches values of FR higher than 40000”/cm in the region of highest absorption but even when the wavelength increases it shows strong FR up to at least 1.2 pm with a structure of the spectrum which is quite detailed with maxima at 0.8. 0.95 and a broad one around 1 pm. The other samples provide evidence of some of these maxima but with intensity strongly reB.V.
Fig. 3. Faraday rotation versus wavelength measured at 300 K (higher part of the figure) and 77 K (lower part). (0) is for Slh. ( x) for SIZ, ( t) for S13 and (*) for Sl7.
variations
in intensity
for
the
transitions
can
he considered
when
observed nearly
changes.
the sample
over
I .4 km
independent
the
while intcnitv
of the sample.
3. Conclusions
The variation the composition 4
we note
that
ing
highest
the
the surface We
1’1g. 2. Magnrtlc circular dichroism vt‘r\us wavelength on dlfferent samplea measured at 300 K (higher part of the figure) and 77 K (lowr part). (0) IS for S16. ( X) for S12. C+ ) for Sl3 :rnd (*) for Sl7.
duced.
Over
1.4 FITI the differences
the samples a structure
measured
S12 has them km. at
at 1.5 and
S17 reached 1.7 and
attributed transitions
are smaller:
of the spectrum
with 1.95 pm.
the maxima
2.5 pm.
These
shifts
wavelengths
spectra
all the samples two
negative
have
of the tails which
2.05
2.4 pm and
in wavelength
of
maxima:
S13 at 1.55 and
at 1.6 and
to the superposition at lower
in the FR
S16
can
be
of the strong
show
very
large
lengths with above
(table
quantity
of the iron that
attributed just
below
different
tahedral).
may
of the samples S16 is the most
recall
transitions
in FR intensity
show 1.4 pm
local
he correlated
concentrated
of oxidized
with
1). In particular one contnin-
Fe ions
placed
at
particles.
in other
materials,
containing
to Fe’ r tom are observed 1 km.
On the other
coordination
transitions
which
hand
(tetrahedral are located
iron. at wave-
Fe” and
ions.
oc-
at I.1 and
[4.5].
.J.L. Dormann. D. Fioranl. J.L. Tholence Lund C’. Sella. J. Magn. Magn. Mat. 35 (1983) 117. D. Fiorani. J.L. Dormann. J.L. Tholence, L. Behsai\ and G. Villers. this conference. paper\ 5B2. E Papararzo. J.L. Dormann and D. Floranl, Phv\. Kev. B 2x (1983) 1154. F. Lucari. C. Maatrogluaeppe and G. Tomassrtt~. J. f’hw. C‘. 10 (1977) 4869. F. D’Oru~o. F. Lucarl. E. Turenrm and C;. Tomn~wtt~. fEEb Trans. on Magn. MAGI9 (1983) 1775.
of Magnetism
OPTICAL
and Magnetic
Materials
54-57
AND MAGNETOOPTICAL
F. D’OkAZIO,
J.L. DORMANN
1309
(1986) 1309-1310
STUDY
OF Fe-Al *03 GRANULAR
*, D. FIORANI
THIN
FILMS
** and F. LUCARI
The optical absorption, magnetic circular dichroism and Faraday rotation on four samples of Fe-Al :O, granular film\ have been measured in the near infrared. One of these shows large FR values. Many optical transitions are observed and they are connected with Fe’+ and Fe’+ 10”s present inside the films.
1. Introduction Fe-Al,O,
granular
Table 1 thin
films
are composite
materi-
of small iron particles (50 A < $J < 80 A) dispersed in an amorphous Al,O, insulating matrix. Their magnetic properties have been extensively studied [1,2] and have found to be strongly dependent on the volume of the metallic particles and their concentration. The dynamic properties, investigated by AC susceptibility and Mbssbauer measurements, show some similarities with respect to the spin-glass behaviour. while significant differences are observed in the static ones [2]. als consisting
2. Experiments and results Four samples were deposited by cosputtering Fe and Al,O, as thin films (thickness about 0.5 pm) onto a silica substrate. X-ray photoemission spectroscopy and Massbauer spectra indicate that at the surface the iron is in a fully oxidized 3 + state, while the “bulk” is constituted by metallic iron and iron aluminate. The Fe’+ aluminate, which indicates an interaction between the metal particles and the alumina matrix, shows increasing percentages with decreasing particle size [3]. The bulk weight percentages were determined by EMB analysis and the particle size was measured by electron microscopy (table 1). On these samples we made measurements with the apparatus described in ref. [4] improved and connected on-line with a microcomputer. The magnetic field used in the measurements of magnetic circular dichroism and Faraday effect is static and its value is approximately 2.5 kOe. We devoted our attention principally to the sample S16 which has the highest iron content and which shows the highest blocking temperature of the magnetization III. In fig. 1 the optical absorption of this sample is reported. The measurements were made at room temperature in the range of wavelength 0.6-2.7 pm. The figure shows that this sample absorbs light strongly in the visible region, where our apparatus reaches its limit, * Laboratoire de Magnetisme, CNRS, 1 Place A. Briand, 92190 Meudon Bellevue. France. ** ITSE, CNR. CP 10, 00016 Monterotondo
0304-8853/86/$03.50
0 Elsevier
Stazione,
Science
Italy.
Publishers
Percentages in weight of iron and mean particles different samples
diameter
Sample
0(A)
Fe”
FeLi
Fe”
s12 s13 s17 Slh
50 60 70 80
33 36 37 38
7 8 10 14
10 12 13 14
for
while in the near infrared the absorption decreases rapidly as the wavelength increases. However. above this strong decrease it is possible to identify four small enhancements of the curve placed around 1.0. 1.2. 1.8 and 2.2 km indicative of the presence of optical transitions of the atoms composing the film. Fig. 2 shows the results of MCD measurements carried out at room and liquid nitrogen temperatures. The results are affected by a strong noise but is appears that the absolute differences in the absorption originated by the magnetic field are very small because the samples are very thin. However, we note that the signal intensity nearly doubles with the decrease in temperature and that at both temperatures the sample S16 has values a little higher than the other samples especially in the interval of wavelength I-1.6 pm. In addition it is evident that the tails of the stronger transitions are nearly exhausted over 1.8 Km. The strong magnetic dichroism signal indicates that the transitions. present in this wavelength range, originate from magnetic ions. Fig. 3 shows the results of the measurements of Faraday rotation (FR) at room and liquid nitrogen temperatures. Despite the small thickness of the samples the measurements do not show a particularly high noise and many interesting experimental conclusions can be obtained. The increase observed on lowering the temperature is almost the same as in the MCD measurements. The differences among the samples here are more evident especially looking at the results for Sl6. This sample reaches values of FR higher than 40000”/cm in the region of highest absorption but even when the wavelength increases it shows strong FR up to at least 1.2 pm with a structure of the spectrum which is quite detailed with maxima at 0.8. 0.95 and a broad one around 1 pm. The other samples provide evidence of some of these maxima but with intensity strongly reB.V.
Fig. 3. Faraday rotation versus wavelength measured at 300 K (higher part of the figure) and 77 K (lower part). (0) is for Slh. ( x) for SIZ, ( t) for S13 and (*) for Sl7.
variations
in intensity
for
the
transitions
can
he considered
when
observed nearly
changes.
the sample
over
I .4 km
independent
the
while intcnitv
of the sample.
3. Conclusions
The variation the composition 4
we note
that
ing
highest
the
the surface We
1’1g. 2. Magnrtlc circular dichroism vt‘r\us wavelength on dlfferent samplea measured at 300 K (higher part of the figure) and 77 K (lowr part). (0) IS for S16. ( X) for S12. C+ ) for Sl3 :rnd (*) for Sl7.
duced.
Over
1.4 FITI the differences
the samples a structure
measured
S12 has them km. at
at 1.5 and
S17 reached 1.7 and
attributed transitions
are smaller:
of the spectrum
with 1.95 pm.
the maxima
2.5 pm.
These
shifts
wavelengths
spectra
all the samples two
negative
have
of the tails which
2.05
2.4 pm and
in wavelength
of
maxima:
S13 at 1.55 and
at 1.6 and
to the superposition at lower
in the FR
S16
can
be
of the strong
show
very
large
lengths with above
(table
quantity
of the iron that
attributed just
below
different
tahedral).
may
of the samples S16 is the most
recall
transitions
in FR intensity
show 1.4 pm
local
he correlated
concentrated
of oxidized
with
1). In particular one contnin-
Fe ions
placed
at
particles.
in other
materials,
containing
to Fe’ r tom are observed 1 km.
On the other
coordination
transitions
which
hand
(tetrahedral are located
iron. at wave-
Fe” and
ions.
oc-
at I.1 and
[4.5].
.J.L. Dormann. D. Fioranl. J.L. Tholence Lund C’. Sella. J. Magn. Magn. Mat. 35 (1983) 117. D. Fiorani. J.L. Dormann. J.L. Tholence, L. Behsai\ and G. Villers. this conference. paper\ 5B2. E Papararzo. J.L. Dormann and D. Floranl, Phv\. Kev. B 2x (1983) 1154. F. Lucari. C. Maatrogluaeppe and G. Tomassrtt~. J. f’hw. C‘. 10 (1977) 4869. F. D’Oru~o. F. Lucarl. E. Turenrm and C;. Tomn~wtt~. fEEb Trans. on Magn. MAGI9 (1983) 1775.