The effect of deposition and annealing conditions on textured growth of
sputterdeposited strontium ferrite films on different substrates
B. Ramamurthy Acharya, Shiva Prasad, N. Venkataramani, S. N. Shringi, and R. Krishnan
Citation: J. Appl. Phys. 79, 478 (1996); doi: 10.1063/1.360854
View online: http://dx.doi.org/10.1063/1.360854
View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v79/i1
Published by the American Institute of Physics.
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The effect of deposition and annealing conditions on textured growth
of sputter-deposited strontium ferrite films on different substrates
B. Ramamurthy Acharya,a) Shiva Prasad,a),b) N. Venkataramani,c) and S. N. Shringia)
Indian Institute of Technology, Powai, Bombay 400 076, India
R. Krishnan
Laboratoire de Magnétisme et Optique, CNRS, F-92195 Meudon, France
~Received 29 March 1995; accepted for publication 21 September 1995!
M -type strontium ferrite films were prepared by radio-frequency ~rf! sputtering on fused quartz
substrates using different deposition conditions and were subjected to two different types of
annealing treatments. The study showed that in addition to the deposition conditions such as rf
power, oxygen to argon ratio in the sputtering gas, and target to substrate distance, the
postdeposition annealing conditions also play an important role in determining the texture and
properties of the films. The films with random orientation or with preferred c-axis orientation either
normal to the film plane or in the film plane could be deposited depending on the process parameters
chosen. The study carried out by depositing these films on different substrates such as Si~100!,
Si~111!, sapphire~110!, and Gd3Ga5O12~111! showed that though the nature of the substrates plays
a role in determining the texture and properties of the films, such effects are less dominant in
comparison to the effect of deposition and annealing conditions in the case of strontium ferrite
films. © 1996 American Institute of Physics. @S0021-8979~96!05601-1#
I. INTRODUCTION
In recent years, M -type hexagonal ferrite films
~BaFe12O19 and SrFe12O19! have been extensively investigated due to their potential applications in magnetic and
magneto-optic recording, millimeter and microwave
devices.1–18 M -type ferrites have a large uniaxial magnetocrystalline anisotropy and generally the hexagonal c axis is
the easy axis of magnetization. Hence, there has been interest
in depositing these films having c-axis orientation. M -type
barium ferrite ~BaM! films with c-axis oriented normal to the
film plane have been reported by different workers.1–9 These
films have been deposited using different deposition techniques such as dc and rf diode sputtering, targets facing type
sputtering, laser ablation, etc.4 –7 These films have been crystallized either by in situ heating of substrates or postdeposition annealing of the films. In the case of in situ heating, the
dependence of the texture and the properties of the films on
deposition conditions and the nature of the substrates has
been studied to a certain extent.4 On the other hand, in the
case of the postdeposition annealing technique, no systematic work has been reported. Interestingly, in the case of postdeposition annealing, BaM films with in-plane anisotropy
have also been reported by some workers.10–14 Recently a lot
of interest has been focused on the films with in-plane anisotropy for realizing these films as longitudinal recording
media.10–14
In a recent article we reported the rf sputter deposition of
strontium ferrite ~SrM! films on amorphous fused quartz substrates with the c-axis normal to the film plane.17 We showed
that the rf power and argon to oxygen ratio in the sputter gas
play an important role in determining the texture and propa!
Department of Physics.
Electronic mail:[email protected]
Advanced Center for Research in Electronics.
b!
c!
478
J. Appl. Phys. 79 (1), 1 January 1996
erties of the films. For instance, by using a suitable combination of the above parameters, it was possible to deposit
SrM films with c-axis orientation normal to the film plane
even on amorphous fused quartz substrates. We have also
shown18 that the dependence of texture and properties of
SrM films on deposition conditions holds true even in the
case of crystalline Si~111! substrates. By varying deposition
conditions, films with c-axis orientation in the plane could
also be deposited on Si~111! substrates. In this paper, we
describe our detailed study which shows that in addition to
the deposition conditions, the postdeposition annealing conditions also play an important role in determining the texture
of the SrM films. We also report a comparative study of SrM
films deposited on various substrates such as amorphous
fused quartz, Si~100!, Si~111!, sapphire~110!, and
Gd3Ga5O12~111!@GGG~111!#.
II. EXPERIMENT
The films were prepared by rf sputtering in a Leybold
Z400 system. A disk of 3-in-diam cut from a commercial
M -type strontium ferrite was used as the target. Sputtering
gas was a mixture of argon and oxygen. Oxygen to argon
ratio ~R! was kept at 1.5% or 15%. The total gas pressure
was 631023 mbar. The rf power ~P! was varied from 60 to
330 W. The target to substrate distance was kept at 50 mm
except when mentioned otherwise. The substrates were neither heated nor water cooled during sputtering. Though the
temperatures of the substrates during deposition were not
measured, we do not expect the temperatures to be very high.
Our earlier studies using in situ heating of substrates had
shown that the films crystallize only at substrate temperatures higher than 800 °C.19 The deposition rates were of the
order of 8 –12 Å/min at P560 W to 35– 45 Å/min at P5330
W, depending on the R values. Presputtering of the targets
was carried out for 90 min prior to each deposition.
0021-8979/96/79(1)/478/7/$6.00
© 1996 American Institute of Physics
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FIG. 1. X-ray diffraction patterns for strontium ferrite films prepared with different rf power ~P! values and oxygen to argon ratios ~R!, and subjected to fast
annealing @~a! and ~b!#, and to slow annealing @~c! and ~d!#.
Two different types of annealing conditions were employed, which are referred to here as fast annealing and slow
annealing. For fast annealing, the films were introduced in a
furnace which was maintained at 900 °C. The annealing was
carried out for 2 h in air, after which the samples were furnace cooled. For slow annealing, the samples were heated to
900 °C in air at a rate of 100 °C/h. The temperature was then
maintained at 900 °C for 2 h after which cooling was also
carried out at a rate of 100 °C/h. The films deposited on
Si~100! and Si~111! substrates were annealed at 800 °C for 3
h instead of annealing at 900 °C for 2 h.
The crystal structure and texture of the films were determined using x-ray diffractometry. To evaluate the c-axis orientation of the films, the factor20
f c ~ 001! 5
C2C 0
12C 0
~1!
was calculated where C5SI(00l)/SI(hkl), where I(hkl) is
the intensity of ~hkl) peaks for the specimen films, and
C 0 5SI 0 (00l)/SI 0 (hkl) where I 0 (hkl) is the intensity of
the (hkl) peaks for a SrM powder diffraction pattern.21
f c (001) will be 1 for the film with complete c-axis orientation normal to the film plane and 0 for the films with complete random orientation. Similarly, f c (110) is calculated for
evaluating ~110! orientation in the film.
The magnetization and M – H loops were measured using the vibrating sample magnetometer ~VSM!. The thicknesses of the films were measured using a profilometer. The
films with thicknesses of the order of 2000 Å were used in
this study. The composition for the films were studied using
inductively coupled plasma analysis ~ICP! and energy dispersive x-ray analysis ~EDAX!.
J. Appl. Phys., Vol. 79, No. 1, 1 January 1996
III. RESULTS
A. The effect of annealing conditions
We shall first confine our discussions to the films deposited on fused quartz substrates. As has been reported earlier,
the as-deposited films did not show any x-ray diffraction
~XRD! peaks and magnetic order. When annealed above
800 °C, XRD peaks corresponding only to SrM were
observed.17–19 In Fig. 1 we show the XRD patterns for the
films deposited with four different values of rf power ~P! and
two different oxygen to argon ratios ~R!, and subjected to
fast and slow annealing.
1. Fast annealing
a. The effect of rf power. The XRD patterns corresponding to fast annealed samples are shown in Figs. 1~a! and 1~b!.
As seen in these figures, the films with lower P, e.g., P560
and 95 W show prominent ~00l! peaks along with a few
other peaks of less intensity. The XRD of the film with
P560 W and R515% shows the presence of ~006!, ~008!,
~001I 0I !, and ~001I 4I ! lines along with a less intense peak corresponding to ~107!. When P is increased for the same R
value, ~114! and ~203! peaks also start appearing along with
~00l! and ~107! lines. In the case of P5230 W, the films
show prominent SrM peaks other than ~00l!, indicating random orientation of the film. This indicates that as P increases
the c-axis orientation normal to the film plane decreases.
This is also clear from the f c ~001! values plotted against P in
Fig. 2~a!, wherein the f c ~001! changes from 0.8 to 0 as P
increases from 60 to 230 W.
At P5330 W, the fast annealed films show prominent
~110! and ~220! peaks, indicating that these films also have
texture but with the c axis in the film plane. Thus this study
shows that, in the case of fast annealed films, as P increases
Ramamurthy Acharya et al.
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479
FIG. 2. Preferential orientation factors f c ~100! and f c ~110! plotted against rf power values for fast annealed @~a! and ~b!# and slow annealed @~c!# strontium
ferrite films.
from 60 to 330 W, the texture of the film changes from ~00l)
orientation to random orientation and then to ~110! orientation.
In Figs. 3~a! and 3~b! we show the perpendicular and
in-plane M – H loops for two fast annealed films with P560
W, R515% and P5330 W, R515%, respectively. The perpendicular loop for the film with P560 W is rectangular
with a remanance ratio of nearly one, confirming the conclusion that the film has an easy axis ~c axis! normal to the film
plane. A fairly open in-plane loop with remanance of 0.41 is
observed which indicates the presence of a nonoriented fraction in the film, as was also indicated by the presence of a
few peaks other than ~00l!. The film with P5330 W shows
nearly rectangular loops both in perpendicular and in-plane
configuration with remanance ratios of 0.59 and 0.65, respectively. The loops measured, applying the fields in different
directions in the film plane, were similar indicating that there
is no preferential axis within the plane of the film. Hence in
these films the easy axis is oriented randomly within the film
plane. Such films which have the hard axis lying normal to
the film plane are of particular interest in the case of longi-
FIG. 3. M – H loops of strontium ferrite films deposited with ~a! rf power
~P!560 W, and oxygen to argon ratio ~R!515%, fast annealed; ~b! P5330
W, R515%, fast annealed; ~c! P560 W, R51.5%, slow annealed; and ~d!
P5330 W, R51.5%, slow annealed.
480
J. Appl. Phys., Vol. 79, No. 1, 1 January 1996
tudinal recording.10–14 The M s values were of the order of
70%–90% of the M s value for bulk SrM, which are similar
to the values reported in case of BaM films.4,10–11
b. The effect of oxygen to argon ratio. From Fig. 2~a!, it
is clear that for a particular value of P, the films with
R515% showed better f c ~001! values than the corresponding ones with R51.5%. As seen from Fig. 2~b!, for a particular value of P, fc~110! values are also higher for the films
with R515% than for those with R51.5%.
2. Slow annealing
a. The effect of rf power. XRD patterns for the slow
annealed films are shown in Figs. 1~c! and 1~d!. The slow
annealed films with P560 and 95 W showed c-axis orientation normal to the film plane as was observed in the case of
fast annealed films. As P is increased to 230 W the films
become randomly oriented as observed in the case of fast
annealing. However, no further c-axis orientation parallel to
the film plane takes place at 330 W. These results can also be
understood from the plot of f c (001) values against P for
these films shown in Fig. 2~c!.
In the case of slow annealed films, the films deposited at
P values of 60 and 95 W were of single phase SrM, but this
is not the case for the films deposited at 230 and 330 W.
Some additional peaks in XRD which are not indexed to
SrM appear for samples with higher P values, suggesting the
formation of some other phases. For example, the sample
with P5330 W and R51.5% shows all the prominent peaks
of a-Fe2O3 along with ~110!, ~107!, ~203!, ~205!, and ~220!
peaks of SrM @peaks indexed with ‘‘F’’ in Figs. 1~c! and
1~d!, such as F~110! correspond to a-Fe2O3#. This indicates
that the samples prepared with higher power when subjected
to slow annealing at 900 °C contain a-Fe2O3 as a second
phase.
In Figs. 3~c! and 3~d! we show the perpendicular and
in-plane M – H loops for slow annealed films with P560 W,
R51.5% and P5330 W, R51.5%, respectively. The M – H
loops for the slow annealed film with P560 W are similar to
corresponding fast annealed film. These films also showed
M s values of the order of 70%–90% of the bulk value. The
slow annealed film with P5330 W showed an M s value of
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FIG. 4. ~M r /M s )' , M r /M s ) i and H c values plotted against rf power for
strontium ferrite films prepared with two different oxygen to argon ratios,
and subjected to fast annealing @~a! and ~c!# and slow annealing @~b! and
~d!#.
53 emu/cc which is only 14% of the bulk value. As mentioned earlier, XRD of this film showed the presence of
a-Fe2O3 as a second phase. Since a-Fe2O3 is nonmagnetic,
this low value of M s can be expected.
b. The effect of oxygen to argon ratio. At lower P values
~P560 and 95 W!, unlike fast annealing, the films with
R51.5% showed better ~00l! orientation than the corresponding films with R515%.
3. The comparison
The higher P films when slow annealed showed two
distinct structural differences in comparison to corresponding
fast annealed samples.
~1! The films with P5330 W showed ~110! orientation
when fast annealed, whereas they did not show any such
preferential orientation when subjected to slow annealing.
~2! None of the fast annealed films showed the presence
of any phase other than SrM. On the other hand, slow annealed films with higher P values showed the presence of a
second phase.
The lower P films, on the other hand, showed c-axis
orientation normal to the film plane in the case of both slow
and fast annealing.
In Figs. 4~a! and 4~b! we show the M r /M s values plotted
against P for both fast annealed and slow annealed films. In
both cases the ~M r /M s )' values decrease with an increase in
P, showing a similar trend as observed in the case of f c (001)
values.
The fast annealed films showed coercivities in the range
of 4 –5.5 kOe, whereas slow annealed films showed coercivities in the range of 4 – 6.5 kOe. These coercivity values are
much higher in comparison to the one obtained by others in
the case of BaM films on similar substrates.1,4,10,11 In Figs.
J. Appl. Phys., Vol. 79, No. 1, 1 January 1996
FIG. 5. X-ray diffraction patterns for fast annealed strontium ferrite films
with different target to substrate distances for ~a! rf power ~P!560 W and
oxygen to argon ratio ~R!515%, and ~b! P5330 W and R515%.
4~c! and 4~d! we show ~H c )' values plotted against P. We
observe in the case of fast annealed films the ~H c )' values
increase with the decrease in P. In the case of slow annealed
films, ~H c )' decreases in general with a decrease in P.
Thus these studies show that the postdeposition annealing conditions also play an important role in determining the
texture and properties of SrM films.
B. The effect of target to substrate distance
We noted in the previous section that the fast annealed
SrM film with P560 W and R515% showed a good c-axis
orientation normal to the film plane. Also the film with
P5330 W and R515% showed a good degree of c-axis
orientation in the film plane. In order to see the effect of
target to substrate distance on the texture and properties of
the films, a study was carried out with changing target to
substrate distance ~d t-s ! to 25 and 50 mm for the above two
cases.
In Figs. 5~a! and 5~b! we showed the XRD of the films
deposited with different d t-s . For P560 W and R515%, the
c-axis orientation improves with the decrease in d t-s . At
d t-s 525 mm very sharp peaks corresponding to only ~00l!
planes are observed. The absence of any peaks other than
~00l! indicates that these samples are having a very high
degree of orientation with c-axis normal to the film plane. In
Fig. 6~a! we show the variation ~M r /M s )' and ~M r /M s ) i for
these samples. All the samples showed ~M r /M s )' as unity.
~M r /M s ) i decreases as d t-s decreases, indicating that the
samples with lowest d t-s has a better degree of orientation. In
Fig. 6~b! we show the variation of ~H c )' and (H c ) i with
d t-s . As seen from the figure coercivity in both the configuration decreases with a decrease in d t-s .
In Fig. 5~b! we show the XRD of the films prepared at
different values of d t-s but at P5330 W and R515%. For
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481
FIG. 7. X-ray diffraction patterns for fast annealed strontium ferrite films on
different substrates with ~a! rf power ~P!560 W and oxygen to argon ratio
~R! 15%, and ~b! P5330 W and R515%.
FIG. 6. Plots of M r /M s and H c values against target to substrate distances
for fast annealed strontium ferrite films with rf power560 W and oxygen to
argon ratio of 15%.
the sample with d t-s 550 mm, prominent peaks of ~110! and
~220! are seen along with low intensity peaks corresponding
to ~107! and ~114!. As d t-s is decreased, the relative intensity
of peaks other than ~110! and ~220! increases. This indicates
that for P5330 W, the c axis in-plane orientation is better in
the case of a higher value of d t-s .
The films prepared with d t-s 525 and 35 mm showed M s
values similar to those shown by the films with d t-s 550 mm.
C. The effect of substrate materials
In the case of BaM films prepared with in situ substrate
heating, it was reported that the nature of the substrates play
an important role in determining the texture of the films.2,3
As mentioned earlier, our study on amorphous fused quartz
and Si~111! substrates showed that the films prepared on two
substrates under similar conditions showed similar
properties.17,18 To further check this point we show a comparative study of the films deposited on different kinds of
substrates.
In Fig. 7~a! we show the XRD of the films deposited on
fused quartz, Si~100!, Si~111!, sapphire ~110!, and GGG~111!
prepared with P560 W and R515% and subjected to fast
annealing. As it is clear from these XRD patterns, all the
films show prominent ~00l! SrM peaks indicating a good
c-axis orientation normal to the film plane.
Similarly in Fig. 7~b! we show the XRD of the films on
different substrates with P5330 W and R515% and further
subjected to fast annealing. The films on Si~100! and Si~111!
482
J. Appl. Phys., Vol. 79, No. 1, 1 January 1996
showed prominent ~110! and ~220! SrM peaks like the films
deposited on fused quartz indicating that these films are oriented with the c axis in the film plane. However, the films on
sapphire were found to have random orientation even at 330
W.
These studies thus showed that though the nature of the
substrates play some role in determining the texture and
properties of the films such effects are less dominant in comparison to the effect of deposition and annealing conditions.
D. The other characterizations
Polar Kerr loops were measured on some of these films.
The films deposited on fused quartz substrates showed polar
Kerr rotation ~u k ! values in the range of 0.04°–0.11° and the
films deposited on Si substrates showed u k values in the
range of 0.04°–0.27°. One of the typical polar Kerr loops has
been shown in Fig. 8.
The torque magnetometer measurements were carried
out for some of the films and some of these torque curves
have already been reported.17,19 The values of uniaxial anisotropy constants obtained for different films were of the
order of 1.53106 to 2.63106 ergs/cc which were similar to
those reported for BaM films.5,11
FIG. 8. Polar Kerr rotation loop obtained for a strontium ferrite film on the
Si~100! substrate, which was deposited with rf power of 60 W, oxygen to
argon ratio of 15%, and slow annealed at 800 °C for 3 h.
Ramamurthy Acharya et al.
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We tried to analyze the film compositions using different
techniques and were successful in such an analysis only using ICP and EDAX. The detailed analyses using these techniques showed that the film compositions did not vary with
different parameters like rf power, oxygen to argon ratio, etc.
The actual variations measured were of the order of the accuracy of the methods employed. As reported by us earlier,
the conversion electron Mössbauer spectroscopic studies also
indicated that the films deposited under different conditions
retain the bulk composition.18 In the case of rf sputtered
BaM films, the change in composition of the films in comparison to the target composition has been reported by some
workers.4 This change has been attributed to the preferential
resputtering of Ba atoms due to higher mass ratio of Ba to Fe
atom.4 The mass ratio of Sr to Fe atom is much smaller and
might be one of the reasons for not observing any significant
change in the film composition with deposition parameters.
Also we had carried out a presputtering of the targets before
each deposition as mentioned in Sec. II. One of our important conclusions is the effect of annealing conditions. Since
the films which show different characteristics when subjected to two different annealing conditions were essentially
deposited under identical conditions, we do not expect composition changes in these films.
IV. DISCUSSIONS
Morisako et al.4,22 have reported a systematic work on
the effect of deposition conditions on the properties of BaM
films prepared by rf sputtering. They employed in situ heating of the substrates in their work, unlike our case where
postdeposition annealing of the films was carried out. There
has been no systematic work reported relating the texture and
properties to deposition and annealing conditions in the case
of hexagonal ferrite films prepared by postdeposition annealing. We make a comparison of our results with those reported
by Morisako et al.4,22
~1! Morisako et al.4,22 have maintained a constant power
density of 1.5/cm2 during sputtering. Hence they do not report any study on the effect of change of power density on
the texture and properties of the films. In our study we have
varied the P value from 60 to 330 W which correspond to a
power density change from 1.4 to 7.5 W/cm2.
~2! In a study to see the effect of oxygen partial pressure
on the film properties, they have changed the oxygen partial
pressure from 0 to 431023 mbar. They find that for a certain
range of low oxygen partial pressure, only a spinel phase
~Fe3O4! is formed. In a middle range of oxygen partial pressure, BaM grows on a spinel layer of ;300 Å. They reported
that their films showed the presence of XRD peaks corresponding to spinel structure along with BaM lines. At higher
oxygen partial pressures, they obtained a single phase BaM
film. For all the values of oxygen partial pressure, their films
showed f c (001).1. However, from the c-axis dispersion
angle obtained from the x-ray rocking curve, they show that
the c-axis orientation is better in the range where the BaM is
formed over the spinel layer, as the spinel layer promotes
c-axis oriented growth of a BaM. In comparison, our XRD
results did not show any peaks corresponding to the spinel
J. Appl. Phys., Vol. 79, No. 1, 1 January 1996
phase. Also, f c (001) values showed larger changes with the
change of R, even of the order of 20% in some cases.
~3! Morisako et al.4 studied the effect of target to substrate distance changing it from 35 to 67 mm. They showed
that the c-axis orientation normal to the film plane improves
for the samples with higher target to substrate distance. This
they attributed to decrease of bombardment of the films by
energetic ions during sputtering. However, our results were
contradictory to them. As shown above, the c-axis orientation normal to the film plane improved with decrease of d t-s
in our case.
The above discussions highlight the fact that in the case
of substrate heated BaM films, Morisako et al.4,22 have also
observed a dependence of texture and properties of the films
on deposition parameters. However, in general, these dependences are not similar to our case. We would, nevertheless,
like to mention that though it may not look surprising that
the properties of the films prepared with in situ heating show
a dependence on the process parameters, it does look interesting that the films crystallized only during postdeposition
annealing show a dependence of texture and properties on
deposition parameters. These results indicate that the deposition conditions leave their signature on the as-deposited
films which determine the texture and properties of the films
when annealed.
It is clear from the above discussions that the various
process parameters and postdeposition annealing do affect
the properties of the M -type ferrite films. It is, however, not
possible to provide physical reasoning for such effects, as the
processes involved here are very complicated. The sputtering
process involves a complex series of collisions involving a
series of angular deflections and energy transfers between
many atoms. It is very difficult to model the sputtering process to obtain the dependences of sputtering process on various experimental parameters.23 In the present case, the situation becomes more complicated since postdeposition
annealing conditions are also involved. Also, the structure of
the material is fairly complicated involving three different
atoms @Sr, Fe, and O#. However, the fact that similar postdeposition annealings lead to different properties of films
depending on deposition conditions, one feels that during
deposition, crystallites not detectable by XRD are formed
which act as nucleation centers for further crystallization
during postdeposition annealing. The microstructural studies
on as-deposited films using transmission electron microscopy are in progress which might lead to better understanding of the growth mechanism in these films.
ACKNOWLEDGMENTS
One of the authors ~B.R.A.! acknowledges the Council
of Scientific and Industrial Research, New Delhi, for financial support. The help rendered by Dr. S. Kumar and
Dr. K. V. Suryanarayana, Metallurgical Engineering Department, in x-ray diffraction measurements has been gratefully
acknowledged. We also acknowledge Dr. L. S. Mombasawala, Regional Sophisticated Instrumentation Center, for
ICP measurements and Dr. V. R. Palkar, Materials Science
Division, Tata Institute for Fundamental Research, Bombay,
for EDAX measurements.
Ramamurthy Acharya et al.
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Ramamurthy Acharya et al.
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