Conversion electron Mössbauer studies on strontium ferrite films with inplane and perpendicular anisotropies
Antony Ajan, B. Ramamurthy Acharya, Shiva Prasad, S. N. Shringi, and N. Venkataramani
Citation: J. Appl. Phys. 83, 6879 (1998); doi: 10.1063/1.367566
View online: http://dx.doi.org/10.1063/1.367566
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JOURNAL OF APPLIED PHYSICS
VOLUME 83, NUMBER 11
1 JUNE 1998
Conversion electron Mössbauer studies on strontium ferrite films
with in-plane and perpendicular anisotropies
Antony Ajan,a) B. Ramamurthy Acharya,b) Shiva Prasad, and S. N. Shringi
Department of Physics, Indian Institute of Technology, Bombay-400 076, India
N. Venkataramani
ACRE, Indian Institute of Technology, Bombay-400 076, India
Hexagonal strontium ferrite thin films with in-plane and perpendicular magnetic anisotropies have
been studied using conversion electron Mössbauer spectroscopy. The hyperfine parameters of the
films were found to be similar to the one obtained for bulk strontium ferrite. However, the intensity
ratios of the first and second lines of the Mössbauer spectra were found to be different for films
deposited at different rf powers. This indicated a change in the anisotropy of the deposited film. The
resultant change in the magnetization direction and crystallization texture was compared to other
measurements. © 1998 American Institute of Physics. @S0021-8979~98!20111-3#
I. INTRODUCTION
deposited at two different rf powers of P560 and 330 W
~power densities of 1.3 and 7.5 W/cm2, respectively! and two
different O2 /Ar gas ratios of R51.5% and 15%. These films
were prepared on 2 in. diam Si~111! wafers, and were enriched with 57Fe by placing a small quantity of enriched
Fe2O3 on the strontium ferrite target. The use of films with
larger areas and with enrichment by 57Fe is for enhancement
of the signal to noise ratio in the Mössbauer spectra. The
distance between the target and the substrate was maintained
at 50 mm. At P560 W, the deposition rates were 13 and 10
Å/s for R51.5% and 15%, while for P5330 W, they were
40 and 37 Å/s respectively. The films were prepared at ambient temperatures and were then annealed at 800 °C for 3 h
in air. Films of '240 nm thickness were used in this study.
The Mössbauer spectra for the target material were recorded in transmission mode by taking a small portion of the
material from the target used for sputtering. Mössbauer spectra of the thin films were recorded using the scattering mode
~CEMS!. A 10 mci 57Co source on a Rh matrix was used as
the Mössbauer source.
Recently a number of materials has been investigated for
application as thin film media for high density recording.
Hexagonal ferrite ~BaFe12O19 and SrFe12O19! thin films are
investigated due to their possible application in longitudinal
and perpendicular recording media.1–4 In addition to their
chemical stability and mechanical hardness, hexagonal ferrite media in both particulate and thin film mode have been
shown to exhibit lower recording noise.1–3 In thin film form
it is important to control the crystallographic texture, microstructure, and magnetic properties for such applications. The
properties of these thin films can be significantly varied by
changing the deposition conditions.
Mössbauer studies have been used in past to give information about the magnetic ordering, cation distribution, valence levels, impurity, etc. in ferrites. Mössbauer studies on
bulk polycrystalline strontium ferrite has also been
reported.5–7 In strontium ferrite, Fe31 is located at five different types of sites which are denoted in literature as 12k,
4 f 1 , 4 f 2 , 2a, and 2b. 5–7 Since Fe31 in different sites has
different hyperfine parameters, the resultant Mössbauer spectrum is a superposition of five subspectra corresponding to
five different Fe31 sites. The ratio of the area under the subspectra gives the ratio of occupation of these sites. In the
case of the reported bulk Mössbauer spectra only four sixline Mössbauer patterns have been resolved. The hyperfine
parameters of 4 f 1 and 2a are close to each other and hence
not resolved for polycrystalline materials. In this article we
report on the conversion electron Mössbauer spectroscopy
~CEMS! study of strontium ferrite films deposited by rf sputtering under various conditions. The results obtained were
compared with the Mössbauer studies on bulk.
III. RESULTS AND DISCUSSIONS
In Fig. 1 a Mössbauer spectrum for the bulk target material is shown. The hyperfine parameters were obtained
from the least square fit method. The hyperfine parameters
ob-
II. EXPERIMENTAL DETAILS
The strontium ferrite films were deposited by rf sputtering using a Leybold Z400 sputtering unit. The films were
a!
Electronic-mail: [email protected]
Present address: Toyota Technological Institute, 2-12-1 Hisakata,
Tempaku-ku, Nagoya 468, Japan
b!
0021-8979/98/83(11)/6879/3/$15.00
FIG. 1. Transmission Mössbauer spectrum for the bulk strontium ferrite
powder. The fitted spectra are shown as solid lines.
6879
© 1998 American Institute of Physics
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6880
Ajan et al.
J. Appl. Phys., Vol. 83, No. 11, 1 June 1998
TABLE I. The results of Mössbauer analysis of the strontium ferrite bulk
sample and thin films.
Sample
Site
Field
IS
QS
Intensity Relat.
~kOe! ~mm/s! ~mm/s! ratio, b intensity
Bulk
12k
4 f 1 , 2a
4f2
2b
412
491
512
422
0.37
0.27
0.40
0.32
0.42
0.16
0.29
2.47
2.0
48.6
27.5
19.7
4.2
Film A
60 W, 1.5%
12k
4 f 1 , 2a
4f2
2b
409
487
512
432
0.37
0.27
0.39
0.36
0.42
0.21
0.29
2.60
1.2
50.1
30.6
15.0
4.3
Film B
60 W, 15%
12k
4 f 1 , 2a
4f2
2b
409
487
517
427
0.37
0.27
0.39
0.32
0.42
0.21
0.29
2.68
0.9
49.4
31.8
16.6
2.2
Film C
12k
330 W, 1.5% 4 f 1 , 2a
4f2
2b
409
487
512
432
0.37
0.27
0.39
0.23
0.472
0.21
0.29
2.42
2.2
51.1
29.7
15.6
3.6
Film D
330 W, 15%
411
487
513
432
0.37
0.27
0.39
0.40
0.46
0.21
0.29
2.60
2.8
51.1
26.8
19.3
2.8
12k
4 f 1 , 2a
4f2
2b
ond lines of subspectra ~b! were iterated to obtain the best fit.
The intensity of the second and fifth lines, ~b!, was taken to
be the same for all the subspectra. This is because of the spin
structure of SrFe12O19, where spins of Fe31 at different sites
are parallel to the c axis. The various parameters of the fitted
Mössbauer spectra are listed in Table I. A comparison of
these parameters with the bulk strontium ferrite values shows
that the ratios of the area under different subspectra match
well with bulk values within an error of 64%. This shows
that the strontium ferrite films have a nearly stoichiometric
composition and a cationic distribution like that of the bulk.
The presence of any second phase would have led to a modification in the spectra from the bulk. Hyperfine field values
obtained for all the samples agree well with the bulk sample.
The isomer shift ~IS! as well as the quadrupole shift ~QS!
values are also found to agree within the experimental error
tained for the bulk sample are listed in Table I. The hyperfine
parameters obtained agree well with the reported values for
bulk strontium ferrite.
The Mössbauer spectrum for one of the as-deposited
films is shown in Fig. 2. The as deposited films show a
paramagnetic doublet spectrum. Magnetization data on as
deposited samples show a similar result.8 Mössbauer spectra
of the crystallized films deposited at different rf powers and
different O2 /Ar ratios are shown in Fig. 3. All the spectra
look identical except the second and fifth peaks which are
positioned at '24 and 14 mm/s, respectively. The intensities of the second and fifth lines are comparatively high for
the film which is deposited at higher power, i.e., 300 W
compared to the film deposited at a lower power ~60 W!.
The hyperfine parameters were obtained from the least
square fit method by fitting Lorentzian lines. The values of
different parameters like hyperfine field, isomer shift, quadrupole splitting, and the intensity ratios of the first and sec-
FIG. 2. Conversion electron Mössbauer spectrum for the as deposited strontium ferrite film on Si~111!, deposited at rf power 5 60 W and on O2 /Ar
ratio of 15%.
FIG. 3. Conversion electron Mössbauer spectra for strontium ferrite films on
Si~111! with different powers and O2 /Ar ratios.
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Ajan et al.
J. Appl. Phys., Vol. 83, No. 11, 1 June 1998
TABLE II. Magnetic properties of strontium ferrite thin films deposited
under different conditions.
Hc(kOe)
Sample
Power
~W!
O2 /Ar
~%!
i
A
B
C
D
60
60
330
330
1.5
15
1.5
15
3.9
3.1
3.9
3.6
M r /M s
'
Ms
~emu/cc!
i
'
4.1
3.4
3.7
3.5
240
270
280
275
0.50
0.45
0.60
0.70
0.94
0.98
0.55
0.61
~60.04 mm/s!. indicates the chemical as well as the structural environments in thin film form are identical with the
bulk strontium ferrite.
The b value, which indicates the local magnetic moment
direction with respect to the incident g ray direction, was
found to vary significantly with the rf power and the O2 /Ar
ratio. For film deposited at the 60 W the b value decreases
from 1.2 to 0.9 as the O2 /Ar ratio increases from 1.5% to
15%. This b value, lower than 2, indicates that the magnetic
moment direction tends to lie perpendicular to the plane of
the film. The orientation is better for a higher O2 /Ar ratio.
For a film deposited at 330 W the b value increases from 2.2
to 2.8 as the O2 /Ar ratio increases from 1.5% to 15%. This b
value, higher than 2, indicates that the magnetic moment
direction tends to lie in the plane of the film. Again the
orientation is better for a higher O2 /Ar ratio. Thus our results
indicate that for the same rf power the orientation improves
with a higher O2 /Ar ratio. A value of b52 indicates that a
random orientation was found from fitting the bulk polycrystalline powder sample. The x-ray diffraction ~XRD! studies8
carried out on Sr ferrite films deposited at 60 W and at different O2 /Ar ratio show a predominant c-axis orientation
perpendicular to the film plane. The sample deposited at
higher power ( P5330, W) and with a different O2 /Ar ratio
had a predominant c-axis orientation in the film plane. The
easy axis was randomly distributed within the film plane.
The magnetic properties of the films used in this study
are listed in Table II.
The coercivity (H c ) values decrease from 4.1 kOe for
the sample deposited at 60 W with a 1.5% O2 /Ar ratio to 3.4
6881
kOe for the sample deposited at 60 W with a 15% O2 /Ar
ratio. Similarly, the H c values decrease from 3.7 kOe for the
sample deposited at 330 W with a 1.5% O2 /Ar ratio to 3.4
kOe for the sample deposited at 330 W with a 15% O2 /Ar
ratio. Other properties such as M r /M s , etc. also show a
considerable change with the change in the O2 /Ar ratio, as is
evident from Table II. Large M r /M s confirms that samples
with 60 W power have a perpendicular orientation.
It is interesting to note that even though the orientation is
better for a higher O2 /Ar ratio the coercivity is comparatively small. A study by Acharya et al.9 on these samples
indicated that intergranular interaction increases with the
O2 /Ar ratio. This intergranular interaction could be the cause
for the reduced coercivity in these films even though the
orientation has improved with increasing O2 /Ar ratio.
IV. CONCLUSION
Hyperfine parameters obtained from Mössbauer analysis
were found to be similar in bulk and in thin film samples.
The b value, on the other hand, is found to be different from
the bulk and was also dependent on power and the O2 /Ar
ratio. This indicates textured growth in these films which
was confirmed by XRD studies.9 These studies indicate that
by varying the depostion condition we could achieve films
with either the easy axis parallel to or perpendicular to the
plane of the film. This information can be very useful for
longitudinal and transverse magnetic recording.
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