Observation of Thickness Dependent Properties
in Novel Multiferroic Thin Films
K. Prashanthi, S.P.Duttagupa, R.Pinto, and V. R Palkar
Abstract— In this paper we report the thickness
dependent multiferroic properties of Bi0.7Dy0.3FeO3 films
grown on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition
technique. In these films, magnetic anisotropy is developed
non-linearly with the thickness. It is correlated to stress
developed during growth process due lattice mismatch,
difference in thermal coefficient, internal defects etc. The
lattice cell parameter c also changes arbitrarily with the
thickness of the film but correlates with stress. The saturation
polarization (Ps) values scale with c parameter. The
information obtained by this study would be significantly
useful in innovative devices planned with this advanced
material.
Index Terms— Multiferroics, Pulsed Laser Deposition,
Thin film.
I
I. INTRODUCTION
N recent years there is growing interest in developing
new systems that are exhibiting multiferroic properties
at room temperature. It is mainly due to their potential for
remarkable device applications and fascinating physics.
Moreover, most of the known multiferroic systems are
typically antiferromagnetic with transition temperatures
below room temperature [1,2]. Recently we have been
successful in achieving room temperature multiferroicity in
bulk as well as thin films of PbTi1-xFexO3 system [3,4]. We
have also demonstrated the existence of similar properties
in bulk and thin films of Bi0.9-xMxLa0.1FeO3 ( M - Tb, Dy)
[5-7]. More importantly, our recent unpublished work on
bulk has further indicated that in presence of Dy, additional
substitution with lanthanum at Bi site is not essential either
for stabilizing perovskite phase or to reduce leakage
current as in case of pure BiFeO3 [8]. Interestingly, the
removal of La leads to orders of magnitude enhancement
in magnetization value of the sample without increasing
the leakage current. However, in the interest of utilizing
such remarkable properties in device applications, it is
essential to reproduce the similar properties in thin film
form. In fact, various factors could influence the
characteristics of thin films. Hence there is need to
understand these parameters to achieve improved and
controlled multiferroic properties useful for the future
Manuscript received August 19, 2007.
The authors are with the Department of Electrical Engineering, Indian
Institute of Technology Bombay, Mumbai 400076, India, (email:
[email protected], [email protected])
devices. Thickness is one of the key factor which would
affect the physical properties of thin films [9] and hence
essential to study in detail. In this article, we are reporting
variation in multiferroic properties with thickness of
Bi0.7Dy0.3FeO3 thin films deposited on Pt/TiO2/SiO2/Si
substrate by using pulsed laser deposition (PLD) technique.
II. EXPERIMENTATION
The powder sample of Bi0.7Dy0.3FeO3 was prepared using
partial co-precipitation route described elsewhere [5]. The
powder material was compacted in the pellet form and
sintered at 800 0C for 2 hours. This highly dense pellet thus
obtained was used as target for PLD technique. The PLD
technique used for growing films is described earlier in
detail [10]. The parameters such as laser energy, target to
substrate distance, substrate temperature and oxygen
pressure in the chamber were optimized to achieve
insulating thin films of Bi0.7Dy0.3FeO3 with maximum
possible phase purity on Pt/TiO2/SiO2/Si substrate. Only
the deposition time was varied between 15-60 min. to get
films with different thickness. The thickness of the film
was estimated using profilometer (Ambios, USA). The
characterization of these films was done by various
techniques. Phase purity and crystal structure were
determined by X-ray diffraction (Fig. 1). Magnetic
properties (Fig. 2) were studied with the help of vibrating
sample magnetometer (Lakeshore, USA).
Using X Pert (PRO, USA) the elastic strain of the crystal
lattice, caused by the internal stress, is measured. It is done
by measuring the spacing d of lattice plane <100> and
comparing it to the lattice spacing of a stress-free material.
The diffraction data is represented as d-sin2 ϕ curve and
the stress is calculated from the slope. The ferroelectric
hysteresis loop measurement was carried out by using Pt as
bottom electrode and 100 × 100 µm chrome-gold pads as
top electrode using ferroelectric loop tracer from Radiant
Technology, USA. The hysteresis loops obtained for films
with different thickness are shown in fig. 3. The variation
of lattice cell parameter c and stress as a function of film
thickness is shown in fig.4
III. RESULTS AND DISCUSSIONS
Fig.1 indicates that all films with different thickness
ranging between 100nm – 500nm are polycrystalline in
nature. The presence of very small amount of Bi2Fe4O9
type of impurity phase has been detected in all films.
978-1-4244-1728-5/07/$25.00 ©2007 IEEE
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024,Pt
Pt substrate
110
*
Si
104
012
Intensity (arb.units)
1300
0
500 nm
*
650
*
-650
100 nm
30
40
50
2θ (deg)
Fig.1. X-ray diffraction patterns obtained for Bi0.7Dy0.3FeO3 thin films
grown on Pt/TiO2/SiO2/Si substrate with different thickness. ‘*’
indicates the presence of impurity phase
However, the percentage of impurity phase increases with
the thickness. The impurity phase is neither magnetic nor
ferroelectric so its presence could at the most reduce the
overall percentage of ferromagnetic and ferroelectric
component in the films. The films exhibit perovskite
structure and belong to space group of R3c.
Fig. 2 shows the magnetic behavior (M-H curve) of the
films with different thickness in // and ⊥ geometry. There
are various possible reasons that are known to cause
magnetic anisotropy in the films, for example, magnetocrystalline structure of the material and texturing, stress
developed during growth of the film, grain size
(crystalanity) etc. Out of which magnetocrystalline
anisotropy is an intrinsic property of a magnetic material
independent of grain size and shape. Depending on the
crystallographic orientation of the sample in the magnetic
field, the magnetization reaches saturation value at
different fields. Magnetocrystalline anisotropy is the
energy necessary to deflect the magnetic moment in a
single crystal or epitaxial thin films from the easy to the
hard direction. The easy and hard directions arise from the
interaction of the spin magnetic moment with the crystal
lattice (spin-orbit coupling). The same explanation is also
applicable in determining the magnetic properties of the
polycrystalline films if textured in particular direction [11].
In the presence of internal stress, the magnetization vector
attempts to orient itself to minimize the stress energy.
Since the overall domain structure is affected by the
presence of stresses, it affects the shape of the hysteresis
loop and its associated parameters. In addition, in
polycrystalline films owing to randomly oriented microstresses there may be many localized regions in which the
anisotropy is uniaxial. It may give rise to overall uniaxial
resultant stress. During magnetization, it could impede
domain wall moment and magnetization values [12].
Magnetic properties are also affected by grain size. Due to
pinning of magnetization at the grain boundaries Hc
increases as the grain size decreases [13]. In the present
case, since the films are polycrystalline with hardly any
texturing, anisotropy caused by the magetocrystalline
structure is not expected. In addition, as the grain size does
not change with the thickness, grain boundary contribution
M (emu/cc)
220 nm
20
// to field
⊥ to field
-1300
-10
1200
600
100 nm
-5
0
5
10
// to field
⊥ to field
0
-600
-1200
-10
1200
600
220 nm
-5
0
5
10
// to field
⊥ to field
0
-600
-1200
-10
500 nm
-5
0
5
10
Magnetic field (KOe)
Fig.2. Magnetization vs. applied magnetic field (M-H) curves obtained
for Bi0.7Dy0.3FeO3 thin films grown on Pt/TiO2/SiO2/Si substrate with
different thickness [(a) 100 nm, (b) 220 nm and (c) 500 nm] using VSM.
also remains same for all films. As a result, the effect of
crystallanity on magnetic properties remains the same. The
stress measurements revealed randomness in values with
increase in thickness. Large stress (294 MPa) observed for
100 nm thick film seems to be obvious since there is a
large lattice mismatch between top platinum layer of the
substrate (a-3.92Å) and Bi0.7Dy0.3FeO3 (a -5.6Å).
Interestingly, there is decrease in stress value to 99 MPa
with further increase in thickness to 200nm. However, it is
expected that as the thickness increases further growth of
the film get support from parent material itself. Due to
close lattice match, relaxation of the stress largely is
possible. Surprisingly further increase in thickness to 500
nm sets stress in opposite direction (235 MPa). For getting
films with the thickness of the order of 500 nm, the
ablation has to be carried out for nearly 1 hour. In practice,
it is very difficult to maintain the formation of exactly
same plume all through out the ablation process for such a
long duration, also, the target surface keeps being modified
with the time. Overall, it is likely to cause internal defects
and disorders in the films resulting into a heterogeneous
stress distribution, not only from one crystallite to another,
but even inside each crystallite. When the effect of the
stress distribution of various sub-microscopic defects is
taken in to account, it would of course increase the
heterogeneity of the stress distributions even more.
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2
Polarization (µC/cm )
be different from conventional ferroelectric systems [17]. It
is therefore interesting to observe that stress causes similar
effect here also.
30
5 00 nm
15
220 nm
0
100 n m
-1 5
-3 0
-70
-35
0
35
70
E le c t r ic fie ld ( K V / c m )
IV. CONCLUSION
The trend in variation of magnetic as well ferroelectric
properties could be mainly ascribed to stress induced during
film growth process. Since the stress does not vary
proportionately with the thickness mulltiferroic properties
also change abruptly. In our opinion, the information
obtained by this study could be significantly useful during
realization of devices using this novel room temperature
multiferroic system.
ACKNOWLEDMENT
Fig.3. Ferroelectric hysteresis loops obtained for Bi0.7Dy0.3FeO3
thin films grown on Pt/TiO2/SiO2/Si substrate with different thickness
13 .28
The authors wish to thank Prof. S. Bhattacharya (TIFR,
Mumbai) for discussion, Sahu and Kanu (IITB) for
experimental help.
4 00
3 00
13 .24
2 00
13 .20
Stress (M Pa)
c parameter (Å)
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1 00
13 .16
0
1 50
30 0
4 50
F ilm T hickn ess (nm )
Fig.4. Variation of lattice parameter, c and stress as a function of
thickness for Bi0.7Dy0.3FeO3 thin films grown on Pt/TiO2/SiO2/Si
substrate
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thick films. Jiang et al [9] have earlier observed similar
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effect can lead to substantial increase in spontaneous
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