APPLIED PHYSICS LETTERS 99, 042503 (2011)
Evolution of magnetic bubble domains in manganite films
S. R. Bakaul, W. Lin, and T. Wua)
Division of Physics and Applied Physics, School of Physical and Mathematical Sciences,
Nanyang Technological University, Singapore 637371, Singapore
(Received 19 April 2011; accepted 2 July 2011; published online 26 July 2011)
We report a thickness-dependent evolution of magnetic domains from long stripe-like to bubblelike entities in La1xSrxMnO3 (x 0.3) (LSMO) films grown on LaAlO3 substrates. By using 2-D
fast Fourier transformation of magnetic force microscopy images and power spectral density
function, we accurately determine the domain width in LSMO films with a wide range of thickness
(50–325 nm). We find that the domain size scales with the Kittel’s square root law [C. Kittel, Phys.
Rev. 70, 965 (1946).] only when reduced film thicknesses are used, which suggests the critical role
C 2011 American Institute of Physics.
of substrate-film interaction in domain formation. V
[doi:10.1063/1.3615708]
As a model complex oxide with strong correlations
between charge, spin, orbital, and lattice, mixed-valence perovskite manganites have attained a lot of attention because
of not only the rich physics but also the application potentials.1 A prominent material in the manganite family is
La1xSrxMnO3 (x 0.3) (LSMO), owing to its half metallicity, colossal magnetoresistance (CMR), and high Curie temperature (Tc).2,3 To make use of this versatile material in
nanoscale devices, understanding its magnetic properties, in
particular domain characteristics, is of utmost importance.
However, there has been a lack of insight on the domain
physics in manganite thin films although it is a consensus
that the magnetic domain properties of manganites are markedly different from the conventional metal counterparts.
Magnetic domains in manganite thin films are often believed
to be elusive, and even invisible if the appropriate deposition
conditions are not satisfied.
In addition, both substrate strain and film thickness often
play crucial roles in determining the domain physics, producing convoluted effects. For instance, LSMO films with a
thickness larger than a critical value and grown under a compressive strain on LaAlO3 (LAO) substrates show perpendicular labyrinthine domains, whereas those grown under a
tensile strain on SrTiO3 (STO) substrates exhibit “featherlike” in-plane domain structures.4,5 Furthermore, the domain
size appears to increase with film thickness.6,7 However, it is
unclear whether the manganite thin films follow the
“universal” Kittel’s square root law which governs most ferromagnetic thin films.8 Basically, the Kittel’s law predicts
that the domain size should increase in a square root fashion
with the film thickness, as a result of minimization of the
sum of magnetic energy inside domains, anisotropy energy,
and domain wall energy. However, the extricate interactions
between manganite thin films and underlying substrates often
lead to thickness-dependent phenomena, contributing to the
complexity of this problem. Thus, understanding the domain
physics in manganites entails systematic precise measurements on the domain characteristics of LSMO films with a
wide range of thickness.
a)
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0003-6951/2011/99(4)/042503/3/$30.00
In this letter, we report on the thickness (50–325 nm) dependent evolution of magnetic domains in LSMO thin films
grown on LAO. Thinner samples exhibit long and continuous stripe-like domains with various degrees of local orientational order, whereas the domains in thicker samples curve
up and break into bubble-like entities. By using 2-D fast
Fourier transformation (FFT) and power spectral density
(PSD) techniques, we accurately determined the domain
sizes in the LSMO films. The clear deviation from the Kittel’s square root law, particularly in thinner films, suggests a
reduced effective thickness due to the substrate effect.
LSMO films were grown on (100) LAO substrates in a
pulsed laser deposition system, and the growth was monitored in situ using reflection high-energy electron diffraction
(RHEED). The depositions took place at 750 C and under
an oxygen pressure of 0.27 mbar, followed by a controlled
cooling down to room temperature at a rate of 5 C/min and
in 100 mbar O2. The structure and strain states of the thin
films were characterized by using an x-ray diffractometer
(Rigaku, SmartLab, Japan). The film thickness was complimentarily checked by using x-ray reflectivity and examining
the sample cross section in a scanning electron microscope.
Surface topography and magnetic domain images were captured at room temperature using tapping mode atomic force
microscopy (AFM) and magnetic force microscopy (MFM),
respectively. Low moment tips (Veeco MESPLM, 3 1014
emu, 15 nm CoCr reflex/tip coated) and a lift height of 30
nm were used to avoid any tip induced magnetic
perturbation.
The MFM images shown in Fig. 1 unveil an interesting
and so far unreported thickness dependent evolution of magnetic bubble domains in LSMO films. The samples thinner
than 50 nm do not exhibit any MFM contrast, which corroborates with previous works.9 As the thickness increases, MFM
contrast becomes enhanced and dense labyrinthine stripe
domains locally emerge (Fig. 1(a)). With further increase of
film thickness, e.g., 95 and 105 nm sample shown in Figs.
1(b) and 1(c), these labyrinthine stripe domains attain more
visibility, indicating enhanced magnetization component orienting towards the perpendicular direction. Although, the
stripe domains do not have any preferred in-pane orientation
which is reflected by the symmetric angular distribution in
99, 042503-1
C 2011 American Institute of Physics
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042503-2
Bakaul, Lin, and Wu
Appl. Phys. Lett. 99, 042503 (2011)
FIG. 1. (Color online) MFM images of LSMO films with ten different thicknesses from 50 to 325 nm. The scan size of all images is 5 lm 5 lm. Insets show
2-D FFT patterns of the corresponding MFM images. In (d) and (e), the arrows mark the preferred orientations of stripe domains.
the corresponding FFT patterns (insets of Figs. 1(b) and 1(c)),
they are locally parallel, long, and unbroken. Interestingly, the
stripe domains become more “straight” as film thickness further increases, as shown in Figs. 1(d) and 1(e) taken on the
117 nm and 135 nm samples, leading to asymmetric FFT patterns (insets of Figs. 1(d) and 1(e)). Similar observations were
reported in Co films grown on mica substrates,10 where the
less straightness of stripe domains in films with reduced thickness was attributed to island-prone growth behaviors. Similarly, in thinner LSMO films (t < 105) the morphological
roughness and inhomogeneous distribution of strain may
curve up the long domains, erasing the directionality.
As the films grow thicker beyond 135 nm, the domains
start to break into bubble-like branches (Figs. 1(f)-1(j)), giving rise to symmetric FFT patterns. Conversion of stripe
domains into bubbles was previously achieved by applying a
perpendicular magnetic field in epitaxial Co films and was
attributed to the change of Zeeman energy.11 In contrast, our
samples were not exposed to any external magnetic field,
and instead an increase in film thickness leads to the effect.
Interestingly, the LAO substrate underneath has long straight
terraces with a width of 80–100 nm,12 commensurate with
the dimension of stripe domains. Similar lock-in of stripe
domains to substrate features was previously reported for
GdFe films.13 However, as the films grow thicker, the interaction between the film and the substrate steps gradually
weakens, and the free energy minimization breaks up stripe
domains into bubble like features in MFM images.
While the angular distribution of FFT patterns describes
the directionality of the stripe domains, their 1-D PSD (see
inset of Fig. 2) provides a better measure of the domain sizes
than the commonly used stereological method. The peak of
the PSD curve unambiguously points out the wavelength of
the most predominant feature in an image, where multiple
frequency components may be present. Thus the domain
sizes were accurately determined, and they systematically
increase from 45 6 10 nm (the 50 nm sample) to 165 nm
(the 325 nm sample), as shown in Fig. 2.
Surprisingly, the domain size vs. thickness data does not
follow the Kittel’s square root law. As shown in Fig. 2, a
simple direct fitting to this basic scaling law overestimates
the domain width for the thinner samples. At lower film
thickness, deviations from theoretical prediction of magnetic
properties such as saturation magnetization, uniaxial anisotropy coefficient and Curie temperature have been reported
and attributed to the presence of substrate effect.2,14–17 In
addition, the presence of oxygen vacancies may also play
important roles in magnetic exchange interactions.18 In line
with these observations, we also found that the saturation
magnetization decreases from 313 emu/cm3 for the 325 nm
sample to 269 emu/cm3 for the 50 nm sample. Similarly, the
substrate strain influences the LSMO layer, affecting the domain formation. This motivated us to fit the data by applying
Kittel’s law with a reduced effective film thickness, t tr.
Interestingly, tr ¼ 34 nm gives the best fitting result.
To understand the substrate strain effect, we measured
the lattice constants of LSMO films (Fig. 3). The samples
FIG. 2. (Color online) Thickness (t) dependence of domain width (W) in
LSMO films. Also shown are the square root fitting results, where the modified Kittel’s law with reduced film thickness can explain the data. Inset
shows the normalized PSD function for the 105 nm sample whose peak position gives the domain width.
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042503-3
Bakaul, Lin, and Wu
Appl. Phys. Lett. 99, 042503 (2011)
FIG. 4. (Color online) (a) Room temperature in-plane and out-of-plane M-H
loops for (a) 50 nm and (b) 325 nm LSMO samples. (c) Thickness dependent
out-of-plane coercivity.
FIG. 3. (Color online) Thickness-dependent evolution of lattice constants in
LSMO films with different thicknesses. For comparison, the lattice constant
of LAO is also shown.
thinner than 135 nm appear to be more strained. This is consistent with the observation of isotropic bubble domains in
Fig. 1. Importantly, a very recent study on LSMO thin films
revealed that the growth substrate can lead to more profound
effects than mere strain or magnetic dead layers.19 Due to
doping instability and electronic reconstruction in the Mn eg
orbitals, the Mn3þ moments at the near interface region may
couple antiferromagnetically, and the superexchange coupling between Mn3þ and Mn3þ sites can affect the double
exchange interaction.19 Such a substrate-induced long range
interaction may perturb the domain formation in our LSMO
films, giving rise to the reduced effective thickness as well
as the rich domain characteristics.
The room temperature M-H curves in Figs. 4(a) and 4(b)
for both 50 and 325 nm samples clearly indicate that the
magnetic easy axes lie in plane irrespective of film thickness.
These results are in line with previous works on LSMO/LAO
films,4,6 which suggest that the formation of bubble domains
in manganite thin films is uncorrelated to the direction of
magnetic easy axis. At zero magnetic field, the magnetization is oriented perpendicular to the substrate plane, but
when a finite magnetic field is applied, the strong demagnetization field in the perpendicular direction results in a hard
magnetic axis. On the other hand, the thin films are easy to
be magnetized in the in-plane direction, but the magnetization is too soft to retain a notable in-plane remnant
magnetization.
As shown in Fig. 4(c), the out-of-plane coercivity of
LSMO films monotonously decreases from 55 to 10 Oe as
the film thickness increases from 50 to 325 nm. The
enhanced coercivity in films with reduced thickness was previously reported in LSMO films grown on other substrates,20,21 which was attributed to domain wall pinning at
the substrate-film interface. Similarly, in thinner LSMO
films, the surface features of LAO substrates could serve as
centers for pinning domain walls, but this substrate-induced
effect would gradually diminish as the films grow thicker,
which is also in line with the thickness-dependent evolution
of domain characteristics as we detailed above.
To summarize, we report a continuous thickness dependent evolution of long stripe to bubble magnetic domains in
compressively strained LSMO films. It is clear that LAO substrates play important roles in not only stabilizing the bubble
domains in thicker films but also giving rise to stripe domains
as well as enhanced coercivity in thinner films. Furthermore,
we found that the size of magnetic domains in LSMO films
follow the Kittel’s square root scaling law only if reduced film
thicknesses are used, underlining the critical role of substratefilm interactions in the intricate domain formation.
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