Complex Epitaxial Oxides: Synthesis and Scanning Probe Microscopy
Goutam Sheet,1 Udai Raj Singh,2 Anjan K. Gupta,2 Ho Won Jang,3 Chang-Beom Eom3 and Venkat Chandrasekhar1
1Department
of Physics and Astronomy, Northwestern University, Evanston, IL,USA
2Dept. Of Physics, Indian Institute of Technology, Kanpur, India
3Dept. Of Materials Science and Engineering, University of Wisconsin-Madison, Wisconsin, USA
Perovskite oxides exhibit exotic physical properties like ferroelectricity, ferromagnetism, and
superconductivity. Heterostructures of more than one such oxide with different physical
properties may be tailored leading to multifunctional properties. We have synthesized
nanometer scale heterostructures of ferroelectrics and ferromagnets, and ferromagnets and
superconductors to study fundamental physics including the interplay of multiple physical
phenomena. We employ scanning probe techniques including atomic force microscopy
(AFM), magnetic force microscopy (MFM), electrostatic force microscopy (EFM) and
scanning tunneling microscopy (STM) at varying temperatures and magnetic fields to study
such phenomena. Here we report on two issues related to our research: fabrication of
multiferroic heterostructures and their characterization using magnetic force microscopy, and
observation of a pseudogap in the metallic state of the broad bandwidth manganite
La0.7Sr0.3MnO3 using scanning tunneling microscopy and spectroscopy.
Scanning Tunneling Microscopy and Spectroscopy on La0.7Sr0.3MnO3:
On as-grown samples Evidence for a Pseudogap
295 K
dlnI/dlnV
4
158 K
4
94 K
3.0x10
3
H = 10 G
2.5x10
3
2.0x10
3
1.5x10
3
1.0x10
3
5.0x10
2
1.5
1.0
0.5
Before Annealing
-1.0
-0.5
0.0 0.5
Bias (V)
1.0
0
1.5
100
200
300
The spectra do not show a strong
temperature dependence
There is not much temperature
The surface is granular due
evolution.
to a fast growth rate.
After annealing at 8000C in air
310 K
255 K
dlnI/dlnV
78 K
Height (in nm)
(a)
-1.0
1.2
Terraces are formed during post-growth annealing and we do not see
regular step-terrace morphology, most likely due to the substrate on
which these films are grown.
We have performed room temperature magnetic force
microscopy (MFM) on CFO nano-pillars to study their
magnetic domain structures and anisotropy. At room
temperature, CFO is a ferromagnetic oxide which
shows 450 out-of-plane magnetization in thin films.
No clear contrast is observed at the top of a single nanopillar, which indicates that each pillar is a single-domain
magnet. Most possibly, the magnetization in the pillar is
completely out-of-plane. However, the possibility of a
canted magnetization cannot be ruled out from these
data.
0
-1
-2
100
200
300
T (K)
(b)
(a)
as grown
annealed
50
100
150
200
250
300
350
(a) Topography and (b) MFM images of CFO nano-pillar arrays
Blown up image showing the single domain structures
(MFM lift height = 30 nm)
The pseudogap observed in the metallic state might be the signature of localized polarons arising from the
strong coupling between electrons and dynamic Jahn-Teller distortions while a finite DOS at the Fermi energy
indicates presence of the delocalized carriers in the metallic state.
-4
-0.5
0.0
0.5
Bias (V)
78 K
-0.4
1.0
The pseudogap could also originate from the dynamic phase separation as suggested by recent Monte Carlo
simulations.
(b)
-0.2
0.0
0.2
Bias (V)
0.4
Ongoing research and future goals:
8
All the spectra at different temperatures were captured at the same tunneling
resistance (bias voltage = 1 V 205
andKcurrent
4 = 0.1 nA).
The spectra below 150 K are
gap-like: A pseudogap in the
152 more
K
0
310 K
4 metallic state!!
100 K
78 K
2
(a)
0
-1.0
I (pA)
dlnI/dlnV
Height (in nm)
The terraces are formed with single unit cell height (~ 0.4 nm).
STO
(a) STS spectra on the annealed film at310
different
temperatures. (b) Representative I-V curves
K
6
40
80
120
Length (in nm)
STO
(6)
1
8
-8
255 K
0
(3)
2
152 K
We may not see the inhomogeneities
in the STM-S data310
asK
0
100 K
the STM technique
is very surface sensitive
152 K
4
0
On the samples annealed at 8000C in air
Topography of the annealed film at 295 K.1.4Scale: 500x500 nm2 The
line profile corresponding to the green line on the topography image
1.2
is shown.
STO
3
205conductance
K
The variation in the
is less than 5 % .
4
6
1.4
1.6
STO
(5)
Figure 5: Topography (left) and conductance (right) image of the
annealed film at 78 K.
8
1.6
8
(2)
T (K)
2
40
80
120
Length (in nm)
STO
Magnetization and transport measurements on a LSMO thin film
-1.0 -0.5 0.0 0.5 1.0 1.5
Figure 4: STS spectra
on as-grown
film at
Bias
(V)
three different temperatures.
Figure 4: STS spectra on as-grown film
at three different temperatures.
0
(1)
4
T(K)
Before Annealing
STO
(4)
5
0
400
The substrate (STO [001])
Deposition of CFO or LSMO with a SRO underlayer
e-beam patterning
900 ion milling
Deposition of the matrix layer
Planarization by low angle ion milling
SRO
CFO
Ferroelectric/Piezoelectric
6
0.0
0.0
2
0
Figure 3: Topography of as-grwon
The surface is granular
film at 78 K.
94 K
2
0
Topography of as-grown film at 78 K.
Scan range is 205 nm x 205 nm
158 K
H in plane
H out of plane
1.
2.
3.
4.
5.
6.
SEM images of (a) LSMO and (b) CFO nano-pillar arrays
I (pA)
dlnI/dlnV
295 K
3
 (-cm)
6
M (emu) x 10
6
-4
2.0
3.5x10
-7
On as-grown samples
(d / dT) 10
2.5
Multiferroic heterostructures: Fabrication & Characterization
-4
-8
-0.5
0.0
0.5
Bias (V)
1.0
152 K
78 K
(b)
-0.4
-0.2
0.0
0.2
Bias (V)
The variation in the conductance is less than 5 % .
0.4
1. Imaging the intrinsic inhomogeneities in manganites and studying their temperature and
magnetic field dependence using our homebuilt variable temperature EFM and MFM.
2. Probing the Andreev bound states between d-wave superconductors and ferromagnets as
a function of temperature, magnetic field and the spin polarization of the ferromagnet
using our variable temperature STM.
3. Controlling the magnetization axis of the nanostructured ferromagnetic pillars in a
ferroelectric matrix by electric field.
References:
Inhomogeneities in the bulk are not probed as STM is very surface
sensitive
Topography (left) and conductance (right) image of the annealed film at 78 K.
Scan range is 524 nm x 524 nm.
1. E. Dagotto et. al., Phys. Rep. 344, 1 (2001).
2. A. Chikamatsu et. al., Phys. Rev. B 76, 201103 (2007).
3. Yu. et. al., Phys. Rev. B 77, 214434 (2008).
4. Mannella et. al., Nature 438, 474 (2005).