Oakridge 1
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Understanding Phase Coexistence in Transition
Metal Oxide Thin Films
Transition metal
oxides (TMO) exhibit
a strong spin-charge-lattice interaction that
can lead to electronic
phase separation
(PS). This
phenomenon carries a number of fascinating electronic and magnetic phases while
maintaining a single crystalline structure. We are motivated to study artificially layered
and spatially confined manganite thin films by the number of unanswered questions
concerning the mechanisms that give rise to the diverse range of exotic electronic and
magnetic phases that can coexist within a single crystal of manganese oxide.
Simultaneously acquired morphology (left) and
spectroscopy (center) images from a (La5/8-0.3Pr0.3)Ca3/8MnO3 epitaxial thin film.
Perovskite structure (right).
La1-x+3 Ax MnO3 A+2 =Ca, Sr, Ba, Pb
Re s u l t s :
1. Visualizing Localized Holes:
The magnetic and transport behaviors of manganites are critically related to the spatial
distribution and correlation of doped holes. Using in situ scanning tunneling microscopy,we have imaged both occupied and unoccupied states simultaneously in hole-doped
(La5/8-0.3Pr0.3)Ca3/8MnO3 epitaxial thin films. Doped holes localized on Mn+4 ion
sites were directly observed with atomic resolution in the paramagnetic state at roomtemperature. In contrast to a random distribution, these doped holes show strong
short-range correlation and clear preference of forming nanoscale CE-type charge-
order-like clusters. The results provide direct visualization of the nature of intriguing
electronic inhomogeneity in transition metal oxides.[1]
Figures: 20nm x10nm dual bias
STM images
obtained
simultaneously in
the same area at
paramagnetic state
of a 120nm LPCMOfilm. (a) Occupied-state image (Vbias=1.5V, It=0.020nA) and (b) unoccupied-state
image (Vbias=2.0V, It=0.050nA). Both (a) and (b) reveal the same square lattice of Mn
ions. In the unoccupied-state image, the brighter and darker lattice sites correspond to
Mn+4 (localized hole) and Mn+3 ions, respectively. The relative contrast between Mn+4
and Mn+3 ions is reversed in the occupied-state image.
2. Substrate Effects:
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Large scale phase separation
between ferromagnetic metallic
and charge-ordered insulating
states in La1-x-y PryCaxMnO3
LPCMO crystals and thin films is
very sensitive to structural and
magnetic changes and isresponsible for the enhanced
magnetoresistance in LPCMO
compared to its parentcompounds. By epitaxially
growing LPCMO thin films on
different substrates, the strain on
the LPCMO thin films can be
changed, thereby controlling the
energy balance between the two
phases. LPCMO films of several
different thicknesses have been
grown on NdGaO3 (NGO), SrTiO3
(STO), SrLaGaO4 (SLGO), and
LaAlO3 (LAO) substrates. The
compressive strain from the LAOand SLGO substrates suppresses
the long-range charge ordering in
these samples and enhances
magnetoresistance and magnetic
hysteresis. Conversely, the
tensile strain from the STO and
NGO substrates enhances the long-range charge ordering and reduces the
magnetoresistance and magnetic hysteresis. [2]
3. Spatial Confinement:
Optical lithography is used to fabricate LPCMO wiresstarting from a single
La5/8-0.3Pr0.3Ca3/8MnO3 (LPCMO) film epitaxially grown on a LaAlO3(100) substrate.
As the width of the wires is decreased, the resistivity of the LPCMO wires exhibits giant
and ultrasharp steps upon varying temperature and magnetic field in the vicinity of the
metal-insulator transition. The origin of the ultrasharp transitions is attributed to the
effect of spatial confinement on the percolative transport in manganites.[3]
Figures: Left:
SEM images of
LPCMO wires
fabricated from a
single
LPCMO/LAO(100)film with different
sizes. Inset:enlarged image of
wire. Right:
Diagram
illustrating phase
separation in LPCMO wires. Notice that the scale of the wires is on par with that of the
phase separation.
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Plots: Up-Left:
Resistivity vs
temperature (R-T)
curves for the
LPCMO wires under
a 3.75 T magneticfield. Arrows
indicate the
direction of the
temperature ramp.
The R-T curves all
exhibit hysteresis
behavior in
cooling-warming
cycles, which is
consistent with the
coexistence of FM
and CO domains in
the LPCMO system.The MIT is rather
smooth for both
the 20mm and the
5mm wires.
Ultrasharp and
giant steps are
clearly visible for
the 1.6mm wire;
Up-right: resistivity
vs magnetic field curves for the LPCMO wires measured at 110 K. Sudden steplike
jumps are again visible in the 1.6mm wire. Arrows indicate the sweeping directions of
the magnetic field for each curve. (a) R-T curves of the 1.6mm wire measured
repeatedly in three temperature cycles under the same magnetic field (3.75 T). Whilesharp jumps appear in all three cases, their location and magnitude are clearly random.
(b) R-T curves of the 1.6mm wire measured at different magnetic fields. The sudden
jumps disappear at 6 T and higher fields.
[1] J. X. Ma, D. T. Gillaspie, E.W. Plummer, and J. Shen, Phys. Rev. Lett. 95, 237210
(2005).
[2] Dane Gillaspie, J.X. Ma, H.Y. Zhai, T.Z. Ward, H.M. Christen, E.W. Plummer and J.
Shen, J. App. Phys 99, 08S901 (2006).
[3] Hong-Ying Zhai, J.X. Ma, D.T. Gillaspie, X.G. Zhang, T.Z. Ward, E.W. Plummer and
J. Shen, Phys. Rev. Lett. 97, 167201 (2006).