oakridge 1

7/27/2019 Oakridge 1

http://slidepdf.com/reader/full/oakridge-1 1/3

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:



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.



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).

oakridge 1

Documents