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.

La1-x+3 Ax MnO3 A+2 =Ca, Sr, Ba, Pb

Simultaneously acquired morphology (left) and spectroscopy (center) images from a (La5/8-0.3Pr0.3)Ca3/8MnO3 epitaxial thin film. Perovskite structure (right).

Results:

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 room temperature. 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 x 10nm dual bias STM images obtained simultaneously in the same area at paramagnetic state of a 120nm LPCMO film. (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 is responsible for the enhanced magnetoresistance in LPCMO compared to its parent compounds. 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 LAO and 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 magnetic field. 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). While sharp 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).