Micro and Nanotechnology Laboratory | U of I

Highlights

Ph.D. Physics Stanford Univ, Stanford CA 1978

Research Statement:

Superconducting and Magnetic Materials

We have been studying the physics of colossal magnetoresistance using manganite films grown by atomic layer-by-layer molecular beam epitaxy. The films are atomically flat and have low-temperature transport rivaling that of single crystal samples. In our best samples, we have observed low-temperature residual resistivity of less than 50 ìÙ W cm.
We have studied simple patterned transport samples extensively. We have observed a large anisotropy in magnetic behavior and magneto-transport that we attribute to spin orbit scattering. We have also seen that the metal insulator transition in our samples is independent of the magnetic state of the sample. That is, when the sample magnetization is fixed, the metal insulator transition occurs at a fixed temperature, independent of magnetization.
More recently, we have begun to study tunneling into these materials by assembling single-crystal tunneling structures, which consist of a base manganite film, covered epitaxially with a titanate barrier layer. Since the two species are well lattice matched, it is possible to grow such an interface continuously. The barrier is followed by an in-situ deposited gold film. The trilayers are patterned into mesa structure.
Interpretation of the tunneling is complicated by the spreading resistance of the manganite, which can be large. When the manganite is the most conducting, however, it is possible to obtain tunneling spectra that are predominantly caused by the tunnel junction. These spectra show metallic behavior in the manganite and no significant sign of the gap structure recently reported in samples investigated by ex-situ scanning tunneling spectroscopy. A small reduction in conductivity is observed near V = 0, but this appears to be unrelated to the data shown in the STS report. We propose that the metallic tunneling obtained in these structures grown with epitaxial and in-situ interfaces is due to a metallic and ungapped density of states at the Fermi energy in the manganites.
Colossal Magnetoresistance Spin-valves Grown by Molecular Beam Epitaxy

The large conduction electron spin polarization in the manganite magnetic oxides such as La_0.67Sr_0.33MnO₃ (LSMO) make them candidates for high sensitivity spin-valves. Previous manganite ferromagnet-insulator-ferromagnet tunnel junctions grown by laser deposition techniques (J. Z. Sun et al., Appl. Phys. Lett. 73 1008 [1998]; M. Viret et al., Europhys. Lett. 39, 545 [1997]) did indeed exhibit record magnetoresistance (MR) sensitivity at 4.2 K. However, this MR was found to decrease with temperature to decidedly "un-colossal" levels at room temperature, despite the fact that 295 K is well below the LSMO Curie temperature, and the spin-polarization is expected to be large.
Interfacial disorder (structural or compositional) leading to suppressed ferromagnetism is a likely explanation for the reduction in spin-valve magnetoresistance prior to loss of bulk ferromagnetic order. To address this issue, we have fabricated manganite spin-valve structures with crystalline SrTiO₃ or CaTiO₃ insulating barriers using molecular beam epitaxy for high quality interfaces, and control of doping and stoichiometry at the atomic level. We will discuss the extent to which "colossal" magnetoresistance in manganite spin-valves can be preserved at higher temperatures and low fields by atomic layer engineering of the interface.

Research Interests:

• Electronic and quantum electronic properties of complex oxide materials. Thin film-based devices using heterostructures of different complex oxide phases. III-V semiconductor materials grown by MBE. Nanoscale electronic devices with superconducting and magnetic components. Electro-optic devices incorporated into integrated optic circuits.
• Ph.D. thesis title: High Resolution Spectroscopy Using Multiple Coherent Interactions

Honors, Recognition, and Outstanding Achievements:

• Bernd T. Matthias Prize (2012) Awarded at the 2012 Materials and Mechanisms of Superconductivity Conference, August 2012, Washington, DC.
• Fellow, American Physical Society (2005)
• Arnold O. Beckman Research Award, Research Board, University of Illinois at Urbana-Champaign (2001)
• Dupont Graduate Fellow (1973-1975)
• National Merit Scholar (1969-1973)

Honors, Recognition, and Outstanding Achievements for Teaching:

• Incomplete Lists of Teachers Ranked as Excellent, Physics 487 - Quantum Mechanics II, Fa2004