Nanomagnetism in Emergent Oxide Materials: Top-Down Meeting Bottom-Up

Tom WU

Assistant Professor

Physics and Applied Physics Division,

School of Physical and Mathematical Sciences

Nanyang Technological University, Singapore

Transition metal oxides are strategically important and versatile materials for both basic sciences and potential applications. Their magnetic properties are especially fascinating and have been intensively pursued, highlighted by the discoveries of colossal magnetoresistance manganites and room-temperature diluted magnetic semiconductors. As a key ingredient for high-density device integration, nanoscale geometrical confinement often brings about novel phenomena and functionalities. In this talk, I will focus on two particular materials: mixed-valent manganite and transition-metal-doped zinc oxide, as examples of top-down and bottom-up approaches to nanomagnetism, respectively.

For the first example, in manganites, the competition between ferromagnetic metallic and charge-ordered insulating phases leads to the formation and coexistence of nano to micro-scale metallic domains within an insulating matrix. I will demonstrate the impact of geometrical confinement using mesoscopic structures fabricated by top-down electron-beam lithography. Negative differential resistance was observed, as a result of heating-induced local annihilation of conducting filaments. For the second example, we prepared Cu-doped ZnO nanowires using different bottom-up approaches and carried out a comparative study of their magnetic properties. Compared with the conventional vapor transport method, annealing a ZnO core/Cu shell structure leads to significantly enhanced room-temperature ferromagnetism. We suggest that the structural inhomogeneity boosts the ferromagnetism by promoting the formation of bound magnetic polarons. Therefore, in both examples, rooted in the emergent materials physics, novel physical functionalities were created in nanostructures.