My current research involves three interdisciplinary projects, including epitaxial exchange bias, epitaxial patterning, and ‘top-down’ nanoparticles.
Epitaxial exchange bias: The exchange bias effect, particularly in the form of a ferromagnetic/antiferromagnetic bilayer, has been widely studied due to its applications in magnetic storage technologies. Since the essential behavior of exchange bias critically depends on the atomic level chemical and spin structure at the interface between the ferromagnetic and antiferromagnetic components, epitaxial AF/F systems in which the quality of the interface and the crystalline coherence are optimized and well known are ideal candidates for a better understanding of the underlying physics of exchange bias. The dependence of exchange bias on the spin conﬁgurations at the interfaces can be accomplished by selecting diﬀerent crystallographic orientations. The role of interface roughness can also be understood from thin-ﬁlm systems by changing the growth parameters, and correlations between the interface structure and exchange bias can be made. ('New trends in magnetic exchange bias', Eur. Phys. J. B 45 155 (2005))
Epitaxial patterning: Patterned structures from epitaxial singlecrystalline magnetic thin films allow us to study more technologically important magnetic properties, not easily accessible in their polycrystalline counterparts, such as magnetocrystalline anisotropy, magneto-elastic coupling, and ferromagnetic/antiferromagnetic interface spin structures. Additional functionality in magnetic memory devices and spintronics devices can be achieved by epitaxial growth (tuning the magnetocrystalline anisotropy) and patterning wire arrays (lithographically-induced shape anisotropy). Therefore, reliable and economical methods to fabricate large area, high quality epitaxial submicron wires are greatly desired for model studies and technological developments.
‘Top-down’ nanoparticles: Recently developed nanoimprint lithography (NIL) methods are capable of large area patterning with high throughput and resolution, and are promising for patterning arrays of interest in magnetism. We have developed a series of recipes, including the bilayer resist and undercut profiles, to fabricate patterned magnetic nano-elements on Si and MgO wafer with very high quality. We have also developed a defect-free NIL process by using a soft, flexible and economic ETFE plastic nano-stamp. With the above attributes, we are working on a much improved process to fabricate disc-shaped SAF nanoparticles directly on Si wafer, and to release them via a dry etching procedure, which can avoid all the related pitfalls associated with the sacrificial layer that are used in conventional methods. This process could potentially enable better functional coating which is done before particle release. The nanoparticles can be released directly to water without any etchant transfer. By using an ethylene tetrafluoroethylene (ETFE) mold that is replicated from a master Si mold and a bilayer resist lift-off, defect-free imprints, and subsequently high-quality nanoparticles have been achieved over the whole mold size (> 1 × 1 cm2).