William Bailey

Modern ultrahigh vacuum deposition techniques have made it possible to create 'heterostructures,' ultrathin films (dozens of atoms thick, or thinner) of one material stacked upon another. Composition can be controlled routinely to the level of ~three atomic layers, with the potential for control of single monolayers. Heterostructures of ferromagnets with other sorts of materials (metals, insulators, semiconductors) offer entirely new functional properties due to the presence of the interface, going far beyond a simple averaging of the two components. 

A canonical example of a new property engineered in a heterostructure is giant magnetoresistance (GMR) in ferromagnet/noble metal heterostructures, topic of the 2008 Nobel Prize in Physics for Albert Fert and Peter Gruenberg. The key idea is that the spin of the electrons that carry charge in a conductor can be controlled through magnetization.  This property had direct application as a magnetic field sensor in magnetic recording heads; GMR went from first laboratory discovery (1988) to incorporation as a 'spin valve' in mass-produced magnetic recording heads (1998) in just 10 years, something of a record in applied science.

Many new spin-transport-related phenomena have been discovered in the intervening years, with spin torque transfer (and magnetic random access memory, MRAM) devices serving as a focal point.  A relatively new direction for these studies, and one which has interested my group particularly in the past several years, is the possibility of controlling the flow of 'pure' spin currents (without charge, or electrical currents) in ferromagnetic heterostructures.  

The particular specialty of our lab is high-speed / high-frequency magnetization dynamics in sheet-level films.  We have developed some unique tools in magnetic materials deposition (UHV sputtering; see picture of chamber above) to optimize dynamical properties, and ferromagnetic resonance (FMR) characterization of films in some novel configurations, in our labs at Columbia.  In collaboration with other groups, we have extended FMR to synchrotron pump-probe measurements, which allows us to probe the dynamics of different constituents of heterostructures at GHz frequencies.

The work done in our group is primarily experimental, with quantitative models (usually phenomenological) often used to interpret phenomena in better-developed materials systems.  The research area offers opportunities to work in materials development, fundamental science, and applications in magnetic information storage.

Research Areas

• Nanoscale Magnetic Films and Heterostructures 
• Materials Issues in Spin-Polarized Transport
• Materials Engineering of Magnetic Dynamics 
• Materials