Title: Nanoparticle-based magnetic materials and nanocomposites
Abstract: Nanoparticles (and nanoclusters) are proving to be a rich playground for both the discovery of new structures and as a means to exploit (and explore) nanoscale (especially nanomagnetic) phenomena. Nanoclusters can, among other things, lead to the formation of extended solid solutions in immiscible systems, novel non-equilibrium atomic structures, and core-shell structures. Additionally, the confined scale of the systems can produce phase separation at a very fine scale. These new structures and phase distributions produces new magnetically ordered structures and/or enhanced magnetic properties. One area of interest is exchange-coupled high-energy permanent magnets. Here, combining hard and soft magnetic phases at the nanoscale results in enhanced remanence and high energy products. We use geometrically confined phase separation in nanoclusters to significantly increase the volume fraction of the soft magnetic phase, and the exchange coupling produces an energy product more than double that of the single hard magnetic phase (25 MGOe v. 12 MGOe). Other examples of exchange-coupled materials will also be presented. Additionally, alloy clusters with unique structures, and consequently magnetic properties, are also being investigated. For instance, we have made a series of Fe-Au alloy clusters that form the ordered L12 or L10 structures, which are not observed in the equilibrium system. These clusters are either ferromagnetic or antiferromagnetic, depending on Fe:Au ratio. Additionally, we have observed ferromagnetic behavior in the Laves phase of Fe2W and antiferromagnetic behavior in L10 MnAu (as well as core-shell structures). Finally, we have observed increased magnetocrystalline anisotropy in W-doped hcp Co. Details of the cluster formation process, as well as structures and magnetic properties, will be presented.
Speaker Bio: Jeff Shield received his B.S. in metallurgical engineering from the South Dakota School of Mines and Technology, and his Ph.D. in materials science and engineering from Iowa State University (1992). He was an assistant and then associate professor of materials science and engineering at the University of Utah. Since 2001, he has been in the Department of Mechanical & Materials Engineering at the University of Nebraska. He served as interim chair from 2008-2011, and chair since 2011. His research background includes the structure and properties of quasicrystals, and for the past 15 years magnetic materials. The work in magnetic materials has focused on permanent magnets, including rare earth-based materials and, recently, non-rare earth materials.