Magnetism in reduced dimensions: Nanoparticles, thin films and quantum spin systems

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Nanomagnetism in nanoparticles, thin films, and quantum spin systems: Metals-based crystallites with sub-100 nm sizes, called nanoparticles, exhibit a wide range of physical properties that are strikingly different from their bulk counterparts. Many novel properties arise from the large fraction of atoms that reside on the surface and the finite number of atoms in each nanoparticle core. Our research program is dedicated to answering the fundamental question: what overall effect does the surface have on the nanoparticle magnetism. The magnetic frustration effects from the surface and core magnetism dichotomy in a nanoparticle can be "peeled" away onto a 2D surface. A thin film that is a ferromagnetic layer interfaced to an antiferromagnetic layer is an ideal example. Just like a nanoparticle, such a film exhibits the exchange bias effect. Exchange bias is the result of magnetic moments at the interface which are "locked-in". The physical origins of this "locking" are currently unknown. With a unique combination of nanoscale structural characterization, and a comprehensive description of the nanomagnetism from atomic to the bulk length scales, we intend to enable new insights into the physics of the "locking". A further reduction in the degrees of freedom is possible in certain structures where interactions are possible in only one crystallographic direction. A 1D quantum spin system is then created, an example of which are Haldane gap materials. These materials exhibit novel magnetism. Quantum spin systems present a unique theoretical description whose robustness still requires rigorous experimental tests. Our research group's significant experience measuring and modeling static and dynamic magnetism will provide unique insights into the physics of these materials. There is an unmistakable, pressing need for knowledge in the above research areas. First, the next generation of magnetic media hinges on shrinking device sizes where successful implementation depends on understanding the nanomagnetism. Second, new medical technologies based on nanoparticle drug delivery require a comprehensive understanding of the magnetism of single and collections of nanoparticles.