Studies in Nanoscale Magnetism  On the Molecular/Solid State Boundary Matter of nanoscale dimensions (10-9m) exhibits novel physical properties (electronic, magnetic and optical) that lie between those of atomic and molecular systems and the bulk state of solids. The magnetic properties of large molecular clusters and nanometer size particles are being investigated, in order to probe the transition from atomic-scale to bulk magnetism and fundamental properties of magnetic nanolattices.

Magnetic Ferrofluids, Magnetic /Quantum-Dot and Magnetic/Noble-Metal Hybrid Nano-Structures The magnetic properties of isolated magnetic particles and their assemblies are investigated in novel structures of well controlled composition and high crystallinity. Hybrid nano-maghemite/CdSe-Quantum Dot structures and nano-magnetite/Ag-noble metal bifunctional particles are under investigation for potential bio-imaging and bio-medical applications. Novel magnetic ferrofluids for hyperthermia cancer treatment are also studied.

Multiferroic Nanoparticles Coupling of electric, magnetic and structural order parameters give rise to simultaneous ferroelectricity, ferromagnetism and ferroelasticity. Magnetization and dielectric polarization can be modulated and activated by the application of an electric and a magnetic field, respectively. Such systems that exhibit coupled magnetic and electrical properties are of interest for advanced device applications. Ferroelectric nanostructures of BiFeO3 are under investigation in order to understand the fundamental electronic and magnetic properties leading to multiferroic behavior and the enhancement of multiferroic properties at the nanoscale.

Biological Iron Utilization: Biomineralization in Recombinant and Mutant Ferritins Iron is a required element necessary as an enzyme co-factor and as a substrate for heme biosynthesis.  However, iron is also toxic. Malregulated iron accumulation can be responsible for organ dysfunction. Ferritin, the iron storage protein, has evolved to bind ferrous ions in solution and convert them into a ferric biomineral, ferrihydrite, within a protein shell.  Thus, ferritin performs the dual function of iron detoxification and iron storage.  The mechanism of oxidative iron deposition in wild-type, recombinant and mutant ferritins, iron nucleation and polymerization processes and the structural and magnetic properties of the resulting nano-bio-mineral cores are under investigation. Additionally, the iron-induced protein aggregation in ferritins containing the mutant L-chain variant 460InsA (Mt-FTL) will be investigated. 

Interparticle Interactions in Core/Shell Iron-Oxide Nanoparticle Assemblies Magnetic relaxation in nanoparticle assemblies is poorly understood due to complex long-range, dipole-dipole (d-d) interactions.  Yet, relaxation mechanisms are of great importance in technological and biomedical applications of magnetic nanoparticles. With increasing magnetic volume fraction an ensemble would pass from that of isolated, superparamagnetic (SP) particles to a collective spin-glass-like (SG) system. We are using γ-Fe₂O₃ nanoparticles derived by reverse microemulsion and coated with silica shells of precisely controlled thickness as ideal experimental test systems to study d-d interactions as a function of inter-particle distance and magnetic core size. Experimental studies are being complemented with theoretical/computational (Monte Carlo) methods at the nanoscale.

Broader Impact to Society, Research and Education:

Our studies on nanoferrites impact on magnetic device development and catalytic applications.

Our basic research on native, recombinant and mutant ferritins impacts on our understanding of biological iron regulation and iron-related diseases, cytotoxicity and abnormal accumulation of iron on vital organs including the central nervous system.

Studies on interparticle interactions impact broadly on the design of magnetic nanoparticle assemblies for high density information storage; and on our understanding of the effects of agglomeration in the relaxation mechanisms and thermo-magnetic properties of ferrofluids for hyperthermia cancer therapy.

 Our studies of biocompatible nanoparticles impact on nano-bio-medicine and the advancement of novel diagnostic and treatment modalities. 

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Nanostructured Fe-Pd Films

Palladium is an exchange-enhanced metal close to the ferromagnetic transition, which makes it very sensitive to the introduction of 3d-transition metal impurities. In the bulk, iron forms continuous solid solutions with palladium, FexPd1-x for 0 < x <1, which have unique magnetic properties, due to long-range-spin-polarization of the Pd matrix. We are investigating size dependent effects on long range-spin-polarization in nanostructured Fe-Pd, and the stability of the nanostructured state for applications to hydrogen separation nanotechnologies. (In collaboration with Y.J. Ying, MIT, Dept. of Chemical Eng.).

Ferrite-based Magnetic Nanocomposite Materials.

Ferrite spinels exhibit combined electrical and magnetic properties that have found numerous applications in high-frequency devices, memory cores and magnetic recording media. Currently, the properties of nanometer size ferrites are under intense investigation due to the broad range of magnetic behavior that may be engineered into such structures. We are studying the magnetic properties of CoFe2O4 nanoparticles self-assembled within the spherical nanodomains of microphase separated diblock co-polymers. Mossbauer spectroscopy, TEM and SQUID magnetometry are combined to probe the internal magnetism, superparamagnetic relaxation, size distribution, low temperature coercivities, and spin reversal mechanisms in this nanocomposite system. (In collaboration with P. Kofinas, U. of Maryland, College Park, dept. of Materials and Nuclear Eng.).

Phase Transitions and Properties of Metastable Wuestite (FexO) Nanocrystals

The large contribution of surface energy in nanoscale materials can stabilize unknown crystallographic phases, which are thermodynamically unstable in the bulk. The crystal structure of the nanoparticle determines its properties and to some extent its shape. Therefore, control over phase and phase-purity is crucial in nanoscale synthesis in addition to the basic requirements of tunable size and narrow size- distribution. The target of our investigation is the synthesis and characterization of wuestite and the study of its decomposition mechanism into metallic iron α-Fe and magnetite, Fe3O4 which can be oxidized to maghemite, γ-Fe2O3. All three oxides are based on an fcc structure of oxygen, only the content and distribution of iron differs, which explains the occurrence of non-stoichiometry in all three phases and the gradual transition between them. The three oxides have vastly different magnetic and electrical properties. The decomposition of FeO affords a non-aqueous route to magnetite synthesis yielding stoichiometric magnetite. Mixtures of the oxides exhibit tunable magnetic properties under investigation. (In collaboration with C. Murray, IBM Watson Labs and S. O'Brien, Columbia University Dept. of Chemistry)

Deuterium Structural Effects in Inorganic and Bioinorganic Aggregates

Deuterium isotopes are routinely used in molecular structure determination and reaction kinetics studies. In general, deuterium substitution is thought to have minimal or no effect on the structural organization and thermodynamic stability of molecules, because of the similar energies of O-H and O-D bonds which result in hydrogen bonded O---O distances varying by 0.03 A or less. However, the importance of small differences in hydrogen bond strength to the solid-state structural organization of fully deuterated molecular aggregates has not been recognized. We are investigating the effect that H2O to D2O substitution has on the structural organization and thermodynamic stability of iron bioinorganic aggregates (In collaboration with S. Gorun, Brown University Department of Chemistry, and N.D. Chasteen, U. of New Hampshire Department of Chemistry)

Fe and Fe-Ni Based Catalysts for Carbon Nanofiber Formation

The discovery of C60 clusters and carbon nanotubes has created great interest in the synthesis and reactivity of carbon based materials for potential applications in nanotechnology. We are investigating the synthesis of carbon nanofibers (CNF) produced by the decomposition of H2 and CO and/or selected hydrocarbons. This decomposition and subsequent growth of the CNF's is promoted by the catalytic action of metallic Fe or bi-metallic Fe-Ni surfaces. It has been shown that the composition of the growth catalyst affects significantly the structure of the resultant CNF. The type and perfection of the CNF lattice are critical for various applications in nanotechnological materials. Metal iron catalysts promote the production of CNF platelets while Fe-Ni catalysts result in ribbon CNF. We are investigating the physical properties of Fe and Fe-Ni catalysts before and after CNF growth, in order to elucidate the mechanism responsible for the promotion of fibers with distinctly different physical characteristics and properties. (In collaboration with C.A. Bessel Dept. of Chem. Villanova Univ.) 

Iron Metabolism in Yeast Cells

We are using the genetics and molecular biology of the budding yeast, s. cerevisiae, in order to study the mechanism and regulation of iron metabolism in eucaryots. Genes involved in intracellular iron transport have been identified. We are using Mossbauer spectroscopy to characterize and compare the state of the iron and distribution in yeast mutants that have defects in specific iron trafficking functions. (In collaboration with O. Ardon, U. of Utah, Medical School, dept. of Pathology).