Doctor of Philosophy (PhD)


Physics and Astronomy

Document Type



TGa3 (T = Fe, Ru & Os) and RuIn3 are a class of isostructural intermetallic binary compounds with interesting physical and structural properties. These materials are non-magnetic, narrow band gap semiconductors with large n-type thermopower below room temperature. Chemical doping drastically changes physical properties and thermoelectric properties enhances by large percentage over the pure compounds. An important structural feature of FeGa3 is the fact that the four Fe atoms per unit cell exist as two Fe-Fe dimers. The nonmagnetic (NM) ground state of FeGa3 may be a consequence of the dimer spins being aligned in an antiparallel singlet state. We observe that FeGa3 becomes a low temperature ferromagnet (FM) by chemical doping with Ge, and the Curie temperature is suppressed toward absolute zero temperature. A model where the extrinsic electrons from the Ge doping creates a mixed valence Fe-dimer with a net effective spin, agrees well with the experimental data. Such a model provides a novel mechanism for a FM-QCP. We also show evidence for a magnetic field-induced critical point in FeGa3. The Ge-doped sample with x = 0.05 is a paramagnetic metal, which shows typical FL behavior at low temperature. However, NFL behavior is observed with applying magnetic field, and a FM-QCP is confirmed by field dependant data. Ru substitution in FeGa3 shows that an unexpected ferromagnetic insulating state develops immediately (0.1% Ru), and it disappears above 25% Ru. This behavior agrees well with the model of spin creation on the transition-metal dimers via conduction electrons, and the enhanced insulating behavior in the electrical resistivity suggests the Ru acquires a 2+ state. Interestingly, Mn doped FeGa3 shows an apparent antiferromagnetic insulating state above 5% Mn. This result provides further evidence that the disruption of the Fe-Fe dimer in the FeGa3 structure can lead to ordered ferro/antiferro-magnetic states. Superconducting and critical current properties of thin films of NiBi3 formed on the surface of carbon microfibers and on sapphire substrates were studied. The critical current density (Jc) was measured below the transition temperature and is well described by the Ginzburg-Landau power-law.



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Committee Chair

Young, David