Photoemission and X-ray absorption studies of magnetic materials
- The works presented in this thesis investigate magnetic materials using various synchrotron techniques. Since the magnetic properties of a material are due to the behavior of the electrons, it is fruitful to measure the electronic structure of magnetic material with synchrotron techniques and make comparisons with theoretical model. Our main interest is in the 3d transition metal oxides, Silicides, and intermetallic compounds. The issues studied are that of colossal magneto-resistance, narrow band insulators, and half-metals. The first phenomenon discussed is CMR (colossal magneto-resistance). It simply means that the electrical conductivity is very sensitive to an applied magnetic field. Most magnetic materials possess some magneto-resistance (less than a few percent), however certain perovskite manganite show many orders of magnitude change in conductivity near a ferro-paramagnetic transition temperature with a strong applied magnetic field. Using photoemission and x-ray absorption, we have tried to extract the electronic structure of the material, understand its temperature and doping dependence, and compare our results with leading theoretical models. In particular, the physics of double exchange, polaron mechanism and charge inhomogeneuity are discussed. We have also determined many of the key parameters of these compounds such as the one electron bandwidth, the Hund's rule coupling energy and the Jahn-Teller energy. These parameters are keys to microscopic models. Second interesting magnetic material system we focused on is FeSi. The unusual magnetic susceptibility of FeSi over a wide temperature range has puzzled physicists for a long time. Various theoretical models have been proposed to explain the phenomenon. By discovering evidence of a narrow band near the Fermi level from angle-resolved photoemission spectra, we support the existence of a sharp density of states between a semiconductor gap in this compound. This may put FeSi into a class of narrow band insulator that is seen only in rare earth compounds. Another interesting family magnetic compound is the Heusler alloys. According to band calculations, this class of alloy is metallic for spin-up electrons and semiconducting for spin-down electrons ("Half-metallic"). However it lacks definite experimental evidence to prove or disprove the calculation. We have performed resonance photoemission, X-ray magnetic circular dichroism, and X-ray photoemission spectroscopy studies of the Heusler alloy and have compared these results with the predictions of band calculations. These results gave qualitative information about the density of states of various orbital/elemental contributions, and found them to be consistent with the band calculation. However, since we do not measure the spin directly, our data can't provide a definitive test.
|Type of resource
|electronic; electronic resource; remote
|1 online resource.
|Park, Chul Hong
|Stanford University, Department of Physics.
|Raghu, Srinivas, 1978-
|Raghu, Srinivas, 1978-
|Statement of responsibility
|Chul Hong Park.
|Submitted to the Department of Physics.
|Thesis (Ph.D.)--Stanford University, 2013.
- © 2013 by Chul Hong Park
- This work is licensed under a Creative Commons Attribution 3.0 Unported license (CC BY).
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