Open Access Dissertation
Date of Award
Physics and Astronomy
Boris E. Nadgorny
It is well accepted that spin polarization of materials has a major role in spintronics to improve the efficiency of spintronic devices. Point contact Andreev reflection (PCAR) spectroscopy, one of the most popular and reliable techniques, was used to obtain the conductance and by analyzing them by modified BTK model, spin polarizations of various materials, generally with low ferromagnetism, were extracted in order to understand the transport properties of spin polarized current and find the relation of spin polarization with other parameters such as saturation magnetization or the Curie temperature so that better spintronic materials can be identified and developed. We investigated how the spin polarization of itinerant ferromagnet MnSi changes as it undergoes magnetic phase transition from helical to conical to induced ferromagnetic. Also, unconventional conductance with enhanced Andreev reflection with amplitude more than two, and conductance oscillations outside superconducting gap indicate the signature of triplet superconductivity in Nb/MnSi contacts made by e-beam lithography and sputtering. Another system studied was Pd1-xNix which is an example of the strong ferromagnetic susceptibility enhancement of nearly ferromagnetic Pd by Ni impurities. The transport spin polarizations of Pd1-xNix with different Ni concentrations were measured and their correlation with saturation magnetization was studied. PCAR was applied to a superconducting sample: Co doped BaFe2As2 which revealed the presence of pseudogap in this material. Studies by SQUID, PCAR and neutron scattering, Pt thin films were found ferromagnetic which was supported by the first principle calculation indicating that its source is the surface roughness of the films, and the surface magnetization is more or less independent of the film thickness. MnBi of stable phase and high Curie temperature showed high perpendicular anisotropy and high transport spin polarization (P) which was found to be due to disparity in Fermi velocity in electron split bands. Their P is proportional to saturation magnetization, and this material is a potential candidate for spin injection to semiconductors. Room temperature ferromagnetism and finite low temperature spin polarization was observed in dilute magnetic semiconductor: undoped and Cr doped InN thin films so that it is another potential material for spintronic devices. Unlike predicted spin torque in FM/NM hybrid structure, freestanding Si3N4 microstructures with coating of Au or Ag or Ni showed torsion while passing dc current. The quadratic dependence of the torsion angles with the dc current were measured and such effect was accounted for the Joule heating of the metal coating and the residual strain of Si3N4 structures. Such electro-thermal actuators could find applications in MEMS technology.
Thapa, Pushkal, "The study of weak ferromagnetism by andreev reflection spin spectroscopy and development of bimorph electro-thermal actuators" (2011). Wayne State University Dissertations. Paper 398.