PhD Defense: "Heusler Compound Heterostructures and Epitaxial Magnetic Tunnel Junctions for Spintronic Applications"

Anthony McFadden

July 21st (Friday), 2:00pm
Engineering Science Building (ESB), Rm 2001

This dissertation focuses on materials growth and integration as well as on the magnetic and electrical characterization of Heusler compounds and heterostructures for magnetic memory and spintronic applications. Magnetic random access memory (MRAM) is an emerging memory technology based on the magnetic tunnel junction (MTJ), which desires densely packed (nanoscale), stable memory cells with a large output (read) signal that can be written electrically. This requires ferromagnetic compounds with high tunneling spin polarization, low Gilbert damping, and ideally with perpendicular magnetic anisotropy (PMA). Heusler compounds are a promising candidate for MRAM applications as these desirable properties have been demonstrated, (though not all three simultaneously), and are of further interest to the field of spintronics due to their ability to adopt a wide range of magnetic and electrical properties which include metallic, superconducting, semiconducting, topologically insulating, and half-metallic (a material with 100\% spin polarization of the conduction electrons).

This work details research of single-crystal, epitaxial ferromagnetic Heusler compounds and heterostructures which were grown by molecular beam epitaxy. Materials growth was studied and optimized for a variety of Heusler compounds and heterostructures which were characterized using electron and X-ray diffraction, in-situ X-ray photoelectron spectroscopy (XPS), SQUID magnetometry, atomic force microscopy, and electrical measurements of device structures which were fabricated using ex-situ processing techniques. Fully epitaxial MgO(001)/Co2FeAl/Pt heterostructures were extensively studied in order to understand magnetic anisotropy and spin dynamics in this single-crystal heavy metal(Pt)/ferromagnet(Co2FeAl) system motivated by the drive to develop PMA materials that may be switched electrically though spin-Hall effects in the adjacent heavy metal. Structural and magnetic characterization is performed at UCSB while FMR and transport measurements are conducted at the University of Minnesota in collaboration with Professor Paul Crowell’s group. MBE growth of fully epitaxial Heusler based magnetic tunnel junctions with MgO tunnel barriers is also developed and studied in detail. An emphasis is placed on understanding the interface formation of single-crystal MgO/ferromagnet structures and its impact on magnetic anisotropy and the performance of MTJs.

About Anthony McFadden:

Anthony came to UCSB in 2010 after earning dual degrees in electrical engineering and applied mathematics at the University of New Mexico. He joined Chris Palmstrøm's group in 2012 and has been working on materials growth by molecular beam epitaxy, fabrication, and measurement of epitaxial thin films and heterostructures.

Hosted by: Professor Chris Palmstrøm