Electrical & Computer Engineering
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Item RESEARCH AND DEVELOPMENT OF THIN GARNET FILM BASED MAGNETO-OPTICAL IMAGERS.(2011) Tkachuk, Sergiy; Mayergoyz, Isaak D; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation deals with the use of magnetic single-crystal bismuth-substituted iron garnet thin films for magneto-optical imaging (MOI) of stray magnetic fields. The main advantages of such garnet-based imagers are their high sensitivity, high contrast and possibility to design films for imaging of magnetic fields in a wide range of magnitudes. The garnet films have been grown by the liquid phase epitaxy method from a flux melt on (210)- and (100)-oriented substrates and extensively characterized using various magnetic and optical methods. Specific melt compositions are identified that allow the growth of high sensitivity and high contrast indicator films on (210)-oriented substrates. Very low saturation field and high sensitivity of such garnet films are attributed to the existence of the so-called "easy plane of magnetization", a plane for which the magnetic free energy density is at a minimum for any orientation of the magnetization vector within this plane. Etching has been extensively used to investigate the effect of intrinsic film domain structures on the quality of MO imagers. It has been determined that the size of the domains reduces as the thickness of the film gets smaller. Below 1μm film thickness, the domains start evolving towards the "single domain" state which is beneficial for imaging purposes. The comparison of the imaging capabilities of the etched films grown on (210)- and (100)-oriented substrates has been performed and the resolution of the (100)-oriented imagers has been found to be inferior to the imagers based on the (210)-oriented samples with an easy plane of magnetization. The possibilities to enhance magneto-optic effects by strong local electric fields from optically induced plasmon resonances in gold nanoparticles embedded in garnet media are analyzed. The experimental investigation of the plasmon resonance enhancement of the Faraday effect has been performed and about 50% increase in Faraday rotation angle at a wavelength of 633 nm has been measured for the samples of interest. It is expected that under ideal conditions of the plasmon resonance excitation as well as proper garnet film parameters, an increase in Faraday rotation up to 400% could be achieved.Item Atomic-level Characterization of Fe(001)/MgO(001)/Fe(001) Tunneling Magnetoresistance Structures and Spin-polarized Scanning Tunneling Microscopy(2010) Lee, Jookyung; Gomez, Romel D.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis seeks to understand the Fe-MgO-Fe system through a series of atomic level studies of the topographic, electronic, and magnetic properties of these epitaxial films. This multilayer system is uniquely important because of its huge tunneling magnetoresistance (TMR) arising from spin coherence and strong spin filtering through the structure. MgO-based magnetic tunnel junctions have been actively investigated and are now successfully applied to commercial products such as non-volatile magnetic random access memories and read-write heads for hard disk. However, despite its popularity most work has been done on macroscopic samples and has focused on the device-level performance. Yet very little effort has been devoted towards the understanding at the atomic length scales including the effects of atomic steps and local variation in stoichiometry. The primary goal of this work is to elucidate the interplay between morphology, stoichiometry, local magnetism, and local electronic properties. To this end a multifaceted approach was used involving atomic/magnetic force microscopy (AFM/MFM), scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), Auger electron spectroscopy, and low energy electron diffraction (LEED), which were operated in the cleanest possible conditions under an ultra-high vacuum. I linked the morphology directly to the formation of different magnetic domain configurations as a function of growth temperature and film thickness. I also correlated these atomic-level properties to the device-level performance. By investigating the topography and the surface electronic density of states with length scales in the nanometer regime, I found that the films had extremely inhomogeneous surface states. Because the structural defects such as surface steps, deep trenches and grain boundaries, as well as the existence of chemical impurities can perturb the spin-coherent tunneling, our observation of the electronic inhomogeneity can provide a direct clue for explaining the diminished TMR phenomenon on real systems compared to the theoretical expectation, which is one of longstanding problems to achieve high TMR in actual devices. In addition to the Fe/MgO/Fe work, I also demonstrated spin polarized STM which revealed the anti-ferromagnetic spin-structure of single crystal chromium and the magnetic domains structure of permalloy film on silicon oxide.