Browsing by Subject "Magnetic force microscopy"
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Item Low temperature magnetic structure studies of La₂₋₂xSr₁₊₂xMn₂O₇ using scanning probe microscopy(2013-12) León Brita, Neliza; de Lozanne, Alejandro L.The high degree of modification through chemical substitution afforded by the perovskite crystal structure and its related counterparts allows a systematic study of structure-property relationships critical to understand the wide variety of exotic phenomena observed in these materials where the spin, charge, orbital, and lattice degrees of freedom are highly correlated. From the multiple phenomena observed in these materials, which includes multiferroicity, catalytic activity, and high temperature superconductivity, this study is concerned with a material that displays colossal magnetoresistance (CMR), La₂₋₂xSr₁₊₂xMn₂O₇; this is a naturally bilayered manganite that exhibits CMR at a paramagnetic to ferromagnetic transition that coincides with an insulator to metal transition. The strong correlation between different degrees of freedom in the material leads to considerable variation in its magnetic properties due to doping even in the small range studied of 0.32 [less than or equal to] x [less than or equal to] 0.4, where the easy axis of magnetization changes from the c-axis to the ab plane. Magnetic force microscopy (MFM) was used for this part of the work, to visualize the local variation of the out of plane (c-axis) magnetization or magnetic microstructure of La₂₋₂xSr₁₊₂xMn₂O₇ for 0.32 [less than or equal to] x [less than or equal to] 0.4 at the exposed ab surface and its evolution due to an applied magnetic field at 4 K. For the x = 0.32 composition, which is close to the out of plane to in plane magnetization transition, a strong preferred magnetization direction within the ab plane or magnetocrystalline anisotropy was observed. The stray magnetic field of the MFM tip perturbs the magnetic microstructure of low coercivity materials like diluted magnetic semiconductors, making it unsuitable for the study of such materials. For this reason, as part of this project a scanning Hall probe microscope (SHPM), a magnetic imaging technique complementary to MFM that uses a Hall sensor that provides a magnetically non-invasive calibrated measurement of the stray fields at the surface of a sample with good resolution (~ 1 [micrometer]), was designed. The construction of a compact cryogenic variable-temperature (77 - 300 K) SHPM, highlighting its features, is described.Item Low temperature scanned probe microscopy studies of magnetic oxides(2011-05) Lee, Alfred K.; de Lozanne, Alejandro L.; Shih, Chih-Kang; Markert, John; Demkov, Alexander; Ruoff, RodneyThis dissertation is divided into two parts. In the first, the general paradigm of scanned probe microscopy is outlined with a focus on atomic force microscopy and a few of its variations. Magnetic force microscopy is covered in detail as it forms the basis of the second part of this dissertation. The core elements and extra features of the instrument are described with attention paid to the upgrades made by the author. In the second part of this dissertation, background information on perovskite oxides and the inverse spinel system, magnetite, is given. Magnetic force microscopy studies were done on three thin film systems and are detailed. In the first study, ferromagnetic manganite films were subjected to discontinuous changes in strain due to structural transitions in their barium titanate substrates. The resulting effect on the magnetic domains was observed. In the second study, the ferromagnetism of a tensile-strained LaCoO₃ film was studied across temperatures from 4.3 K to 90 K and applied fields up to [mu]₀H=1.1 T. Finally, the properties of antiphase domains in magnetite films of varying film strain due to transition metal buffer layers was probed by imaging the magnetic domains which are pinned to the antiphase boundaries.Item Magnetic force microscopy studies of magnetic domain structure in LaCoO₃ and UMn₂Ge₂(2014-12) Berg, Morgann Elizabeth; de Lozanne, Alejandro L.Magnetic force microscopy studies in varying temperature and applied external magnetic field of magnetic thin films of LaCoO₃ under strain and single crystal UMn₂Ge₂ have been performed. In the case of LaCoO₃ thin films the aim is an understanding of the response of the magnetic microstructure to different signs and degrees of strain and a further attempt to distinguish the effect of defects from strain-induced effects. In UMn₂Ge₂ the magnetic microstructure is imaged for the first time and signatures of a possible phase transition at 150 K and crystalline anisotropy are explored. The first portion of this dissertation focuses on the synthesis methods used to produce the samples investigated and the critical role of synthesis in producing high-quality samples. This is followed by a discussion of characterization techniques used to obtain local and global magnetic and structural characteristics, with particular emphasis on magnetic force microscopy including noise characteristics and a discussion of achieving a high force gradient sensitivity by optimizing the fiber-optic interferometer used for cantilever deflection detection. Design elements and features of the multi-mode variable-temperature atomic force microscope used to obtain magnetic force microscopy images are presented and results for LaCoO₃ and UMn₂Ge₂ are discussed.