Browsing by Author "Dahlberg, E.D."

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    Domain structures in single-crystal magnetite below the Verwey transition as observed with a low-temperature magnetic force microscope
    (Geophysical Research Letters (American Geophysical Union), 1996) Moloni, Katerina; Moskowitz, Bruce; Dahlberg, E.D.
    The magnetic domain structures on the {110} plane of magnetite (Fe3O4) below the Verwey transition (Tv=120K) were studied using a Low-Temperature Magnetic Force Microscope (LTMFM). At 298K, domain structures consisted of arrays of 180°, 109° and 71° walls, typical for magnetite with cubic anisotropy. At 77K (below Tv), the cubic style patterns disappeared and transformed into uniaxial patterns consistent with the uniaxial magnetocrystalline symmetry of the low-temperature monoclinic phase of magnetite. We also observed two distinct styles of domain patterns below Tv: (1) wide domains separated by straight 180° walls along the in-plane [100] easy axis; and (2) intricate wavy walls with reverse spike domains characteristic of out-of-plane easy axes. This intimate mixture of domain styles within adjacent areas of the crystal reflects variations in the direction of the magnetic easy axes in different regions produced by c-axis twinning of the crystal below Tv The thermal dependence of planar and wavy-wall patterns show little change from 77K until 110K, where patterns disappear. Upon cooling back to 77K, domain structures are different from the initial 77K states, indicating that renucleation of different domain states occurs by cycling near Tv.
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    Domain walls in single-crystal magnetite investigated by magnetic force microscopy
    (Journal of Geophysical Research (American Geophysical Union), 1999) Foss, Sheryl; Moskowitz, Bruce; Proksch, Roger; Dahlberg, E.D.
    Domain walls in bulk single-crystal magnetite were studied using a variable magnetic field magnetic force microscope (MFM). Classical configurations of 180°, 109°, and 71° walls were observed on (110) surfaces. Magnetostatic effects on these different walls were compared. Profiles of the MFM response above the walls were measured with the MFM tip magnetized in different directions. The contribution to the profiles from the z component of the sample field was distinguished from the in-plane components. An asymmetry of the z component of the response profiles for all wall types was observed, consistent with the existence of Néel caps which terminate the interior Bloch walls near the surface. The wall profiles of the non-180° walls were more asymmetric than that of the 180° walls. The 180° walls were observed to be subdivided into alternating polarity segments of average length 15 μm. These walls formed a characteristic zig-zag structure in which the Bloch lines separating segments were located at the corners of the zig-zag. Only unusually long 109° walls were observed to contain a single Bloch line, and the 71° walls, although the longest, were never observed to be subdivided. An applied field perpendicular to the sample plane moved the Bloch lines within the walls without translating the walls themselves. Multipolar walls were converted to unipolar in perpendicular applied fields from O to 100 mT. Profiles of opposite polarity segments of a subdivided wall indicated that the Néel cap formation does not alternate sides of the wall from segment to segment. Alignment of opposite polarity segments of parallel subdivided walls provided an example of long range magnetostatic interactions between walls and possibly their Néel caps.
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    Localized micromagnetic perturbation of domain walls in magnetite using a magnetic force microscope
    (Applied Physics Letters (American Institute of Physics), 1996) Foss, Sheryl; Proksch, Roger; Dahlberg, E.D.; Moskowitz, Bruce; Walsh, B.
    Magnetic force microscope(MFM) profiles of domain walls (DWs) in magnetite were measured using commercially available MFM tips. Opposite polarity profiles of a single DW segment were obtained by magnetizing the MFM tip in opposite directions perpendicular to the sample surface. During a measurement, the field of the tip locally magnetized the DW, resulting in a more attractive tip‐sample interaction. The difference between opposite polarity DW profiles provided a qualitative measurement of the reversible changes in DW structure due to the localized field of the MFM tip.
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    Magnetic fine structure of domain walls in iron films observed with a magnetic force microscope
    (Journal of Applied Physics (American Institute of Physics), 1994) Proksch, Roger; Foss, Sherry; Dahlberg, E.D.; Prinz, G.
    The submicron magnetic structure of domain walls in a single‐crystal ironfilm has been studied using a magnetic force microscope(MFM). The MFM tip was sensitized to the component of the field perpendicular to the film plane. The sample examined was a 500‐nm‐thick single‐crystal film of iron,grown by molecular‐beam epitaxy(MBE). Before it was imaged, the film was magnetized along its (in‐plane) easy axis in a 2000‐Oe field. Studies of the domain structure at numerous locations on the filmsurface revealed a rich variety of micromagnetic phenomena. Parallel domain walls, determined to be Bloch walls with a width of 70–100 nm, were seen along the easy axis, spaced roughly 30 μm apart. These appeared to be Bloch walls. Bloch lines were also observed in the walls with an average periodicity of 1.5 μm. This is a value smaller than that predicted for Bloch wall‐line structures. In addition, a pronounced zig–zag structure was observed, as expected from previous Fe whisker observations.
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    Magnetic force microscopy of the submicron magnetic assembly in a magnetotactic bacterium
    (Applied Physics Letters (American Institute of Physics), 1995) Proksch, Roger; Schäffer, T.E; Moskowitz, Bruce; Dahlberg, E.D.; Bazylinski, Dennis; Frankel, R.B.
    A magnetic force microscope(MFM) was used to image topography and magnetic forces from a chain of submicron single magnetic domain particles produced by and contained in isolated magnetotactic bacteria. The noncontact magnetic force microscope data were used to determine a value for the magnetic moment of an individual bacterial cell, of order 10−13 emu, consistent with the average magnetic moment of bacteria from the same sample, obtained by superconducting quantum interference device magnetometry. The results represent the most sensitive quantification of a magnetic force microscope image to date.
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    Measurement of the effects of the localized field of a magnetic force microscope tip on a 180° domain wall.
    (Journal of Applied Physics (American Institute of Physics), 1997) Foss, Sheryl; Dahlberg, E.D.; Proksch, Roger; Moskowitz, Bruce
    Opposite polarity magnetic force microscope(MFM) profiles of domain walls (DWs) in magnetite were measured with a commercial MFM tip magnetized in opposite directions perpendicular to the sample surface. The influence of the tip field on a DW resulted in an overall more attractive interaction. The difference between opposite polarity DW profiles provided a qualitative measurement of the reversible changes in DW structure due to the localized field of the MFM tip. The dependence of the measured alteration on tip-sample separation was fit with a power law at different positions across the DW. The rate of decay of the alteration with tip-sample separation, quantified by the exponent of the power law fit, varied across the DW and was much slower than expected from a simple model.