Sheryl Foss

Localized micromagnetic perturbation of domain walls in magnetite using a magnetic force microscope

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.

Quantitative magnetic field measurements with the magnetic force microscope

We have developed a technique to quantitatively image the magnetic field above a magnetic specimen using a modified magnetic force microscope (MFM). The technique depends on the nonlinear response of a magnetically soft MFM tip to the sample field and to an externally applied field, similar in principle to fluxgate magnetometry. We demonstrate the technique with high resolution, quantitative images of the magnetic field above a sample of longitudinal recording media. The magnetic field resolution is on the order of 1 Oe, with sub‐100 nm spatial resolution comparable to standard MFM techniques.

High field magnetic force microscopy

Magnetic force microscope (MFM) studies of high‐density thin‐film recording media have been performed in the presence of an applied magnetic field. In particular, the erasure behavior of bit transitions in the media have been investigated. For these studies a compact, high‐field dc magnet has been constructed that fits the laser head of a Nanoscope III multimode microscope. Because of space constraints and concern over thermal drifts which could affect the stability of the MFM, a rotating permanent magnet was used instead of an electromagnet. The magnet is mounted in a yoke which guides varying amounts of flux to the sample. This was used to observe the erasure of bits in a magnetic hard disk. The applied field also magnetized the MFM cantilever, making it possible to magnetically characterize both the sample and the probe.

Domain wall structures in single-crystal magnetite investigated by magnetic force microscopy

Domain walls in bulk single-crystal magnetite were studied using a variable magnetic field magnetic force microscope (MFM). Classical configurations of 180 o, 109 o, and 71 o 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 N6el caps which terminate the interior Bloch walls near the surface. The wall profiles of the non- 180 o walls were more asymmetric than that of the 180 o walls. The 180 o walls were observed to be subdivided into alternating polarity segments of average length 15 gm. 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 o walls were observed to contain a single Bloch line, and the 71 o 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 0 to 100 mT. Profiles of opposite polarity segments of a subdivided wall indicated that the N6el 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 N6el caps.

Magnetization reversal processes in perpendicular anisotropy thin films observed with magnetic force microscopy

Abstract We have carried out studies of the magnetic reversal process of a rare earth–transition metal thin film with perpendicular magnetic anisotropy using a magnetic force microscope (MFM) capable of applying in situ magnetic fields. The magnetization of the microscopic area shown in MFM images was determined for a number of field values comprising a complete hysteresis loop. This microscopic hysteresis loop was found to be nearly identical to a bulk hysteresis loop. Changes in the magnetization of the film as the hysteresis loop was traversed can be linked to individual microscopic domain changes evident in the MFM images. These studies show that the magnetization of this film was characterized by a two-stage process – fast and slow rates of change of magnetization with applied field. A second experiment in which the film was incompletely saturated and brought back to zero field showed that domain nucleation was not responsible for the rate of the fast process, but rather all magnetization changes were primarily limited by the low domain wall mobility. These observations are linked to previous work on magnetization processes in similar magnetic systems.

MEASUREMENT OF THE EFFECTS OF THE LOCALIZED FIELD OF A MAGNETIC FORCE MICROSCOPE TIP ON A 180 DOMAIN WALL

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.

Magnetic fine structure of domain walls in iron films observed with a magnetic force microscope

The submicron magnetic structure of domain walls in a single‐crystal iron film 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 film surface 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.

Localized magnetostrictive magnetization reversal using scanning probe tips

Localized magnetic reversal of a perpendicular anisotropy thin film has been performed using the magnetostrictive response of the film to a force applied by probe tips of scanning force microscope cantilevers. Non-magnetic and magnetic cantilever tips were used to apply local stresses which alter the local magnetization through magnetostriction. The magnetic field of the tip, if any, and the local demagnetizing field of the film reverse the stressed area for stresses exceeding a critical value. These findings were in agreement with a simple model.

Force gradient mapping of domain wall structures in magnetite (abstract)

We have used a magnetic force microscope (MFM) in a new imaging mode to study domain walls in magnetite. The oscillation amplitude and phase of a vibrating cantilever were recorded versus cantilever tip‐sample separation at each point in an x‐y raster scan of a conventional MFM image. Amplitude and phase changes of the MFM cantilever depend on the topographic, interferometric, damping, and magnetic force gradients effects the cantilever experiences. For small scan areas, the magnetic force gradient acting between the tip and sample could be separated from the other interactions. This allowed quantitative extraction of the magnetic force gradients into a three‐dimensional dataset called a force gradient map (FGM). FGMs were made over a number of samples; the work reported here focuses on domain wall structures in single crystal magnetite. The results of previous conventional MFM images and FGM images will be compared. One result was that the apparent resolution of the MFM tip was strongly dependent on the ...

Domain structure of iron single crystals grown on Si(111) investigated by magnetic force microscopy (abstract)

Single‐crystal ironfilmsgrown epitaxially on (111) silicon were studied using a magnetic force microscope(MFM). The crystalline anisotropy easy axes (〈100〉), directed 35° out of the film plane, result in a significant magnetic charge density at the film surface and a sixfold symmetry in the energy minima of the system. To reduce the magnetostatic energy of this configuration, an alternating, stripe domain pattern formed in samples of thickness ranging from 1240 to 315 nm. Because of the wavelike form of the MFM response across the stripes, two‐dimensional Fourier transforms of the MFM images were used to give measures at the domain period, i.e., the wavelength of the stripes, as well as the complexity of the patterns. The stripe domain period was found to be approximately equal to the film thickness which is consistent with previous theoretical predictions. The domain patterns of the films in various remanent states and progressive stages of the magnetization process were investigated using the MFM and an in situ, variable magnetic field. A MFM tip that is magnetically soft compared to the iron is most suitable for this. Measurements of the domain period and pattern complexity as a funciton of applied field were correlated with bulk hysteresis measurements.