M. Dreyer

Pulsed-current-induced domain wall propagation in Permalloy patterns observed using magnetic force microscope

Pulsed-current controlled wall motion in 20 /spl mu/m wide/spl times/200 /spl mu/m long/spl times/160 nm thick patterned Permalloy strips was studied using magnetic force microscopy. By sequential imaging, the displacement of Bloch walls as far as 200 /spl mu/m along the strip was observed. The direction of motion was in the same direction as the carrier velocity, which reversed with current polarity. The displacement per pulse was dependent upon the sample thickness and current density, which suggests that the mechanism is a combination of s-d exchange and hydromagnetic domain drag forces.

Variable-temperature MFM/EFM study of (La/sub 1-x/Pr/sub x/)/sub 0.67/Ca/sub 0.33/MnO/sub 3/ thin films

Both ferromagnetic (FM) and charge-ordered domain structures of (La/sub 1-x/Pr/sub x/)/sub 0.67/Ca/sub 0.33/MnO/sub 3/ thin film were detected by magnetic force microscopy (MFM) and electric force microscopy (EFM) operated at low temperatures near the peak resistance temperature (T/sub p/). The in-plane-like FM domains of submicrometer size emerge below T/sub p/, and their magnetic interaction with an MFM tip becomes stronger as the temperature is reduced. Charge-ordered insulating regions have the strong electrostatic interaction with an EFM tip near T/sub p/, and the interaction correlates well with the temperature dependence of resistivity of the film.

Resolution enhancement by applying MFM under UHV conditions

The enhancement in signal-to-noise ratio and lateral resolution in MFM in going from ambient pressure to UHV is demonstrated. The performance of several cantilevers is evaluated using a patterned 50 nm thick permalloy film, with cross-tie as well as /spl sim/90/spl deg/ domain walls, and a 200 nm thick permalloy film with perpendicular magnetization. The increase in the quality factor of the cantilever oscillation in UHV improves the sensitivity, consequently allowing less magnetic material on the tip to achieve the same signal-to-noise ratio. This reduction in magnetic volume sharpens the lateral resolution. We also demonstrate that the magnetic interaction can be so weak that a magnetic contrast is visible only under UHV conditions.

A low temperature scanning tunneling microscopy system for measuring Si at 4.2 K

In this paper we describe our low temperature scanning tunneling microscopy system with ultrahigh vacuum sample preparation capabilities. The main focus lies on the specialized silicon preparation facility which is the most unusual part. Other special solutions such as sample transport will also be described in detail. Finally, we demonstrate the ability to prepare high quality silicon (111) and (100) surfaces.

Low temperature behavior of magnetic domains observed using a magnetic force microscope

A commercial atomic force microscope/magnetic force microscope (MFM) was modified to cool magnetic samples down to around 100 K under a high vacuum while maintaining its routine imaging functionality. MFM images of a 120 nm thick La0.7Ca0.3MnO3 film on a LaAlO3 substrate at low temperature show the paramagnetic-to-ferromagnetic phase transition. Evolution of magnetic domains and magnetic ripples with decreasing temperature are also observed near the edge of a 20 nm thick patterned Co film on a Si substrate.

Comparison of mfm/stm data of patterned ultrathin iron films grown on Si[001], SiO/sub 2/, and NiO i

The topographic and magnetic structures of patterned 20-nm-thick Fe films grown on Si(001), polycrystalline NiO, and SiO/sub 2/ substrates are compared by scanning tunneling microscopy/magnetic force microscopy measurements under ultrahigh vacuum conditions to investigate the influence of the different substrates. Iron grows as a polycrystalline film. The size of the crystallites decreases from Si to NiO and SiO/sub 2/ substrates, respectively, while the surface roughness increases. The increased disorder in the film structure does not necessarily lead to an increased disorder in the magnetic domain structure. Instead, the magnetic structure of iron on NiO is the most complex, which can be attributed to the exchange coupling to the substrate. The antiferromagnetic NiO crystallites exhibit no long range crystallographic ordering which leads to random magnetic orientation. More evidence of the influence of the exchange coupling is found when comparing the magnetic structures of iron on NiO and SiO/sub 2/ as a function of film thickness in the range of 0-10 nm.

Ultrahigh vacuum scanning tunneling microscopy/magnetic force microscopy study of ultrathin iron films grown on polycrystalline nickel oxide films

The thickness dependence of the topographic and magnetic structure of ultrathin Fe films grown on polycrystalline NiO films under ultrahigh vacuum (UHV) conditions was studied to investigate the growth mechanism of the ferromagnetic film and the corresponding magnetic interaction with the antiferromagnetic substrate. Externally prepared NiO films of 60 nm thickness were cleaned by heating in UHV. Ultrathin layers of Fe in the range of 1–27 nm were deposited on top of the NiO film and were analyzed at specific coverages. Iron grows as a polycrystalline film with the grains increasing in size with the thickness. The contours of the underlying NiO crystallites were evident at low coverages but gradually disappeared as the Fe grains coalesced at thicker coverages. Magnetic force microscopy images of the 1 nm thick film show randomly oriented magnetic grains with an average domain size of 30 nm. With an increase in film thickness the size of the domains grows to about 200 nm at 15 nm of iron. At a film thickne...

Investigation of the magnetic interaction of small Permalloy particles

Permalloy island arrays and undulated lines were systematically studied by MFM to clarify the effect of neighbor magnetic interaction. The islands are 860 nm /spl times/320 nm and 35 nm thick, single domains and patterned to have preferential interaction along the easy and hard axes. The switching field for easy axis interaction is relatively high, i.e., 370 Oe with a 32 Oe spread; while the hard axis interaction produces a switching field of 130 Oe and a slightly wider distribution of over 40 Oe. A third array is comprised of undulated lines with connected segments. Magnetic charges accumulate at the ends of each segment, and yield the strongest possible interaction along the easy axis. In this case all segments on one line simultaneously switch at a specific field. The switching field increases linearly from 150 Oe with the number of segments up to a maximum number of 50, and approaches a constant field of 290 Oe. The results favorably compare with the prediction for parallel rotation of the Jacobs-Bean chain of spheres model.

Magnetoresistance of ferromagnetic point junctions from tunneling to direct contact regimes

The behavior of magnetoresistance in NiFe (permalloy) point junctions was investigated using an STM set-up, where the junction resistance was continuously varied from 10/sup 8/ /spl Omega/-10/sup 3/ /spl Omega/. Our results reveal an enhancement of magnetoresistance of more than 80% at one conductance quantum, and the decaying magnetoresistance as one moves away from 12.9 k/spl Omega/ to either the diffusive regime or the tunneling regime, as supported by independent theories on spin-dependent transport. The suppression of magnetoresistance with incorporation of a 35-nm-thick Au barrier was observed, as well as the dependence of magnetoresistance on the relative orientations of the electrodes.

Sr adatoms on As bridge positions on SrFe2As2 observed by scanning tunneling microscopy at 4.2?K

We used a scanning tunneling microscope to image the surface of SrFe(2)As(2) single crystals at 4.2 K. We found, besides the commonly reported row structures and some disordered areas, also maze-like regions. Atomically resolved images of the maze show that the atoms on the surface sit on As bridge positions of the underlying Fe(2)As(2) layer. Examination of the corner positions within the maze-like reconstruction reveals the presence of adatoms rather than As dimers. Hence, the surface atoms on these samples are most likely to be Sr atoms.