A. Bischof

Direct Observation of Domain-Wall Configurations Transformed by Spin Currents

Direct observations of current-induced domain-wall propagation by spin-polarized scanning electron microscopy are reported. Current pulses move head-to-head as well as tail-to-tail walls in submicrometer Fe20Ni80 wires in the direction of the electron flow, and a decay of the wall velocity with the number of injected current pulses is observed. High-resolution images of the domain walls reveal that the wall spin structure is transformed from a vortex to a transverse configuration with subsequent pulse injections. The change in spin structure is directly correlated with the decay of the velocity.

Determining magnetic anisotropies from hysteresis loops

A novel method for determining magnetic anisotropies from hysteresis loops is presented. While sweeping the loop, a magnetic field is applied perpendicularly to the sweep-field axis. This causes the magnetization to rotate reversibly in a wide field range and still reach saturation at finite fields. An example is given whereby surface and volume anisotropies are determined from magneto-optical Kerr effect loops in Co films grown on stepped Cu(001).

Spin-polarized secondary electrons from a scanning tunneling microscope in field emission mode

A new technique has been developed which opens the way to magnetic imaging with nm resolution. A narrow electron beam produced with a scanning tunneling microscope operating in field emission mode impinges on the magnetic surface, and the spin polarization of the emitted secondary electrons is monitored. As a first result, a hysteresis loop from an Fe‐based metallic glass shows that the low‐energy secondary electrons excited with this technique are spin polarized.

Velocity of vortex walls moved by current

Current-induced domain-wall motion experiments in 27nm thick and 200–500nm wide Fe20Ni80 wires are reported. By imaging the domain-wall position after current injections, the mean wall velocities are determined. The initial velocity is found to be constant for pulse lengths between 2 and 25μs but decays after about ten injections. For samples with an increasing wire width the initial velocity is reduced.

Cu adsorption on Co films: edge decoration versus intermixing

Abstract Heteroepitaxial growth of the CoCu system is investigated by scanning tunneling microscopy. Cu deposited on Co Cu (001) decorates the step edges of Co islands and forms nanometer-sized rings. Subsequent deposition of Co transforms the film morphology from compact to heterogeneously intermixed. We relate this growth asymmetry to the largely different surface free energies of Co and Cu, which impose a diffusion barrier at CoCu boundaries, and to atomic exchange processes that occur in the presence of Co adatom gas.

Local modification of magnetic properties by an electron beam

Magnetic anisotropies can be modified locally by intense electron beams with keV energy. In ultrathin epitaxial Co films grown on Pt(111), switching of the easy axis from parallel to perpendicular to the surface can be induced, and the domain size can be enlarged by one order of magnitude.

Growing thin magnetic films with a mask: Distinguishing between magnetic and instrumental asymmetries

A simple technique is presented which allows a very small spin polarization to be distinguished from a vanishing one in spin polarized scanning electron microscopy (spin‐SEM). Ferromagnetic thin films are evaporated through a mask onto a nonmagnetic substrate, thus producing a patterned area. Spin polarization and the element specificity of the secondary electron yield are combined to determine a local ‘‘asymmetry zero’’ where the spin polarization vanishes. With this technique it is possible to distinguish between a single domain thin film and a nonmagnetic one in spin‐SEM. This is exemplified by monolayer Co/Cu (100) films.

Probing depth of threshold photoemission electron microscopy

The probing depth of photoelectron emission microscopy has been determined for photon excitation energies of 5 eV. For Ag overlayers deposited on Fe, it is found that the probing depth is extremely large; that is, 16.2 nm. This result is compared with probing depths of related techniques, and the physical mechanisms responsible for this bulk sensitivity are discussed.

Recording at Large Write Currents on Obliquely Evaporated Medium and Application to a Multilevel Recording Scheme

We have studied the recording process of an obliquely evaporated Co-CoO medium. The experimental readback waveforms and the spin-SEM images of the recorded magnetization reveal an interesting magnetization process upon increasing the write current in one of the two recording directions. With larger write currents, isolated readback pulses drastically change their shape and the remanent magnetization of the written domains decreases. We qualitatively reproduce this behavior using simple write models that include the three-dimensional (3-D) granular nature of the medium and a distribution of anisotropy axes for the evaporated Co grains. Based on these results, we propose a recording scheme in which a patterned multilevel tilted recording medium is used advantageously for 3-D magnetic recording.

MAGNETIC ANISOTROPY OSCILLATIONS (INVITED)

The magnetic anisotropy in Co films epitaxially grown on Cu(001) is investigated by measuring magneto-optical Kerr hysteresis loops during film growth. As a function of Co thickness the magnetic anisotropy oscillates with a period of 1 monolayer. These oscillations are attributed to the periodic variations of the film morphology alternating between filled and incompletely filled atomic layers. Upon coverage of the Co films with Cu overlayers the anisotropy of the magnetic film is found to oscillate with increasing Cu layer thickness. We relate these oscillations to quantum interferences due to the confinement of electrons in ultrathin films.