Rolf Allenspach

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.

Ultrathin films: magnetism on the microscopic scale

Abstract The basic concepts associated with two-dimensional ferromagnetism are illustrated from an experimentalists viewpoint. Ultrathin epitaxial films are the prototype systems for investigating magnetism in reduced dimensions. and spin-polarized scanning electron microscopy is a powerful technique for such experiments. The topics covered include a discussion on the formation of magnetic domains in these films, the microscopic investigation of spin fluctuations, and the key role that anisotropies play in these systems.

Intermixing and growth kinetics of the first Co monolayers on Cu(001)

We report on the interfacial intermixing of ultrathin Co films on a Cu(001) single crystal, two materials that are immiscible in the bulk. With increasing deposition rate we find a crossover from layer-by-layer to bilayer growth due to a hindered diffusion from the first layer to the substrate. Above two monolayers, growth proceeds in a layer-by-layer fashion. These effects are explained in terms of surface free energies.

Spin-polarized scanning electron microscopy

In this paper, a review is presented of a powerful technique for studying magnetic microstructures: spin-polarized scanning electron microscopy, denoted as spin-SEM, or SEMPA. When the beam of a scanning electron microscope traverses a ferromagnetic sample, secondary electrons are emitted whose spin polarization contains information on the magnitude and direction of the magnetization of the surface. Various illustrative examples are presented which describe the main features of the technique, such as its very high surface sensitivity, its suitability for achieving complete separation of relevant magnetic and topographic information, and its high lateral resolution.

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).

Magnetism with picosecond field pulses

Ultrashort magnetic field pulses of 6 ps duration surrounding the focus of 50 GeV electron bunches are shown to write a magnetic pattern into both perpendicular and in-plane metallic magnetic recording films. High-resolution magnetic contrast images reveal the elementary processes necessary to induce magnetization reversal as well as rotation of the magnetization into the field direction.

Magnetologic devices fabricated by nanostencil lithography.

We present magnetic quantum cellular automata (MQCA), fabricated by means of nanostencil lithography, i.e., using a resistless shadow masking technique in ultra-high vacuum. The nanostencil tool allows the fabrication and in situ investigation of structures using atomic force microscopy (AFM) and magnetic force microscopy (MFM). We analyze the error distribution within the structures to shed light on the performance and challenges of magnetic cellular logic devices. Simulations are performed to corroborate an improved concept for these devices which makes use of fourfold magnetic anisotropy.

EXCHANGE COUPLING ACROSS CU(100): A HIGH-PRECISION STUDY

Oscillatory exchange coupling is investigated in epitaxially grown magnetic structures of the type M/Cu/Co/Cu(100), where M = f.c.c.-Fe, b.c.c.-Fe, f.c.c.-Co, and f.c.c.-Ni. Both long- and short-period oscillations are identified for Cu wedge thicknesses up to 60 monolayers. The corresponding periods, amplitudes, and relative antiferromagnetic peak shifts are determined and compared to available theories.

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.

Proposal for a Standard Problem for Micromagnetic Simulations Including Spin-Transfer Torque

of micromagnetic simulation tools. The work is based on the micromagnetic model extended by the spin-transfer torque in continuously varying magnetizations as proposed by Zhang and Li. The standard problem geometry is a permalloy cuboid of 100 nm edge length and 10 nm thickness, which contains a Landau pattern with a vortex in the center of the structure. A spin-polarized dc current density of 10 12 A/m 2 ows laterally through the cuboid and moves the vortex core to a new steady-state position. We show that the new vortex-core position is a sensitive measure for the correctness of micromagnetic simulators that include the spin-transfer torque. The suitability of the proposed problem as a standard problem is tested by numerical results from four dierent nite-dierence and nite-element-based simulation tools.