J. Kirschner

Influence of the growth conditions on the magnetic properties of fcc cobalt films: from monolayers to superlattices

Abstract The growth and magnetic properties of films of fcc cobalt on Cu(100) substrates has been characterized by a multitechnique approach. The films are ferromagnetically ordered in-plane at temperatures below Tc. The Curie temperature of the films displays a linear dependence with the coverage reaching bulk-like behaviour at coverages of 5–6 monolayers. Spin-polarized photoemission shows that the band structure is already close to that of the bulk at 5 ML. Crystalline Co/Cu sandwiches and superlattices have been grown on Cu(100) substrates. The magnetic ordering of Co slabs across Cu layers of varying thicknesses, as explored by SMOKE, changes from ferromagnetic to antiferromagnetic and back to ferromagnetic. The antiferromagnetic ordering has been confirmed by polarized neutron diffraction. As a function of the external magnetic field, the magnetic ordering changes to ferromagnetic with a complex intermediate behaviour.

Recent progress in photoemission microscopy with emphasis on chemical and magnetic sensitivity

Abstract With the improved access to synchrotron radiation sources photoemission electron microscopy is developing into a versatile analytical tool in surface and materials science. The broad spectral range and the well-defined polarization characteristics of synchrotron light permit a unique combination of topographic, chemical, and even magnetic investigations down to a mesoscopic scale. The potentiality of photoemission electron microscopy is demonstrated by several experiments on surfaces and microstructured thin film systems, which have been carried out with a newly designed instrument. We discuss its different modes of operation with respect to both microscopy and spectroscopy. A combination of elemental selectivity and magnetic sensitivity is achieved by using circularly polarized soft X-rays and exploiting the effect of magnetic circular dichroism. This way one obtains information about the magnetic state of individual chemical components within the sample.

In situ observation of epitaxial growth of Co thin films on Cu(100)

Abstract The magnetic and electronic properties of ultrathin Co films have been found to depend sensitively on their growth and structure. Therefore, we studied the evolution of Co films during their deposition onto Cu(100) substrates under UHV conditions by STM. Our STM-design offers a simple way to retrieve particular positions on a sample even after preparations like sputtering or annealing, which require multiple sample transfers to a preparation stage. We have observed the nucleation and growth of individual Co islands during the growth of films from the sub-monolayer regime up to several monolayers. At room temperature, the system exhibits epitaxial growth in a nearly ideal layer-by-layer mode. The density of island nucleation is significantly higher in the growth of the first monolayer, compared with additional layers. The onset of growth of the second layer is well before the completion of the first layer, contrasting the more perfect growth of subsequent layers.

Spin-polarized scanning tunneling microscopy on ferromagnets

A straightforward approach to spin-polarized scanning tunneling microscopy based on the magnetotunneleffect between a ferromagnetic tip and a ferromagnetic sample is demonstrated. By periodically changing the magnetization of the tip in combination with a lock-in technique, topographic and spin-dependent parts of the tunnel current are separated and the topography and the magnetic structure of the sample are recorded simultaneously. Results are given for polycrystallineNi and single crystalline Co(0001) surfaces, revealing a high spin contrast, low data acquisition times, and a resolution down to 10 nm. Potentials and limitations of this technique are discussed.

Magnetoelectric Charge Trap Memory

It is demonstrated that a charge-trapping layer placed in proximity to a ferromagnetic metal enables efficient electrical and optical control of the metals magnetic properties. Retention of charge trapped inside the charge-trapping layer provides nonvolatility to the magnetoelectric effect and enhances its efficiency by an order of magnitude. As such, an engineered charge-trapping layer can be used to realize the magnetoelectric equivalent to todays pervasive charge trap flash memory technology. Moreover, by supplying trapped charges optically instead of electrically, a focused laser beam can be used to imprint the magnetic state into a continuous metal film.

Spin-Dependent Quantum Interference Within a Single Magnetic Nanostructure

Wave-Particle Duality The dual-wave nature of particles is nowhere more evident than in a confined space, where standing waves are formed with wavelengths that depend on particle energy. This so-called quantum interference has been observed in nanostructures using surface probes such as scanning tunneling microscopy. Now, Oka et al. (p. 843) use the spin-polarized version of this technique to study spin-dependent quantum interference on a triangular nanoscale cobalt island deposited on a copper surface. They observe the modulation of the magnetization, with the pattern depending on the energy of the interfering electrons. The experimental results are in good agreement with simulations, which indicate that the magnetization at a given energy and position largely depends on which of two electron spin states present dominates. Magnetization modulation is observed on a cobalt nanoisland using spin-polarized scanning tunneling microscopy. Quantum interference is a coherent quantum phenomenon that takes place in confined geometries. Using spin-polarized scanning tunneling microscopy, we found that quantum interference of electrons causes spatial modulation of spin polarization within a single magnetic nanostructure. We observed changes in both the sign and magnitude of the spin polarization on a subnanometer scale. A comparison of our experimental results with ab initio calculations shows that at a given energy, the modulation of the spin polarization can be ascribed to the difference between the spatially modulated local density of states of the majority spin and the nonmodulated minority spin contribution.

A simple technique to measure stress in ultrathin films during growth

We demonstrate an easy implementation of the cantilever bending beam approach to measure stress during film growth in ultrahigh vacuum. Using a simple and compact optical deflection technique, film stress with sub‐monolayer sensitivity can be detected. A stress measurement during FeSi2 formation on Si(111) is presented.

High efficiency electron spin polarization analyzer based on exchange scattering at Fe∕W(001)

We report on a compact electron spin analyzer based on exchange scattering from a magnetic surface. The heart of the detector is an Fe(001) thin film grown on W(001) with chemisorbed oxygen in the p(1 x 1) structure. The device is mounted at the exit of an energy dispersive analyzer and works at a scattering energy of about 13.5 eV. Its figure of merit is 2 x 10(-3), combined with an excellent stability of more than 2 weeks in UHV.

Experimental method for separating longitudinal and polar Kerr signals

A new procedure is presented which can be easily applied to separate longitudinal and polar Kerr signals. The method is advantageous particularly in systems where in-plane and out-of-plane states of magnetization are involved in the reversal process. The feasibility of the method is demonstrated at the spin-reorientation transition in Co/Au(1 1 1) films.

Magnetic spectromicroscopy from Fe(100)

Magnetic domains on an Fe(100) surface have been imaged by means of energy‐resolved photoemission microscopy. We excited the photoelectrons with circularly polarized synchrotron radiation in the soft x‐ray region, and employed the effect of magnetic circular dichroism in the emitted photoelectrons in order to obtain contrast between differently oriented magnetic domains. This new approach offers a surface sensitive way to combine chemical and magnetic information on a microscopic scale.