G. Schönhense

Improvement of structural, electronic, and magnetic properties of Co2MnSi thin films by He+ irradiation

The influence of 30 keV He+ ion irradiation on structural, electronic, and magnetic properties of Co2MnSi thin films with a partial B2 order was investigated. It was found that room temperature irradiation with light ions can improve the local chemical order. This provokes changes of the electronic structure and element-specific magnetization toward the bulk properties of a well-ordered Co2MnSi Heusler compound.

Hard x-ray photoelectron spectroscopy of buried Heusler compounds

This work reports on high energy photoelectron spectroscopy from the valence band of buried Heusler thin films (Co2MnSi and Co2FeAl0.5Si0.5) excited by photons of about 6?keV energy. The measurements were performed on thin films covered by MgO and SiOx with different thicknesses from 1 to 20?nm of the insulating layer and additional AlOx or Ru protective layers. It is shown that the insulating layer does not affect the high energy spectra of the Heusler compound close to the Fermi energy. The high resolution measurements of the valence band close to the Fermi energy indicate a very large electron mean free path of the electrons through the insulating layer. The spectra of the buried thin films agree well with previous measurements from bulk samples. The valence band spectra of the two different Heusler compounds exhibit clear differences in the low lying s bands as well as close to the Fermi energy.

Correlation in the transition-metal-based Heusler compounds Co2MnSi and Co2FeSi

Half-metallic ferromagnets like the full Heusler compounds with formula X2YZ are supposed to show an integer value of the spin magnetic moment. Calculations reveal in certain cases of X = Co based compounds non-integer values, in contrast to experiments. In order to explain deviations of the magnetic moment calculated for such compounds, the dependency of the electronic structure on the lattice parameter was studied theoretically. In local density approximation (LDA), the minimum total energy of Co2FeSi is found for the experimental lattice parameter, but the calculated magnetic moment is about 12% too low. Half-metallic ferromagnetism and a magnetic moment equal to the experimental value of 6µB are found, however, only after increasing the lattice parameter by more than 6%. To overcome this discrepancy, the LDA+U scheme was used to respect on-site electron correlation in the calculations. Those calculations revealed for Co2FeSi that an effective Coulomb-exchange interaction Ueff = U J in the range of about 2eV to 5eV leads to half-metallic ferromagnetism and the measured, integer magnetic moment at the measured lattice parameter. Finally, it is shown in the case of Co2MnSi that correlation may also serve to destroy the half-metallic behavior if it becomes too strong (for Co2MnSi above 2eV and for Co2FeSi above 5eV). These findings indicate that on-site correlation may play an important role in the description of Heusler compounds with localized moments.

Slater-Pauling rule and Curie temperature of Co2-based Heusler compounds

A concept is presented serving to guide in the search for materials with high spin polarization. It is shown that the magnetic moment of half-metallic ferromagnets can be calculated from the generalized Slater-Pauling rule. Furthermore, it was found empirically that the Curie temperature of Co2-based Heusler compounds can be estimated from a seemingly linear dependence on the magnetic moment. As a successful application of these simple rules, it was found that Co2FeSi is, actually, the half-metallic ferromagnet exhibiting the highest magnetic moment and the highest Curie temperature measured for a Heusler compound.

Photoemission electron microscopy as a tool for the investigation of optical near fields.

Photoemission electron microscopy was used to image the electrons photoemitted from specially tailored Ag nanoparticles deposited on a Si substrate (with its native oxide SiO(x)). Photoemission was induced by illumination with a Hg UV lamp (photon energy cutoff homega(UV) = 5.0 eV, wavelength lambda(UV) = 250 nm) and with a Ti:sapphire femtosecond laser (homega(l) = 3.1 eV, lambda(l) = 400 nm, pulse width below 200 fs), respectively. While homogeneous photoelectron emission from the metal is observed upon illumination at energies above the silver plasmon frequency, at lower photon energies the emission is localized at tips of the structure. This is interpreted as a signature of the local electrical field therefore providing a tool to map the optical near field with the resolution of emission electron microscopy.

Direct observation of half-metallicity in the Heusler compound Co2MnSi

Ferromagnetic thin films of Heusler compounds are highly relevant for spintronic applications owing to their predicted half-metallicity, that is, 100% spin polarization at the Fermi energy. However, experimental evidence for this property is scarce. Here we investigate epitaxial thin films of the compound Co2MnSi in situ by ultraviolet-photoemission spectroscopy, taking advantage of a novel multi-channel spin filter. By this surface sensitive method, an exceptionally large spin polarization of () % at room temperature is observed directly. As a more bulk sensitive method, additional ex situ spin-integrated high energy X-ray photoemission spectroscopy experiments are performed. All experimental results are compared with advanced band structure and photoemission calculations which include surface effects. Excellent agreement is obtained with calculations, which show a highly spin polarized bulk-like surface resonance ingrained in a half metallic bulk band structure.

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.

Microspectroscopy and imaging using a delay line detector in time-of-flight photoemission microscopy

A method for microspectroscopy and energy-selective imaging using a special photoemission electron microscope (PEEM) is presented. A modified commercial PEEM was combined with a delay line device as x, y, t detector serving as the basic arrangement for spectromicroscopy. One can measure the time of flight of the electrons passing a drift section in order to analyze the energy distribution of photoelectrons in PEEM. The time of flight is referenced to the time structure of the synchrotron radiation from an electron storage ring. At electron kinetic energies of less than 20 eV within the drift region a spatial resolution of about 100 nm has been obtained. Fast counting electronics (instead of a camera) delivers an image for real-time monitoring on an oscilloscope screen or for image acquisition by a computer. A time resolution of about 500 ps has been obtained with the potential of further improvement. The spatial resolution of the delay line detector is about 50 μm in the image plane corresponding to 1000 ...

Investigating surface magnetism by means of photoexcitation electron emission microscopy

The imaging of surfaces by means of photoexcitation electron emission microscopy (PEEM) has recently received considerable interest. This is mainly due to the extended use and availability of brilliant synchrotron radiation in the soft x-ray regime which generally facilitates studies with surface specificity and chemical selectivity. The most popular application of the x-ray PEEM (XPEEM) technique concerns studies of magnetic systems and phenomena. By exploiting the high degree of circular or linear polarization of the synchrotron light, the magnetic microstructure in both ferromagnets and antiferromagnets can be visualized. In this contribution we demonstrate the unique potential and the versatility of the PEEM approach, and review the current status with a certain emphasis on experiments with soft x-ray excitation. In some cases, the high-energy excitation studies can be complemented by laboratory experiments employing threshold photoemission with ultraviolet light (UV-PEEM). Current limitations and future developments and perspectives of the PEEM technique applied to magnetic systems are discussed.

Electronic structure and spectroscopy of the quaternary Heusler alloy Co2Cr1−xFexAl

Quaternary Heusler alloys Co2Cr1 xFexAl with varying Cr to Fe ratio x were investigated experimentally and theoretically. The electronic structure and spectroscopic properties were calculated using the full relativistic Korringa-Kohn-Rostocker method with coherent potential approximation to account for the random distribution of Cr and Fe atoms as well as random disorder. Magnetic effects are included by the use of spin dependent potentials in the local spin density approximation. Magnetic circular dichroism in X-ray absorption was measured at the L2,3 edges of Co, Fe, and Cr of the pure compounds and the x = 0.4 alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. Resonant (560eV - 800eV) soft X-ray as well as high resolution - high energy (� 3.5keV) hard X-ray photo emission was used to probe the density of the occupied states in Co2Cr0.6Fe0.4Al.