Transmission X-ray microscopy using X-ray magnetic circular dichroism

X-ray magnetic circular dichroism (X-MCD) was used as a large, element-specific and quantitative magnetic contrast mechanism in the soft X-ray microscopes at BESSY I (Berlin) and the ALS (Berkeley). The present state and potential of magnetic transmission X-ray microscopy (MTXM) is outlined. The possibility to record images in varying magnetic fields and the high spatial resolution down to 25 nm were used to image out-of-plane magnetized (4 ?Fe / 4 ?Gd)×75 systems. Magnetic domains could be studied in arrays of circular and square dots with lateral dimensions down to 180 nm. Hysteresis loops of individual dots were deduced using the direct proportionality of the X-MCD contrast to the sample magnetization. Images of a 3 nmCr / 50 nmFe / 6 nmCr film demonstrate for the first time that MTXM is also able to observe in-plane magnetized domains. In the future the possible applications of MTXM will be extended with regard to the strength of the external field, the available energy range and the sample conditions by building a dedicated transmission X-ray microscope for magnetic imaging at BESSY II. Received: 22 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Magnetic surfaces

The magnetic ground state, magnetic anisotropies and spin excitations of surfaces, interfaces and ultra-thin films of ferromagnetic 3d-metals are discussed. Enhanced magnetic ground state moments and altered hyperfine fields as predicted by ab initio band calculations have not been conclusively verified by experiments up to now. Future calculations should take into account dipolar fields and the role of interface roughness. Very large magnetic anisotropies are observed at magnetic surfaces and interfaces. In Ni/Cu multilayered films, the superposition of surface and stress-induced anisotropies was used to switch the easy axis of magnetization from the film plane to a perpendicular orientation by a proper choice of the Ni layer thickness. This could be an attractive possibility to develop new magnetic materials for technical applications. The temperature dependence of the spontaneous magnetization at surfaces and in ultra-thin films deviates from the behaviour of bulk material. Size effects as well as surface effects of spin wave excitations are discussed, comparing theoretical and experimental results. The need for more complete theories including surface exchange, surface anisotropy and realistic surface structures is emphasized.     

Stability of magnetic tunnel junctions

When magnetic tunnel junctions (MTJ) are built into a memory device they will be arranged in a matrix; therefore some of the not addressed elements will be exposed to a significant field during the switching of one element. It has to be avoided that the state of not selected MTJ is changed during this process. Here we present data on the stability of MTJ against small fields which occur during a writing process.     

Influence of ripple and skew on the fall-back angle α50 in thin uniaxial permalloy films

The overall fall-back angle ₅₀ of thin uniaxial ferromagnetic films was calculated including the effect of the magnetization ripple and the skew. Both, ripple and skew angle were assumed to show a Gaussian distribution with different standard deviations. Ripple and skew are superimposed in a nonlinear manner to give the fall-back angle of the entire film. By direct measurements of the ripple fall-back angle, ₅₀ ⁰ , for very small spots of the film, of the fall-back angle, ₅₀, of the entire film, and of the distribution and standard deviation of the skew, the calculation could be tested, which gave good agreement between experiment and theory. As long as the skew is unknown the fall-back angle alone is not the best measure for the quality of an uniaxial film.The authors are indebted to the Deutsche Forschungsgemeinschaft which supported this work by parts of the equipment. They also thank Dr. K. Kempter for providing his results.     

Operation of a cryogenic conversion electron proportional counter for Mössbauer spectroscopy

The use of He-filled proportional counters for CEMS studies with⁵⁷Fe is very attractive because of their high efficiency. Stability problems at low temperature have been solved by using an electronic feed back which controls the anode voltage to keep the gas gain constant. Long term stable operation has been achieved between 370 K and 50 K. The potential of the counter is demonstrated on epitaxial and amorphous magnetic thin films.     

Darstellung und Eigenschaften von Chromorganylen aus Phillips‐Katalysatoren und Ethylen

Surface Compounds of Transition Metals. XLI [1] Preparation and Properties of Organochromium Compounds by Reaction of Phillips Catalysts with Ethylene Reaction of reduced Phillips catalysts with ethylene at 300 °C deactivates the catalyst; supported organochromium compounds are formed. These can be cleaved from the silica support by HCl and other acids, and transferred into solution by extraction with CH₃OH. Chromatography yields fractions of organochromium compounds which differ by CH₂ moieties. XPS, ^1/2H NMR, and mass spectra as well as magnetic measurements prove that an ensemble of (R_nCp)CrCl₂(CH₃OH) (R_nCp = alkylated cyclopentadienyl) has been formed. The R_nCp ligand results from a chromium‐assisted oxidative coupling of the olefin with or without CC‐cleavage. According to UV/Vis and mass spectroscopy Cl^– and CH₃OH can be substituted for other anions and donor molecules. Without a donor dinuclear, Cl‐bridged molecules are obtained, of which [(1,2,3‐Me₃Cp)CrCl₂]₂ was established by crystal structure analysis. Reaction with O₂ reversibly leads to chromium(V) compounds of the type (R₂Cp)Cr(O)Cl₂.     

Magnetization reversal of sub-micron ferromagnetic tunnel junctions in external magnetic fields

Sub-micron sized magnetic tunnel junctions are fabricated by electron beam lithography. Magnetoresistance measurements were done at crossed easy- and hard-axis fields and the critical switching curves for 3 different sub-μm junctions are discussed. Single domain like switching according to the Stoner and Wohlfarth model can be achieved, but Néel coupling effects and AAF stray field effects have to be controlled.     

Micromagnetic investigation of sub-1 0 0-nm magnetic domains in atomically stacked Fe(0 0 1)/Au(0 0 1) multilayers

Continuous films and nanostructures of atomically stacked epitaxial Fe(0 0 1)/Au(0 0 1) multilayers have been studied by soft X-ray and Lorentz microscopy as well as micromagnetic simulations. Domain imaging shows about 65 nm wide magnetic stripe domains, in which the magnetization is oriented perpendicular to the film plane. These results are confirmed by micromagnetic simulations, which also yield additional information about the internal structure of domains and walls.