Jan M. Luning

Thermal stability of IrMn and MnFe exchange-biased magnetic tunnel junctions

The thermal stability of exchange-biased magnetic tunnel junctions is explored using near-edge x-ray absorption fine-structure spectroscopy. Structures with Ir–Mn antiferromagnetic exchange bias layers are much more thermally stable than similar structures with Fe–Mn exchange bias layers. In both cases, diffusion of Mn from the antiferromagnetic layer through thin exchange-biased ferromagnetic layers to the tunnel barrier is observed at elevated temperatures. This observation explains the diminished magnetoresistance of these structures on annealing even though the resistance of the tunnel junctions is hardly changed.

Liquid Crystal Alignment on Surfaces with Orientational Molecular Order: A Microscopic Model Derived from Soft X-ray Absorption Spectroscopy

To investigate the microscopic origin of liquid crystal alignment on rubbed polymer surfaces several novel experimental techniques have been developed, which allow probing of the liquid crystal-polymer interface on a molecular level. The understanding of the alignment mechanism presented in this Chapter is based on the detection of the molecular orientation at rubbed polymer surfaces by surface sensitive, polarization dependent soft x-ray absorption spectroscopy resolving the near edge x-ray absorption fine structure (NEXAFS) of the absorption coefficient. NEXAFS spectroscopy is a powerful experimental technique for the investigation of ordering phenomena in thin films and at surfaces with element specificity, chemical selectivity, and sensitivity to the presence of orientational order. Its sampling depth can be tuned from the outermost surface layer only to a few ten nanometers deep into the film bulk. Also, this technique is especially powerful for the investigation of ordering phenomena in the absence of long-range order, as often the case in polymers, due to the local nature of the x-ray absorption process.

Photoemission electron microscopy for the study of ferromagnetic and antiferromagnetic materials

Photoemission electron microscopy (PEEM) is a full field imaging technique where x-ray exited electrons are used to form an image of the sample surface as a function of the x-ray photon energy and polarization. Contrast in PEEM can be due to a number of mechanisms including topographical, work function, elemental, chemical, polarization, x-ray magnetic circular and linear dichroism contrast. This wide range of contrast mechanisms together with the surface sensitivity and high spatial resolution make PEEM a very useful tool for the study of magnetic materials. PEEM-II is a new microscope installed at the bending magnet beamline 7.3.1.1 of the Advanced Light Source. In the present paper we describe the design and features of PEEM-II, and show results of our recent studies. Using PEEM and its elemental specificity, it is possible to investigate the various layers in magnetic multilayer structures independently. The experiments described here include the investigation of the switching behavior of magnetic mul...