M. Rennhofer

Interplay between structural and magnetic properties of L10-FePt(001) thin films directly grown on MgO(001)

We present a detailed study of the magnetic and structural properties of L10-FePt thin films. The films are prepared via molecular beam epitaxy directly onto MgO(001) substrates, i.e., without buffer layer. Despite the large lattice misfit between the in-plane lattice parameters of L10 FePt and MgO, highly ordered thin films are obtained with the easy magnetization c axis perpendicular to the film plane. Via high resolution transmission electron microscopy and Rutherford backscattering measurements we focus on the FePt/MgO interface to study the misfit relaxation and the defect density. Further, the influence of elevated substrate temperatures and of postgrowth high temperature annealing on the structural and magnetic properties is discussed.

An ultrahigh vacuum system for in situ studies of thin films and nanostructures by nuclear resonance scattering of synchrotron radiation.

A multifunctional ultrahigh vacuum (UHV) system has been set up at the nuclear resonance beamline ID18 of the European Synchrotron Radiation Facility (ESRF). Thin and ultrathin films, nanoislands and -wires, multilayers, and stoichiometric oxides can be prepared by molecular beam epitaxy and characterized by low-energy electron diffraction, Auger electron spectroscopy, and reflection high-energy electron diffraction. Upon characterization the sample is transferred under UHV conditions to the chamber for experiments with the synchrotron beam. Electronic and magnetic properties, vibrational dynamics, and diffusion phenomena can be investigated by several synchrotron radiation based techniques, such as nuclear forward scattering, nuclear inelastic and quasielastic scattering, synchrotron radiation based perturbed angular correlations, and nuclear and electronic reflectivity. In addition, two portable UHV chambers serve to transfer the sample to other beamlines profiting from the available experimental techniques at the ESRF.

Diffusion mechanism of iron in ternary (Ni,Fe)Al alloys with B2-structure

Abstract We have directly determined the atomistic diffusion mechanism of iron atoms in Ni40Fe10Al50 and Ni45Fe5Al50 intermetallic B2 alloys with quasielastic Mossbauer spectroscopy. Our measurements show a clear nearest-neighbour jump of Fe with an activation energy of 4.0(3) eV for both compositions in agreement with recent tracer diffusion results for Ni diffusion in the NiAl compound with B2 structure. Direct next-nearest neighbour jumps can be definitely excluded as a jump mechanism for iron. Jump frequencies and the diffusion coefficient of iron have been measured. The diffusion of iron in ternary (Ni,Fe)Al is about two times faster than diffusion of nickel in the Ni50Al50 alloy.

Atomic migration and ordering phenomena in bulk and thin films of FePd and FePt

Abstract “Order–order” kinetics was studied by means of “in situ” and quasi-residual (REST) resistometry in bulk and thin films of L10-ordered FePd and FePt intermetallics. Substantial effect of magnetic ordering on the activation energy for chemical ordering was revealed in FePd. A discontinuous change of ordering dynamics was detected in FePt between 800 and 830 K. The results are consistent with the data of Fe* diffusion in FePt multilayer examined by means of nuclear resonant scattering in grazing-incidence geometry (GINRS). Monte Carlo (MC) simulations of “order–order” processes in L10-ordered bulk FePd and FePt and nano-layered FePt have been carried out using Glauber dynamics with vacancy mechanism of atomic jumps. Multi-time-scale “order–order” relaxations observed in the bulk were predominated in nanolayers by a reorientation of the initial z-variant L10 superstructure into a mixture of x- and y-variants.

Optical Characterization of Different Thin Film Module Technologies

For a complete quality control of different thin film module technologies (a-Si, CdTe, and CIS) a combination of fast and nondestructive methods was investigated. Camera-based measurements, such as electroluminescence (EL), photoluminescence (PL), and infrared (IR) technologies, offer excellent possibilities for determining production failures or defects in solar modules which cannot be detected by means of standard power measurements. These types of optical measurement provide high resolution images with a two-dimensional distribution of the characteristic features of PV modules. This paper focuses on quality control and characterization using EL, PL, and IR imaging with conventional cameras and an alternative excitation source for the PL-setup.

Magnetism of ultra-thin iron films seen by the nuclear resonant scattering of synchrotron radiation

Conversion electron Mossbauer spectroscopy proved in the past to be very useful in studying surface and ultrathin film magnetism with monolayer resolution. Twenty years later, its time-domain analogue, the nuclear resonant scattering (NRS) of synchrotron radiation, showed up to be by orders of magnitude faster and more efficient. The evolution of the spin structure in epitaxial 57Fe films on a tungsten W(110) was studied via the accumulation of the NRS time spectra directly during Fe film deposition. In the 0.5 – 4 monolayers Fe thickness range, the complex non-collinear magnetic structure was derived from the NRS data, resulting from the deviation from the layer by layer growth mode. For thicker Fe films, the in-plane thickness induced spin reorientation transition could be clearly identified. Based on the NRS analysis it is shown that SRT process originates at the Fe/W(110) interface and proceeds through a transient fan-like magnetization structure.

Atomic Migration Phenomena in Intermetallics with High Superstructure Stability

Most of the contemporary materials based on intermetallic phases are either multiple bulk phases, or nanostructured layers deposited on appropriate substrates. In each case, the desired properties of the materials are due to chemical order and the preparation technology consists of a generation of specific processes mediated by atomic migration. It is shown how a nanoscopic (atomistic) image of the atomic migration phenomena results from an indirect experimental technique in combination with Monte Carlo (MC) and Molecular Dynamics (MD) simulations. “Order-order” relaxations were observed in phases representing three typical cubic superstructures of high stability: L12 (Ni3Al), B2 (NiAl), and L10 (FePt).

Investigation of the effect of contrails on direct and diffuse irradiance

In the present study we investigate the effect of contrails on global shortwave radiation. This investigation is performed using continuous hemispherical fish eye photographs of the sky, diffuse and direct shortwave measurements. These measurements have been performed at the solar observatory Kanzelhohe (1540 m.a.s.l) located in the southern part of Austria during a period of one and half year. The time resolution of the measurements is one minute, which allows to accurately follow the formation-eventually the disappearance or the movement - of the contrails in the sky. Using the fish eye photographs we identified clear sky days with a high contrail persistence. We especially look at situations where the contrails were obstructing the sun. First results show that contrails moving between sun and observer/sensor may reduce the global radiation by up to 68%. In general we however observe that during days with a high contrail persistence the diffuse irradiance is slightly increased. Finally a statistic of th...

Atomic Ordering in Nano-layered FePt: Multiscale Monte Carlo Simulation

Combined nano- and mesoscale simulation of chemical ordering kinetics in nano-layered L10 AB binary system was performed. In the nano- (atomistic) scale Monte Carlo (MC) technique with vacancy mechanism of atomic migration was implemented with diverse system models. The mesoscale microstructure evolution was, in turn, modeled by means of MC procedure simulating antiphase boundary (APB) motion as controlled by APB energies evaluated within the nano-scale simulations. The study addressed FePt thin layers considered as a material for ultra-high density magnetic storage media and revealed metastability of the L10 c-variant superstructure with monoatomic planes parallel to the (001) free surface and off-plane easy magnetization. The layers, initially perfectly ordered in the L10 c-variant, showed homogenous disordering running in parallel with a spontaneous re-orientation of the monoatomic planes into a mosaic-microstructure composed of L10 a- and b-variant domains with (100)- and (010)-type monoatomic planes, respectively. The domains nucleated heterogeneously on the Fe free surface of the layer, grew discontinuously inwards its volume and finally relaxed generating an equilibrium microstructure of the system. Two �atomistic-scale� processes: (i) homogenous disordering and (ii) nucleation of the L10 a- and b-variant domains showed characteristic time scales. The same was observed for the meso-scale processes: (i) heterogeneous L10 variant domain growth and (ii) domain microstructure relaxation. The above phenomena modelled within the present study by means of multiscale MC simulations have recently been observed experimentally in epitaxially deposited thin films of FePt.