Browsing by Author "Herr, U."

Nanocrystalline soft magnetic materials can have superior properties such as low coercivity combined with a high saturation magnetization. Whereas these materials are usually made by thermal crystallization of amorphous precursors, mechanical crystallization using ball milling provides an alternative route for the generation of nanocrystalline materials. The focus of this study is the development of the magnetic properties and the microstructure at the early stages of the transformation of initially amorphous Fe86Zr7B6Cu1. Controlled deformation of amorphous starting material by cold rolling is performed to clarify the origin of the coercivity maximum. Optical and magneto-optical investigations reveal the influence of the microstructure on the coercivity. The shear band density obtained from the magneto-optical observations is directly correlated with the increase in coercivity.

Nanocrystalline Fe-base soft magnetic materials with high saturation magnetization have been obtained by crystallization of FeZrBCu amorphous precursors. In contrast to the conventional thermal crystallization, mechanical crystallization provides an alternative route for the generation of nanocrystalline materials using a high energy ball mill. Special emphasis is put here on the early stages of the transformation. X-ray diffraction, Mossbauer spectroscopy, and magnetization measurements are used to characterize the material. We find a continuous increase of the saturation magnetization accompanied by an increase of the crystalline fraction. The coercivity shows a different behavior with a large increase after very short milling within the amorphous state and a decrease for long milling duration. The influence of the deformation process on the magnetic anisotropy and the coercivity is discussed in terms of stresses and shear band formation. The results are compared with the phase formation and magnetic properties of thermally crystallized FeZrBCu samples and mechanical crystallization studies of other materials. (C) 2000 American Institute of Physics. [S0021-8979(00)04505-9].

The luminescence of nanocrystalline cubic yttria has been measured for 300 and 80 K. A blueshift and a broadening of the absorption edge, both depending on the particle size were found. The energy of the radiation emitted due to the recombination of a bound exciton does not depend on particle size. The observations are not caused by quantum or phonon confinement. Using a quantum mechanical configurational coordinate diagram an explanation can be given. Hydrostatic pressure increasing the phonon energy of the excited state of the bound exciton, or an increase of the exciton-phonon coupling may generate the size dependent optical properties of nanocrystalline yttria. (C) 2001 American Institute of Physics.

Yttria is a very important material in the lighting industry where it is used as host material for phosphors. Nanocrystalline undoped and europium doped yttria was synthesized using a chemical vapour technique in order to study the influence of particle size on the structural and optical properties. The synthesized powder was characterized by X-ray diffraction, transmission electron microscopy and ultraviolet spectroscopy. The as prepared powder was single phase and only weakly agglomerated. Whereas nanocrystalline yttria prepared by a gas condensation technique crystallizes in a monoclinic structure, our samples prepared by chemical vapours synthesis crystallize in the cubic equilibrium structure Ia3 with an average particle size of 10 nm. The cubic structure is necessary to obtain phosphors with high efficiency. The reflection spectra of nanocrystalline samples show blue shifted absorption bands with respect to a microcrystalline reference sample. Annealing the samples Leads to grain growth accompanied by a reduction of the observed optical shifts. The results can be understood by modified optical properties of the yttria host material.

Fe-100-xZrx thin films (0 < x < 7) grow on top of a Zr base layer in the amorphous phase up to a concentration dependent critical film thickness. At this critical thickness a polymorphous phase transformation of the Fe-100-x-Zrx layer takes place. Zr concentrations higher than 7 at% lead to an amorphous phase for all thicknesses. Here we report on in-situ mechanical stress measurements using an optical deflection technique. Different substrate thicknesses and alloy compositions up to 10 at% Zr were investigated. It is observed that stress builds up only after crystallization. No discontinuity of the deflection is observed during the polymorphous crystallization. The behaviour of stress development is well described by an analytical equation discussed by Cammarata [11].