Professor Thorsten Hesjedal FInstP

Abstract:

We provide the first direct evidence of interactions in a self-organized pattern of coexisting ferromagnetic and paramagnetic MnAs stripe domains as a function of the coupling strength between the stripes (inter) and within one stripe (intra). The value of the intra exchange constant A = 17.7 × 1010 erg/cm is determined unambiguously from spin wave resonance. Additionally, in-plane ferromagnetic resonance measurements reveal the presence of a strong coupling between the neighboring stripes, which can dominate the weak intra-stripe interaction.

Abstract:

High power applications of Surface Acoustic Wave (SAW) devices may lead to acoustomigration in their thin metal electrodes, which deteriorates the performance or may even destroy the SAW device. It is confirmed in this paper that the mechanism of acoustomigration is caused by the SAW-induced stress in the metal. The quantitative calculation of this stress will be shown in detail, starting from the widely used P-Matrix model as a standard analysis tool. The combination with the partial wave method (PWM) yields the stress distribution inside the metal. This approach provides the flexibility to determine the stresses for any given point in a SAW device, for any input power, frequency, wavetype, device geometry, or metal layer. In order to confirm the absolute values of the stress components, we calculated and measured displacements as a function of input power and frequency.

Abstract:

The Si-doped n-type AIN/GaN distributed bragg reflectors grown on 6H-SiC(0001) were discussed. It was observed that the structures were crack-free and have a stopband centered around 450 nm with a full width at half maximum between 40 and 50 nm. It was also observed that the maximum measured reflectance was ≥99%. It was found that the vertical conductance measurements at room temperature on the samples shown on ohmic I-V behavior in the entire measurement range.

Abstract:

Thin films of MnAs grown on GaAs(001) show a self-organized structure of coexisting ferromagnetic α- and paramagnetic β-MnAs stripes in the temperature interval from 10 to 40°C. We quantify the magnetic anisotropies of the α-stripes via ferromagnetic resonance and superconducting quantum interference device magnetometry for samples with thicknesses of 57 and 165nm. The easy axis of magnetization is found to be located perpendicular to the stripe direction, whereas the direction parallel to the stripes is a hard one. While the intrinsic anisotropies show a bulk-like behavior and explain the direction of the hard axis, the key to understanding the direction of the easy axis is given by the demagnetizing fields due to the stripe formation. © 2003 Elsevier B.V. All rights reserved.