Professor Thorsten Hesjedal FInstP

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.

Abstract:

A simple surface acoustic wave sensor system was discussed. This system combines the advantages of multifrequency/multimode operation and GHz frequencies in a single acoustic device structure. The sensor system is based on floating electrode unidirectional transducers that allow efficient excitation of two modes and up to 48 harmonics. The basic operation of sensor system was tested by applying it to humidity sensing without a sensitive layer.

Abstract:

The domain patterns of ferromagnetic MnAs wires on GaAs(001) were investigated by atomic force microscopy (AFM), magnetic force microscopy (MFM) and SQUID magnetometry. The MnAs films break up into ordered arrays of submicron-sized ferromagnetic α and paramagnetic β wires on GaAs(001) substrates. A variety of complex domain patterns were found in micromagnetic investigations. A strong uniaxial in-plane anisotropy with the easy and hard axis of magnetization being in plane and perpendicular to each other was exhibited by MnAs.

Abstract:

The fast sound propagation in AlN layers is demonstrated to enable generation of surface acoustic waves (SAWs) by interdigital transducers at a frequency beyond 30 GHz. While scaling down the wavelength of the transducers fabricated on AlN/SiC structures and the thickness of the AlN top layer to raise the operation frequency, the excitation of SAWs becomes intricate due to the weak electromechanical coupling in SiC. We examine the dependence of the feasibility of SAW generation on the AlN layer thickness.

Abstract:

We have investigated epitaxial MnAs/GaAs(0 0 1) films in the phase transition region between 10°C and 40°C where hexagonal, ferromagnetic α-MnAs and paramagnetic, orthorhomic β-MnAs coexist. Quantitative stress measurements with a cantilever beam magnetometer reveal that during the phase transition the atomic distances change only in the vertical direction providing insight into the nature of the exchange coupling mechanism. © 2003 Elsevier B.V. All rights reserved.