Professor Thorsten Hesjedal FInstP

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.

Abstract:

In the bulk the structural and magnetic phase transitions between a- and β-MnAs are coupled and proceed abruptly by first order. In heteroepitaxially constrained MnAs/GaAs(001) films, on the other hand, the two phases coexist at 10-40°C and the magnetization decreases continuously with temperature. We present detailed magnetic measurements on MnAs/GaAs(001) in the phase transition region covering the ferromagnetic a phase, the paramagnetic β phase and the α + β coexistence region. The coexistence of two magnetically distinguishable types of a-MnAs around the onset of the coexistence region at 10°C, the occurrence of metastable magnetic properties, and the absence of a Curie-Weiss peak in the susceptibility point to a first order of the magnetic phase transition also in thin films. The two types of a-MnAs are characterized by different coercivity and magnetic anisotropy, the latter due to a change in shape anisotropy of the striped phase. © 2004 The American Physical Society.