Thin film quantum materials

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.

Abstract:

Based on recent reports about the acoustical power distribution in SAW resonators we present an analytical method to determine the distribution of the dynamic strain and stress components in SAW resonators on LiTaO3. This enables us to calculate the absolute strain and stress values for each point in the layer of a resonator for any driving condition and frequency. The SAW resonator is described by a P-Matrix based model, which gives us the distribution of the potential power and the resulting energy density. For calculation of the relative strain and stress values we used the Partial Wave Method. Using the correlation between the total acoustic power and the energy density distribution normal to the substrate surface, we can calculate the strain and stress values for a given input power. For the direct experimental verification of our calculations we measured the SAW induced displacements as a function of input power.