Professor Thorsten Hesjedal FInstP

Abstract:

The magnetization reversal of MnAs nanowires was studied by magnetic force microscopy (MFM) imaging in conjunction with superconducting quantum interference device magnetometry and micromagnetic simulations. MnAs films on GaAs(001) exhibit a submicron-sized regular array of ferromagnetic and nonmagnetic stripes, where the width of the stripes can be tuned by the temperature. The investigated thin samples show squarelike hysteresis loops, and the corresponding field-dependent MFM measurements confirm a collective flipping of the domains at the coercive field. Thicker samples, as well as thinner samples at higher temperatures, generally exhibit a rounded magnetization curve with a very low remanent magnetization. Based on three-dimensional micromagnetic simulations, the micromagnetic structure as well as the magnetic hysteresis of MnAs films on GaAs(001) is explained in a consistent way. © 2006 The American Physical Society.

Abstract:

The micromagnetic properties of MnAs thin films grown on the (111)B-oriented GaAs surface have been investigated. Compared to films grown on (001) surfaces, these films exhibit completely different domain patterns, as the c axis of the hexagonal unit cell is oriented normal to the surface. In the course of the first order phase transition, ferromagnetic α -MnAs forms a network of quasihexagonal areas separated by Β -MnAs. We present an analysis of the micromagnetic properties based on imaging and simulations. We observe closure domains that either appear as a vortex-like state or a stripe structure. © 2006 American Institute of Physics.

Abstract:

Manganese arsenide is one of the few ferromagnetic metals that can be grown on semiconductor substrates as a thin film with high structural perfection. The coupled magnetic and structural phase transition around 40°C leads to a variety of different phenomena such as the self-organized stripe formation on GaAs(001) substrates or the anisotropic lattice shrinkage. By investigating the domain pattern in the phase coexistence region we provide experimental evidence that the magnetic order is due to competing ferromagnetic double-exchange and antiferromagnetic direct exchange interactions. This scenario corroborates recent theoretical calculations and may explain the frequently observed angle of 38° in the domain pattern of epitaxial MnAs films. © 2005 The American Physical Society.

Abstract:

Low-loss surface acoustic wave (SAW) filters are widely used in modern mobile phones, especially for front-end applications. Therefore high power durability of the SAW-structure is required. In order to develop an appropriate metallization the mechanism of acoustomigration has to be investigated in more detail. For this purpose, we designed a two-port test resonator which enables us to investigate acoustomigration in thin films. With a standard measuring procedure to quantify the power durability of metallization it is possible to investigate acoustomigration effects in thin layers systematically. For a better understanding of acoustomigration, we analysed the defect structure by different microscopy techniques as a function of the driving power, testing time and temperature. By in situ tests it was possible for the first time to investigate the growing of extrusions and cracks in real time on a submicron scale. We found a correlation between the loading and the measured number of extrusions as well as the frequency shift of the SAW device. © 2005 Elsevier B.V. All rights reserved.