Professor Thorsten Hesjedal FInstP

Abstract:

We investigate the domain structure of submicrometer sized ferromagnetic stripes exhibiting in-plane and out-of-plane magnetized areas with magnetic force microscopy (MFM). Two simulation approaches are used to calculate the observed MFM response. The first relies on an analytical solution for the stray field of a bar magnet and the subsequent modeling of the sample as an arrangement of bar magnets. The MFM response is calculated for a realistic tip shape incorporating a distribution of magnetic dipoles. The second, numerical approach is based on a discretization scheme, breaking the tip-sample problem up into cells and then calculating the energy of the magnetic tip-sample interaction. The MFM responses obtained for the ferromagnetic stripe structure are compared. A discussion of the advantages and limitations of the two methods is given in terms of precision, computing time, and flexibility. The numerical method offers shorter computing times and greater flexibility, opening the door for realistic three-dimensional MFM response simulations. The advantage of the analytical method is the investigation of small structures, as its precision is higher for the comparable computational effort. © 2006 American Institute of Physics.

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.