Thin film quantum materials

Abstract:

Strained MnAs films on GaAs(001) grown by molecular beam epitaxy exhibit unique micro-magnetic properties due to the strain-mediated coexistence of hexagonal ferromagnetic a-MnAs and orthorhombic paramagnetic β-MnAs arranged in selforganized periodic stripe patterns. To explore the internal structure of the magnetization, which is not accessible by the magnetic imaging techniques AFM and XMCDPEEM, detailed micromagnetic simulations are needed. Otherwise, physically unreasonable models would be developed. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

Abstract:

We report the high-aspect ratio patterning of epitaxial MnAs-on-GaAs(0 0 1) films. The control of strain is key since MnAs-on-GaAs(0 0 1) exhibits a strain-stabilized coexistence of two chemically, elastically and magnetically distinct phases forming a self-organized stripe structure over a temperature range of 10-40 °C. Anisotropic plasma etching allows for high-aspect ratios and good reproducibility. Using Ti films as an etch mask, arbitrarily oriented structures can be transferred into films of up to 300 nm thickness. The removal of the masking material is challenging as MnAs reacts with all common acids, alkalis and even water. Optimum results are obtained by etching the Ti mask in hydrofluoric acid at elevated temperatures (>50 °C), where MnAs is entirely in its β-phase. © 2006 IOP Publishing Ltd.

Abstract:

We report on apparently charged domain walls in MnAs-on-GaAs(0 0 1) layers. For head-on domains, described as two domains facing each other with opposite magnetization, the domain walls of ≳ 200 nm thick films exhibit a zig-zag pattern. Depending on the width of the ferromagnetic stripes, which is a function of temperature and thus the strain in the easy axis direction, the zig-zag angle 2 θ increases from 90^{{ring operator}} in the case of wide stripes to 180^{{ring operator}} (i.e., to a straight wall) for narrow stripes. The underlying domain structure was calculated using a three-dimensional micromagnetic simulator. The calculations reveal a number of distinct domain patterns as a result of the system's attempt to reduce its energy through the formation of closure domain-like patterns in the easy plane. A diamond-like state consisting of two intersecting sub-surface domain walls is the underlying magnetic structure resulting in the observed, apparently charged domain walls. The zig-zag pattern of the domain boundary is explained by stray field minimization of the diamond state along the stripe. © 2006 Elsevier B.V. All rights reserved.

Abstract:

We studied the magnetic coupling of ferromagnetic, submicron-sized stripes in the material system MnAs on GaAs. A specific coupling state, determined by stripe period and stripe width, can be tuned via film thickness and temperature, respectively. Micromagnetic imaging - in combination with micromagnetic simulations - reveals two coupling regimes. Strong magnetic coupling between the stripes creates micromagnetic domains extending across several stripes, whereas weak coupling allows for demagnetization within individual stripes. This behavior is observed for all investigatedfilm thicknesses, since a stripe geometry leading to a given coupling scenario is a function of temperature.

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.