Professor Thorsten Hesjedal FInstP

Abstract:

Stress-induced material transport in surface acoustic wave devices, so-called acoustomigration, is a prominent failure mechanism, especially in high-power applications. We used scanning probe microscopy techniques to study acoustomigration of metal structures in-situ, i.e., during the high-power loading of the device. Scanning acoustic force microscopy (SAFM) allows for the simultaneous measurement of the acoustic wavefield and the topography with submicron lateral resolution. High-resolution microscopy is essential as acoustomigration is a phenomenon that not only results in the formation of more macroscopic voids and hillocks but also affects the microscopic grain structure of the film. We present acoustic wavefield and topographic image sequences giving a clear insight into the nature of the film damage on a submicron scale. The 900 MHz test structures were fabricated on 36 degrees YX-lithium tantalate (YX-LiTaO3) and incorporated 420-nm thick aluminium (Al) electrodes. By correlating the acoustic wavefield mapping and the local changes in topography, we confirmed model calculations that predict the correspondence of damage and stress (i.e., hillocks and voids) are preferentially formed in areas of high stress. The way the film is damaged does not significantly depend on the applied power (for typical power levels used in this study). Furthermore, acoustomigration leads to smoother surfaces via lateral grain growth. Another contribution to the grain dynamics comes from the apparent grain rotation in the highly anisotropic stress field of an acoustic wave. Thus, through in-situ scanning probe microscopy techniques, one can observe the initial changes of the grain structure in order to obtain a more detailed picture of the phenomenon of acoustomigration.

Abstract:

Strain in epitaxial MnAs thin films on GaAs(001) substrates plays an important role in the coupled magnetostructural phase transition. As a result of strain, the phase transition from the ferromagnetic α phase to the paramagnetic Β phase proceeds over a wide temperature range and the coexisting phases form a periodic stripe array. Employing suitable wet chemical etchants, the two MnAs phases can be etched selectively. Perpendicular to the α-Β -stripe structure, the built-up strain relaxes in the course of the etching process by the formation of cracks. The combination of both strain relaxation mechanisms allows for the defined patterning of two-dimensional arrays of nanomagnets. Through micromagnetic investigations, it is possible to identify the location of α - and Β-MnAs which helps to clarify two major aspects of the etching process. First, it is possible to determine the etch rates of α - and Β-MnAs and follow the complex interplay of strain and phase composition during the etching process. Second, as strain reflects itself in a shifted phase-transition temperature, temperature-dependent micromagnetic studies allow to determine the strain environment of the cracks. © 2005 American Institute of Physics.

Abstract:

This book presents the latest research in this frontier field.

Abstract:

We present an analytic solution for the magnetic field of a bar-shaped permanent magnet. Assuming a constant magnetization, we derive expressions for the stray field in three dimensions. The analytic solutions can be readily applied to field calculation problems for magnetic force microscopy simulations without the need for finite element methods. © 2005 American Institute of Physics.

Abstract:

Manganese arsenide is a promising candidate for new spintronics applications since it is ferromagnetic at room temperature and can be grown with high epitaxial quality on semiconductors. However, the transition temperature of ∼40 °C is a limiting factor for device applications. Since the coupled magnetic and structural transition is of first order, it is in principle possible to shift the transition temperature by changing external parameters. Here we show that by either applying an external magnetic field or by growing the MnAs films on GaAs(1 1 1), i.e., by changing the epitaxial constraints which are equivalent to external pressure, ferromagnetic order can be stabilized well above the bulk-phase transition temperature. © 2004 Elsevier B.V. All rights reserved.