Thin film quantum materials

Abstract:

MnAs films were deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. Imaging of the temperature-dependent magnetic structure by x-ray magnetic circular dichroism photoemission electron microscopy, and the comparison with magnetization measurements by superconducting quantum interference device (SQUID) magnetometry, is used to study the impact of the different strain state of MnAs/GaAs(001) and of MnAs/GaAs(111)B films on the phase transition between ferromagnetic α -MnAs and paramagnetic Β -MnAs, the spatial distribution of the two structural and magnetic phases, and the transition temperature. For the isotropically strained MnAs/GaAs(111)B films, the phase coexistence range is much wider than for the anisotropically strained MnAs/GaAs(001) films. The characteristic change of the saturation magnetization with film thickness is found to be a universal property of films with different epitaxial orientation, if at least one MnAs 〈11 2- 0〉 direction is in the film plane. For MnAs/GaAs(001) films this variation is related to the striped coexistence of α and Β MnAs and the changing intra- and inter-stripe magnetic interaction with film thickness and temperature. The magnetic structure of MnAs/GaAs(111)B is more complex due to the existence of three symmetry-equivalent α -phase domains superimposed by a honeycomb-like network of the coexisting Β phase. The magnetic properties (saturation magnetization, domain size) of thin MnAs/GaAs(001) films can be improved by postgrowth annealing. Above a certain film thickness this is inhibited by the complex magnetic structure of the film. © 2005 American Vacuum Society.

Abstract:

We investigated the stress evolution in single-crystal MnAs films on GaAs(001) upon applying high external magnetic fields in the α/β phase transition regime (10-40 °C) and beyond. Our stress measurements reveal large field-induced lattice distortions at temperatures, where β-MnAs is present, even well above the phase transition (> 40 °C). A quantitative comparison with the field-induced increase of magnetization reveals that the changes in the lattice dimensions can be fully explained by the (reversible) back-transformation of β-MnAs to α-MnAs. Our direction-dependent experiments identify the structural distortions at the phase transition as a volume magnetostriction effect and - due to the persisting magnetocrystalline anisotropy above 40 °C - strongly support an antiferromagnetic state for β-MnAs. © EDP Sciences.

Abstract:

Magnetic force microscopy (MFM) studies of epitaxial MnAs films on GaAs(001) have been performed as a function of the applied magnetic field and the sample temperature. For this purpose, we combined a stable variable-temperature sample stage with a compact magnet assembly to fit a commercial magnetic force microscope. In order to keep the thermal drift that affects MFM measurements low, we employed a permanent magnet that can be rotated in a yoke assembly guiding the magnetic flux to the sample. © Springer-Verlag 2005.

Abstract:

We have studied the domain behavior of submicrometer wide ferromagnetic stripes by magnetic force microscopy (MFM) in the presence of an in situ magnetic field. MFM images in the demagnetized state show alternatingly magnetized domains fully extended across the stripe. Moreover, domain structures are found to exhibit a substructure across the stripe. Increasing fields drive out the domain walls of the complex domains first, leaving the alternating domains behind. The remaining magnetization process aligns increasing parts of the domains along the field direction by gradually shrinking the width of oppositely magnetized domains rather than by flipping larger areas at once. Micromagnetic simulations confirm the observed behavior. The simulations reveal that flipping of the domains occurs only when a magnetic pinning center is involved. © 2005 American Institute of Physics.

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.