Micromagnetic investigation of MnAs thin films on GaAs surfaces

This work presents the study of the micromagnetic domain structure and the coupled magneto-structural phase transition of MnAs thin films on GaAs. In particular, the influence of substrate orientation, film thickness and external magnetic field on the magnetic and structural properties are investigated, employing the complementary measurement techniques atomic force microscopy-AFM / magnetic force microscopy-MFM and low energy electron microscopy-LEEM / X-ray magnetic circular dichroism photoemission electron microscopy-XMCDPEEM for the characterization of the structural / micromagnetic properties of the materials system. MnAs exhibits a first-order phase transition from the hexagonal, ferromagnetic α-phase to the orthorhombic, paramagnetic β-phase at around 40 ◦C. In epitaxial MnAs films on GaAs, it is known that the ferromagnetic and the paramagnetic phases coexist over a wide temperature range of about 30 ◦C below the phase transition temperature. In this temperature range, MnAs films on GaAs (001) and (311)A substrates show a regular array of ferromagnetic stripes due to the involved strain. The width of the ferromagnetic stripes can be tuned by varying the temperature, whereas the periodicity of the stripe pattern is found to be a linear function of film thickness. The in part complex micromagnetic domain structure strongly depends on the width and the distance of the ferromagnetic stripes, as it directly affects the shape anisotropy and magnetic coupling, respectively. The films exhibit a strong uniaxial magnetic anisotropy, where the magnetic easy axis is the a-axis (along MnAs [112̄0]) and the magnetic hard axis is the c-axis (along MnAs [0001]) and both axes lie in the film plane. For MnAs films grown on GaAs (111)B, the c-axis is normal to the surface and the [2̄110] and [011̄0] directions of MnAs are parallel to the [1̄10] and [112̄] directions of GaAs, respectively. The epitaxy also leads to a different strain state of the film and a higher phase transition temperature of roughly 56 ◦C compared to films grown on GaAs (001) and (311)A. Over a large temperature range of more than 30 ◦C below the phase transition temperature the αand β-phase coexist, however, now forming polygonal ferromagnetic structures embedded in a honeycomb-like paramagnetic network. Based on the observation of the micromagnetic domain structure of MnAs films on GaAs (001), a classification of the domain patterns was carried. Depending on the number of subdomains along the easy a-axis direction on a ferromagnetic stripe, up to three basic domain types can be distinguished. More complicated domain patterns are explained by the respective combinations of the three basic domain types. The measured MFM contrast is simulated based on this domain classification scheme and a good agreement was found. Moreover, XMCDPEEM measurements are able to confirm the domain classification scheme that was initially derived from MFM observations. As the employed micromagnetic imaging techniques have a different information depth, it is possible to gain information about the domain structure in the vertical direction as well. Whereas MFM and XMCDPEEM results agree well for thinner films (< 300 nm), the observation of thicker films leads to a discrepancy of the deduced domain patterns. The apparent differences are explained by the assumption of domains in the vertical direction that reflect themselves in the MFM images, but not in the XMCDPEEM images. MnAs films exhibit a delicate strain balance that directly affects their magnetic and structural properties. By using wet-etching techniques, αMnAs and β-MnAs show different etch rates. Selective removal of Mn atoms from the MnAs layer destroys the strain balance of the film, which results in the formation of regularly spaced cracks running along the c-axis direction. Further etching leads to the formation of 2-dimensional arrays of islands. Using temperature-dependent AFM, MFM and XMCDPEEM, it is shown that the local strain relaxation in the vicinity of the cracks results in a locally increased phase transition temperature. In order to study magnetization reversal processes on a microscopic scale, as well as the influence of the magnetic field on the micromagnetic domain structure, a variable-magnetic field set-up is added to the existing variabletemperature scanning probe microscope. By field-dependent MFM measurements, the magnetization curve is measured microscopically and a good agreement is found with integral SQUID magnetometry measurements. Furthermore, by varying the temperature in an external magnetic field a deeper insight into the coupled structural and magnetic phase transition in MnAs films is gained, as the coupling between the ferromagnetic stripes can be tuned. It was found that the ferromagnetic order is extended to higher temperatures by applying an external magnetic field.