Abstract
MnAs epitaxial thin films on GaAs(001) single crystalline substrates crystallize at room temperature (RT) in a mixture of two crystalline phases with distinct magnetic properties, organized as stripes along the MnAs [0001] direction. This particular morphology is driven by anisotropic epitaxial strain. We elucidate here the physical mechanisms at the origin of size reduction effect on the MnAs crystalline phase transition. We investigated the structural and magnetic changes in MnAs patterned microstructures (confined geometry) when the lateral dimension is reduced to values close to the periodicity and width of the stripes observed in continuous films. The effects of the microstructure’s lateral size, shape and orientation (with respect to the MnAs mathrm{[11}bar{2}mathrm{0]} direction) were characterized by local probe synchrotron X-ray diffraction (μ-XRD) using a focused X-ray beam, X-ray Magnetic Circular Dichroïsm - Photo Emission Electron Microscopy (XMCD-PEEM) and Low Energy Electron Microscopy (LEEM). Changes in the transition temperature and the crystalline phase distribution inside the microstructures are evidenced and quantitatively measured. The effect of finite size and strain relaxation on the magnetic domain structure is also discussed. Counter-intuitively, we demonstrate here that below a critical microstructure size, bulk MnAs structural and magnetic properties are restored. To support our observations we developed, tested and validated a model based on the size-dependence of the elastic energy and strain relaxation to explain this phase re-distribution in laterally confined geometry.
Highlights
MnAs is a promising candidate for electrical spin injection into GaAs and Si based semiconductors[1,2,3]
In the specific case of the MnAs system several important questions remain open: How the crystalline phases will organize when the associated energies become smaller than the thermal energy? What is the effect of the lateral finite size on the strain release, and on the coexistence regime of the α/β crystalline phases?
We have used an original combination of two local probe X-ray methods: μ-probe X-Ray Diffraction (μ-XRD) and XMCD-PEEM to investigate the finite size effect on the magneto-structural properties. μ-XRD allows to directly access the local lattice parameters and unambiguously identify and quantify the presence of the two crystalline phases for microstructured MnAs thin films
Summary
MnAs is a promising candidate for electrical spin injection into GaAs and Si based semiconductors[1,2,3]. To corroborate our experimental observations, we propose and validate a model originally inspired by that of Kaganer et al.[11,12], which consists in taking into account the size-dependence of the elastic energy stored globally in the microstructure to describe the α/β phases coexistence diagram in a 300 nm thick patterned MnAs thin film. This model is found to reproduce fairly well our observations
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