Optical and Magneto-Optical Properties of Ferromagnetic InMnAs Thin Films and its Alloys

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The optical and magneto-optical properties of the ferromagnetic semiconductors, InMnAs and its alloy InMnAsP were investigated to determine the band structure and nature of the ferromagnetism in these materials. Alloys were grown by metal-organic vapor phase epitaxy. Infrared absorption of InMnAs was investigated to determine the presence and properties of the Mn related impurity bands. Infrared absorption indicates the formation of shallow and deep impurity bands that result from substitutional Mn and atomic scale cubic MnAs clusters respectively. At room temperature, the shallow Mn acceptors contribute free holes to the valence band while the deep band binds the majority of holes within the clusters. X-ray absorption spectroscopy and magnetic circular dichroism spectra were measured to determine the local environment and electronic state of Mn. By fitting the experimental spectra with calculated atomic spectra, we determined that Mn atoms are substitutional with a d5 ground state configuration An inter-cluster exchange mechanism responsible for global ferromagnetism was proposed and investigated using reflection magnetic circular dichroism over the visible spectral range. A broad featureless transition observed in all samples was tentatively attributed to transitions between an exchange split deep Mn impurity band and conduction band. A fraction of the InMnAs films also exhibit a negative dichroism peak that was ascribed to transitions between spin split valence and conduction bands. Exchange splitting of the deep impurity band and valence band was measured that results in spin polarized bound holes and itinerant holes respectively. Since only the deep impurity band was spin-split in all films, we propose that only bound holes stabilize the global ferromagnetism. To determine matrix effects on exchange in disordered materials, In1-xMnxAs1-yPy alloys with x ranging from 0.01 to 0.04 and y ranging from 0.11 to 0.21 were grown. X-ray diffraction established the presence of second phase, hexagonal MnAs clusters, with the fraction of Mn incorporated into MnAs clusters varying between 33-98%. Magnetic properties were measured over the temperature range of 5-350 K. A Curie temperature of 325 K was observed. The Neel model was used to explain the observed irreversibility in the magnetization versus temperature dependence. The room temperature ferromagnetism was attributed to the presence of large clusters with radii greater than 5.5 nm

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  • 05/22/2018
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