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In this talk we present (i) the first photoluminescence (PL) data ever obtained from dilute ferromagnetic (Ga,Mn)As layers [1] and (ii) a new way to achieve ferromagnetic III-V semiconductors with high Tc by Gd-doping of wurtzite GaN layers [2].
Hot-electron PL from ferromagnetic (Ga,Mn)As has provided direct experimental evidence for the absence of free holes in this material. Instead, the holes are bound to the Mn impurity band. Therefore, ferromagnetism in this material cannot be explained in terms of the RKKY model. Our analysis of the polarization of the impurity-band holes allowed to deduce the ground state of the system which is formed by a split-off level caused by internal stress. Finally, our findings demonstrate that ferromagnetic(Ga,Mn)As layers are microscopically not uniform and contain paramagnetic regions. Percolation-based theories are thus more appropriate to account for the origin of ferromagnetism in this material. Gd-doped GaN layers in the range 7x1015 to 2x1019 cm-3, grown by reactive MBE on 6H-SiC(0001) substrates, are found to be ferromagnetic with an in-plane easy axis upto temperature far above room temperature. An even more striking result is that the effective magnetic moment per Gd atom in GaN, obtained from the value of the saturation magnetization Ms and the Gd concentration [Gd], reaches values which are two orders of magnitude larger than the pure moment of Gd. This colossal magnetic moment can be explained in terms of a very effective spin-polarization of the GaN matrix by the Gd atoms. We have developed a simple phenomenological model to account for this long-range spin-polarization, which is supported by additional magneto-photoluminescence measurements on the Gd-doped GaN layers. [1] V. F. Sapega et al., Phys. Rev. Lett. 94 (2005) 137401 [2] S. Dhar et al., Phys. Rev. Lett. 94 (2005) 037205 |