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Abstract:
Half-metallic Co2MnSi-based Heusler compounds have attracted attention
because they yield very high tunnelling magnetoresistance (TMR) ratios.
Record TMR ratios of 1995% (at 4.2 K) are obtained from
off-stoichiometric Co2MnSi-based magnetic tunnel junctions. This work
reports on a combination of band structure calculations and
spin-resolved and photon-polarisation-dependent photoelectron
spectroscopy for off-stoichiometric Heusler thin films with the
composition Co2Mn1.30Si0.84. Co and Mn are probed by magnetic dichroism
in angle-resolved photoelectron spectroscopy at the 2p core level. In
contrast to the delocalised Co 3d states, a pronounced localisation of
the Mn 3d states is deduced from the corresponding 2p core level
spectra. The valence states are investigated by linear dichroism using
both hard x-ray and very-low-photon-energy excitation. When a very low
photon energy is used for excitation, the valence bands exhibit a spin
polarisation of about 30% at the Fermi energy. First principles
calculations reveal that the low spin polarisation might be caused by a
spin-flip process in the photoelectron final states.
