La0.66Sr0.33MnO3 (LSMO) has been at the center of great interests in the last decades, due to the subtle and exotic phenomena (such as CMR and half-metallicity) having their origin in the delicate interplay between charge, spin and orbital degrees of freedom. The LSMO peculiar transport properties in thin films, joined to the ferromagnetic ordering persisting up to about 350 K, make it technologically very attractive in the context of spin-injection, given a large spin-polarization at the Fermi level reaching about 100% below the Curie temperature. However, it is known that the control of the oxygen content is crucial in order to obtain good transport and magnetic properties, and can greatly vary, depending on the film growth parameters and post-deposition treatments, leading to the likely formation of oxygen vacancies. The underlying electronic structure is obviously at the basis of the LSMO complex physics: anyway, despite the large number of photoemission spectra (PES) reported for LSMO, there is little consensus on what is the effect of oxygen vacancies in the LSMO electronic structure. In this work, PES experiments performed on epitaxial thin films of LSMO with different content of oxygen vacancies, grown by Pulsed Laser Deposition and examined in-situ, will be presented. The experimental results are interpreted by means of ab-initio calculations within the density functional formalism. We find that the introduction of oxygen vacancies causes a shift of the valence band features towards higher binding energies, and an increase of the degree of covalency of Mn bondings. The Mn magnetic moments undergo some changes, keeping however the situation relatively close to the ideal non-defective system: in none of the different vacancy configurations, a drastic charge or spin rearrangement occurs. There is, though, an important vacancy-induced drawback: half-metallicity, typical of the perfectly stoichiometric system, is generally lost, due to defective crossing the Fermi level. Our findings clearly indicate that the control over oxygen deficiency should therefore be experimentally achieved, to avoid unwanted consequences in terms of spin-injection efficiency [1].
[1] Phys. Rev. B, accepted for publication
|