Ab initio electronic and magnetic structure in La0.66Sr0.33MnO3:: strain and correlation effects

The effects of tetragonal strain on the electronic and magnetic properties of strontium-doped lanthanum manganite, La2/3Sr1/3MnO3 (LSMO), are investigated by means of density-functional methods. As far as the structural properties are concerned, the comparison between theory and experiments for LSMO strained on the most commonly used substrates shows an overall good agreement: the slight overestimate (at most of 1-1.5%) for the equilibrium out-of-plane lattice constants points to possible defects in real samples. The inclusion of a Hubbard-like contribution on the Mn d states, according to the so-called 'LSDA + U' approach, is rather ineffective from the structural point of view, but much more important from the electronic and magnetic point of view. In particular, full half-metallicity, which is missed within a bare density-functional approach, is recovered within LSDA+U, in agreement with experiments. Moreover, the half-metallic behaviour, particularly relevant for spin-injection purposes, is independent of the chosen substrate and is achieved for all the considered in-plane lattice constants. More generally, strain effects are not seen to crucially affect the electronic structure: within the considered tetragonalization range, the minority gap is only slightly (i.e. by about 0.1-0.2 eV) affected by a tensile or compressive strain. Nevertheless, we show that the growth on a smaller in-plane lattice constant can stabilize the out-of-plane versus in-plane eg orbital and significantly change their relative occupancy. Since eg orbitals are key quantities for the double-exchange mechanism, strain effects are confirmed to be crucial for the resulting magnetic coupling.