In spintronics, to obtain large effects it is of paramount importance to master sources of highly spin-polarized charge carriers. The simplest way to achieve this is to use half-metallic materials, like mixed-valence manganites (A1-xA'xMnO3). When integrated as electrodes in magnetic tunnel junctions (MTJs), manganites have proved to yield large tunnelling magnetoresistance (TMR) values, which underscores their usefulness for spintronics applications. In this talk, we will review recent results obtained at Orsay in manganite MTJs. At low temperature, a TMR of 1800 % is obtained in a La2/3Sr1/3MnO3 / SrTiO3 / La2/3Sr1/3MnO3 junction. This corresponds to a spin-polarization of 95% for La2/3Sr1/3MnO3 (LSMO) at the interface with SrTiO3 (STO). The bias voltage dependence of the TMR shows a very peculiar behaviour, from which quantitative information on the spin-dependent DOS above EF can be extracted. We have also studied the temperature dependence of the TMR in MTJs with STO and TiO2 barriers. In both systems, the TMR vanishes in the 280K range. Even if, at low temperature, the spin-polarization of the LSMO / TiO2 interface is only 70 % as opposed to 95 % in the case of LSMO / STO), the decay upon rising T of both PLSMO/STO and PLSMO/TiO2 is very similar, and differs from that of the spin-polarization of a free LSMO surface. We will discuss the possible reasons for this behaviour.
A possible candidate to obtain TMR at room temperature is Sr2FeMoO6 (SFMO), which has a TC above 420K. The growth of SFMO films is problematic and the films usually show a morphology incompatible with a classical lithography process. We have circumvented this difficulty by defining junctions with areas in the 10-100 nm2 range, thanks to a new technology of nanolithography, based on nanoindentation. At low temperature, a positive TMR of 45% is obtained in a SFMO / STO / Co nanojunction. This confirms the large negative spin-polarization of SFMO predicted by band-structure calculations.