An insight on magnetic metal/insulater interfaces

S. CARREIRA; M. AGUIRRE; J. BRIATICO; M. SIRENA; L.B. STEREN

Glasgow

Conferencia; 8th Joint European Magnetic Symposia; 2016

Artificial heterostructures consisting of complex oxides have become a special issue in recent years, as much for the fundamental physics involved as for the technological prospects. Novel phenomena emerging at the interfaces of transition metal oxides can give us an alternating and new pathway of controlling the magnetic and transport properties of magnetorresistive devices, like magnetic tunnel junctions. Particularly important is the case of La0.66Sr0.33MnO3 (LS0.33MO) as it is ferromagnetic (FM) with the highest transition temperature (Tc ~ 370 K) and a half-metal, so becoming an excellent candidate to use as electrode in magnetic tunneling devices. However and despite its room temperature FM order, the tunneling magnetoresistance (TMR) ratios measured in junctions built with this material are vanishing small at room temperature. The decrease of the TMR ratios is usually attributed to the existence of a so-called dead layer, a few unit cells wide, at the interface between the electrode and the tunnel barrier. The physics inherent to the reduction of the spin polarization at the interface is usually related to surface symmetry breaking effects, preferential orbital occupation, new magnetic phases, strain and charge transfer. In this work, we studied series of LSxMO/LS0.33MO bilayers grown by pulsed laser deposition on SrTiO3 (001) substrates, where the tunnel barriers are low doped manganites with Sr doping 0≤x≤0.1 and thicknesses from 2 nm to 6 nm. These barriers are Mott insulators and can exhibit different magnetic phases in thin films depending on the Sr doping. Therefore, the magnetic coupling between layers plays an important role. By combining surface sensitive techniques such as XAS, XMCD (Fig.1) and XLD, we described the magnetic order, oxidation states, the electronic orbital occupation and the spin polarization at interfaces. The quality of the films has been probed by STEM with HAADF techniques. All the heterostructures are grown coherently, with no apparent formation of misfit dislocations at the interface. In terms of the barrier thickness, we can see a non trivial dependence of the in-plane spin polarization at the interface. This would suggest an optimal barrier thickness in terms of the reduction of the spin polarization (Fig. 3). In this work, we will also discuss about the hybridization between transition metal d and oxygen p orbitals, which are responsible for the functional properties of transition metal oxides.