No abstract

BaO has been introduced as a novel tunnel barrier material in magnetic tunnel junctions (MTJs). Due to the good agreement regarding lattice constants and crystal structure, we believe BaO to be particularly suitable as barrier in MTJs containing Heusler compound electrodes. Co FeSi/BaO/Fe MTJs have been fabricated by molecular beam epitaxy (MBE) and investigated in terms of microstructure, transport properties and tunnel magneto resistance (TMR). A TMR amplitude as high as 104% at room temperature (RT) has been achieved for very small bias voltages and a strong dependence on could be observed as the TMR ratio decreases with increasing to about 14% at mV.

We succeed to integrate BaO as a tunneling barrier into Co2FeAl based magnetic tunnel junctions (MTJs). By means of Auger electron spectroscopy it could be proven that the applied annealing temperatures during BaO deposition and afterwards do not cause any diffusion of Ba neither into the lower Heusler compound lead nor into the upper Fe counter electrode. Nevertheless, a negative tunnel magnetoresistance (TMR) ratio of -10% is found for Co2FeAl (24 nm) / BaO (5 nm) / Fe (7 nm) MTJs, which can be attributed to the preparation procedure and can be explained by the formation of Co- and Fe-oxides at the interfaces between the Heusler and the crystalline BaO barrier by comparing with theory. Although an amorphous structure of the BaO barrier seems to be confirmed by high-resolution transmission electron microscopy (TEM), it cannot entirely be ruled out that this is an artifact of TEM sample preparation due to the sensitivity of BaO to moisture. By replacing the BaO tunneling barrier with an MgO/BaO double layer barrier, the electric stability could effectively be increased by a factor of five. The resulting TMR effect is found to be about +20% at room temperature, although a fully antiparallel state has not been realized.