Heusler Alloyed Electrodes Integrated in Magnetic Tunnel-Junctions

Abstract

As a consequence of the growing theoretically predictions of 100% spin polarized half– and full–Heusler compounds over the past 6 years, Heusler alloys are among the most promising materials class for future magnetoelectronic and spintronic applications. We have integrated Co₂MnSi as a representative of the full–Heusler compound family as one magnetic electrode into technological relevant magnetic tunnel junctions. The resulting tunnel magnetoresistance at 20 K was determined to be 95% corresponding to a Co₂MnSi spin polarization of 66% in combination with an AlO_x barrier thickness of 1.8 nm. For magnetic tunnel junctions prepared with an initially larger Al layer prior to oxidation the tunnel magnetoresistance at 20 K increases to about 108% associated with a Co₂MnSi spin polarization of 72% clearly proving that Co₂MnSi is already superior to 3d-based magnetic elements or their alloys. The corresponding room temperature values of the tunnel magnetoresistance are 33% and 41%, respectively. Structural and magnetic properties of the Co₂MnSi AlO_x – barrier interface have been studied with X-ray diffraction, electron and X-ray absorption spectroscopy and X-ray magnetic circular dichroism and it is shown that the ferromagnetic order of Mn and Co spins at this interface is only induced in optimally annealed Co2MnSi layer. The underlying atomic ordering mechanism responsible for achieving about its theoretical magnetic moment could be assigned to the elimination of Co-Si antisite defects whereas the reduction of Co-Mn antisite defects results in large tunnel magnetoresistance. The presence of a step like tunnel barrier which is already created during plasma oxidation while preparing the AlO_x tunnel barrier has been identified as the current limitation to achieve larger tunnel magnetoresistance and hence larger spin polarization and is a direct consequence of the oxygen affinity of the Co₂MnSi - Heusler elements Mn and Si.