Which states contribute to the tunneling current for large barrier thicknesses?
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0941-Q03-05
Which states contribute to the tunneling current for large barrier thicknesses? Christian Heiliger, Peter Zahn, and Ingrid Mertig Department of Physics, Martin Luther University, D-06099 Halle, Germany
ABSTRACT The transport properties of planar Fe/MgO/Fe tunnel junctions are investigated theoretically by means of ab initio calculations. In particular, the k||-resolved conductance in dependence on the barrier thickness, the interface structure, and the magnetic configuration is studied. The results show that the number of states in the k||-space contributing significantly to the overall current is decreasing with increasing barrier thickness as expected. In contrast to simple parabolic band models the contribution of states in the vicinity of k||=0, however, is only involved for a few considered configurations of the system Fe/MgO/Fe.
INTRODUCTION Epitaxially grown planar Fe/MgO/Fe tunnel junctions show a very high tunneling magneto resistance (TMR) ratio of more than 220 % [1,2]. Therefore, these junctions are promising candidates for future applications like hard disk read/write heads and magnetic RAM (MRAM). The influence of structure and chemical composition is intensively studied experimentally and theoretically. The strongest impact on the tunneling current is caused by the interface structure between the Fe leads and the MgO barrier. Experimental investigations of both interfaces, namely Fe/MgO [3] and MgO/Fe [4], obtained a partially occupied FeO layer. The consideration of these layers in the calculations leads to drastic changes of the TMR ratio [5]. A detailed analysis of the bias voltage dependence of the conductance and the TMR ratio as a function of interface structure [6] enlightens the variety of experimental results [1,7]. In earlier calculations [6] the authors considered a barrier thickness of 4 MgO layers. The question is: How does the current voltage characteristic change at larger barrier thicknesses? To answer this question the tunneling current is studied as a function of barrier thickness resolved into individual contributions of the electronic states in the k||-space.
THEORY The influence of the interface structure is investigated considering three different interface geometries. All of them have the experimentally measured interlayer distances [4] including the relaxation of the Fe interface layer. The FeO layer [4,5] was included in the calculations in the following way. The ideal junction structure contains no FeO layer at all. The symmetric junction has a fully occupied FeO layer at both interfaces. The third configuration takes an FeO layer at one interface into account causing this junction to be asymmetric. The same interface geometries were already considered in our previous calculations [6]. Self-consistent calculations within the framework of density functional theory by means of a Screened KKR (Korringa-Kohn-Rostoker) Green’s function method were performed to obtain the electronic structure of the systems. For this purpose a superlattice with 6 MgO layers sandwic
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