How Many Fe layers Cause TMR?
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0941-Q01-02
How Many Fe layers Cause TMR? Christian Heiliger, Peter Zahn, and Ingrid Mertig Department of Physics, Martin Luther University, D-06099 Halle, Germany
ABSTRACT The influence of the structural properties of the leads of planar tunnel junctions on the tunneling current is investigated by means of ab initio electronic structure calculations. In particular, a NM/Fe/MgO/Fe/NM tunnel junction with non-magnetic (NM) leads and finite Fe spacer layers between the leads and the MgO barrier is discussed. The conductance is calculated as a function of the number of Fe layers. The results show that even one iron layer next to the barrier is sufficient to obtain a high spin polarization and a high TMR ratio. This finding implies that similar results can be expected for tunnel junctions with nonmagnetic and even amorphous leads, if states of ∆1 symmetry are provided.
INTRODUCTION A very high tunneling magneto resistance (TMR) ratio of more than 220 % [1,2] was obtained in planar Fe/MgO/Fe tunnel junctions consisting of crystalline leads and a crystalline barrier. However, in recent experiments [3] very high TMR ratios have been reported even in junctions with amorphous leads but with crystalline barrier. The fabrication of amorphous leads is less demanding than the fabrication of high quality crystalline leads. For this reason, tunnel junctions with amorphous leads would be better candidates for future applications like read/write heads in hard disks and magnetic RAM (MRAM). This is a sufficient motivation to study the influence of the properties of the leads on the tunneling current. Other aspects like the interface structure between the leads and the barrier have been widely studied experimentally [4-6] as well as theoretically [7,8]. The theoretical description of amorphous leads, however, is cumbersome and time consuming. Though, we decided to investigate a simplified model. We assume nonmagnetic leads acting as reservoirs to provide electronic states which can tunnel through the barrier. Magnetic polarization, however, is caused by ferromagnetic spacer layers between the leads and the barrier. The main question is: How many magnetic layers are necessary to achieve a remarkable TMR signal?
THEORY The idea of the non-magnetic leads is realized in the following way. We consider a NM/Fe/MgO/Fe/NM tunnel junction with a barrier of six MgO layers embedded between Fe spacers of varying thickness and non-magnetic leads with bcc crystal structure and a core charge corresponding to Cu. The density of states of this lead material is shown in figure 1. Ideal interfaces without an FeO layer but with the experimentally obtained interlayer distances [4] including the relaxation of the interface Fe layer are considered. The number of atomic Fe layers is varied between 0 and 20. For every configuration a self-consistent supercell calculation within the framework of density functional theory was performed. A screened KKR (Korringa-KohnRostoker) multiple scattering Green’s function method was applied to obtain the electronic a
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