Theory of nonspecular tunneling in magnetic tunnel junctions

X.-G. Zhang,^1,2 Yan Wang,³ and X. F. Han³

¹Center for Nanophase Materials Sciences, Oak Ridge National Laboratory
²Computer Science and Mathematics Division, Oak Ridge National Laboratory
³Institute of Physics, Chinese Academy of Science

Barrier thickness dependence of the scattering rates calculated from experimental resistance for two independent terms: (a) scatter-out; (b) scatter-in. The agreement between the two terms validates the model,

Achievement:
A new theoretical model for tunneling in magnetictunnel junctions is developed. This model accounts for nonspecular (k_// nonconserving) scattering by defects within the barrier layer. It is based on a differential master equation that explicitly includes the scatter-in terms (vertex corrections). This model combined with the first-principles band structure and quantum transport calculations, makes predictions of barrier thickness dependences of tunneling resistance and magnetoresistance, oscillatory magnetoresistance, all in good agreement with experimental measurements. In addition, the model is used to extract a temperature dependent interface magnon scattering rate from experimental data.

Significance:
The first-principles quantum transport calculation predicts that the tunnelling resistance for P (magnetic moments of the two electrodes aligned parallel) and AP (moments aligned antiparallel) increase with the barrier thickness at different rates, and that the magnetoresistance should increase with the barrier thickness. Experimental measurements have always shown an essentially constant magnetoresistance as a function of barrier thickness. This is a long standing puzzle in the field of spintronics that is finally conclusively resolved by the new model. The new model predicts that in the presence of defect scattering, all tunnelling channels contribute to the current with the same exponential dependence on the barrier thickness, in agreement with experiments.

This work has been accepted for publication in Physical Review B.