Resonant Tunneling in Magnetic Nanojunctions

Experiments: A. Sokolov, I.F. Sabirianov, and B. Doudin

Theory: E.Y. Tsymbal

In collaboration with X.Z. Li, and J. Redepenning

The objective of this project is to elucidate the influence of resonant tunneling in magnetic tunnel junctions (MTJs) of a small area on tunneling magnetoresistance (TMR) and to understand the dependence of TMR on applied bias voltage and temperature. Funded by NSF, NRI and ONR.

Disorder and impurities in MTJs create localized electronic states in the barrier resulting in a resonant tunneling. The resonant tunneling leads to the inversion of TMR: 

The inversion of TMR can be found in MTJs of a small area under conditions at which the energy of the localized state lies close to the Fermi energy and resonant tunneling dominates direct tunneling. We have observed this phenomenon in nanowire junctions synthesized by electrochemical techniques. We use polyester track-etched membranes with cylindrical holes of 80 nm diameter as templates. The membrane is half filled by electrodeposition of Ni in the holes. A film of NiO of 2nm thickness is subsequently obtained by anodization process. The top ferromagnet is made by Co electrodeposition in a nonaqueous bath.

Measurements of magnetoresistance involving many samples show a broad distribution of TMR values with the highest peak lying at positive values close to zero. Some tunnel junctions display very large values of TMR up to 40%. The remarkable fact is the appearance of the inverse magnetoresistance with the largest negative value of -25%.

Calculations of TMR are  performed using Landauer-Büttiker theory within a simple tight-binding model which includes random variations in on-site atomic energies and inelastic scattering in the barrier. These calculations explain the measured distribution of magnetoresistance in terms of disorder-driven statistical variations of TMR with a finite probability of inversion due to resonant tunneling.

 

Papers:

E. Y. Tsymbal, A. Sokolov, I. F. Sabirianov, and B. Doudin, "Resonant Inversion of Tunneling Magnetoresistance", Physical Review Letters 90, 186602 (2003).

A. Sokolov, I. F. Sabirianov, E. Y. Tsymbal, and B. Doudin, X.Z. Li, and J. Redepenning, "Resonant tunneling in magnetoresistive Ni/NiO/Co nanowire junctions", Journal of Applied Physics 93, 7029-7031 (2003).

 

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