User Project Brings New Understanding to Magnetic Tunnel Junctions

Yan Wang1, X.-F. Han1, and X.-G. Zhang2,3
1Institute of Physics, Chinese Academy of Sciences,
2Center for Nanophase Materials Sciences, Oak Ridge National Laboratory,
3Computer Science and Mathematics Division, Oak Ridge National Laboratory

Achievement:

The material of choice for spintronics device today is Fe/MgO/Fe tunnel junction. The quality of the electrode/barrier interfaces is crucial for avoiding the formation of FeO at Fe/MgO interfaces to achieve a giant tunneling magnetoresistance (TMR) effect. First-principles theory shows that the interfacial resonance (IR) states, also called hot spots, which mainly contribute to the conductance of minority-spin channel electrons for the parallel configuration and both spin channels for the antiparallel configuration, can dramatically reduce the TMR effect. Thus eliminating the IR contribution by modi?cation of the interface is an important topic in spintronics research.

In our work, the effect of Co interlayers in Fe(001)/Co/MgO/Co/Fe magnetic tunnel junctions is studied by first-principles calculation using the layer-KKR code developed at ORNL. We confirm that the Co layers inserted at the two Fe/MgO interfaces strongly influence the interfacial resonance (IR) states and the tunneling magneto resistance (TMR). The effect is not monotonic. Strongest IR occurs at Co layer thickness of 0.5 monolayer (ML). With 1 ML Co, the IR is dramatically reduced and TMR ratio is maximized.

Significance:

The 2001 prediction of giant TMR in Fe/MgO/Fe tunnel junctions by ORNL researchers led to a breakthrough in spintronics, allowing a much higher density for magnetic storage than previous technologies. Today, all computer hard drives sold utilize the giant TMR effect. That prediction was made using the layer-KKR code developed at ORNL. Today, the layer-KKR code attracts users from around the world to the CNMS to study magnetic tunnel junctions. The current work is initiated by the Institute of Physics in China. Experimentalists have long known that adding Co to the Fe/MgO interface can increase the TMR. This was mostly understood for two reasons, a stronger exchange coupling in Co that minimizes loose spins that cause spin-flip scattering, and the prevention of the formation of FeO layer. Our work gives a different, strong theoretical reason for the effect of Co, and provides a path for optimization of the TMR. Through the suppression of the IR states, a ML of Co maximizes the TMR.

Reference:
Y. Wang, X. F. Han, and X.-G. Zhang, "Effect of Co interlayers in Fe/MgO/Fe magnetic tunnel junctions," Appl. Phys. Lett. 93, 172501 (2008).