L10-ordered MnAl, a perpendicularly magnetized film material, has attracted much attention due to its high magnetic anisotropy, low saturation magnetization, and low magnetic damping for spin-transfer-torque magnetic random access memory (STT-MRAM) applications. However, in magnetic tunnel junctions (MTJs) with a single MnAl ferromagnetic layer, the TMR ratio has not been confirmed. This is due to the large lattice mismatch between MgO tunneling barrier and MnAl layer, which prevents the coherent tunneling of Δ1 electrons (1). In this work, we utilized Co-doped MnAl and MgAl2O4 tunneling barrier to reduce lattice mismatch and inserted a very thin Fe layer into (MnCo)Al/MgO interface to improve TMR effect.
The structure of the MTJs prepared in this study is shown in Fig.1. The MTJ films were prepared using the ultrahigh vacuum magnetron sputtering method. (MnCo)Al layers with low roughness and high (001) orientation by adding 2% Co. The composition of MgAl2O4 was measured using X-ray photoelectron spectroscopy (XPS). Compositional analysis showed that the composition of MgAl2O4 was Mg : Al : O =9.2 : 18.9 : 41.5 (atom%). MTJ devices were microfabricated and characterized their TMR properties by DC 4-probe method.
We succeeded in observing a TMR ratio of 8% at 300K without an insertion layer by using (MnCo)Al electrode and MgAl2O4 tunneling barrier as shown in Fig. 2(a). In addition, TMR ratio was improved to 9.4% by insertion of ultra-thin Fe layer into the (MnCo)Al/MgO interface as shown in Fig. 2(b). This translates to 18.8% in terms of magnetization parallel state. Since the RA product was decreased from 1.2 MΩμm2 to 223 kΩμm2 by insertion of Fe, coherent tunneling process was thought to be promoted by the interfacial modification.
This work was supported by the Center for Advanced Spintronics Research and Development, the Center for Spintronics Cooperative Research and Education and Development, and the Center for Spintronics Cooperative Research and Education, Tohoku University.
(1) H.Saruyama, M.Oogane, Y.Ando, 2013, Jpn. J. Appl. Phys., 52, 063003