MgGa2O4 spinel barrier for magnetic tunnel junctions: Coherent tunneling and low barrier height

Abstract

Epitaxial Fe/magnesium gallium spinel oxide (MgGa2O4)/Fe(001) magnetic tunnel junctions (MTJs) were fabricated by magnetron sputtering. Tunnel magnetoresistance (TMR) ratio up to 121% at room temperature (196% at 4 K) was observed, suggesting a TMR enhancement by the coherent tunneling effect in the MgGa2O4 barrier. The MgGa2O4 layer had a spinel structure and it showed good lattice matching with the Fe layers owing to slight tetragonal lattice distortion of MgGa2O4. Barrier thickness dependence of the tunneling resistance and current-voltage characteristics revealed that the barrier height of the MgGa2O4 barrier is much lower than that in an MgAl2O4 barrier. This study demonstrates the potential of Ga-based spinel oxides for MTJ barriers having a large TMR ratio at a low resistance area product. * Electronic mail: sukegawa.hiroaki@nims.go.jp 2 Magnetic tunnel junctions (MTJs) have played a central role in spintronic devices such as read heads of hard disk drives and non-volatile magnetoresistive random access memories (MRAMs) for the last two decades, and many efforts have been made to improve their performance. 1 One of the most prominent achievements which accelerated the practical applications was the realization of giant tunnel magnetoresistance (TMR) ratios by using rock-salt type MgO crystalline barrier. [2][3][4][5] The giant TMR effect is attributed to the spin-dependent coherent tunneling through the 1 Bloch state in MgO(001). Toward ultra-high density spin transfer torque (STT)-MRAM applications, MTJs with low resistance area (RA) product of around 1 m 2 are needed as well as high TMR ratios. In MgO-based MTJs, the thickness of the MgO barrier must be reduced to a few monoatomic layers for achieving such a low RA, which causes substantial reduction of TMR ratios. This means the necessity of alternative barriers having a low barrier height. Doping other elements into MgO has been known to reduce the MgO barrier height; e.g., Zn 6,7 and Ti 8 dopings were reported to provide lower RA values although such dopings are likely to cause reduction of TMR ratios.Spinel oxide MgAl2O4(001) barrier also exhibits the coherent tunneling effect, 9,10 and over 300% TMR ratios at room temperature (RT) were reported in MgAl2O4-based MTJs. 11,12 Very recently, a TMR ratio at RT of 92% in an MgO/spinel-type -Ga2O3(001) bilayer barrier MTJ 13 and that of 120% in a Li-Mg-Al-O quaternary spinel-based barrier MTJ were reported, 14 showing the capability of the spinel based barriers for MTJs. The lattice spacing of MgAl2O4 is 4% smaller than that of MgO. In 3 addition, the lattice spacing can further be tuned by the Mg/Al composition, leading to the highly lattice-matched interfaces with various ferromagnetic materials. 9,12 On the other hand, the experimental band gap of MgAl2O4 is reported to be 7.8 eV, 15 which is similar to that of MgO (7.587.8 eV); 15 therefore, no reduction in RA values using an MgAl2O4 barrier was observed. 16 Some of spinel oxides other than MgAl2O4, such as ZnAl2O4, SiMg2O4 and SiZ...

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