Magnetoresistance (MR) ratio up to 230% at room temperature (294% at 20 K) has been observed in spin-valve-type magnetic tunnel junctions (MTJs) using MgO tunnel barrier layer fabricated on thermally oxidized Si substrates. We found that such a high MR ratio can be obtained when the MgO barrier layer was sandwiched with amorphous CoFeB ferromagnetic electrodes. Microstructure analysis revealed that the MgO layer with (001) fiber texture was realized when the MgO layer was grown on amorphous CoFeB rather than on polycrystalline CoFe. Since there have been no theoretical studies on the MTJs with a crystalline tunnel barrier and amorphous electrodes, the detailed mechanism of the huge tunneling MR effect observed in this study is not clear at the present stage. Nevertheless, the present work is of paramount importance in realizing high-density magnetoresistive random access memory and read head for ultra high-density hard-disk drives into practical use.

The giant tunneling magnetoresistance effect has been achieved in low-resistance CoFeB∕MgO(001)∕CoFeB magnetic tunnel junctions (MTJs) at room temperature. A magnetoresistance (MR) ratio as high as 138%, seven times that of state-of-the-art MTJs for magnetic sensor application, was obtained at room temperature in MTJs with a resistance-area product (RA) as low as 2.4Ωμm2. Such a high MR ratio at such a low resistance was made possible by introducing an ultrathin Mg metal layer with a thickness of 4 Å between the CoFeB bottom electrode layer and the MgO(001) tunnel barrier layer. The Mg layer was slightly but not fully oxidized, which resulted in a reduction in MR for a thicker MgO barrier (high RA) region and in an increase in MR for a thinner barrier (low RA) region. The Mg layer improves the crystalline orientation of the MgO(001) layer when the MgO(001) layer is thin. These MTJs will accelerate the realization of highly sensitive read heads for ultrahigh-density hard-disk drives.

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