  The tunnel magnetoresistance (TMR) effect observed in magnetic tunnel
junctions (MTJs) is commonly used in many spintronic applications because the
effect can easily convert from local magnetic states to electric signals in a
wide range of device resistances. In this study, we demonstrated TMR ratios of
up to 631% at room temperature (RT), which is two or more times larger than
those used currently for magnetoresistive random access memory (MRAM) devices,
using CoFe/MgO/CoFe(001) epitaxial MTJs. The TMR ratio increased up to 1143% at
10 K, which corresponds to an effective tunneling spin polarization of 0.923.
The observed large TMR ratios resulted from the fine-tuning of atomic-scale
structures of the MTJs, such as crystallographic orientations and MgO interface
oxidation, in which the well-known Delta1 coherent tunneling mechanism for the
giant TMR effect is expected to be pronounced. However, behavior that is not
covered by the standard coherent tunneling theory was unexpectedly manifested;
i.e., (i) TMR saturation at a thick MgO barrier region and (ii) enhanced TMR
oscillation with a 0.32 nm period in MgO thickness. Particularly, the TMR
oscillatory behavior dominates the transport in a wide range of MgO
thicknesses; the peak-to-valley difference of the TMR oscillation exceeded 140%
at RT, attributable to the appearance of large oscillatory components in
resistance area product (RA). Further, we found that the oscillatory behaviors
of the TMR ratio and RA survive, even under a +-1 V bias voltage application,
indicating the robustness of the oscillation. Our demonstration of the giant
RT-TMR ratio will be an essential step for establishing spintronic
architectures, such as large-capacity MRAMs and spintronic artificial neural
networks. More essentially, the present observations can trigger us to revisit
the true TMR mechanism in crystalline MTJs.
