Quantum size effect in magnetic tunnel junctions with single-crystal ultrathin electrodes

Control of coherent electron transports in spintronic devices is an essential issue to realize highly functional spin-devices such as spin-dependent resonant-tunneling transistors. The spin dependent resonant tunneling via the quantum-well states in the electrode gives us many clues to understand and to control transport property in magnetic tunnel junctions (MTJs). To observe the quantum-well oscillations in TMR effect, we prepared the magnetic tunnel junctions with single-crystal ultrathin electrode or insertion layers. The MTJs with an ultrathin Fe [001] electrode shows the oscillation of TMR ratio as a function of the biasing voltage. In the case of an ultrathin Cu [001] layer inserted between a Co electrode and Al₂O₃ barrier layer, we found large oscillations of TMR with respect to the thickness of the Cu electrode. These results clearly show a feasibility of the coherent spintronic devices using metallic systems.