The spin-polarized tunnel current and the spin-transfer torque in triple-barrier junctions with external electrodes and quantum wells made of ferromagnetic materials

The electron transport in triple-barrier junctions consisting of external ferromagnetic layers and three non-magnetic barriers separated by two inner ferromagnetic layers is analysed. It is assumed that the magnetic moments in the external electrodes are oriented in the same direction, whereas the magnetic moments of the inner layers can form any angle with this direction. The possibility of the gigantic enhancement of the normal torque component has been found in the junctions with a non-colinear orientation of magnetic moments in the inner layers. This enhancement may be much larger than in the double junctions. The current and the in-plane torque are most amplified when the quantum-well states in both inner layers have similar energies and can form the complex resonant states located slightly below the Fermi level. When the tunnelling by these complex resonant states occurs, the torque is greatest in the magnetic configuration other than that where the current is maximally enhanced. Additionally, the angular dependence of the torque differs significantly from that observed in single tunnel junctions.