Manipulation of magnetic moments in a nano-scale ferromagnet
by a current is one of the most important techniques for the future spintronics
devices. Especially, current induced magnetization switching in MgO-based
magnetic tunnel junctions (MTJs) has been expected for a method of writing
in high density magnetoresistive random access memory (MRAM).
Conducting electrons flowing through a fixed magnetic layer
(F1 in Fig. 1) in a magnetoresistive device are spin polarized along the
magnetization of F1. When these spin-polarized electrons pass through another
nanomagnetic layer (F2), the polarization direction may have to change
depending on relative orientation of F1 and F2. In this repolarization
process, the nanomagnet experiences a torque (spin torque) associated with
the transfer of spin angular momentum from conducting electrons. For large
current, the spin torque amplifies the cone angle of spin precession and
leads magnetization switching in the case that the spin torque overcomes
magnetic damping. The magnetization of nanoscaled free layer is controllable
by the flowing current direction (experimental result is shown in Fig.2).
The necessary critical current for spin transfer switching
decreases as a free-layer volume decreases. From this scalability, the
spin transfer switching can reduce a writing current in MRAM, while conventional
writing method using magnetic field generated by a current needs a large
Fig. 1 Schematic illustration of spin transfer torque.
Fig. 2 Typical TMR loop driven by a pulse current.