5.3 Spin-Transfer Torque (STT)-Driven Magnetization Dynamics
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Consequently, the associated magnetic moment of the free FM layer would tend to
attain its lowest energy. Now, the lowest energy corresponds to the alignment of the
magnetic moment along the direction of the applied magnetic field, i.e., along z-axis.
Therefore, it is evident that the damping torque should point towards the z-axis in
a manner, as shown in Fig. 5.3. Hence, it comes out that in absence of any term
related to spin-transfer torque, the magnetization associated with the free FM layer
will spiral towards the z-axis and eventually finish up by attaining the lowest-energy
configuration directing along z-direction.
5.3.2 Magnetization Dynamics in Presence of STT
This case corresponds to the action of an additional torque that can point either
in the same or opposite direction to the damping torque, depending on the sign of
the current that produces this spin-transfer torque (with reference to Sect. 5.3.2).
Recalling Sect. 5.3.2, under the application of negative current, the spin-transfer
torque would be so generated that it will simply strengthen the damping torque.
Consequently, the magnetization associated with the free FM layer would spiral or
relax even faster towards the z-direction, compared to the case when there is no STT.
On the other hand, with the application of small positive currents, the generated
spin-transfer torque would be such that it will oppose the effective damping, i.e.,
spin-transfer torque will be acted opposite to the direction of damping torque, as
shown in Fig. 5.3. As a result, the magnetization of the free FM layer would spiral
or relax rather slowly towards the z-direction, compared to the case when there is no
STT.
Furthermore, depending on the magnitude of the applied magnetic field and
the detailed mutual angular dependence of the spin-transfer and damping torques,
magnetization dynamics can be of diversified forms. Let us discuss these cases
separately.
(a) Under the application of appropriate magnitude of positive currents, it is
feasible for the magnetization of the free FM layer to spiral up to ever-increasing
values of θ all the way to θ = π. This implies that the magnetic moment associated with the free FM layer can attain a stable static state with magnetization
antiparallel to the fixed FM layer. Experimentally, such simple spin-transfer
torque-driven magnetization reversal is achievable under the application of low
applied magnetic fields. Noteworthy, the magnetization of this free FM layer
can be controllably switched back to the parallel alignment with the fixed layer
moment by applying a sufficiently large negative current. This phenomenon and
the related device are under current investigation for applications in magnetic
memory devices (Myers et al. 1999; Katine et al. 2000).
(b) In a similar scenario as described above, with the optimization of parameter
values in some different ways, the magnetization of free FM layer may not
spiral all the way to θ = π. Rather, it might attain some dynamical equilibrium
133
Consequently, the associated magnetic moment of the free FM layer would tend to
attain its lowest energy. Now, the lowest energy corresponds to the alignment of the
magnetic moment along the direction of the applied magnetic field, i.e., along z-axis.
Therefore, it is evident that the damping torque should point towards the z-axis in
a manner, as shown in Fig. 5.3. Hence, it comes out that in absence of any term
related to spin-transfer torque, the magnetization associated with the free FM layer
will spiral towards the z-axis and eventually finish up by attaining the lowest-energy
configuration directing along z-direction.
5.3.2 Magnetization Dynamics in Presence of STT
This case corresponds to the action of an additional torque that can point either
in the same or opposite direction to the damping torque, depending on the sign of
the current that produces this spin-transfer torque (with reference to Sect. 5.3.2).
Recalling Sect. 5.3.2, under the application of negative current, the spin-transfer
torque would be so generated that it will simply strengthen the damping torque.
Consequently, the magnetization associated with the free FM layer would spiral or
relax even faster towards the z-direction, compared to the case when there is no STT.
On the other hand, with the application of small positive currents, the generated
spin-transfer torque would be such that it will oppose the effective damping, i.e.,
spin-transfer torque will be acted opposite to the direction of damping torque, as
shown in Fig. 5.3. As a result, the magnetization of the free FM layer would spiral
or relax rather slowly towards the z-direction, compared to the case when there is no
STT.
Furthermore, depending on the magnitude of the applied magnetic field and
the detailed mutual angular dependence of the spin-transfer and damping torques,
magnetization dynamics can be of diversified forms. Let us discuss these cases
separately.
(a) Under the application of appropriate magnitude of positive currents, it is
feasible for the magnetization of the free FM layer to spiral up to ever-increasing
values of θ all the way to θ = π. This implies that the magnetic moment associated with the free FM layer can attain a stable static state with magnetization
antiparallel to the fixed FM layer. Experimentally, such simple spin-transfer
torque-driven magnetization reversal is achievable under the application of low
applied magnetic fields. Noteworthy, the magnetization of this free FM layer
can be controllably switched back to the parallel alignment with the fixed layer
moment by applying a sufficiently large negative current. This phenomenon and
the related device are under current investigation for applications in magnetic
memory devices (Myers et al. 1999; Katine et al. 2000).
(b) In a similar scenario as described above, with the optimization of parameter
values in some different ways, the magnetization of free FM layer may not
spiral all the way to θ = π. Rather, it might attain some dynamical equilibrium