142
5 Spin-Transfer Torque
5.6 Possible Applications of Spin-Transfer Torques
A plethora of research activities are presently being carried out on the application
of STT (Krivorotov et al. 2005; Zhu and Zhu 2004; Nazarov et al. 2002; Kato et al.
2004; Kikkawa and Awschalom 1999; Weber et al. 2001; Joly et al. 2006a, b).
5.6.1 Magnetic Random Access Memory
It is now evident from our discussion that spin-transfer torque effect has potentials
of controllable switching of magnetic moments of two ferromagnetic layers back
and forth between a high-resistance antiparallel and a low-resistance parallel state.
This in turn proposes that STT might be employed to write information within nonvolatile magnetic random access memories. In fact, switching produced by spintransfer torque mechanism has been found to be more effective than that obtained by
current-induced magnetic fields to control magnetic bits. Other potential advantages
of STT-driven magnetization switching over magnetic field induced switching are
as follows:
1. Since there is no need of magnetic field to realize spin-transfer torque effect,
accordingly in this case, there is no requirement for designing and fabrication
of extra bit lines;
2. Again absence of any magnetic field causes decrease of perturbation, coming
from neighbouring magnetic elements while writing of bits in array;
3. While independently maintaining the magnetic-anisotropy barriers needed for
thermal stability, it is possible to minimize the currents responsible for spintransfer torque switching. Therefore, there is feasibility to continuously scale
the size of the magnetic bit to the lithography limits of silicon processing;
4. Compared to magnetic field switching, in case of spin-transfer architectures,
the switching characteristic is not necessarily required to be so uniform. This
in turn leads to much less demanding device tolerances.
Challenges for applications of spin-transfer torques
The two main challenges that restricted the applications of spin-transfer torque in
memory technologies are summarized below:
1. All-metal spin valve devices are characterized by very low resistances in the
range of 1–10 . This is pretty less than the range 1–10 k, which is required
to yield reasonable signal-to-noise ratio in case a silicon circuit is employed to
read the magnetic configuration (Zhu and Zhu 2004; Nazarov et al. 2002).
2. It should be noted that critical currents, needed to excite magnetic switching,
have decreased steadily as a result of improvements in device processing. For
example, critical currents have reduced from 5 mA in Co devices (used earlier) to
approximately 0.3 mA in the Py samples (existing devices) (Katine et al. 2000).
5 Spin-Transfer Torque
5.6 Possible Applications of Spin-Transfer Torques
A plethora of research activities are presently being carried out on the application
of STT (Krivorotov et al. 2005; Zhu and Zhu 2004; Nazarov et al. 2002; Kato et al.
2004; Kikkawa and Awschalom 1999; Weber et al. 2001; Joly et al. 2006a, b).
5.6.1 Magnetic Random Access Memory
It is now evident from our discussion that spin-transfer torque effect has potentials
of controllable switching of magnetic moments of two ferromagnetic layers back
and forth between a high-resistance antiparallel and a low-resistance parallel state.
This in turn proposes that STT might be employed to write information within nonvolatile magnetic random access memories. In fact, switching produced by spintransfer torque mechanism has been found to be more effective than that obtained by
current-induced magnetic fields to control magnetic bits. Other potential advantages
of STT-driven magnetization switching over magnetic field induced switching are
as follows:
1. Since there is no need of magnetic field to realize spin-transfer torque effect,
accordingly in this case, there is no requirement for designing and fabrication
of extra bit lines;
2. Again absence of any magnetic field causes decrease of perturbation, coming
from neighbouring magnetic elements while writing of bits in array;
3. While independently maintaining the magnetic-anisotropy barriers needed for
thermal stability, it is possible to minimize the currents responsible for spintransfer torque switching. Therefore, there is feasibility to continuously scale
the size of the magnetic bit to the lithography limits of silicon processing;
4. Compared to magnetic field switching, in case of spin-transfer architectures,
the switching characteristic is not necessarily required to be so uniform. This
in turn leads to much less demanding device tolerances.
Challenges for applications of spin-transfer torques
The two main challenges that restricted the applications of spin-transfer torque in
memory technologies are summarized below:
1. All-metal spin valve devices are characterized by very low resistances in the
range of 1–10 . This is pretty less than the range 1–10 k, which is required
to yield reasonable signal-to-noise ratio in case a silicon circuit is employed to
read the magnetic configuration (Zhu and Zhu 2004; Nazarov et al. 2002).
2. It should be noted that critical currents, needed to excite magnetic switching,
have decreased steadily as a result of improvements in device processing. For
example, critical currents have reduced from 5 mA in Co devices (used earlier) to
approximately 0.3 mA in the Py samples (existing devices) (Katine et al. 2000).