62
W. C. Law and S. De W. Wong
Fig. 8 Torque terms acting
upon M due to the influence
of an effective field and a
spin-polarized current. Green
arrows indicate the STT
terms due to spin-down
current transferring its
angular momentum to M
where s refers to the unit directional vector of the spin current, while τ || and τ ⊥
correspond to the in-plane and perpendicular torque terms, respectively. In Fig. 8,
the vector representations of the four terms within the right hand side of LLGS
equation are illustrated, in which STT terms in green depend on the direction of
the spin current (against the damping term in this case). Therefore, a large enough
spin polarized current (i.e. the third term greater than the second term) can induce
magnetization reversal of the storage layer even in the absence of external magnetic
field. This leads to much more compact design in the subsequent generation of
MRAM as the current can be injected via the top or bottom electrodes instead of
relying on external fields.
A variety of potential spintronics applications have since opened up due to a
greater understanding of the magnetization dynamics over the years. For example,
by injecting an appropriate level of DC current (which will subsequently be polarized
either via STT or SOT), steady-state magnetization precession on the GHz scale can
be achieved. As such, a new class of spintronics product known as the Spin-Torque
Nano-Oscillator (STNO) can be developed to generate high-frequency microwaves
[69, 70, 71]. Alternatively, incident microwaves may also be rectified as the precession of the free layer induces a high and low resistance state every half a cycle,
leading to another class of spintronics application known as the spin diode that could
potentially be used for energy harvesting technology [72, 73, 74, 75, 76, 77].
4 Performance Indicators in STT-MRAM
While MRAM has been claimed to potentially be the universal memories due to its
overall superiority in every aspect required of a memory system, there remains a
significant amount of effort needed to address further downscaling for future technology nodes. A pentalemna exists as the requirements of a functional memory
W. C. Law and S. De W. Wong
Fig. 8 Torque terms acting
upon M due to the influence
of an effective field and a
spin-polarized current. Green
arrows indicate the STT
terms due to spin-down
current transferring its
angular momentum to M
where s refers to the unit directional vector of the spin current, while τ || and τ ⊥
correspond to the in-plane and perpendicular torque terms, respectively. In Fig. 8,
the vector representations of the four terms within the right hand side of LLGS
equation are illustrated, in which STT terms in green depend on the direction of
the spin current (against the damping term in this case). Therefore, a large enough
spin polarized current (i.e. the third term greater than the second term) can induce
magnetization reversal of the storage layer even in the absence of external magnetic
field. This leads to much more compact design in the subsequent generation of
MRAM as the current can be injected via the top or bottom electrodes instead of
relying on external fields.
A variety of potential spintronics applications have since opened up due to a
greater understanding of the magnetization dynamics over the years. For example,
by injecting an appropriate level of DC current (which will subsequently be polarized
either via STT or SOT), steady-state magnetization precession on the GHz scale can
be achieved. As such, a new class of spintronics product known as the Spin-Torque
Nano-Oscillator (STNO) can be developed to generate high-frequency microwaves
[69, 70, 71]. Alternatively, incident microwaves may also be rectified as the precession of the free layer induces a high and low resistance state every half a cycle,
leading to another class of spintronics application known as the spin diode that could
potentially be used for energy harvesting technology [72, 73, 74, 75, 76, 77].
4 Performance Indicators in STT-MRAM
While MRAM has been claimed to potentially be the universal memories due to its
overall superiority in every aspect required of a memory system, there remains a
significant amount of effort needed to address further downscaling for future technology nodes. A pentalemna exists as the requirements of a functional memory