134
5 Spin-Transfer Torque
state at an arbitrary intermediate angle, where it perform precession about zdirection. In this case, the energy gained from spin-transfer torque and the
energy lost due to damping torque are balanced over each cycle of precession
of free layer magnetic moment. Significantly, this experimental observation
explores an intriguing fact, in which spin-transfer torque exploits a DC applied
current to produce steady-state magnetic precession oscillations at GHz or tens
of GHz frequencies (Kiselev et al. 2003; Slonczewski 2002).
5.4 Experimental Results
In order to realize experimentally detectable spin-transfer torque effect in a device,
the prime condition is that the flow of electronic current should be restricted to a small
diameter. This in turn implies that to attain spin-transfer torque, device fabrication
would be very much critical.
To summarize some crucial conditions
1. Following the basic idea of spin-transfer torque mechanism, the amount of
current required to induce magnetic excitations, employing spin-transfer torque,
scales with the total magnetic moment associated with the free FM layer. This
is realizable by constructing the free FM layer a few nanometres thick and a few
hundreds of nanometres in diameter. Because of this nanometric dimension of
the device, free layer magnetic moment will be pretty less and correspondingly
less current is needed to induce magnetic excitations by spin-transfer torque
mechanism.
2. Another crucial point of concern for employing nanoscopic devices is that in
this case spin-transfer torque effect dominate over the effects of the magnetic
field, which is supposed to be pretty small because of very less current has
produced it.
3. Spin-transfer torque effects are very easily attainable in devices, which are
small enough that the free FM layer becomes single magnetic domain. It is well
known that in case of single magnetic domain, movement of magnetic moment
is favourable for the spin-transfer torque effect to be observed. It should be
mentioned that the dimension of the devices should be close to 100 nm scale.
Here we present some illustrations (Fig. 5.4) of various kinds of magneticmultilayer devices that can demonstrate spin-transfer torque effects.
5.4.1 Point Contact Device
Point contact device is a mechanical device, in which a magnetic multilayer is
contacted by a sharp metal tip. Such contact region is formed on the scale of few
tens of nanometres. Point contact devices, produced lithographically, have also been
employed to produce similar device geometry. In these both types of point contact
5 Spin-Transfer Torque
state at an arbitrary intermediate angle, where it perform precession about zdirection. In this case, the energy gained from spin-transfer torque and the
energy lost due to damping torque are balanced over each cycle of precession
of free layer magnetic moment. Significantly, this experimental observation
explores an intriguing fact, in which spin-transfer torque exploits a DC applied
current to produce steady-state magnetic precession oscillations at GHz or tens
of GHz frequencies (Kiselev et al. 2003; Slonczewski 2002).
5.4 Experimental Results
In order to realize experimentally detectable spin-transfer torque effect in a device,
the prime condition is that the flow of electronic current should be restricted to a small
diameter. This in turn implies that to attain spin-transfer torque, device fabrication
would be very much critical.
To summarize some crucial conditions
1. Following the basic idea of spin-transfer torque mechanism, the amount of
current required to induce magnetic excitations, employing spin-transfer torque,
scales with the total magnetic moment associated with the free FM layer. This
is realizable by constructing the free FM layer a few nanometres thick and a few
hundreds of nanometres in diameter. Because of this nanometric dimension of
the device, free layer magnetic moment will be pretty less and correspondingly
less current is needed to induce magnetic excitations by spin-transfer torque
mechanism.
2. Another crucial point of concern for employing nanoscopic devices is that in
this case spin-transfer torque effect dominate over the effects of the magnetic
field, which is supposed to be pretty small because of very less current has
produced it.
3. Spin-transfer torque effects are very easily attainable in devices, which are
small enough that the free FM layer becomes single magnetic domain. It is well
known that in case of single magnetic domain, movement of magnetic moment
is favourable for the spin-transfer torque effect to be observed. It should be
mentioned that the dimension of the devices should be close to 100 nm scale.
Here we present some illustrations (Fig. 5.4) of various kinds of magneticmultilayer devices that can demonstrate spin-transfer torque effects.
5.4.1 Point Contact Device
Point contact device is a mechanical device, in which a magnetic multilayer is
contacted by a sharp metal tip. Such contact region is formed on the scale of few
tens of nanometres. Point contact devices, produced lithographically, have also been
employed to produce similar device geometry. In these both types of point contact