Could you explain the mechanism of spin transfer torque and spin-orbit torque in ferromagnetic materials? when it is happening and how ?
Spin transfer torque and spin orbit torque both happen when the non-equilibrium spin density (the spin quantity carried by the electronic flux) is not collinear to the direction of the magnetization. Both torques are given by the cross product between the spin density and the magnetization tau=s \times m. As it is the notion of all torques and following this expression you can easily figure out that this torque is non zero only when the two vectors are not collinear. Depending on the strength of the non-equilibrium spin density compared to the magnetization, one can have either alignment of the itinerant spins with the magnetization direction (in the case the magnetization is large) or excitation of the magnetization (in the case the spin density is large).
Now the difference between the spin transfer torque and the spin orbit torque resides in the origin of the spin density in each of which. The spin transfer torque - spin density originates from the polarization by a ferromagnetic layer, where the spin orbit torque - spin density originates from the spin orbit coupling which favors one direction of the spin angular momentum in a particular region of space.
Spin transfer torque is a term used to label phenomena associated to changes of the magnetization state induced by spin polarized currents.
Consider an electric current through some magnetically polarized material, it will in general exhibit some finite spin polarization. If this current flows through an area where the state of magnetizaton corresponds to a different orientatin of the spin polarization, then somehow the carriers have to adapt to this. This comes down to change in the spin expectation value (polarization and/or orientation) being exerced onto them.
The foregoing kind of description considers the (different) magnetization states in the material or device as given, and their role would be to impose boundary contitions to which the carriers have to adapt. But by the reaction principle there will also be an effect on the magnetic polarization states of the material, causing some rotation of the magnetization and/or the motion of domain walls. This can be understood as the effect of some "transferof spin" from the carriers to the magnetization states.
This is what I understand by the notion of spin transfer torque. My understanding may be incomplete, though, so take it with a grain of salt.
people.ccmr.cornell.edu/~ralph/papers/JMMM-320-1190-2008.pdf please read this paper for better understanding on STT
Spin transfer torque and spin orbit torque both happen when the non-equilibrium spin density (the spin quantity carried by the electronic flux) is not collinear to the direction of the magnetization. Both torques are given by the cross product between the spin density and the magnetization tau=s \times m. As it is the notion of all torques and following this expression you can easily figure out that this torque is non zero only when the two vectors are not collinear. Depending on the strength of the non-equilibrium spin density compared to the magnetization, one can have either alignment of the itinerant spins with the magnetization direction (in the case the magnetization is large) or excitation of the magnetization (in the case the spin density is large).
Now the difference between the spin transfer torque and the spin orbit torque resides in the origin of the spin density in each of which. The spin transfer torque - spin density originates from the polarization by a ferromagnetic layer, where the spin orbit torque - spin density originates from the spin orbit coupling which favors one direction of the spin angular momentum in a particular region of space.
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