Chapter 5
Spin-Transfer Torque
5.1 Introduction on Spin-Transfer Torque
Spin-transfer torque, generally observed in magnetic multilayers, is a very intriguing
phenomenon. In fact, magnetic multilayers, exhibiting GMR effect, in which conduction current flows perpendicular to the interfaces, could demonstrate spin-transfer
torque effect under suitable experimental conditions.
5.1.1 What Is Spin-Transfer Torque (STT)?
At the onset, let us suppose a spin-polarized electron current flows through a ferromagnetic (FM) layer, magnetized to saturation. In this case, a fraction of the spin
angular momentum, associated with the spin-polarized electron current, can possibly
be transferred from the spin polarization of the incident electron current to the magnetization of the FM material. From elementary mechanics, it is well known that the rate
of change of angular momentumwith time is equal to torque, given by the following
equation:
τ =
d
L
dt
,
(5.1)
where τ = torque,
L = angular momentum. Therefore, this phenomenon can be interpreted as if spin-polarized electrons applying torque directly to the magnetization
of the FM layer. Hence, this interaction is referred to as spin-transfer torque (STT).
If the density of the spin-polarized current is sufficiently high, we may obtain
precession of the magnetization of the FM materials. An even higher density of the
spin-polarized current may result in reversal of the magnetization of the FM materials. This concept of magnetization reversal of a FM layer induced by STT effect was
© Springer Nature Singapore Pte Ltd. 2021
P. Dey and J. N. Roy, Spintronics,
https://doi.org/10.1007/978-981-16-0069-2_5
127
Spin-Transfer Torque
5.1 Introduction on Spin-Transfer Torque
Spin-transfer torque, generally observed in magnetic multilayers, is a very intriguing
phenomenon. In fact, magnetic multilayers, exhibiting GMR effect, in which conduction current flows perpendicular to the interfaces, could demonstrate spin-transfer
torque effect under suitable experimental conditions.
5.1.1 What Is Spin-Transfer Torque (STT)?
At the onset, let us suppose a spin-polarized electron current flows through a ferromagnetic (FM) layer, magnetized to saturation. In this case, a fraction of the spin
angular momentum, associated with the spin-polarized electron current, can possibly
be transferred from the spin polarization of the incident electron current to the magnetization of the FM material. From elementary mechanics, it is well known that the rate
of change of angular momentumwith time is equal to torque, given by the following
equation:
τ =
d
L
dt
,
(5.1)
where τ = torque,
L = angular momentum. Therefore, this phenomenon can be interpreted as if spin-polarized electrons applying torque directly to the magnetization
of the FM layer. Hence, this interaction is referred to as spin-transfer torque (STT).
If the density of the spin-polarized current is sufficiently high, we may obtain
precession of the magnetization of the FM materials. An even higher density of the
spin-polarized current may result in reversal of the magnetization of the FM materials. This concept of magnetization reversal of a FM layer induced by STT effect was
© Springer Nature Singapore Pte Ltd. 2021
P. Dey and J. N. Roy, Spintronics,
https://doi.org/10.1007/978-981-16-0069-2_5
127