3.2.1 Reversal speed in the switching process

We consider, as a measure of the switching speed, the time instant $t_0$ at which the average $x$ component $<m_x>$ ($<\cdot>$ means spatial average) is zero after the application of the external field (the external field strength is the same in both the simulations):

$$t_0=\min\{t>0:<m_x>=0\} \quad.$$ (3.22)

Figure: Numerical results. Comparison between damping (dashed line) and precessional (solid line) switching: time for average $m_x$ component to reach zero from the starting configuration for $H_a=19.51$ kA/m.

In figure 3.5 one can observe the behavior of the average $m_x$ component until it reaches zero, showing that the precessional switching dynamics is much faster ( $t_0=0.09$ ns) than damping switching's ( $t_0=0.17$ ns). This is due to the different nature of the mechanism driving magnetization motion in the two processes: in conventional switching there is only one equilibrium configuration after the application of the external field, namely the reversed state, so the switching process is a kind of relaxation process towards the equilibrium and therefore the damping process is crucial. Conversely, in precessional switching the main role is played by the magnetic torque acting on the magnetization, which causes a fast precessional motion around the effective field driving the magnetization back and forth between the initial and the reversed state. Therefore, in most cases this process is so fast that dissipative effects can be neglected.