3.7 Magnetization dynamics of an FePt/FeRh bilayer

In the magnetic recording, the writing process of a bit requires the application of a field of opposite direction to invert the magnetization. The recovery of the magnetization after the field pulse has to last few nano seconds and phenomena as ringing [Crawford 00] are completely undesired. A realistic simulation of magnetic media needs the inclusion of the writing head. However, we will include an homogeneous applied field to the study the dynamical switching, which is by itself an interesting question. We have simulated an FePt/FeRh thin film under a constant magnetic field using the LLG equation. The parameters of the LLG equation are: the damping $ \alpha=
0.05$ and the integration time step $ \Delta t=1.56564\cdot10^{-16} s$. This time step does not allow to simulate large system sizes and in this case we have chosen the in-plane dimensions $ 45 nm \times 45 nm$ (periodic boundary conditions) and the thicknesses $ 15 nm$ FePt and $ 30 nm$ FeRh. The interfacial exchange parameter is $ J_s/J=0.1$ and the anisotropy $ K^{FePt} = 2 \cdot 10^7 erg/cm^3$. The field can not take any value immediately, due to physical limitations, and needs some time to reach its final value. The field rise time of the pulse was $ 0.25 ns$ and the maximum field $ H_{max}= 0.55 H_k = 2 T$, which is slightly larger than the corresponding static coercivity for the film. To compare, the maximum field obtained from the current writing heads is $ 1.7 T$ [Kanai 05].

Figure 3.29: Magnetization profile in an FePt/FeRh film under a field pulse for different times. The field rise is plotted in the right figure.

Fig. 3.29 represents the temporal evolution of the magnetization of the film. Before applying the field the initial state is the remanence, which includes a $ 90^\circ $ domain wall, with the magnetization of the FePt pointing in the Z positive direction. The magnetization reversal process is found to essentially involve domain wall propagation, but is somewhat complex, and takes place in three distinct stages. In stage 1 there is a gradual propagation of the domain wall into the hard FePt phase. During this stage, the domain wall slowly changes its nature from $ 90^\circ $ to $ 180^\circ$ domain wall. The complete reversal of the FeRh layer, which is equivalent to the $ 180^\circ$ domain wall, is not necessary to induce propagation of the domain wall into the FePt phase. During the second stage of the magnetization reversal process the magnetization in the FePt becomes more negative than that in the FeRh. This establishes a reverse domain wall, which propagates back into the FeRh layer, resulting in complete reversal of the whole system. The third stage consists in the relaxation of the magnetization to its new equilibrium value. In case the field is removed, this last stage includes the creation of a domain wall. The duration of this last stage has to be minimized in order to design a good magnetic recording media.

Additionally, the soft/hard bilayer presents very interesting dynamic properties. For example, if for a particular mode or spin-wave in one material there are no modes in the coupled material, then the interface between the two materials acts to pin that particular mode in the first material, reducing the amplitude and changing the mode profile. This effect is known as dynamic pinning [Hoffmann 70] and can be tuned with an applied field. However, the study of spin waves in composite media is beyond the scope of this thesis and will be the focus of future work.