Nano and picosecond magnetization dynamics of weakly coupled CoFe/Cr/NiFe trilayers studied by a multitechnique approach

We present results on the magnetization dynamics in heterostructures of the CoFe/Cr/NiFe type. We have employed a combination of different layer-selective methods covering a broad range from quasistatic hysteresis measurements by x-ray magnetic circular dichroism (XMCD), over time-resolved photoemission electron microscopy (PEEM) at subnanosecond timescales to high-frequency ferromagnetic resonance (FMR) experiments. With increasing driving frequency, we found a different influence of the coupling between the two ferromagnetic layers on the dynamic behavior. Employing the spatial resolution of the PEEM method, we have been able to discern various dynamic responses in different regions of the sample that could be attributed to magnetodynamic processes with a different degree of coupling. In conjunction with the complementary FMR and XMCD measurements, we attribute the inhomogeneous influence of interlayer coupling to a shift from domain-wall-motion-dominated dynamics at low frequencies to precession-dominated dynamics at higher frequencies.

Figure 1

Layer-resolved hysteresis loops of 5-nm CoFe and 2-nm NiFe for (a) single layered films and (b) parts of a trilayer sample incorporating a 2.5-nm-thick Cr interlayer.

Figure 2

FMR measurements on CoFe/Cr/NiFe trilayer: (a) FMR spectra at three in-plane angles and (b) in-plane angular dependence of the FMR signal. ${\phi }_B=0$ corresponds to measurements along the [110] direction of the GaAs substrate and the CoFe films, for ${\phi }_B={45}^{\circ }$, the field is directed toward the [010] direction.

Figure 3

Peak-to-peak FMR linewidth ($\Delta B_{pp}$) as a function of the frequency for CoFe (black circles) and NiFe (red circles). The experimental error of this method for determining $\alpha$ is $\Delta \alpha =0.01$.

Figure 4

XMCD images of the layer-resolved magnetization dynamics. (a) Response of the CoFe layer measured at the Co-$L_3$ edge, (b) response of the NiFe layer measured at the Ni-$L_3$ edge, and (c) temporal profile of the excitation pulse. The light-incidence and positive magnetic field directions are indicated by the arrows marked with $h\nu$ and $H$, respectively. The domain configuration in the ground state is marked by the black and white arrows and the black-white dashed lines are marking the domain walls. The numbers (1)–(3) mark positions where magnetization rotation (1), domain nucleation (2), and domain-wall rotation (3) are observed.

Figure 5

Dynamic response in the CoFe (open symbols) and NiFe layer (closed symbols) measured in three regions of interest of the $8\times 8$ $\mu$m${}^2$ magnetic microstructure (marked in the inset) reproducing the temporal characteristics of magnetization rotation (R1), domain nucleation (R2), and domain-wall motion (R3). The lines provide a guide to the eye (data offset for clarity).

Figure 6

Time-resolved displacement of the domain wall measured along the red line (inset) under the influence of the bipolar magnetic field pulse with $\pm$5 mT amplitude. In both layers (closed symbols: NiFe, open symbols: CoFe) the domain-wall motion is synchronous.