Laser-Induced Magnetization Dynamics of Lanthanide-Doped Permalloy Thin Films

We investigate the effect of Ho, Dy, Tb, and Gd impurities on the femtosecond laser-induced magnetization dynamics of thin Permalloy films using the time-resolved magneto-optical Kerr effect. Varying the amount of Ho, Dy, Tb content from 0% to 8%, we observe a gradual change of the characteristic demagnetization time constant from $\approx 60$ to $\approx 150\mathrm{fs}$. In contrast, Gd concentrations up to 15% do not influence the time scale of the initial photoinduced magnetization loss. We propose a demagnetization mechanism that relies on strong magnetic inertia of the rare-earth dopant which stabilizes the ferrimagnetic ordering and thereby delays the demagnetization.

Figure 1

(a) Illustration of the energy levels of the $3d$, $5d$, and $4f$ levels of the RE-doped ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ films. In the pump-probe experiment we predominantly excite the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ $3d$ electrons. (b) The variation of the Gilbert damping parameter $\alpha$ with the concentration of Ho and Gd impurities, as measured by FMR. The black lines are guides to the eye.

Figure 2

Pump-induced changes in the linear reflectivity measured on the pure ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ sample. The pronounced oscillatory component is the signature of the coherent artifact. Its contribution is separated by differentiating the transient LR signal as displayed in the inset. The solid line is a Gaussian fit to the data which gives a pulse length of 50 fs.

Figure 3

Time-resolved MOKE response measured on pure ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ and Ho- and Gd-doped ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ samples. The dash-dotted horizontal lines indicate the zero level of the transient signal, while the dashed line indicates the unpumped signal measured at negative delays. The black dotted lines illustrate the gradual tendency of the extremum position of the TR MOKE transients to longer pump-probe delays for Ho, while for Gd the position remains unchanged.

Figure 4

Change of the demagnetization time constant ${\tau }_M$ with Ho, Dy, Tb, and Gd impurity content. The empty symbols represent the calculated values of ${\tau }_M$ for the case of Ho and Gd doping according to 12. All curves are guides to the eye. The yellow triangular area corresponds to the region which can be expected from the slow relaxing impurity model for Ho, Dy, and Tb.