Finite-size effects in ultrafast remagnetization dynamics of FePt

We investigate the ultrafast magnetization dynamics of FePt in the $\mathrm{L}{1}_0$ phase after an optical heating pulse, as used in heat-assisted magnetic recording. We compare continuous and nano-granular thin films and emphasize the impact of the finite size on the remagnetization dynamics. The remagnetization speeds up significantly with increasing external magnetic field only for the continuous film, where domain-wall motion governs the dynamics. The ultrafast remagnetization dynamics in the continuous film are only dominated by heat transport in the regime of high magnetic fields, whereas the timescale required for cooling is prevalent in the granular film for all magnetic field strengths. These findings highlight the necessary conditions for studying the intrinsic heat transport properties in magnetic materials.

Figure 1

Static magnetic properties of the FePt samples and sketch of the TR-MOKE setup. (a) Magnetization hysteresis loops and (b) temperature dependence of the magnetization of the granular (open symbols) and continuous (plain symbols) samples. For (b) a stabilizing field of 65 mT was applied for the continuous film, while the granular  film was measured in remanence. The magnetic field in (a) and (b) was applied perpendicular to the plane of the samples. Sketch of (c) the continuous and (d) the granular FePt samples. (e) Sketch of the TR-MOKE setup.

Figure 2

Magnetic field dependence of laser induced magnetization dynamics. (a), (b) Normalized Kerr rotation $\mathrm{\Delta }{\theta }_K$ measured in the (a) continuous and (b) granular samples at different applied magnetic fields for a fixed pump fluence of $5\mathrm{mJ}/{\mathrm{cm}}^2$. The inset (c) shows the magnetic-field dependence of the time $t_{\text{1/2}}$ at which half of the demagnetization amplitude has recovered for this pump fluence of $5\mathrm{mJ}/{\mathrm{cm}}^2$ and an approximately doubled fluence of 9 and $10\mathrm{mJ}/{\mathrm{cm}}^2$ for the continuous and granular sample, respectively. Solid lines are a guide to the eye.

Figure 3

Hysteresis measured in the continuous (a) and granular (b) samples at different pump fluence and delay between the pump and the probe pulses. In all measurements, the applied $B_{\text{ext}}$ ranges between $\pm 0.75\mathrm{T}$. The vertical scaling is fixed for all hysteresis curves and the saturation magnetization is normalized as described in the text (a) to the hysteresis loop without pump and (b) by the signal observed at the highest fluence.

Figure 4

Pump fluence dependence of laser-induced ultrafast magnetization dynamics. (a), (b) $\mathrm{\Delta }{\theta }_K$ measured in the (a) continuous and (b) granular samples as a function of the pump fluence at high external magnetic field of $B_{\text{ext}}=0.75\mathrm{T}$. The dashed dotted and dashed lines in (b) are the $\mathrm{\Delta }{\theta }_{\text{K}}$ signal induced in continuous film by $F_{\text{pump}}$ of 4.5 and $8.5\mathrm{mJ}/{\mathrm{cm}}^2$, respectively. The inset (c) shows the pump fluence dependence of the time $t_{\text{1/2}}$ at which half of the demagnetization amplitude has recovered for 0.75 T with the solid lines as guide to the eye.