Microscopic reversal magnetization mechanisms in CoCrPt thin films with perpendicular magnetic anisotropy: Fractal structure versus labyrinth stripe domains

The magnetization reversal of CoCrPt thin films has been examined as a function of thickness using magneto-optical Kerr effect (MOKE) microscopy and first-order reversal curves (FORC) techniques. MOKE images show differentiated magnetization reversal regimes for different film thicknesses: while the magnetic domains in 10-nm-thick CoCrPt film resemble a fractal structure, a labyrinth stripe domain configuration is observed for 20-nm-thick films. Although FORC distributions for both cases show two main features related to irreversible processes (propagation and annihilation fields) separated by a mostly flat region, this method can nonetheless distinguish which magnetization reversal process is active according to the horizontal profile of the first FORC peak, or propagation field. A single-peak FORC profile corresponds to the fractal magnetization reversal, whereas a flat-peak FORC profile corresponds to the labyrinth magnetization reversal.

Figure 1

Out-of-plane (■) hysteresis loops of 10-nm-thick CoCrPt film along with MOKE images. In images with applied fields of $-72$, $-80$, and $-90\mathrm{Oe}$, magnified images $(20\times 20\mu {\mathrm{m}}^2)$ of the reversed domains are shown in the insets and pointed out by white arrows. Reversed domains (“down”) are highlighted in black.

Figure 2

Out-of-plane $(•)$ hysteresis loops of 20-nm-thick CoCrPt film along with MOKE images. Reversed domains (“down”) are highlighted in black.

Figure 3

FORC distributions of (a) 10-nm-thick and (b) 20-nm-thick CoCrPt films. The color scale of the FORC diagrams ranges from light blue to red for $\rho >0$ and from light blue to black for $\rho <0$. (c),(d) Profiles along the horizontal dashed lines in Figs. 3 and 3 of peak A of the FORC diagrams for (c) 10-nm-thick and (d) 20-nm-thick CoCrPt films. In light blue, and as a guide to the eye, we plotted simplified diagrams of the FORC profiles for both 10-nm-thick (c) and 20-nm-thick (d) films.

Figure 4

Minor hysteresis loops of 10-nm-thick CoCrPt film along with MOKE images at different steps along the minor loop indicated in red ($a=-75\mathrm{Oe}$; $b=-65\mathrm{Oe}$; $c=+34\mathrm{Oe}$; $d=+60\mathrm{Oe}$; $e=+110\mathrm{Oe}$).

Figure 5

Minor hysteresis loops of 20-nm-thick CoCrPt film along with MOKE images at different steps along the minor loop indicated in red ($a=-100\mathrm{Oe}$; $b=-70\mathrm{Oe}$; $c=0\mathrm{Oe}$; $d=+80\mathrm{Oe}$).