Model for multishot all-thermal all-optical switching in ferromagnets

All-optical magnetic switching (AOS) is a recently observed rich and puzzling phenomenon that offers promising technological applications. However, a fundamental understanding of the underlying mechanisms remains elusive. Here we present a model for multishot helicity-dependent AOS in ferromagnetic materials based on a purely heat-driven mechanism in the presence of magnetic circular dichroism (MCD). We predict that AOS should be possible with as little as 0.5% of MCD, after a minimum number of laser shots heat the sample close to the Curie temperature. Finally, we qualitatively reproduce the all-optically switched domain patterns observed experimentally by numerically simulating the result of multiple laser shots on an FePtC granular ferromagnetic film.

Figure 1

The AOS model supposes an (a) $N_x$ by $N_y$ grid of cells, which present (b) temperature-dependent magnetic anisotropy energy barriers $E_{ab}$, resulting in (c) two stable possible states $a$ (up) and $b$ (down). As a (d) circularly polarized laser pulse arrives onto the grid, (e) cells in $a$ and $b$ states will absorb different amounts of energy due to MCD inducing a temperature distribution in the grid of hot and cold cells. This will lead to (f) different energy landscapes for the hot and cold cells, resulting mostly in stochastic switching of the hotter cells.

Figure 2

All-optical switching probability as a function of the laser temperature increase $\mathrm{\Delta }T$ for different numbers of shots $N$. Three different temperature regimes are observed. At low temperature no switching is possible due to strong anisotropy barriers. Close to $T_C$ a certain amount of AOS occurs as the number of shots increases. With enough pulses full switching becomes possible. At higher temperatures the sample gets randomly demagnetized into a multidomain structure.

Figure 3

AOS Probability as a function of temperature and MCD for $N=10$ pulses.

Figure 4

Simulations of the magnetization state after assuming a Gaussian temperature increase induced by the laser intensity profile (profile details in text). (a) Simulation of successive shots with left circularly polarized $\left({\sigma }^{-}\right)$ light on an initially saturated black (“down”) grid. After ten shots a fully switched white ring develops. (b) Simulations scanning the beam with left circular $\left({\sigma }^{-}\right)$, right circular $\left({\sigma }^{+}\right)$, and linearly polarized $\left(\pi \right)$ laser shots at a ten shots/cell sweeping speed. Initially, the grid consisted of a left-half $up$ (white) magnetization and right-half down (black) magnetization. Each helicity favors one magnetization direction, which is determined by the edge of the Gaussian profile, whereas the linearly polarized laser beam only demagnetizes the sample (see video in the Supplemental Material [15]).