Theory of laser-induced demagnetization at high temperatures

Laser-induced demagnetization is theoretically studied by explicitly taking into account interactions among electrons, spins, and lattice. Assuming that the demagnetization processes take place during the thermalization of the subsystems, the temperature dynamics is given by the energy transfer between the thermalized interacting baths. These energy transfers are accounted for explicitly through electron-magnon and electron-phonon interactions, which govern the demagnetization time scale. By properly treating the spin system in a self-consistent random phase approximation, we derive magnetization dynamic equations for a broad range of temperature. The dependence of demagnetization on the temperature and pumping laser intensity is calculated in detail. In particular, we show several salient features for understanding magnetization dynamics near the Curie temperature. While the critical slowdown in dynamics occurs, we find that an external magnetic field can restore the fast dynamics. We discuss the implication of the fast dynamics in the application of heat-assisted magnetic recording.

Figure 1

Temperature dependence of (a) magnetization and (b) specific heat (arbitrary unit) for spin$=1/2$, 1, 2, 8 in the absence of the external field; temperature dependence of (c) magnetization and (d) specific heat for spin$=1/2$, 1, 2, 8 in an external field $H/T_c=0.001$.

Figure 2

Time dependence of the temperatures of the electrons, spins, and lattice after irradiation by a low-intensity laser with $T_e\left(0\right)=0.7T_c$, and $T_s\left(0\right)=T_p\left(0\right)=T_{rm}=0.47T_c$, and $T_c=620$ K. The inset shows the minimum magnetization (or maximum spin temperature) occurs at about 260 fs. The other parameters are $J_{ex}=0.15$ eV, ${\epsilon }_F=8$ eV,, $M/m={10}^5$, and $a_0=0.25$ nm.

Figure 3

Time-dependent magnetization as a function of time in logarithmical scale for various exchange parameters at a fixed laser fluence. The parameters are the same as those of Fig. 2.

Figure 4

Time evolution of three-temperature model for a large laser-fluence case $T_e\left(0\right)=1.6$. The critical slowing down of the spin system is identified as the plateau in the figure. The inset defines a slowdown time ${\tau }_d$. The smaller inset shows the magnified region in the vicinity of the maximum temperature. The other parameters are same as those in Fig. 2.

Figure 5

Log${}_2$-log${}_2$ plot of ${\tau }_d$ versus the reduced temperature for $J_{ex}=0.1$ and 0.2. The exponents are $\delta =0.43$ and $0.34$, respectively. The parameters are the same as those in Fig. 4. The dashed line is for eye guidance.

Figure 6

Magnetization as a function of time with and without the magnetic field. The parameters are same as those in Fig. 4.

Figure 7

Log${}_2$(${\tau }_d$) versus the initial electron temperature for two values of $J_{ex}$ with (open symbols) and without (filled symbols) the magnetic field. $T_e\left(0\right)$ is normalized initial electron temperature.