Femtosecond laser heating induced ultrafast magnetization reversal in TbCo films with different electron-phonon coupling inter-action

TbCo alloy films possess ferrimagnetic structure and high perpendicular anisotropy and have been proven to rapidly reverse their magnetic moments irradiated with an ultra-short pulse laser, which have the potential to use as all optical switching media [1–2]. In this paper, we have adopted a two-temperature model [3] and a general theoretical framework [4] to calculate the temperatures of the electron gas $( T_{\mathrm {e}})$ and the phonon $( T_{\mathrm {p}})$ and the magnetization dynamics of Tb26Co74 films, respectively, and found that electron-phonon coupling factor (G) plays a significant influence on the $T_{\mathrm {e}}$ and magnetization reversal dynamics of TbCo films. In our simulation, the laser wavelength and pulse width, laser fluence are 560 nm, 100 fs and 15 J/m², respectively. The G values vary from 1 to 10×10¹⁷ W/m³. K. Fig 1 shows the time evolution of $T_{\mathrm {e}}$ and $T_{\mathrm {p}}$. $T_{\mathrm {e}}$ rapidly increases to the peak temperature within about 0.3ps and then decreases with different rates until reaching the same value as $T_{\mathrm {p}}$. TbCo films with a smaller G have a higher peak temperature of $T_{\mathrm {e}}$ and exhibit a slower cooling rate, and thus experience a longer heat exchange time between the electron gas and phonon. Fig 2 presents the computed time-resolved magnetization dynamics of Co- and Tb- sublattices. When G is just 1×10¹⁷W/m³. K, only the rapid demagnetization process occurs in Co- and Tb- sublattices and no magnetization reversal is observed due to the higher $T_{\mathrm {e}}$ and slower cooling rate. As G continues to increase, the demagnetization time of Co- sublattice rises slowly, but the initial reversal time of Co- sublattice drops gradually from 1.27ps to 0.66ps, which is attributed to the decreasing $T_{\mathrm {e}}$ and the increasing cooling rate of electron gas. Moreover, Tb- sublattice delays to reverse its magnetic moment with the increase of G perhaps owing to the lower $T_{\mathrm {e}}$ and the slower demagnetization of Tb- sublattice. Furthermore, although Co- and Tb- sublattices both generate magnetization reversal, the total magnetization reversal of Tb26Co74 films does not occur within the timescale of 5ps and will happen after a relatively longer recovery of magnetic moments of Tb- sublattice.