Manipulating magnons via ultrafast magnetization modulation

We demonstrate how fundamental properties of magnons may be manipulated using femtosecond laser pulses by performing ab initio real-time time-dependent density functional theory simulations. To illustrate this, we show how the spin-wave dynamics in ${\mathrm{Fe}}_{50}{\mathrm{Ni}}_{50}$ can be manipulated in three different ways by tailoring the applied laser pulse to excite optically induced intersublattice transfer (OISTR): (1) element-selective destruction of magnon modes depending on the laser intensity, (2) delay-dependent freezing of the magnon mode into a transient noncollinear state (where the delay is in the pulse peak with respect to the start of simulations), and (3) OISTR-driven renormalization of the optical magnon frequency. Harnessing such processes would significantly speed up magnonic devices.

Figure 1

The four observed modes in a four-atom supercell of the ferromagnetic alloy ${\mathrm{Fe}}_{50}{\mathrm{Ni}}_{50}$. (a) Goldstone mode, (b) optical mode, (c) pure iron mode, and (d) pure nickel mode.

Figure 2

Oscillation of the transverse ($x,y$) magnetic moments of the individual nickel and iron atoms in a four-atom supercell of ${\mathrm{Fe}}_{50}{\mathrm{Ni}}_{50}$ for different initial states. These magnons correspond to momenta $q$ = $\mathrm{\Gamma }, \pm 1/2 \mathrm{\Gamma }X$, and $X$. Decoupled, element-specific magnon modes can be seen for (a) nickel and (b) iron. Coupled Goldstone and optical modes can be seen in (c), and all four modes are excited in (d).

Figure 3

(a) The electric field profile of the two laser pulses designed to induce OISTR transitions in ${\mathrm{Fe}}_{50}{\mathrm{Ni}}_{50}$; both have a frequency of 2.19 eV and FWHM of 2.41 fs but different fluences, 9.6807 and $0.9537\mathrm{mJ}/{\mathrm{cm}}^2$. (b) The change in the $z$ magnetic moment for iron and nickel with the strong fluence pulse (solid lines) and weak laser pulse (dashed lines). (c) The unperturbed modes. The response of these modes to the (d) strong and (e) weaker laser pulses. In (d) only the Goldstone and optical modes survive, whereas in (e) the pure Fe mode is destroyed, while the pure Ni persists.

Figure 4

The canting vector is dependent on the delay time of the laser and is shown relative to the pure Fe mode oscillations. A laser of fluence $0.9537\mathrm{mJ}/{\mathrm{cm}}^2$ is applied at (a) 16.8 fs and (b) 8.4 fs on the pure iron mode. The vertical lines represents the peak of the laser pulse.

Figure 5

The resulting change in the coupled Fe-Ni magnon frequency can be seen in the local moment dynamics following the applied laser pulse (the vertical line corresponds to the peak of the applied laser pulse) for two laser pulses with intensities of (a) $0.9537\mathrm{mJ}/{\mathrm{cm}}^2$ and (b) $9.680\mathrm{mJ}/{\mathrm{cm}}^2$. (c) Fourier transforming the dynamics following the laser pulse allows the altered magnon frequency to be extracted. These new frequencies are plotted against the laser intensity, for which a linear dependence was found.