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Based on micromagnetic simulations and experimental observations of the magnetization and lattice dynamics after the direct optical excitation of the magnetic insulator Bi : YIG or indirect excitation via an optically opaque Pt/Cu double layer, we disentangle the dynamical effects of magnetic anisotropy and magneto-elastic coupling. The strain and temperature of the lattice are quantified via modeling ultrafast x-ray diffraction data. Measurements of the time-resolved magneto-optical Kerr effect agree well with the magnetization dynamics simulated according to the excitation via two mechanisms: the magneto-elastic coupling to the experimentally verified strain dynamics and the ultrafast temperature-induced transient change in the magnetic anisotropy. The numerical modeling proves that, for direct excitation, both mechanisms drive the fundamental mode with opposite phase. The relative ratio of standing spin wave amplitudes of higher-order modes indicates that both mechanisms are substantially active.
Controlling the magnetization dynamics at the fastest speed is a major issue of fundamental condensed matter physics and its applications for data storage and processing technologies. It requires a deep understanding of the interactions between the degrees of freedom in solids, such as spin, electron, and lattice as well as their responses to external stimuli. In this paper, we systematically investigate the fluence dependence of ultrafast magnetization dynamics induced by below-bandgap ultrashort laser pulses in the ferrimagnetic insulators BixY3-xFe5O12 with 1 xBi 3. We demonstrate subpicosecond demagnetization dynamics in this material followed by a very slow remagnetization process. We prove that this demagnetization results from an ultrafast heating of iron garnets by two-photon absorption (TPA), suggesting a phonon-magnon thermalization time of 0.6 ps. We explain the slow remagnetization timescale by the low phonon heat conductivity in garnets. Additionally, we show that the amplitudes of the demagnetization, optical change, and lattice strain can be manipulated by changing the ellipticity of the pump pulses. We explain this phenomenon considering the TPA circular dichroism. These findings open exciting prospects for ultrafast manipulation of spin, charge, and lattice dynamics in magnetic insulators by ultrafast nonlinear optics.
Manipulating spin waves is highly required for the development of innovative data transport and processing technologies. Recently, the possibility of triggering high-frequency standing spin waves in magnetic insulators using femtosecond laser pulses was discovered, raising the question about how one can manipulate their dynamics. Here we explore this question by investigating the ultrafast magnetiza-tion and spin-wave dynamics induced by double-pulse laser excitation. We demonstrate a suppression or enhancement of the amplitudes of the standing spin waves by precisely tuning the time delay between the two pulses. The results can be understood as the constructive or destructive interference of the spin waves induced by the first and second laser pulses. Our findings open exciting perspectives towards generating single-mode standing spin waves that combine high frequency with large amplitude and low magnetic damping.