The effect of dynamical compressive and shear strain on magnetic anisotropy in a low symmetry ferromagnetic film

Abstract

Dynamical strain generated upon excitation of a metallic film by a femtosecond laser pulse may become a versatile tool enabling control of magnetic state of thin films and nanostructures via inverse magnetostriction on a picosecond time scale. Here we explore two alternative approaches to manipulate magnetocrystalline anisotropy and excite magnetization precession in a low-symmetry film of a magnetic metallic alloy galfenol (Fe,Ga) either by injecting picosecond strain pulse into it from a substrate or by generating dynamical strain of complex temporal profile in the film directly. In the former case we realize ultrafast excitation of magnetization dynamics solely by strain pulses. In the latter case optically-generated strain emerged abruptly in the film modifies its magnetocrystalline anisotropy, competing with heat-induced change of anisotropy parameters. We demonstrate that the optically-generated strain remains efficient for launching magnetization precession, when the heat-induced changes of anisotropy parameters do not trigger the precession anymore. We emphasize that in both approaches the ultrafast change of magnetic anisotropy mediating the precession excitation relies on mixed, compressive and shear, character of the dynamical strain, which emerges due to low-symmetry of the metallic film under study.

Generation of spin waves via spin-phonon interaction in a buried dielectric thin film

“…Based on these observations, we can conclude that the highest frequency mode in the TR-MOKE is due to the propagation of the longitudinal acoustic pulse in the GGG substrate, since it has the same frequency as the Brillouin oscillation. We note that the observation of Brillouin oscillations in a TR-MOKE signal was previously reported in the literature [29][30][31]56]. It can be understood considering a small difference in the reflection of right (σ + ) and left (σ − ) helicity of light induced by the acoustic pulse during its propagation in the sample [30,56].…”

“…For instance, the propagation of an ultrashort hot-electron pulse in conducting magnets can induce an ultrafast demagnetization [24][25][26] and a full magnetization reversal [25,26] via thermal effects. Picosecond acoustic pulses can trigger a homogeneous collective spin oscillation (k = 0, i.e., ferromagnetic resonance mode) via inverse magnetostriction in conducting [27][28][29], semiconducting [30], and insulating [31] magnetic materials. Very recently, the geometry for exciting spin dynamics indirectly via pumping a Pt/Cu bilayer that protects a magnetic layer from optical excitation [24,25] was investigated by ultrafast x-ray diffraction (UXRD) [32].…”

Picosecond acoustic-excitation-driven ultrafast magnetization dynamics in dielectric Bi-substituted yttrium iron garnet

“…4), substantiate this interpretation. Indeed, a decrease of the magnetocristalline parameters induced by heating effects does not allow exciting the FMR mode when the magnetization is aligned along H ext [25]. Let us also mention that the non-thermal mechanisms based on the Cotton-Mouton effect [55] and photo-induced magnetic anisotropy [12,32] usually used to induce in magnetic garnet a magnetization precession with a linearly polarized light can be excluded in our case.…”

Femtosecond Laser-Excitation-Driven High Frequency Standing Spin Waves in Nanoscale Dielectric Thin Films of Iron Garnets

“…This clearly demonstrates that the pump polarization-dependent mechanisms such as the photoinduced magnetic anisotropy [22][23][24][25] and the Cotton-Mouton effect [28,29] are not at the origin of the excitation. The insensitivity of the excitation on the polarization of the pump suggests that the photons are absorbed and induce an ultrafast change of the magnetic anisotropy via incoherent or coherent phonons, i.e., by ultrafast heating of the lattice [18,[37][38][39] and/or due to inverse magnetostriction induced by a hypersound wave propagating into the film [40][41][42][43][44]. In Bi-substituted iron garnets, the heat energy induced by a pump pulse with a wavelength of 800 nm is caused by the excitation of the phonon assisted 6 S → 4 G and 6 S → 4 P electronic d − d transitions simultaneously by one-and two-photon absorption processes [45].…”