We report about the magnetization dynamics of a ferromagnetic nickel film at room temperature excited by acoustic pulses generated with femtosecond laser pulses. The ultrafast change of magnetization is detected from both the front and back sides of the nickel film. The propagating strain associated with the acoustic pulses modifies the magnetic anisotropy and induces a precession of the magnetization. We model the time-dependent magnetoacoustic response of the metallic film by combining a three temperature model for the temperatures of the charges, the spins, and the lattice, the wave equation for the strain, and the Landau-Lifshitz-Gilbert equation for the magnetization. It is shown that the precession dynamics can be controlled by matching the precession period with the round trip time of the acoustic echoes. The calculation of the time-dependent precession torque $\tau${}=|M\ifmmode\times\else\texttimes\fi{}Heff| allows understanding the underlying physics.

Controlling the angular momentum of spins with very short external perturbations is a key issue in modern magnetism. For example it allows manipulating the magnetization for recording purposes or for inducing high frequency spin torque oscillations. Towards that purpose it is essential to modify and control the angular momentum of the magnetization which precesses around the resultant effective magnetic field. That can be achieved with very short external magnetic field pulses or using intrinsically coupled magnetic structures, resulting in a transfer of spin torque. Here we show that using picosecond acoustic pulses is a versatile and efficient way of controlling the spin angular momentum in ferromagnets. Two or three acoustic pulses, generated by femtosecond laser pulses, allow suppressing or enhancing the magnetic precession at any arbitrary time by precisely controlling the delays and amplitudes of the optical pulses. A formal analogy with a two dimensional pendulum allows us explaining the complex trajectory of the magnetic vector perturbed by the acoustic pulses.

We have investigated the ultrafast magnetization dynamics of $\text{L}{1}_0$ -ordered ${\text{Fe}}_{50}{\text{Pt}}_{50}$ thin film by means of a time-resolved magneto-optical Kerr effect measurement. We have found a high Gilbert damping value of $\alpha \sim 0.26$ , together with a very high precession frequency of $f\sim 85\text{GHz}$ and the shortest relaxation characteristic time of $\tau \sim 6.5\text{ps}$ ever reported. We believe that $\text{L}{1}_0$ -ordered FePt film with the unique property of a very high precession frequency and the shortest relaxation time will be very useful for the realization of picosecond spin switching.

We report the magnetization dynamics excited by picosecond acoustic pulses confined in the acoustic cavity of freestanding nickel films. By detecting both sides of the sample we show that acoustic pulses moving back and forth inside the cavity efficiently control the magnetization precession. The multiple round trips of acoustic pulses, which survive for 12 bounces, enhance the amplitude of the magnetization precession by a factor of 4.5. Even though the conditions of total reflection are met in the freestanding film, the acoustic pulses have a large attenuation as compared to similar Ni films on $\mathrm{Si}{\mathrm{O}}_2$ substrate, for which we speculate that it is related to a stress induced by the substrate. Our results constitute a guideline for simple designs of acoustic cavities and for achieving a very large amplification of the magnetization precession angle. It is potentially useful for high-frequency magneto-optical modulators.

An ultrafast spin demagnetization process of an amorphous ${\text{Tb}}_{35}{\text{Fe}}_{65}$ alloy film has been investigated by means of an all-optical pump-probe technique. Interestingly, steplike demagnetization on a subpicosecond time scale is observed before a much slower change on a time scale of tens of picoseconds. The steplike demagnetization at the subpicosecond scale is explained by the extended three-temperature model considering the interaction between a nonthermal electron and a spin system. The characteristic of subpicosecond demagnetization of TbFe alloy film is expected to be very useful in the manipulation of the spin state in ultrafast regime.

We have experimentally investigated the field- and fluence-dependent terahertz (THz) emission behaviors by a femtosecond optical pump in Co ferromagnetic films, together with the time-resolved magneto-optical Kerr effect measurement. The thickness of Co and Ta capping layers has been systematically varied. For thick films, THz emission behavior is found to be directly matched with the photoinduced ultrafast demagnetization and remagnetization dynamics. The angle-dependent THz emission measurement also confirms a direct relation between the THz emission and the photoinduced demagnetization/remagnetization dynamics in the case of thick films, while THz emission from the inverse spin Hall effect also plays a role in the case of thin Co films.