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We report about the existence of magneto-acoustic pulses propagating in a 200-nm-thick ferromagnetic nickel film excited with 120 fs laser pulses. They result from the coupling between the magnetization of the ferromagnetic film and the longitudinal acoustic waves associated to the propagation of the lattice deformation induced by the femtosecond laser pulses. The magneto-acoustic pulses are detected from both the front and back sides of the film, using the time-resolved magnetooptical Kerr technique, measuring both the time dependent rotation and ellipticity. We show that the propagating acoustic pulse couples efficiently to the magnetization and is strong enough to induce a precession of the magnetization. It is due to a transient change of the crystalline anisotropy associated to the lattice deformation. It is shown that the results can be interpreted by combining the concepts of acoustic pulse propagation and ultrafast magnetization dynamics.The technology of information and communication constantly needs to improve the speed and density of memory devices. Towards that goal, intense researches are being carried out for manipulating the spins in magnetic materials using various excitation methods like the use of external magnetic field pulses [1][2][3]. Alternatively, femtosecond laser pulses have been utilized to induce an ultrafast demagnetization via a sudden and abrupt change of the temperature of the magnetic material [4-10], or using the inverse Faraday effect [11,12]. This new field of magnetism, named "femtomagnetism" [13,14], uses photons to directly manipulate magnetic structures with a temporal resolution of a few femtosecond. The demagnetization can then be used to modify the anisotropy of the magnetic material [15] which leads to a reorientation of the magnetization vector followed by its precession and damping in the direction opposite to the initial one [10,16,17]. In spite of their versatility, due to the various laser wavelengths and pulse durations, magnetooptical methods are limited by the absorption depth of photons. For application purposes, it is a disadvantage for controlling devices at long distances, particularly in opaque materials like ferromagnetic metals.In the present work we explore an alternative way of controlling the magnetization, based on magnetoacoustic performed at room temperature in ferromagnetic films. It is known that strain pulses, corresponding to a lattice deformation of a material, can be generated with a laser pulse, a subject which has been extensively studied theoretically and experimentally [18][19][20], since the pioneering works of Thomsen et al. [21,22]. The relative amplitude of these strain pulses can be as large as 10 −3 . Such acoustic waves have been used for perturbing the magnetic properties of a dilute magnetic semiconductor material, at low temperature and with a very low efficiency [23]. Here we show that one can efficiently use the strain pulses that propagates over long distances in ferromagnetic metals at room temperature and induce very large ...