A.V. Scherbakov : Manipulating magnetization by coherent phonons in nanometer ferromagnetic films

The coupling of elasticity and magnetization in ferromagnetic materials is well known and widely used in many everyday applications since the discovery of magnetostriction by James Joule. In our experiments, we scale the magnetostrictive effects down to nanometers and up to GHz by using the methods of the ultrafast acoustics, which allow generating of the picosecond strain pulses and monochromatic phonons in the GHz-THz frequency range. We demonstrate that this approach provide us with wide abilities to manipulate magnetization on the nanoscale.

In the basic experimental scheme, the picosecond strain pulse injected into a nanometer ferromagnetic layer modifies the magneto-crystalline anisotropy (MCA) of a ferromagnet and launches the precession of magnetization, which we monitor in time domain by the transient Kerr rotation technique. This approach has been approved in experiments with thin films of ferromagnetic semiconductors and metals. The parameters of the excited magnetization precession (e.g. amplitude, spectral content, lifetime etc.) depend on external magnetic field and on the parameters of acoustic excitation. We have found out that by adjusting these two factors and by utilizing of peculiar acoustic properties of a ferromagnetic structure, we can widely manipulate the magnetization precessional response. By exploiting the boundary conditions for both magnetization and strain, we may selectively excite a certain standing spin wave among broad magnon spectra. The picosecond shear strain pulses allow archiving high amplitude of precession, which exceeds 10% of the saturation magnetization. Finally, we demonstrate that monochromatic phonons localized in a ferromagnetic acoustic nanocavity may drive the magnetization precession resonantly. In this experiment, the structure studied is 56-nm ferromagnetic layer of Galfenol (Fe_0.81Ga_0.19) grown on semiconductor superlattice, which play a role of acoustic Bragg mirror. Such a structure possesses several Eigen phonon modes in 10-40 GHz range, which may be excited by a femtosecond optical pulse. By applying an external magnetic field, we tune the precession frequency relative to the frequency of the localized phonons and observe the enormous increase in the amplitude of the magnetization precession when the frequencies of free magnetization precession and phonon modes are equal.