Resonant elastic driving of magnetization precession in ferromagnetic nanogratings
We report the ultrafast experiments with planar metallic ferromagnetic nanostructures, in which we realize the resonant driving of magnetization precession by surface elastic waves of hypersound frequencies (1010 Hz). This work continues our ultrafast magneto-acoustic studies started in 2010  and demonstrates ability to spatially localize the magnetization precession by its coupling with high-frequency coherent elastic excitations in an optically excited ferromagnet.
The planar nanostructures for our studies were produced by focus ion beam nanolithography on a film of ferromagnetic galfenol (Fe0.81Ga0.19) of 100-nm thickness grown on (100)-GaAs substrate. The grooves of 40 nm depth were etched to from a periodic grating. Several structures of 5x5-microns lateral size with different parameters were produced: groove’ direction was either parallel to  or  crystallographic direction, the groove width/grating period were 40/150 and 50/190 nm.
At the experiment performed in a conventional pump-probe scheme, we use femtosecond pulses of an Yb:KGW regenerative amplifier (1030 nm wavelength, 200 fs pulse duration, 5 KHz repetition rate) to excite and detect the magneto-elastic response. The pump pulse was focused to the spot of 100 microns (excitation density was up to 20 mJ/cm2) and, thus, covers both the patterned and non-patterned parts of the film. The linearly polarized probe pulse focused by means of 50x microobjective to the spot of submicron size allows selective detection of the magneto-elastic kinetics either from the patterned structure or the non-patterned film area. We monitor the rotation of the probe polarization plane (magneto-optical Kerr effect) as a function of delay between the pump and probe pulses.
The non-patterned film demonstrates a typical response on optical excitation with an oscillating Kerr rotation signal, which reflects the precession of magnetization excited due to the ultrafast changes of the magneto-crystalline anisotropy (MCA). The dependences of the precession frequency (i.e. the ferromagnetic resonance frequency) and amplitude on direction and strength of external magnetic field, B, are typical for a metal ferromagnetic film and determined by the MCA. At B|| we observe the magnetization precession with frequency linearly increasing with B and with weak nonmonotonic field dependence of the precession amplitude. At B|| we detect no excitation of the magnetization precession. The same results in non-patterned galfenol films were obtained in our previous ultrafast magneto-optical experiments performed without submicron spatial resolution .
The main result of the present work is the drastically different magnetization response observed in the patterned structures. In the structures with the grating oriented along  crystallographic direction at B|| we, contrary, observe the long-living (up to 3 ns) precessional response. The detected frequency (15 GHz for the grating period of 150 nm) is independent of B, while the amplitude demonstrates strong field dependence. The sharp maximum of the precession amplitude is achieved at B=150 mT, which corresponds to the ferromagnetic resonance frequency of 15 GHz. Such a behavior demonstrated before in our experiments with ferromagnetic nanolayer embedded into phononic Fabry-Perot resonator indicates the resonant driving of magnetization precession by localized elastic excitations . The surface acoustic wave (SAW) with the wavevector determined by the grating period is optically excited in a patterned structure. Due to large depth of the grating, the SAW velocity in a patterned area is significantly lower than in non-patterned one  and, thus, the SAW remains localized. This results in resonant driving of the magnetization precession when the resonant conditions controlled by external magnetic field are fulfilled.
This research is result of collaboration between the Laboratory of the Physics of Ferroics “Ferrolab” of the Ioffe Institute (Anastasya Rudkovskaya, Alexey Salasyuk and Alexey Scherbakov), the Institute of Nanotechnology for Microelectronics of the Russian Academy of Science (Polina Nekludova, Sergey Sokolov and Andrey Elistratov), the Lashkaryov Institute of the National Academy of Science, Ukraine (Boris Glavin) and the University of Nottingham, UK (Andrey Akimov and Andrew Rushforth).
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