We demonstrate a variety of precessional responses of the magnetization to ultrafast optical excitation in nanolayers of Galfenol (Fe,Ga), which is a ferromagnetic material with large saturation magnetization and enhanced magnetostriction. The particular properties of Galfenol, including cubic magnetic anisotropy and weak damping, allow us to detect up to 6 magnon modes in a 120nm layer, and a single mode with effective damping α ef f = 0.005 and frequency up to 100 GHz in a 4nm layer. This is the highest frequency observed to date in time-resolved experiments with metallic ferromagnets. We predict that detection of magnetisation precession approaching THz frequencies should be possible with Galfenol nanolayers.

We demonstrate spin pumping, i.e. the generation of a pure spin current by precessing magnetization, without application of microwave radiation commonly used in spin pumping experiments. We use femtosecond laser pulses to simultaneously launch the magnetization precession in each of two ferromagnetic layers of a Galfenol-based spin valve and monitor the temporal evolution of the magnetizations. The spin currents generated by the precession cause a dynamic coupling of the two layers. This coupling has dissipative character and is especially efficient when the precession frequencies in the two layers are in resonance, where coupled modes with strongly different decay rates are formed.The generation of a spin current (SC) by magnetization precession (MP) is known as spin pumping (SP) [1]. Thereby, the precessing magnetization of a ferromagnetic (FM) film transfers angular momentum to an adjacent material, representing a pure SC that is not accompanied by the flow of charges. SCs generated by SP contain an ac-component at the precession frequency and carry also the MP phase. Conceptually, SP offers a new way of building spintronic devices by flexibly combining conducting and insulating materials [2][3][4][5][6][7][8]. This has stimulated intense efforts aimed at demonstrating SCs in a robust way [9].Conventional SP experiments exploit a ferromagnetic resonance (FMR) where the MP is driven by a microwave field [10]. The transfer of angular momentum to the adjacent material results in enhanced damping of the FMR [11,12] and thus to a broadening of the corresponding resonance spectrum [13,14]. In turn, the SC injected into the adjacent layer can be detected by, for example, the inverse spin Hall effect [2][3][4][5][6][7][8][15][16][17][18][19][20][21][22]. In a spin valve structure consisting of two FM layers separated by a nonmagnetic spacer, the SC generated by one layer drives the magnetization precession of the other layer [23][24][25][26]. At resonance, when the precession frequencies of the FM layers coincide, a strongly coupled collective precessional mode forms [27,28].This conventional approach has a drawback, however: applying monochromatic microwave fields for driving the MP lacks the flexibility required for nanoscale applications, it strictly sets the MP and SC phase, and requires exact matching to the FMR frequency. Ultrafast optical excitation, widely used nowadays in ultrafast optomagnetism for launching MPs [29], is a promising alternative. In metallic FMs, ultrashort laser pulses trigger MP by rapidly alternating the magnetic anisotropy [30]. While laser pulses have been utilized for SC generation via the transport of spin-polarized electrons from an opticallyexcited magnetic region [31][32][33][34][35][36], no evidence of pure SCs generated by optically launched MP has been reported.In this Letter, we report optically excited SP in a pseudo spin-valve (PSV) consisting of two FM layers separated by a normal metal spacer. By femtosecond laser pulses we simultaneously excite MP in the two magnetic layers...

Within a new paradigm for communications on the nanoscale, high-frequency surface acoustic waves are becoming effective data carrier and encoder. On-chip communications require acoustic wave propagation along nano-corrugated surfaces which strongly scatter traditional Rayleigh waves. Here we propose the delivery of information using subsurface acoustic waves with hypersound frequencies ~20 GHz, which is a nanoscale analogue of subsurface sound waves in the ocean. A bunch of subsurface hypersound modes is generated by pulsed optical excitation in a multilayer semiconductor structure with a metallic nanograting on top. The guided hypersound modes propagate coherently beneath the nanograting, retaining the surface imprinted information, on a distance of more than 50 μm which essentially exceeds the propagation length of Rayleigh waves. The concept is suitable for interfacing single photon emitters, such as buried quantum dots, carrying coherent spin excitations in magnonic devices, and encoding the signals for optical communications at the nanoscale.

The rate and pathways of relaxation of a magnetic medium to its equilibrium following excitation with intense and short laser pulses are the key ingredients of ultrafast optical control of spins. Here we study experimentally the evolution of the magnetization and magnetic anisotropy of thin films of a ferromagnetic metal galfenol (Fe0.81Ga0.19) resulting from excitation with a femtosecond laser pulse. From the temporal evolution of the hysteresis loops we deduce that the magnetization MS and magnetic anisotropy parameters K recover within a nanosecond, and the ratio between K and MS satisfies the thermal equilibrium's power law in the whole time range spanning from a few picoseconds to 3 nanoseconds. We further use the experimentally obtained relaxation times of MS and K to analyze the laser-induced precession and demonstrate how they contribute to its frequency evolution at the nanosecond timescale.

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