Optical excitation of single-and multi-mode magnetization precession in Fe-Ga nanolayers

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 4-nm 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. Within the last decade magnetization precession has become an actively exploited tool in nanoscale magnetism. The precessing magnetization of a ferromag-net is an effective, tunable and nanoscopic source of microwave signals. Generation of microwave magnetic fields by precessing magnetization is already implemented in magnetic storage technology such as microwave assisted magnetic recording (MAMR) [1] by means of spin-torque nano-oscillators [2]. Spin waves or magnons, i.e. the waves of precessing magnetization, are information carriers and encoders in magnonics [3] aimed to substitute conventional CMOS technology. The precessing magneti-zation is also an effective tool to generate a pure spin current in a nonmagnetic material by means of spin pumping [4]. The common way to excite magnetization precession in a ferromagnet is the technique of ferromagnetic resonance (FMR). A monochromatic microwave magnetic field drives the magnetization precession, the frequency of which is tuned into resonance with the microwaves by an external magnetic field. This technique, which can provide comprehensive information about the main precession parameters, is not adaptable for practical use with nanostructures due to the need of bulky electromagnetic resonators and waveguides. An alternative approach is broad-band excitation induced by dc-current [5], picosecond magnetic field pulses [6], ultrashort laser pulses [7] and strain pulses [8]. In those cases the parameters of the excited magnetization precession, i.e. the spectral content, lifetime, spatial distribution and their dependences on external magnetic field, are determined by the properties of the ferromagnetic material and the design of the nanostructure [9]. The ability to control these dynamical parameters is of crucial importance for nanoscale magnetic applications. For practical use, an ideal combination of dynamical parameters includes a tunable and narrow spectral band in the GHz and THz frequency ranges; large saturation magnetization and high precession amplitude for high microwave power; and ultrafast triggering for high-frequency modulation. Achieving this combination has been an unmet challenge until now. High precession frequency, f ≫ 10 GHz, can be reached by using ferrimagnetic materials [10, 11], but the weak net magnetization limits their functionality. In the case of metallic ferromagnets with large net magne-tization, the direct way to achieve high frequency pre-cession is to apply a strong external magnetic field, B, which, however, drastically decreases the precession amplitude. Earlier experiments on the excitation of magne-tization precession in metallic ferromagnets by femtosec-ond optical pulses [7, 12–18], i.e. the fastest method of launching precession, report also high values of the effective damping coefficient α ef f = (2πτ f) −1 > 0.01 (τ is the precession decay time). Thus, the excitation and detection of sub-THz narrow band precession in metallic ferromagnets remains extremely challenging. In the present letter, we report the results of ultrafast magneto-optical experiments with nanolayers of (Fe,Ga), i.e. Galfenol. This metallic ferromagnet with large net magnetization is considered as a prospective material for microwave spintronics due to the narrow ferromagnetic resonance [19, 20] and enhanced magnetostriction [21], which allows manipulation of the magnetization direction and precession frequency by applying stress, i.e. without changing the external magnetic field [19, 22]. Our study extends significantly the application potential of Galfenol. We show that in a Galfenol layer with a thickness of several nanometers, the femtosecond optical ex-citation leads to the generation of single-mode magneti-zation precession with frequency f > 100 GHz and large amplitude. Despite the strong interaction between the magnetization and the lattice, we observe a weak damping of precession with α ef f ≈ 0.005. Thus, we demonstrate the possibility to achieve the desirable combination of sub-THz magnetization precession with large amplitude and tunable narrow spectral band. Moreover, w