Optically excited spin pumping mediating collective magnetization dynamics in a spin valve structure

Abstract

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...

Effect of magnetic anisotropy relaxation on laser-induced magnetization precession in thin galfenol films

“…Finally, we note that plane and patterned thin galfenol films on GaAs substrates are prospective structures for optically-reconfigurable magnonics [46]. Ultrafast laserinduced heating of galfenol resulting in demagnetization and anisotropy changes has been shown to trigger magnetization precession [32], propagating magnetostatic [33,47] and standing spin [31] waves, coupled magnon-phonon modes [48], and spin currents [30]. The relaxation processes investigated here may both facilitate or hinder such processes.…”

“…Galfenol films grown on GaAs substrates exhibit intrinsic cubic magnetocrystalline anisotropy with easy magnetization axes along < 100 > crystallographic directions. Additionally, there is a growthinduced uniaxial anisotropy with the easy axis along the [110] direction [27,29,30]. The films are in-plane magnetized at equilibrium.…”

Effect of magnetic anisotropy relaxation on laser-induced magnetization precession in thin galfenol films

“…Finally, we note that, for optically reconfigurable magnonics [40], plane and patterned thin galfenol films on GaAs substrates are found to be prospective structures. Ultrafast laser-induced heating of galfenol resulting in demag-netization and anisotropy changes has been shown to trigger magnetization precession [32], propagating magnetostatic [33,42] and standing spin [31] waves, coupled magnonphonon modes [49], and spin currents [30]. The relaxation processes investigated here may both facilitate and hinder such processes.…”

“…Galfenol films grown on GaAs substrates exhibit intrinsic cubic magnetocrystalline anisotropy with easy magnetization axes along < 100 > crystallographic directions. Additionally, there is a growth-induced uniaxial anisotropy with the easy axis along the [110] direction [27,29,30]. The films are in-plane magnetized at equilibrium.…”

Nonreciprocal spin-wave dynamics in Pt/Co/W/Co/Pt multilayers

“…Interestingly, weak iDMI was also present in symmetric Pt/Co/Pt multilayers [19] due to different quality of the Co interfaces. The influence of the surface quality can also have a significant impact on the anisotropy of the sample [20].…”