Abstract
We demonstrate sustained coherent emission of spin waves in NiFe films using rapid demagnetization from high repetition rate femtosecond laser pulse trains. As the pulse separation is shorter than the magnon decay time, magnons having a frequency equal to a multiple of the 1 GHz repetition-rate are coherently amplified. Using scanning micro-Brillouin Light Scattering (BLS) we observe this coherent amplification as strong peaks spaced 1 GHz apart. The BLS counts vs. laser power exhibit a stronger than parabolic dependence consistent with counts being proportional to the square of the magnetodynamic amplitude, and the demagnetization pulse strength being described by a Bloch law. Spatial spin wave mapping demonstrates how both localized and propagating spin waves can be excited, and how the propagation direction can be directly controlled. Our results demonstrate the versatility of BLS spectroscopy for rapid demagnetization studies and enable a new platform for photo-magnonics where sustained coherent spin waves can be utilized.
Highlights
Magnonics has emerged as a central research topic in nanomagnetism, with rich physics and an increasing number of novel phenomena thanks to the unique field-tunable properties of spin waves (SWs) and a wide range of metallic and insulating magnetic materials [1,2,3,4]
To better discern all features of the laser-induced SW intensity, we show in Fig. 3 a detailed plot of the Brillouin light scattering (BLS) counts vs frequency at a field magnitude of 600 mT for four different
The highly efficient SW generation only occurs at higher harmonics of the 1 GHz laser repetition rate, where the SWs that are in phase with the incoming laser pulses are coherently amplified, while all other SWs are left unaffected
Summary
Magnonics has emerged as a central research topic in nanomagnetism, with rich physics and an increasing number of novel phenomena thanks to the unique field-tunable properties of spin waves (SWs) and a wide range of metallic and insulating magnetic materials [1,2,3,4]. As the typical SW decay time in ferromagnetic metals is a few nanoseconds, any attempt at generating continuous spin waves using optical means requires much shorter pulse separation in order to overcome the damping. The inverse Faraday effect was used for excitation with a 10 μm laser spot size, and a conventional time-resolved magneto-optical Kerr effect (TR-MOKE) pump-probe technique was used for detection. We demonstrate how frequency combs driving thermal rapid demagnetization can be used to excite sustained SWs in ferromagnetic metal thin films at length scales down to the laser diffraction limit. Using a unique Brillouin light scattering (BLS) microscope, where we combine a diffractionlimited BLS SW detection scheme with a diffraction-limited high repetition rate (1 GHz) fs laser, we demonstrate continuous and directional coherent SW emission over a wide range of magnetic fields and frequencies. Our results clearly demonstrate the versatility and benefits of using frequency combs and spatially resolved BLS microscopy for the study of rapid demagnetization in ferromagnetic metals
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