Spin Wave Phenomena in THz-Spin Dynamics
An important topic in magnetism is the spin wave phenomena that can lead to practical applications in spin-electronics. Recently, much attention has been paid to optically generate spin waves via inverse Faraday effect or laser-induced local heating. Our recent finite difference time-domain (FDTD) calculations reveal that the resonant plasmonic nanoantennas are capable of generating intense and ultrashort magnetic field pulses in the frequency regime of 100 Terahertz (THz). The THz magnetic field generated by nanoantennas is spatially concentrated within a nanoscale region, with its intensity increased by more than two orders of magnitude compared to that of the incident light. Given the highly localized confinement of the intense magnetic field, the nanoantenna can be used as a new nanoscale source of THz magnetic field pulses, which can locally manipulate the magnetism within a very short time scale. Micromagnetic numerical results demonstrate the excitation of the THz magnetization oscillation in a ferromagnetic nanoelement, which is coherently coupled to the magnetic field. After the termination of THz magnetization dynamics, propagating spin waves emerge from the excitation field region and travel perpendicular to the static magnetization direction. The result demonstrates the potential of resonant plasmonic nanoantennas in optically triggering the THz magnetization dynamics and subsequently generating spin waves in the GHz frequency domain. The result opens up an interesting perspective for applying plasmonic nanoantennas in the ultrafast optical manipulation of magnetism on the nanometer scale.
Ultrafast Optical Manipulation of Magnetism
Plasmonics is a rapidly developing research field. Much attention has been drawing to this field due to the fundamental interest in light-matter interaction and due to a variety of promising applications, such as cancer therapy. Recently, an increasing number of magneto-plasmonic studies have emerged, focusing on combining magnetic and plasmonic functionalities. Of particular relevance to information technology is that the incorporation of plasmonic nanostructures to magnetic systems can lead to a significant enhancement of magneto-optical (MO) response.
Recently, we investigated ultrafast laser-induced magnetization dynamics in ferromagnetic thin films using a femtosecond Ti:sapphire laser in a pump-probe magneto-optic Kerr effect setup. The effect of plasmon resonance on the transient magnetization was investigated by drop-coating the ferromagnetic films with dimensionally-tuned gold nanorods supporting longitudinal surface plasmon resonance near the central wavelength of the pump laser. With ~4% nanorod areal coverage, we observe a > 50% increase in demagnetization signal in nanorod-coated samples at pump fluences on the order of 0.1 mJ/cm2 due to surface plasmon-mediated localized electric-field enhancement, an effect which becomes more significant at higher laser fluences. We were able to qualitatively reproduce the experimental observations using finite-difference time-domain simulations and mean-field theory. This dramatic enhancement of ultrafast laser-induced demagnetization points to possible applications of nanorod-coated thin films in heat-assisted magnetic recording.