High-frequency magnon excitation due to femtosecond spin-transfer torques

Abstract

Femtosecond laser pulses can induce ultrafast demagnetization as well as generate bursts of hot electron spin currents. In trilayer spin valves consisting of two metallic ferromagnetic layers separated by a nonmagnetic one, hot electron spin currents excited by an ultrashort laser pulse propagate from the first ferromagnetic layer through the spacer reaching the second magnetic layer. When the magnetizations of the two magnetic layers are noncollinear, this spin current exerts a torque on magnetic moments in the second ferromagnet. Since this torque is acting only within the sub-ps timescale, it excites coherent high-frequency magnons as recently demonstrated in experiments. Here, we calculate the temporal shape of the hot electron spin currents using the superdiffusive transport model and simulate the response of the magnetic system to the resulting ultrashort spin-transfer torque pulse by means of atomistic spin-dynamics simulations. Our results confirm that the acting spin-current pulse is short enough to excite magnons with frequencies beyond 1 THz, a frequency range out of reach for current induced spin-transfer torques. We demonstrate the formation of thickness dependent standing spin waves during the first picoseconds after laser excitation. In addition, we vary the penetration depth of the spin-transfer torque to reveal its influence on the excited magnons. Our simulations clearly show a suppression effect of magnons with short wavelengths already for penetration depths in the range of 1 nm confirming experimental findings reporting penetration depths below $2\mathrm{nm}$.

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• Laser-induced torques in metallic antiferromagnets
• Effective exchange interaction for terahertz spin waves in iron layers
• Néel-Vector Switching and THz Spin-Wave Excitation in Mn${}_2$Au due to Femtosecond Spin-Transfer Torques
• Fate of entanglement in magnetism under Lindbladian or non-Markovian dynamics and conditions for their transition to Landau-Lifshitz-Gilbert classical dynamics
• Quantum-classical approach to spin and charge pumping and the ensuing radiation in THz spintronics: Example of ultrafast-light-driven Weyl antiferromagnet Mn${}_3$Sn
• Theory of superdiffusive spin transport in noncollinear magnetic multilayers
• Melting of magnetic order in ${\mathrm{NaOsO}}_3$ by fs laser pulses
• Optically induced magnetic inertia and magnons from non-Markovian extension of the Landau-Lifshitz-Gilbert equation
• Optimizing spin-based terahertz emission from magnetic heterostructures