High-frequency magnetoacoustic resonance through strain-spin coupling in perpendicular magnetic multilayers INVITED

Abstract

In recent years, magnon-phonon coupling has attracted renewed interest[1] both from the physics and technological perspectives owing to its efficient conversion of energy between magnon and phonon. In particular, the magneto-acoustic resonance, has become a focal point, partly owing to the possibility for rapid switching, which is significant for applications in cloud storage, advanced spin memory, logic, and other spintronic devices. An ultrafast optical approach, specifically femtosecond laser pulses, is a powerful tool to initiate the coupling and its successive detection. The laser pulse simultaneously generates the phonon and magnon, through thermal expansion and fast demagnetization. The thermal expansion produces a local strain or phonon, whose frequency is tunable by both the material composition and geometry. At the same time, the fast demagnetization excites the magnetization out of equilibrium and creates magnons. The femtosecond time scale of the laser pulse benefits the detection of both magnon and phonon, as their dynamics endure longer than the pulse time scale. However, research reported in the literature mostly launch strain electrically in piezoelectric materials, which leads to a different phonon propagation direction. In addition, the literature study of the coupling is confined to materials unsuitable for spintronic applications. These materials, limit the resonance frequency or require low temperature for the magnetoacoustic resonance. To date, direct demonstration of magnon-phonon coupling remains elusive in materials with high perpendicular magnetic anisotropy, which are capable of achieving ultrahigh frequency resonance at room temperature.Here, we report an extremely high frequency magnetoacoustic resonance up to 60 GHz, originating from magnon-phonon coupling in [Co/Pd]n multilayer with perpendicular magnetic anisotropy. Our theory uncovers the physics of resonance in a coupled system, and is based on the equation of motion for both magnon and phonon, with coupling introduced in the Hamiltonian through the magnetostrictive energy. Owing to the magnon and phonon possessing the same symmetry, the physics at resonance presents an enhanced wave envelope in the time domain, an anticrossing in the frequency domain, and significant hybridization of both magnons and phonons. When the frequencies of magnon and phonon approach each other in the resonance regime, these two modes hybridize into a quasiparticle contributed by both magnon and phonon. This hybridization generates an anticrossing in the dispersion with a frequency gap Δf in Fig. (a), which is quantitatively determined by the coupling coefficient in our model and consistent with our experiment. The amplitudes of both modes increase in the hybridization regime in Fig. (b): the original phonon mode is now highly visible owing to the admixed magnon, the original magnon mode is enhanced owing to the pumping from phonons. In addition, the resonance in the time domain (an enhanced wave envelope) indicates the strain substantially influences the spin dynamics at an ultrafast picosecond time scale.All these physical features are demonstrated in both ultrafast optical measurements and micromagnetic simulations. The ultrafast optical measurements [3] of the time-resolved magneto-optical Kerr effect (TR-MOKE) and time-domain thermoreflectance (TDTR) directly observe the dynamics of magnon and phonon. The frequency and the amplitude of TR-MOKE and TDTR signals are consistent with the modeling, which indicates the transition of the system from hybridization to de-hybridization, controlled by the external applied field. The simulations apply the properties of the [Co/Pd]n multilayers, whose structures are optimized to achieve a maximum resonance frequency allowable with current experimental settings. All the [Co/Pd]n multilayers samples were prepared in an ultra-high vacuum magnetron sputtering system. With an appropriate strain, the simulation predicts that the system is able to undergo a large-angle magnetization precession and achieve a rapid switching.In summary, our work demonstrates an efficient energy transfer among magnon and phonon at an ultrafast picosecond scale, which paves a potential pathway for enabling an ultrahigh speed strain-assisted magnetization switching in a technologically relevant magnetic system. **