Anomalous temperature dependence of phonon pumping by ferromagnetic resonance in Co/Pd multilayers with perpendicular anisotropy

The generation of phonons by ferromagnetic resonance in magnetic thin films and subsequent propagation of phonons into the substrate is an effect known as phonon pumping [1,2]. This effect can be utilized in spintronic devices that rely on magnetoelastic coupling for information transfer. The phonon pumping effect is most efficient in the perpendicular configuration, and can therefore be suitably integrated in devices composed of materials with perpendicular magnetic anisotropy (PMA). Here we demonstrate the pumping of phonons by ferromagnetic resonance in a series of [Co(0.8 nm)/Pd(1.5 nm)]_n multilayers (n = 6, 11, 15, and 20) with large magnetostriction and PMA. The effect is shown using broadband ferromagnetic resonance over a range of temperatures (10 to 300 K), where a resonant damping enhancement is observed at frequencies corresponding to standing wave phonons across the multilayer. The strength of this effect is strongly dependent on temperature—approximately a factor of 4 larger at 10 K compared to room temperature. Lastly, we demonstrate that the damping enhancement coincides with a shift in the ferromagnetic resonance field, and further that it is a complementary effect that can be predicted from the resonant damping enhancements. We emphasize that [Co/Pd]_n multilayers are an ideal platform for studying this effect due to their large magnetostriction and PMA. Importantly, the PMA does not show significant dependence on multilayer thickness, so devices can be engineered that have phonon pumping resonances at zero applied field. 

[1] S. Streib, H. Keshtgar, and G. E. W. Bauer, Damping of Magnetization Dynamics by Phonon Pumping, Phys. Rev. Lett. 121, 027202 (2018).

[2] K. An et al., Coherent long-range transfer of angular momentum between magnon Kittel modes by phonons, Phys. Rev. B 101, 060407(R) (2020).