Superlattices presenting a modulation of the elastic properties appear as a main tool to reach the THz regime in nanoacoustic devices. The exploration of alternative materials with multifunctional properties remains a fertile domain of research. In this work, we have investigated the structural, magnetic, and acoustic properties of Pt/Co-based ferromagnetic superlattices with varying cobalt thicknesses.
The Pt/Co superlattices were deposited by magnetron sputtering on Si substrates. X-ray reflectometry and scanning transmission electron microscopy (STEM) imaging confirmed the periodic structure of the Pt/Co layers with sub-5 nm-thick period thickness. STEM with local compositional analysis revealed that the samples present a modulation in composition instead of sharp interfaces, due to the effect of roughness and atomic interdiffusion. The polycrystalline nature of the superlattices is evidenced by X ray diffraction and TEM. These superlattices exhibit out-of-plane magnetization hysteresis, with a perpendicular magnetic anisotropy strongly dependent on the Co layer thickness.
We employed a pump-probe setup to study the coherent acoustic phonon dynamics in the metallic superlattices. Picosecond acoustic experiments demonstrated the generation and detection of short-lived ultrahigh-frequency acoustic phonons close to 900 GHz, along with up to 7 acoustic echoes at frequencies below 300 GHz. Simulations based on transfer matrix method and finite element method were performed to characterize the acoustic modes and their dynamics, further supporting our experimental results. The lower frequency peaks correspond to acoustic modes related to the total thickness of the sample, which acts as an acoustic resonator with a fundamental frequency of approximately 40 GHz. The high frequency peaks are linked to the superlattice modes at the Brillouin zone center with displacement profiles commensurate with the period of the superlattice. The cobalt thickness dependence of phonon modes highlights the ability of these structures for phonon engineering at the nanoscale, compatible with the stringent nanophononic requirements. Significant acoustic absorption in Pt/Co superlattices remains challenging for extending oscillation cycles of the acoustic response.
Acknowledgement
The authors acknowledge support from the C.N.R.S. International Research Project Phenomenas. E.R.C. de O., C.X., S.S., and N.D.L.-K. acknowledge funding from European Research Council Consolidator Grant No.101045089 (T-Recs). M.G. and J.E.G acknowledge support from MSCA-RISE-H2020 ULTIMATE-I-Project No 101007825 funded by the E. U. The authors acknowledge the French RENATECH network which partly funds the XRD studies, FIB preparation and TEM observations carried out in this work.