Long range coupling of magnetic bilayers by coherent phonons

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Recent progresses in quantum information technologies have demonstrated the benefits of leveraging coherence effects such as collective spin precession in ferromagnets in order to process information more efficiently. This coherent transfer between different waveforms is an important ingredient of the quantum information processing as it allows transport of quantum states without loss of frequency or phase. An efficient hybridization process requires to reach a strong coupling regime, where the interaction rate between two collective states becomes larger than their relaxation rates. While increasing the overlap integral between two spatio-temporal patterns is primarily a design issue, diminishing the internal friction inside an object is a much more complex task, which requires know-how in material research and growth technology. In spintronics, this has revived interest for insulating materials and in particular garnets, which are the magnetic materials benefiting from the lowest magnetic damping. The sound wave attenuation coefficient in garnets is also exceptional, i.e. up to an order of magnitude lower than that in single crystalline quartz.In addition to the low damping of magnetic and sound waves, a strong coupling can be established between spin-waves (magnons) and lattice vibrations (phonons) through the magnetic anisotropy and strain dependence of the magnetocrystalline energy in magnetic garnets. The magnetoelasticity leads to new hybrid quasiparticles (“magnon polarons”) when spin wave and acoustic wave dispersions cross [1]. This coupling has been exploited in the past to produce microwave acoustic transducers [2]. The adiabatic conversion between magnons and phonons in magnetic field gradients proves their strong coupling in yttrium iron garnet (YIG) [3].Here we first demonstrate that the spin waves can be strongly coupled to coherent transverse sound waves that have very long characteristic decay length and propagate ballistically over millimetric distances. The experiment was performed at room temperature with a magnetic field applied perpendicular to the film. Our sample consists of two 200 nm thick YIG layers deposited on both sides of a 0.5 mm thick gadolinium gallium garnet (GGG) substrate. The circularly polarized standing sound waves couple to the magnetization oscillations in both layers. An interference pattern is observed and it is explained as the strong coupling of the magnetization dynamics of the two YIG layers either in phase or out of phase by the standing transverse sound waves [4]. This long range coherent transport of spin by phononic angular momentum can add new functionalities to insulator spintronic circuits and devices.Furthermore, such mediation of coherent spins could induce a novel dynamic exchange coupling in a spin-valve system when the magnetic resonances of two layers are brought within the strength of the magnetoelastic coupling. To detect such coupling, a Pt strip electrode was deposited on one YIG layer to monitor its dynamics through the inverse spin Hall effect (ISHE). This improves the measurement sensitivity of the phonon mediated spin transfer. In our sample, the two YIG layers have initially different spontaneous magnetizations. We tune the difference between two magnetic resonances either i) by varying the polar angle of an external magnetic field or ii) by applying a temperature gradient along the stack direction. The existence of the novel dynamic coupling is evidenced by an emergence of the “bright” and “dark” collective states in both measurements. Our observation of such collective modes resembles the level attraction/repulsion behavior between two hybridized modes [5]. This feature is similar to the coupling control via microwave phase [6]. We show that the observed behavior is in agreement with theoretical predictions.Finally we discuss the detection of heat carried by coherent phonons. We measure thermal voltages across Pt in YIG|GGG|Pt system, where no ISHE voltage should be present. The signal appears only at the acoustic phonon resonances indicating that the heat can be efficiently carried by circularly polarized coherent phonons excited by magnons. When the magnetic field direction is tilted away from the film normal direction, the signal vanishes, suggesting that the heat carried by acoustic phonons becomes less coherent. We perform several control experiments, which shows the universality of the signal and possible enhancement schemes. **