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 magneto crystalline 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]. I will 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 [4]. 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. This long range coherent transport of spin by phononic angular momentum can add new functionalities to insulator spintronic circuits and devices. If time allows, I will also discuss my previous work on the nonequilibrium between magnons and phonons [5]. Here the local nonequilibrium is created optically within a focused laser spot and probed directly via micro-Brillouin light scattering. Through analyzing the deviation in the magnon number density from the local equilibrium value, we obtain the diffusion length of thermal magnons. By explicitly establishing and observing local nonequilibrium between magnons and phonons, this study represent an important step toward a quantitative understanding of various spin-heat coupling phenomena.