## 2 Materials and Methods  
We fabricated the sap/Co (10 nm)/Pt (2 nm) and sap/Co\({}_{50}\)B\({}_{50}\)/(5-15 nm)/Pt (4 nm) structure using a magnetron sputter with a base pressure of <2 \(\times\) 10\({}^{-7}\) Torr. The sap layer was a sapphire substrate with an orientation of (0001). The Co or Co\({}_{50}\)B\({}_{50}\) layer acted as a transducer for acoustic waves. The Pt layer acted as a layer that protected Co or Co\({}_{50}\)B\({}_{50}\) from oxidation. All metal layers were deposited with an Ar pressure of 3 mTorr, a target-to-substrate distance of 100 mm, and a typical deposition rate of 0.02 nm s\({}^{-1}\). The deposition rate of each layer was predetermined with a single, thick layer, whose thickness was determined by X-ray reflectivity measurements; then, the thickness of each layer was controlled by deposition time. We used a pure element target for the deposition of the Co and Pt layers and a CoB alloy target with an equiatomic composition for deposition of the Co\({}_{50}\)B\({}_{50}\) layer. All layers were deposited at room temperature. The lateral sizes of all samples were around 10 mm. We did not use a special treatment, such as chemical cleaning or pre-annealing, for the substrate. Therefore, a thin, soft layer, such as a polymer, naturally formed on top of the substrate. However, such a thin, soft layer is usually destroyed during the sputtering process because of the high kinetic energy of the sputtered elements, and thus a strong coupling between the deposited layer and substrate is usually achieved. The kinetic energy of the sputtered elements depends critically on the surface binding energy, as follows:  
\[f(E)\propto\frac{E}{\left(E+E_{\text{sb}}\right)^{3}}, \tag{1}\]  
where \(f(E)\) is the number fraction of sputtered elements, \(E\) is the kinetic energy of the sputtered elements, and \(E_{\text{sb}}\) is the surface binding energy of the target material with a typical magnitude of a few electronvolts [30]. The schematics for the sputtering process and energy distribution of sputtered elements are shown in Figure 1.  
To investigate the propagation of acoustic waves at the interface, we measured the change of reflectivity (\(\Delta R\)) from the transducer layer as a function of time [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. When a pump pulse thermally excites the metal transducer, a strain pulse is introduced in the transducer. A probe pulse has a time delay from the pump pulse, measured as \(\Delta R\), caused by the changes of temperature (\(\Delta T\)) and strain (\(\Delta\varepsilon\)) of the transducer layer. \(\Delta R\) arising from \(\Delta\varepsilon\) is easily distinguished from that arising from \(\Delta T\) because \(\Delta\varepsilon\) oscillates rapidly, whereas \(\Delta T\) changes slowly. \(\Delta R\) arising from \(\Delta T\) can be employed to investigate the heat transport from the transducer layer to substrate [31]. However, when the transducer is thinner than the penetration depth of the probe light, \(\Delta T\) corresponding to both the transducer layer and substrate affects \(\Delta R\); thus, the thermal analysis becomes extremely complicated. To investigate the heat transport at the interface, we measured the magneto-optic Kerr effect (MOKE) from the transducer layer as a function of time [32; 33]. When the transducer layer is a ferromagnet, the polarization of the reflected probe light rotates, and the rotation angle (\(\Delta\theta\)) is proportional to the change in magnetization (\(\Delta M\)). When \(\Delta M\) is caused by heating and the resulting \(\Delta T\) is small enough, a linear relationship between \(\Delta\theta\) and \(\Delta T\) is achieved [32; 33]. Because the sapphire substrate was not a ferromagnet, only the transducer layer contributed to \(\Delta\theta\) regardless of the transducer thickness. For the pump and probe pulses, we used a Ti:sapphire laser oscillator that produced 0.1 ps laser pulses with a repetition rate of 80 MHz and a wavelength of 785 nm.