## 3 Results  
### Acoustic Propagation at Interface  
We compared the \(\Delta R\) responses of the Co and Co\({}_{50}\)B\({}_{50}\) transducers (Figure 3a). In the case of the Co transducer, \(\Delta R\) showed a typical temperature response (\(\Delta R\) arising from \(\Delta T\)) without any acoustic oscillations (\(\Delta R\) arising from \(\Delta\epsilon\)): a rapid increase in \(\Delta T\) at zero time delay by the pump pulse, followed by a gradual decrease because of slow heat dissipation to the substrate (Figure 3b). In contrast, the Co\({}_{50}\)B\({}_{50}\) transducer showed a strong acoustic oscillation in addition to the slow temperature response (Figure 3c). The magnitude of acoustic oscillation was similar to that of the temperature signal. By subtracting the temperature signal from the raw data, we were able to extract acoustic oscillations. Then, we compared the acoustic oscillations obtained with different  
Figure 3: \(\Delta R\) comparison of the Co and Co\({}_{50}\)B\({}_{50}\) transducers. (**a**) Schematics for detection of \(\Delta R\), which was caused by changes in the strain (\(\epsilon\)). The blue/orange/gray layers represent the transducer/interface/substrate, respectively. \(\Delta R\) results of the (**b**) sap/Co (10 nm)/Pt (2 nm) and (**c**) sap/Co\({}_{50}\)B\({}_{50}\) (10 nm)/Pt (4 nm) sample. The black squares are experimental data. The black line in (**b**) represents the background temperature signal. Figure 2: The pump-probe experimental set-up. A pulsed laser was split to the pump (thick red line) and probe (thin red line) via a polarizing beam splitter (PBS). The pump and probe were modulated via an electro-optic modulator (EOM) and optical chopper, respectively. The time delay between the pump and probe was controlled by a delay stage. The pump and probe were focused on the transducer surface by a lens. The reflection from the probe, after the non-polarizing beam splitter (BS), was collected by a normal detector (ND) for \(\Delta R\) measurement and by a balanced detector (BD) for \(\Delta\theta\) measurement. For \(\Delta\theta\) measurement, a halfwave plate (HWP) and Wollaston prism (WP) were used to control the polarization of the probe beam. \(\mathrm{Co_{50}B_{50}}\) thicknesses (Figure 4). The oscillations were fitted with a damped cosine function, \(A\cos(2\pi ft)\exp(-t/\tau)\), where \(A\) is the oscillation amplitude,\(f\) is the oscillation frequency, \(t\) is the time delay between the pump and probe, and \(\tau\) is the exponential decay time. A smaller \(\mathrm{Co_{50}B_{50}}\) thickness resulted in a larger \(f\) and smaller \(\tau\). Such a strong oscillation of acoustic waves indicated that acoustic waves were reflected from the sap/\(\mathrm{Co_{50}B_{50}}\) interface with a high reflection coefficient (\(r\)). The reflection of acoustic waves was determined by acoustic impedance, \(Z=\rho v_{\mathrm{s}}\), where \(\rho\) is the density of the material. When two materials with \(Z\) of \(Z_{1}\) and \(Z_{2}\) have infinite thicknesses, \(r\) is expressed as [34,35]  
\[r=\frac{Z_{1}-Z_{2}}{Z_{1}+Z_{2}}. \tag{2}\]  
Because the sapphire and \(\mathrm{Co_{50}B_{50}}\) had a similar \(Z\) value of \(\approx\)4 \(\times\) 10\({}^{7}\) kg m\({}^{-2}\) s\({}^{-1}\), direct contact between them was not able to produce a strong acoustic oscillation. In the following, we show that a high reflection of acoustic waves was achieved due to the presence of a soft intermediate layer between \(\mathrm{Co_{50}B_{50}}\) and sapphire (we did not expect any soft layer at the CoB/Pt interface, because Pt was deposited immediately, in a few seconds, after the CoB deposition inside a vacuum chamber). We extracted the thickness of the soft intermediate layer by analyzing acoustic oscillations. The frequency dependence on the transducer thickness is well understood by the speed of sound of the metal transducer (Figure 5a).