Measurements of sound velocity in iron–nickel alloys by femtosecond laser pulses in a diamond anvil cell

By comparing the seismic wave velocity profile in the Earth with laboratory data of the sound velocity of iron alloys, we can infer the chemical composition of materials in the Earth’s core. The sound velocity of pure iron (Fe) has been sufficiently measured using various techniques, while experimental study on the sound velocity of iron–nickel (Fe–Ni) alloys is limited. Here, we measured longitudinal wave velocities of hexagonal-close-packed (hcp) structured Fe up to 29 GPa, Fe–5 wt% Ni, and Fe–15 wt% Ni up to 64 GPa via a combination of the femtosecond pulse laser pump–probe technique and a diamond anvil cell at room temperature condition. We found that the effect of Ni on the sound velocity of an Fe-based alloy is weaker than that determined by previous experimental study. In addition, we obtained the parameters of Birch’s law to be VP = 1146(57)ρ − 3638(567) for Fe–5 wt% Ni and VP = 1141(45)ρ− 3808(446) for Fe–15 wt% Ni, respectively, where VP is longitudinal wave velocity (m/s) and ρ is density (g/cm3).