Ballistic-to-diffusive transition of ultrafast orbital current transport in nickel

Understanding the transport dynamics of orbital angular momentum in solids is crucial for the development of next-generation spintronic devices. Here, we provide experimental evidence of a ballistic-to-diffusive transition of orbital current transport in the ferromagnetic metal Ni. By systematically varying the Ni thickness and probing the resulting terahertz emission, we observe abrupt changes in both the delay and amplitude of the extracted ultrafast charge current. These changes are indicative of a transition from ballistic to diffusive orbital transport within the Ni layer. A numerical model is constructed to explain the observed delay, yielding a critical length of ballistic-to-diffusive transition ($\ensuremath{\sim}4.05\phantom{\rule{0.16em}{0ex}}\mathrm{m}$). Our findings advance the understanding of orbital current transport in ferromagnetic metals and pave the way to designing novel functional orbitronics devices.