Ultrafast THz probing of nonlocal orbital current in transverse multilayer metallic heterostructures

Abstract

THz generation from femtosecond photoexcited spintronic heterostructures has become a versatile tool for investigating ultrafast spin-transport and transient charge-current in a non-contact and non-invasive manner. The equivalent effect from the orbital degree of freedom is still in the primitive stage. Here, we experimentally demonstrate orbital-to-charge current conversion in metallic heterostructures, consisting of a ferromagnetic layer adjacent to either a light or a heavy metal layer, through detection of the emitted THz pulses. Our temperature-dependent experiments help to disentangle the orbital and spin components that are manifested in the respective Hall-conductivities, contributing to THz emission. NiFe/Nb shows the strongest inverse orbital Hall effect with an experimentally extracted value of effective intrinsic Hall-conductivity, ${\left({\sigma }_{SOH}^{\mathrm{int}}\right)}^{eff}~195{\Omega }^{-1}{cm}^{-1}$, while CoFeB/Pt shows maximum contribution from the inverse spin Hall effect. In addition, we observe a nearly ten-fold enhancement in the THz emission due to pronounced orbital-transport in W-insertion heavy metal layer in CoFeB/W/Ta heterostructure as compared to CoFeB/Ta bilayer counterpart.

By optically driving the magnetization in a magnetic system, terahertz emission can be induced from an adjacent normal metal, as a result of spin-to-charge conversion. Here, Kumar and Kumar successfully show the equivalent effect arising from orbital-to-charge conversion.