  The orbital Hall effect provides an alternative means to the spin Hall effect
to convert a charge current into a flow of angular momentum. Recently,
compelling signatures of orbital Hall effects have been identified in 3d
transition metals. Here, we report a systematic study of the generation,
transmission, and conversion of orbital currents in heterostructures comprising
3d, 5d, and 4f metals. We show that the orbital Hall conductivity of Cr reaches
giant values of the order of 5*10^5 Ohm^{-1} m^{-1} and that Pt presents a
strong orbital Hall effect in addition to the spin Hall effect. Measurements
performed as a function of thickness of nonmagnetic Cr, Mn, and Pt layers and
ferromagnetic Co and Ni layers reveal how the orbital and spin currents compete
or assist each other in determining the spin-orbit torques acting on the
magnetic layer. We further show how this interplay can be drastically modulated
by introducing 4 f spacers between the nonmagnetic and magnetic layers. Gd and
Tb act as very efficient orbital-to-spin current converters, boosting the
spin-orbit torques generated by Cr by a factor of 4 and reversing the sign of
the torques generated by Pt. To interpret our results, we present a generalized
drift-diffusion model that includes both spin and orbital Hall effects and
describes their interconversion mediated by spin-orbit coupling.
