Terahertz emission from CoFeB/Cr/Pt trilayers: The role of Cr as both a spin current transporter and generator

Terahertz (THz) emission from CoFeB/Cr/Pt trilayers is investigated with emphases on the role of Cr as both a spin current transporter and generator. Compared with CoFeB/Pt bilayer emitters, THz emission from CoFeB/Cr/Pt exhibits large ellipticity, polarization-dependent magnetic response and temporal profile, and different decay length inside Cr. All these features can be explained by assuming that both the CoFeB layer and uncompensated spins in Cr near the CoFeB/Cr interface generate spin currents upon femtosecond laser excitation, which are subsequently converted to transient charge current via the inverse spin Hall effect in Pt, and thereby generating THz emission with different polarizations. Our results highlight the important role of antiferromagnet as a spin current generator when its magnetic order is weak near the Neel or blocking temperature.