Terahertz Emission From an Exchange-Coupled Synthetic Antiferromagnet

We report on terahertz emission from $\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Pt}/\mathrm{Ru}/\mathrm{Fe}\text{\ensuremath{-}}\mathrm{Mn}\text{\ensuremath{-}}\mathrm{Pt}$ and $\mathrm{Pt}/\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}/\mathrm{Ru}/\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}/\mathrm{Pt}$ synthetic antiferromagnet (SAF) structures upon irradiation by a femtosecond laser; the former is via the anomalous Hall effect, whereas the latter is through the inverse spin Hall effect. The antiparallel alignment of the two ferromagnetic layers leads to a terahertz emission peak amplitude that is almost double that of a corresponding single-layer or bilayer emitter with the same equivalent thickness. In addition, we demonstrate by both simulation and experiment that terahertz emission provides a powerful tool to probe the magnetization reversal processes of individual ferromagnetic layers in a SAF structure, as the terahertz signal is proportional to the vector difference $({\mathbf{M}}_{1}\ensuremath{-}{\mathbf{M}}_{2})$ of the magnetizations of the two ferromagnetic layers.