  We report on terahertz emission from FeMnPt/Ru/FeMnPt and
Pt/CoFeB/Ru/CoFeB/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 for 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 of the magnetizations of the two
ferromagnetic layers.
