  Terahertz emission spectroscopy (TES) has recently played an important role
in unveiling the spin dynamics at a terahertz (THz) frequency range. So far,
ferromagnetic (FM)/nonmagnetic (NM) heterostructures have been intensively
studied as THz sources. Compensated magnets such as a ferrimagnet (FIM) and
antiferromagnet (AFM) are other types of magnetic materials with interesting
spin dynamics. In this work, we study TES from compensated magnetic
heterostructures including CoGd FIM alloy or IrMn AFM layers. Systematic
measurements on composition and temperature dependences of THz emission from
CoGd/Pt bilayer structures are conducted. It is found that the emitted THz
field is determined by the net spin polarization of the laser induced spin
current rather than the net magnetization. The temperature robustness of the
FIM based THz emitter is also demonstrated. On the other hand, an AFM plays a
different role in THz emission. The IrMn/Pt bilayer shows negligible THz
signals, whereas Co/IrMn induces sizable THz outputs, indicating that IrMn is
not a good spin current generator, but a good detector. Our results not only
suggest that a compensated magnet can be utilized for robust THz emission, but
also provide a new approach to study the magnetization dynamics especially near
the magnetization compensation point.
