  Electrical switching of antiferromagnets is an exciting recent development in
spintronics, which promises active antiferromagnetic devices with high speed
and low energy cost. In this emerging field, there is an active debate about
the mechanisms of current-driven switching of antiferromagnets. Harmonic
characterization is a powerful tool to quantify current-induced spin-orbit
torques and spin Seebeck effect in heavy-metal/ferromagnet systems. However,
the harmonic measurement technique has never been verified in antiferromagnetic
heterostructures. Here, we report for the first time harmonic measurements in
Pt/$\alpha$-Fe$_2$O$_3$ bilayers, which are explained by our modeling of
higher-order harmonic voltages. As compared with ferromagnetic heterostructures
where all current-induced effects appear in the second harmonic signals, the
damping-like torque and thermally-induced magnetoelastic effect contributions
in Pt/$\alpha$-Fe$_2$O$_3$ emerge in the third harmonic voltage. Our results
provide a new path to probe the current-induced magnetization dynamics in
antiferromagnets, promoting the application of antiferromagnetic spintronic
devices.
