Strong impact of crystalline twins on the amplitude and azimuthal dependence of THz emission from epitaxial NiO/Pt

Ultrafast generation of spin currents involving antiferromagnets is currently attracting tremendous interest. Here, we demonstrate broadband THz emission from a [111]-oriented NiO/Pt bilayer grown on MgO and Al2O3 substrates. The NiO films are grown by pulsed laser deposition, whereas the Pt films are grown by magnetron sputtering. While we obtained epitaxial films on both substrates, NiO films on the Al2O3 substrate showed the presence of crystalline twins. We show that the existence of crystalline twins reduces the THz amplitude by an order of magnitude while simultaneously dramatically changing the azimuthal dependency of the THz amplitude. The findings have significant implications for antiferromagnetic spintronics.