Near-field infrared imaging of polar domain walls in Ni₃TeO₆

As devices progress into the nanoscale, domain walls have emerged as a leading platform for low-power switching and memory devices. These walls can be created with controllable density, moved and erased. Although extensive imaging has been performed, very little is known about the structure-property relationships of domain walls. To explore this, we employ synchrotron based near-field infrared spectroscopy to look at the phononic response of polar domain walls in multiferroic Ni₃TeO₆. Near-field infrared spectroscopy is a tip-based technique which provides 20 x 20 nm² spatial resolution and when combined with a synchrotron can allow for probing into the far-IR. This novel technique allows for real-space measurements of individual domain walls, previously unrealized through traditional spectroscopic techniques which are constrained to bulk measurements due to diffraction limitations. Ni₃TeO₆ is an incredibly unique platform in that not only does it have interlocked polar and chiral domain walls, but it also offers an opportunity to look at charged and neutral walls in a single sample. In this work we find that in terms of strain, charged walls are nearly double the width of neutral walls and show large frequency shifts in certain vibrational modes across the wall. Additionally, we see the near-field amplitude decrease as the wall is crossed. The contributions from the chirality and polarization are unraveled as well as the phonon lifetimes at these interfaces.