Domain walls are leading platforms for the development of ultra-low power switching and memory devices due to their ability to move, be created and erased in real time, and mitigate heat flux. Interface vs. wavelength size effects unfortunately preclude the measurement of phonons by traditional spectroscopic techniques, making it difficult to unravel the primary excitations of the lattice and the symmetries that they represent across these functional interfaces. In this work, we employed synchrotron-based near-field infrared nanospectroscopy to image 180◦ polar domain walls in multiferroic Ni_3 TeO_6. This is a unique platform because, in addition to hosting polar and chiral domains that are interlocked with one another, Ni_3 TeO_6 displays both charged and neutral interfaces depending upon the direction allowing the development of structure-property relations. Comparison of the contour, fixed distance, and fixed frequency plots reveals that charged walls are twice as wide as (and less stable than) the neutral interfaces due to additional strain created by the on-end chiral helices. Chirality is responsible for much of the interface stiffness and the hardening of certain phonons at the walls. The largest frequency shift, for instance, takes place in a mode consisting of a NiO_6 octahedral contraction and rotation along c, modifying the force constant by approximately 1%. Frequency shifts at walls of both types indicate that polarization switches via an Ising-type mechanism due to structural constraints associated with interlocked chirality. Our estimates also reveal that phonon lifetimes are on the order of 1 ps or less, with marked changes at the charged and neutral walls. The ability to quantify the consequences of charge accumulation at a functional interface in terms of phonon lifetimes opens new avenues for heat management in domain wall-based devices.

Reference: A. M. Sargent, K. A. Smith, X. Xu, K. Du, S.-W. Cheong, L. Wehmeier, G. L. Carr, and J. L. Musfeldt, Near-field infrared imaging of polar domain walls in Ni_3 TeO_6, J. Applied Physics 138, 055302 (2025).