Description:

Topotactic transformation of epitaxial thin films by soft chemistry allows the realization of high crystalline quality metastable phases. In this talk I will discuss two physics topics that can be addressed through harnessing topotactic transformations in transition metal oxides.

First, infinite-layer nickelate superconductors (RNiO2) are synthesised by topotactically reducing their perovskite (RNiO3) counterparts. This enables the unfavorable Ni+ ion and unlocks a novel family of unconventional superconductors [1]. In particular I will focus on results from low-energy muon spin rotation [2] that reveal local magnetism in these materials that 1) onsets at rather high temperature, 2) is independent of the rare earth 4f electrons, 3) appears to be robust to doping 4) is antiferromagnetic and possibly short-range-ordered in nature and 5) coexists with superconductivity at low temperatures. From these conclusions we can make some comparisons with the nominally-similar copper-oxide superconductors.

Secondly, perovskite alkali earth ferrites (AFe4+O3) are fascinating helimagnetic compounds, proposed to host a high density skyrmion lattice despite the chemical centrosymmetry [3]. Topotactic oxidation of the more stable Fe3+ material, AFeO2.5, is necessary to synthesize high quality samples. I will touch briefly on our recent work to engineer the helimagnetism by the thin film environment.

[1] D. Li et al, Nature 572 (2019) 624
[2] J. Fowlie et al, Nature Physics 18 (2022) 1043
[3] S. Ishiwata et al, Phys. Rev. B 101 (2020) 134406

Jennifer Fowlie, Assistant Professor of Materials Science and Engineering, Northwestern University

Host: Venkat Chandrasekhar