Topotactic transformation of epitaxial thin films by soft chemistry allows the realisation 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 (RNiO_2 ) are synthesised by topotactically reducing their perovskite (RNiO_3 ) counterparts. This enables the unfavourable 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 (AFe ^{4+} O_3) are fascinating helimagnetic compounds, proposed to host a high density skyrmion lattice despite the chemical centrosymmetry [3]. Topotactic oxidation of the more stable Fe^{3+} material, AFeO_{2.5}, is necessary to synthesise high quality samples. I will touch briefly on our recent work to map out the helimagnetic phases in temperature, magnetic field and, now, biaxial strain.

[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