Iron-chalcogenide superconductors are a viable platform to understand pairing interactions emerging from competing electronic phases. Bulk FeSe is unusual at ambient pressure, as it hosts a nematic electronic phase but no long-range magnetic order, due to different competing magnetic orders [1]. Nematic phase can be suppressed by both S substitution (x ~ 0.175) and applied hydrostatic pressure, but a SDW is only stabilized under applied pressure. Here, we discover a field-induced incipient SDW in single crystals of FeSe1-xSx inside the nematic B phase (0.11 x 0.17) in magnetic fields up to 70 T. As superconductivity is suppressed, we observe sharp upturns in the longitudinal resistivity, and a dominant slow quantum oscillation frequency consistent with a Fermi surface reconstruction [2,3]. These studies are compared with a nematic system measured under pressure with a similar Ts, where resistivity upturns are significantly amplified in magnetic field due to the stabilization of SDW [4,5]. Additionally, we investigate the sensitivity of these resistivity upturns to applied uniaxial strain in fields up to 30 T. We find that field-induced incipient spin-density phases are stabilized inside the nematic phase of FeSe1-xSx in the vicinity of the nematic end point, challenging the nature of pairing interactions.

[1] Fernandes et al., Nature 2022
[2] Coldea et al., npj QM, 2019
[3] Bristow et al., PRR 2020
[4] Sun et al., Nat Comms 2016
[5] Zajicek et al., PRB 2026