Title: Evidence for short-range magnetic order in the nematic phase of FeSe from anisotropic in-plane magnetostriction and susceptibility measurements,
Abstract: The nature of the nematic state in FeSe remains one of the major unsolved
mysteries in Fe- based superconductors. Both spin and orbital physics have been
invoked to explain the origin of this phase. Here we present experimental
evidence for frustrated, short-range magnetic order, as suggested by several
recent theoretical works, in the nematic state of FeSe. We use a combination of
magnetostriction, susceptibility and resistivity measurements to probe the
in-plane anisotropies of the nematic state and its associated fluctuations.
Despite the absence of long-range magnetic order in FeSe, we observe a sizable
in-plane magnetic susceptibility anisotropy, which is responsible for the
field-induced in-plane distortion inferred from magnetostriction measurements.
Further we demonstrate that all three anisotropies in FeSe are very similar to
those of BaFe2As2, which strongly suggests that the nematic phase in FeSe is
also of magnetic origin.