  Magneto-elastic distortions are commonly detected across magnetic long-range
ordering (LRO) transitions. In principle, they are also induced by the magnetic
short-range ordering (SRO) that precedes a LRO transition, which contains
information about short-range correlations and energetics that are essential
for understanding how LRO is established. However these distortions are
difficult to resolve because the associated atomic displacements are
exceedingly small and do not break symmetry. Here we demonstrate high-multipole
nonlinear optical polarimetry as a sensitive and mode selective probe of SRO
induced distortions using CrSiTe$_3$ as a testbed. This compound is composed of
weakly bonded sheets of nearly isotropic ferromagnetically interacting spins
that, in the Heisenberg limit, would individually be impeded from LRO by the
Mermin-Wagner theorem. Our results show that CrSiTe$_3$ evades this law via a
two-step crossover from two- to three-dimensional magnetic SRO, manifested
through two successive and previously undetected totally symmetric distortions
above its Curie temperature.
