Dimensional crossover in a van der Waals ferromagnet detected by spin correlation driven distortions

Magneto-elasticity – the structural deformation of a crystal in response to a change in its magnetic energy – is commonly detected across magnetic long-range ordering (LRO) transitions and yields insight into magnetic ground state energetics. In principle, distortions are also induced by magnetic short-range ordering (SRO), which provide complementary 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 CrSiTe₃ as a testbed. This compound is composed of van der Waals bonded sheets of ferromagnetically interacting Heisenberg spins that, in isolation, would be impeded from LRO by the Mermin-Wagner theorem. Our results show that CrSiTe₃ evades this law via a two-step crossover from two- to three-dimensional magnetic SRO above its Curie temperature (T_c=31K), manifested through two previously undetected totally symmetric distortions at T_2D~110K and T_3D~60K, respectively. Such data open new avenues for mechanical control of magnetism.