Dimensional crossover in a layered ferromagnet detected by spin correlation driven distortions

TL;DR

This study uses advanced nonlinear optical techniques to detect and analyze the subtle atomic distortions caused by short-range magnetic order in a layered ferromagnet, revealing a dimensional crossover before long-range order sets in.

Contribution

The paper introduces high-multipole nonlinear optical polarimetry as a novel, sensitive method to detect SRO-induced distortions in layered magnets, uncovering a two-step dimensional crossover.

Findings

Detected two successive symmetric distortions above Curie temperature.

Revealed a two- to three-dimensional magnetic SRO crossover.

Demonstrated evasion of Mermin-Wagner theorem via dimensional crossover.

Abstract

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.