Critical behavior and evidence of dimensional crossover in quasi-two-dimensional Li$_2$FeSiO$_4$

TL;DR

This study investigates the critical magnetic behavior and dimensional crossover in quasi-two-dimensional Li$_2$FeSiO$_4$, revealing a transition from 2D to 3D Ising behavior and highlighting the material's magneto-elastic coupling and layered structure.

Contribution

It provides experimental evidence of dimensional crossover in a high-spin $S=2$ system with layered structure, expanding understanding of quasi-2D magnetism in new material classes.

Findings

Crossover from 2D to 3D Ising behavior near $T_N$

Significant magneto-elastic coupling at magnetic transition

Layered structure supports 2D magnetic behavior at higher temperatures

Abstract

We report thermal expansion and heat capacity studies on Li$_2$FeSiO$_4$ single crystals which enable us to investigate the critical behavior in the magnetically quasi-two-dimensional (2D) material. Pronounced $\lambda$-shaped anomalies at the magnetic ordering temperature $T_{\rm N}$ imply significant magneto-elastic coupling. Our analysis of both the thermal expansion and the specific heat data implies the crossover from 2D Ising-like behavior for $|(T-T_{\rm N})/T_{\rm N}|>0.3$ to 3D Ising behavior \rev{below $\simeq 1.3\times T_{\rm N}$. The 2D-like behavior is further supported by density functional calculations which show minimal dispersion perpendicular to the crystallographic $ac$ planes of the layered structure, thereby indicating the 2D nature of magnetism at higher temperatures.} Our results extend the available model materials of quasi-2D magnetism to a high-spin $S=2$ system with tetrahedrally coordinated Fe$^{2+}$-ions, thereby illustrating how magnetic order evolves in a 2D Ising-like system with orbital degrees of freedom.