Abnormal Critical Fluctuations Revealed by Magnetic Resonance in the Two-Dimensional Ferromagnetic Insulators

TL;DR

This study uses magnetic resonance to investigate critical fluctuations in two-dimensional ferromagnetic insulators, revealing anisotropic interactions and suggesting proximity to a quantum critical point, especially in CrSiTe3.

Contribution

The paper introduces a magnetic resonance approach to analyze critical fluctuations in 2D ferromagnetic insulators, highlighting anisotropic effects and the proximity to quantum criticality.

Findings

Anisotropic shift of the g factor indicates anisotropic interactions.

CrSiTe3 shows more 2D character and is closer to quantum criticality.

Enhanced g shift correlates with specific heat and magnetometry data.

Abstract

Phase transitions and critical phenomena, which are dominated by fluctuations and correlations, are one of the fields replete with physical paradigms and unexpected discoveries. Especially for two-dimensional magnetism, the limitation of the Ginzburg criterion leads to enhanced fluctuations breaking down the mean-field theory near a critical point. Here, by means of magnetic resonance, we investigate the behavior of critical fluctuations in the two-dimensional ferromagnetic insulators $\rm CrXTe_3 (X=Si, Ge)$. After deriving the classical and quantum models of magnetic resonance, we deem the dramatic anisotropic shift of the measured $g$ factor to originate from fluctuations with anisotropic interactions. The deduction of the $g$ factor behind the fluctuations is consistent with the spin-only state (${g\approx}$ 2.050(10) for $\rm CrSiTe_3$ and 2.039(10) for $\rm CrGeTe_3$). Furthermore, the abnormal enhancement of $g$ shift, supplemented by specific heat and magnetometry measurements, suggests that $\rm CrSiTe_3$ exhibits a more typical two-dimensional nature than $\rm CrGeTe_3$ and may be closer to the quantum critical point.