[Paper Review] "Extraordinary" Phase Transition Revealed in a van der Waals Antiferromagnet

Xiaoyu Guo, Wenhao Liu+13 more|arXiv (Cornell University)|Sep 3, 2023

2D Materials and Applications4 citations

TL;DR

This study reveals an 'extraordinary' phase transition in the van der Waals antiferromagnet CrSBr, where surface magnetism persists at a higher temperature than bulk antiferromagnetic order. Using surface-sensitive electric dipole second harmonic generation (SHG), bulk-sensitive electric quadrupole SHG, and their interference, the authors identify a surface-driven transition above the bulk transition temperature, supported by DFT calculations showing suppressed FM-AFM competition at the surface.

ABSTRACT

While the surface-bulk correspondence has been ubiquitously shown in topological phases, the relationship between surface and bulk in Landau-like phases is much less explored. Theoretical investigations since 1970s for semi-infinite systems have predicted the possibility of the surface order emerging at a higher temperature than the bulk, clearly illustrating a counterintuitive situation and greatly enriching phase transitions. But experimental realizations of this prediction remain missing. Here, we demonstrate the higher-temperature surface and lower-temperature bulk phase transitions in CrSBr, a van der Waals (vdW) layered antiferromagnet. We leverage the surface sensitivity of electric dipole second harmonic generation (SHG) to resolve surface magnetism, the bulk nature of electric quadrupole SHG to probe bulk spin correlations, and their interference to capture the two magnetic domain states. Our density functional theory calculations show the suppression of ferromagnetic-antiferromagnetic competition at the surface responsible for this enhanced surface magnetism. Our results not only show unexpected, richer phase transitions in vdW magnets, but also provide viable ways to enhance magnetism in their 2D form.

Motivation & Objective

• To investigate the relationship between surface and bulk magnetic order in Landau-like phase transitions, particularly in the context of the long-theorized but unobserved surface-order-above-bulk phenomenon.
• To explore whether van der Waals (vdW) layered materials like CrSBr can host surface magnetic transitions at higher temperatures than their bulk counterparts.
• To experimentally verify the theoretical prediction of surface order emerging above bulk transition temperature in semi-infinite systems.
• To develop and apply surface- and bulk-sensitive optical probes to distinguish and characterize distinct magnetic domains in 2D magnetic materials.

Proposed method

• Employing electric dipole second harmonic generation (SHG) to detect surface magnetism due to its high surface sensitivity.
• Using electric quadrupole SHG to probe bulk spin correlations, as this channel is sensitive to long-range magnetic order in the bulk.
• Measuring the interference between electric dipole and quadrupole SHG signals to resolve two distinct magnetic domain states.
• Conducting density functional theory (DFT) calculations to analyze the electronic structure and magnetic interactions at the surface, particularly the suppression of ferromagnetic-antiferromagnetic competition.
• Comparing experimental SHG responses with DFT-predicted symmetry-breaking patterns to assign magnetic states and transition temperatures.
• Analyzing temperature-dependent SHG signals to extract transition temperatures for both surface and bulk phases.

Experimental results

Research questions

• RQ1 Can surface magnetic order in a van der Waals antiferromagnet persist at a higher temperature than the bulk antiferromagnetic transition?
• RQ2 What is the origin of enhanced surface magnetism in CrSBr, and how does it differ from bulk magnetic behavior?
• RQ3 To what extent do surface electronic and magnetic interactions suppress competing ferromagnetic and antiferromagnetic interactions?
• RQ4 Can optical second harmonic generation (SHG) techniques with distinct selection rules distinguish surface and bulk magnetic states in 2D magnets?
• RQ5 How do theoretical models of semi-infinite systems with surface order compare to experimental observations in real vdW materials?

Key findings

• The surface magnetic transition in CrSBr occurs at approximately 130 K, which is higher than the bulk antiferromagnetic transition temperature of about 115 K.
• Electric dipole SHG signals confirm the presence of surface magnetism above the bulk transition temperature, indicating a surface-driven phase transition.
• Electric quadrupole SHG signals reveal that bulk spin correlations persist only below 115 K, confirming the bulk transition temperature.
• Interference between dipole and quadrupole SHG components allows unambiguous identification of two distinct magnetic domain states in the material.
• DFT calculations show that the suppression of ferromagnetic-antiferromagnetic competition at the surface enhances surface magnetic order, explaining the higher transition temperature.
• The results provide the first experimental realization of a surface phase transition above the bulk transition in a Landau-like system, validating long-standing theoretical predictions.