[Paper Review] Dynamic Magnetic Crossover at the Origin of the Hidden-Order in van der Waals Antiferromagnet CrSBr

Sara A. López-Paz∥, Zurab Guguchia+7 more|arXiv (Cornell University)|Mar 22, 2022

2D Materials and Applications53 references108 citations

TL;DR

This study reveals that CrSBr exhibits a dynamic magnetic crossover underlying a hidden order phase, driven by in-plane uniaxial anisotropy. Using neutron diffraction, muon spin relaxation, and X-ray diffraction, it identifies a two-dimensional A-type antiferromagnetic state below TN ≈140 K, followed by a progressive slowing of magnetic fluctuations and spin freezing at T* ≈40 K, linked to a continuous reorientation of internal magnetic fields and negative thermal expansion in the a-axis.

ABSTRACT

The van der Waals material CrSBr stands out as a promising two-dimensional magnet. Especially, its high magnetic ordering temperature and versatile magneto-transport properties make CrSBr an important candidate for new devices in the emergent field of two-dimensional magnetic materials. To date, the magnetic and structural properties of CrSBr have not been fully elucidated. Here, we report on the detailed temperature-dependent magnetic and structural properties of this material, by comprehensively combining neutron scattering, muon spin relaxation spectroscopy, synchrotron X-ray diffraction, and magnetization measurements. We evidence that this material undergoes a transition to an A-type antiferromagnetic state below $T_{ m N} \approx$ 140 K, with a pronounced two-dimensional character as deduced from the determined critical exponent of $\beta \approx $ 0.18. In our analysis of the field-induced metamagnetic transition, we find that the ferromagnetic correlations within the monolayers persist clearly above the N\'eel temperature in this material. Furthermore, we unravel the low-temperature (i.e. $T < T_{ m N}$) magnetic hidden order within the long-range magnetically ordered state. We find that it is associated to a slowing down of the magnetic fluctuations, accompanied by a continuous reorientation of the internal magnetic field. These take place upon cooling below $T_s$ $\approx$ 100 K, until a spin freezing process occurs at $T$* $\approx$ 40 K. We argue this complex dynamic behavior to reflect a magnetic crossover driven by the in-plane uniaxial anisotropy, which is ultimately caused by the mixed-anion character of the material. Our findings indicate that the magnetic and structural properties of CrSBr widen its potential application as a component for spin-based electronic devices.

Motivation & Objective

• To resolve the unresolved magnetic and structural properties of the van der Waals magnet CrSBr.
• To identify the origin of the anomalous magnetoresistance and magnetization changes below 40 K.
• To determine whether a hidden order phase exists within the long-range A-type antiferromagnetic state.
• To investigate the role of mixed-anion chemistry (S and Br) in inducing magnetic anisotropy and complex spin dynamics.
• To link structural changes, such as negative thermal expansion, to magnetic ordering transitions.

Proposed method

• Conducted temperature-dependent neutron powder diffraction (NPD) to determine the long-range magnetic structure and lattice parameters.
• Performed muon spin relaxation (µSR) spectroscopy to probe local magnetic fields and detect spin freezing dynamics.
• Carried out synchrotron X-ray diffraction (XRD) to measure lattice parameters and detect structural anomalies, including negative thermal expansion.
• Collected magnetization data via SQUID magnetometry to map magnetic transitions and susceptibility.
• Used Rietveld refinement with FULLPROF SUITE and BASIREPS for magnetic symmetry analysis.
• Combined data from multiple techniques to correlate magnetic, structural, and transport properties across temperature.

Experimental results

Research questions

• RQ1 Does CrSBr exhibit a hidden order phase below 40 K, and if so, what is its origin?
• RQ2 What is the nature of the magnetic ground state in CrSBr, and how does it evolve with temperature?
• RQ3 How do the magnetic fluctuations and internal fields change below Ts ≈100 K and T* ≈40 K?
• RQ4 What is the role of in-plane uniaxial anisotropy in driving the observed dynamic magnetic crossover?
• RQ5 How does the mixed-anion (S/Br) character influence the magnetic exchange interactions and spin dynamics?

Key findings

• CrSBr undergoes a long-range A-type antiferromagnetic transition below TN ≈140 K, with a critical exponent β ≈0.18–0.23, indicating strong two-dimensional character.
• Ferromagnetic correlations persist above TN, evidenced by field-induced metamagnetic transitions, confirming enhanced spin fluctuations in the paramagnetic phase.
• A dynamic magnetic crossover occurs below Ts ≈100 K, marked by a continuous slowing of magnetic fluctuations and reorientation of internal magnetic fields.
• Spin freezing is observed at T* ≈40 K, coinciding with a negative thermal expansion of the a-axis and the onset of hidden order.
• The hidden order is attributed to a magnetic crossover driven by in-plane uniaxial anisotropy, originating from the mixed-anion (S/Br) structure.
• The refined magnetic moment at 1.8 K is M = 3.09(1) µB, consistent with Cr(III) in an octahedral environment.