Dynamic magnetic crossover at the origin of the hidden-order in van der Waals antiferromagnet CrSBr

Abstract

The van-der-Waals material CrSBr stands out as a promising two-dimensional magnet. Here, we report on its detailed magnetic and structural characteristics. We evidence that it undergoes a transition to an A-type antiferromagnetic state below TN ≈ 140 K with a pronounced two-dimensional character, preceded by ferromagnetic correlations within the monolayers. Furthermore, we unravel the low-temperature 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 field. These take place upon cooling below Ts ≈ 100 K, until a spin freezing process occurs at T* ≈ 40 K. We argue this complex behavior to reflect a crossover driven by the in-plane uniaxial anisotropy, which is ultimately caused by its mixed-anion character. Our findings reinforce CrSBr as an important candidate for devices in the emergent field of two-dimensional magnetic materials.

Spin Waves and Magnetic Exchange Hamiltonian in CrSBr

“…The single crystal neutron diffraction is shown in Figure and confirms the ground state magnetic order in ref. [7]: below a transition temperature of 132.3(6) K, new Bragg peaks appear at half‐integer ℓ positions in accord with $$(00\frac{1}{2})$$ magnetic order. In the , we refine the Bragg intensities and show they indicate A‐type antiferromagnetism in Figure 1.…”

“…Thus we conclude, as did ref. [7], that the 30 K discontinuity is not associated with a change in the spatially‐averaged magnetic order. This is consistent with the proposal in ref.…”

“…CrSBr forms in 2D layers of magnetic Cr 3 + ions forming a rectangular lattice, as shown in Figure . In bulk, it orders magnetically at T N = 132 K [ 5–7 ] with A‐type antiferromagnetism: ferromagnetic planes polarized along the b ‐axis, alternating in orientation for an overall antiferromagnetic (AFM) order, and becoming ferromagnetic (FM) in the monolayer limit. [ 8 ] This material is air‐stable and has a semiconducting gap low enough to gate with realistic electric fields.…”

Probing the Spin Dimensionality in Single‐Layer CrSBr Van Der Waals Heterostructures by Magneto‐Transport Measurements

“…[ 10 ] The authors used the experimental bulk lattice parameters determined by López‐Paz et al. [ 10 ] between 10 and 270 K and optimized the atomic positions using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithm until the forces on each atom were smaller than 1 × 10 −5 Ry au and the energy difference between two consecutive relaxation steps was ≤1 × 10 −7 Ry. To avoid unphysical interactions between images along the non‐periodic direction, the authors added a vacuum of 18 Å in the z‐ direction.…”

“…[ 8 ] However, in bulk, the layers couple between them antiferromagnetically –A type antiferromagnetism, T N ∼140 K– behaving as a metamagnet, where it is possible to switch the magnetization of the layers from antiparallel to parallel configurations with an external magnetic field (B sat,a ∼1.0 T, B sat,b ∼0.6 T and B sat,c ∼2.0 T at 2 K, where B sat states for the saturation fields; Figure 1d). [ 9–11 ]…”