Altermagnetism and Room-Temperature Metal-to-Insulator Transition in CsCr$_2$S$_2$O

Abstract: Metal-to-insulator transitions (MITs), particularly near room temperature, have been extensively studied in nonmagnetic and conventional ferromagnetic and antiferromagnetic systems, yet the co-emergence of MIT and altermagnetism (AM) remains unexplored. Here, a layered chromium-based compound CsCr$2$S$_2$O that realizes this coexistence was synthesized. It crystalizes in CeCr$_2$Si$_2$C-type structure with Cr moments orders in a C-type antiferromagnetic configuration below $T\mathrm{N}$ = 326 K, constituting a room-temperature d-wave altermagnet. In the altermagnetic state, a subsequent Verwey-type MIT appears at $T_\mathrm{MI}$ = 305 K, driven by a tetragonal-to-orthorhombic structural distortion and stripe charge ordering of Cr$^{+2}$/Cr$^{+3}$ ions, while maintaining its altermagnetic character. First-principles calculations show moment-dependent spin-split electronic structures with maximum splitting energies of ~0.6 eV and ~0.3 eV in the metallic and insulating states, respectively. Our work links the two prominent phenomena, MIT and AM, in a single material, establishing a new platform for potential spintronic applications.

• The paper demonstrates the coexistence of altermagnetism and a room-temperature metal-insulator transition in CsCr₂S₂O through comprehensive structural, magnetic, and electronic analyses.
• The paper employs neutron diffraction and DFT+U calculations to reveal significant spin splitting and charge ordering that drive the transition.
• The paper highlights the potential for spintronic applications by enabling controlled switching between metallic and insulating spin-polarized states.

Altermagnetism and Room-Temperature Metal-to-Insulator Transition in CsCr$_2$S$_2$O

Introduction

The synthesis and characterization of the layered chromium oxysulfide CsCr$_2$S$_2$O introduces a new material platform where altermagnetism and a room-temperature Verwey-type metal-to-insulator transition (MIT) coexist. This system crystallizes in the CeCr$_2$Si$_2$C-type (1221) structure, which is conducive to symmetry environments supporting d-wave altermagnetic order. The unique realization of a MIT at $T_{MI} = 305$ K, closely following the Néel transition at $T_N = 326$ K, enables reversible switching between a metallic and an insulating altermagnetic state near ambient conditions.

Structural, Magnetic, and Electronic Transitions

The crystal chemistry of CsCr$_2$S$_2$O reveals alternately stacked Cr$_2$0OS$_2$1 and Cs layers, with Cr residing in a mixed-valent (+2.5) state coordinated by O and S in a distinctly asymmetric octahedral environment. Upon cooling, CsCr$_2$2S$_2$3O undergoes a sequence of symmetry-lowering transitions. Initial C-type antiferromagnetic (AFM) order with spins aligned along the c-axis emerges below $_2$4. Upon further cooling, a first-order MIT occurs at $_2$5, concomitant with a tetragonal-to-orthorhombic transition, modulation of atomic positions, stripe charge ordering (Cr$_2$6/Cr$_2$7), and strong resistivity enhancement. This charge ordering is reflected in bond valence sum analysis and confirmed through detailed single-crystal and powder diffraction data.

Magnetic Structure and Symmetry Analysis

Neutron diffraction confirms a commensurate C-type AFM order with moments parallel to the c-axis, consistent with strong magnetic anisotropy. The MIT does not alter the AFM configuration but breaks fourfold rotational symmetry, leading to orthorhombic distortion and charge/moment disproportionation between Cr sites. The temperature dependence of the ordered moment and susceptibility points to strong quasi-2D Ising character and significant short-range correlations above $_2$8.

The underlying altermagnetic order is preserved across both metallic and insulating states. In the high-temperature (HT) phase, the symmetry relation between opposite-spin Cr sublattices is dictated by crystalline operators such as $_2$9, inducing pronounced momentum-dependent spin-split bands along specific Brillouin zone directions. In the low-temperature (LT) charge-ordered phase, nonsymmorphic operations involving glides enforce a rotated (by 45$_2$0) spin-splitting pattern, with splitting persisting in the (110)$_2$1 and (1$_2$20)$_2$3 directions.

Electronic Structure and Correlation Effects

First-principles DFT+U calculations (with $_2$4 eV, consistent with experimental activation energy) corroborate the strong electron-correlation-driven transition. In the AFM metallic phase, the density of states at the Fermi level is governed by Cr $_2$5/$_2$6 bands, hybridized with S $_2$7-orbitals and exhibiting momentum-selective spin splitting up to $_2$8 eV. The insulating LT phase exhibits a band gap of $_2$9 meV, mirroring experiment, and features parallel spin-split $_2$0/$_2$1 bands just below and above $_2$2, localized predominantly on Cr$_2$3 and Cr$_2$4, respectively.

The magnitude of spin splitting ($_2$5 eV metallic, $_2$6 eV insulating) is substantial within the class of altermagnets, strongly exceeding typical spin splitting from SOC, and can be directly traced to d-wave symmetry of the underlying magnetic state. The emergent polarization anisotropy enables giant spin Hall current potential with channel selectivity controlled by the MIT.

Implications and Future Prospects

CsCr$_2$7S$_2$8O constitutes the first realized example of a layered 3$_2$9 altermagnetic system exhibiting a room-temperature, reversible MIT of Verwey type, driven by charge ordering and structural symmetry lowering, all while preserving a robust and symmetry-protected spin-split electronic structure. This platform enables unique opportunities for electrically or strain-controlled quantum phase switching between metallic and insulating spin-polarized transport channels, minimized stray fields, and high spin-filtering efficiency. Its behavior directly addresses critical limitations in ferromagnetic- and SOC-derived spintronic devices, promising enhanced integration and device scalability.

Practically, the strong electron–lattice coupling, sensitivity of the MIT to external perturbations (as established in related systems), and large AM-derived spin splitting make CsCr$_2$0S$_2$1O a candidate for non-volatile, low-power, and high-speed spintronic or quantum computation elements. Theoretically, the results motivate systematic exploration of correlated electron physics in altermagnetic systems, particularly in the context of MIT phenomena, unconventional charge and magnetic order, and new manipulation protocols via electronic, structural, or field-induced means.

Conclusion

The demonstration of a coupled MIT and robust altermagnetic order in CsCr$_2$2S$_2$3O at ambient temperature establishes this material as a unique testbed for correlated quantum phenomena with direct implications for future spintronics. The realization of large spin-split energies, momentum-controlled spin textures, and charge-order-driven transitions consolidates the relevance of 1221-type oxysulfides and related motifs for discovering and engineering emergent altermagnetic matter at functional energy scales (2604.02114).