Evidence for multiferroicity in single-layer CuCrSe₂

The exploration of multiferroic materials, which simultaneously exhibit ferroelectric and ferromagnetic orders has been at the forefront of condensed matter physics due to their fascinating fundamental properties and promising applications in spintronics and information technology. In these materials, the interplay between magnetic and electric orders gives rise to cross-correlated phenomena, enabling manipulation of magnetic states by electric fields, and vice versa. With device miniaturization, there is strong interest in two-dimensional (2D) materials and, in particular, in realizing 2D multiferroics for practical devices. However, as thickness diminishes, long-range ferromagnetic and ferroelectric orders are often suppressed by thermal fluctuations and depolarization fields. While 2D ferromagnetic and ferroelectric materials have been realized down to the single-layer limit, their transition temperatures are often reduced. Against this backdrop, the authors investigate CuCrSe₂, a non-van der Waals layered material known for thermoelectric properties, to determine whether intrinsic multiferroicity can persist at the single-layer limit with elevated transition temperatures.

Even in three-dimensional materials, multiferroicity is rare because the origins of magnetism and ferroelectricity are often mutually exclusive. In 2D systems, prior experimental instances of multiferroicity include p-doped SnSe₂ and ε-Fe₂O₃, typically at thicknesses over 3 nm, where multiferroic order diminishes with decreasing thickness. Single-layer NiI₂ was reported to host a type-II multiferroic ground state via optical probes, but its low transition temperature (about 21 K), poor air stability, and reliance on all-optical characterization have drawn skepticism and a need for more direct probes such as PFM and magnetic measurements. Another advance is one-unit-cell-thick multiferroic Cr₂S₃, where ferroelectricity arises from interfacial modulation rather than being intrinsic. Therefore, identifying a single material exhibiting intrinsic 2D multiferroicity at technologically relevant temperatures remains a key challenge.

Materials synthesis and exfoliation: Bulk single crystals of CuCrSe₂ were synthesized via chemical vapor transport. Stoichiometric high-purity Cu, Cr, and Se powders were sealed in evacuated quartz and sintered (1050 °C, 40 h), then heated to 1200 °C and quenched to obtain pure phase, followed by CVT growth using CrCl₃ as a transport agent (temperature gradient 1050 °C hot end/800 °C cold end for 200 h with rapid quenching). Ultrathin nanosheets were prepared by redox-controlled electrochemical exfoliation: a CuCrSe₂ crystal as cathode (between Ti plates) and a Pt anode in 0.1 M tetraoctylammonium bromide in acetonitrile; 5 V applied for 1 h for intercalation, followed by DMF washes and manual shaking to exfoliate. Dispersions were centrifuged (112×g, 3 min) to remove unexfoliated material; thicknesses were selected by AFM. Optical and vibrational characterization: Raman spectra (532 nm excitation, room temperature) were collected with a confocal system, calibrated to Si at 520.7 cm⁻¹. Nonlinear optics: SHG spectra and polarization-resolved SHG were acquired on a WITec alpha 300RA using a 1064 nm ultrafast laser (~15 ps, 80 MHz). A half-wave plate controlled polarization; analyzer was parallel to incident polarization; signals collected by UHTS spectrometer with CCD. Scanning probe microscopy: AFM, PFM, and EFM (Asylum Research Cypher S) at room temperature on heavily doped Si substrates. PFM local switching spectroscopy used DART mode with an AC signal superimposed on DC triangular/sawtooth waveforms; off-field phase and amplitude loops were recorded at zero tip voltage. Domain writing employed ±10 V bias patterns; stability checked over days. STEM: Cross-sectional lamellae were prepared by FIB. HAADF-STEM was performed on JEOL-ARM 300F at 300 kV (convergence semi-angle 22 mrad; collection 64–180 mrad). Elemental maps were obtained by EDS. Magnetometry: Films for SQUID measurements (MPMS-3) were prepared by filtering nanosheet dispersions onto PVDF membranes and vacuum-baked at 80 °C for 1 h. ZFC/FC M–T and M–H loops were measured. Transport/Hall: Hall devices were fabricated by transferring chemically exfoliated flakes onto pre-patterned Au/Ti electrodes and capping with hBN. Hall measurements were carried out in a 9 T PPMS (DynaCool).

• Demonstration of intrinsic multiferroicity in single-layer CuCrSe₂ with room-temperature out-of-plane ferroelectricity and ferromagnetism with TC ≈ 120 K.
• Ferroelectricity:
  • SHG shows a strong 532 nm emission under 1064 nm excitation and a six-fold polarization pattern, confirming broken inversion symmetry consistent with the R3m structure.
  • PFM off-field phase loops for 1L and 2L exhibit clear hysteresis with 180° phase switching and butterfly-shaped amplitude loops; coercive voltage ~4–5 V, larger than several known 2D ferroelectrics (e.g., In₂Se₃ ~1.5 V, SnS ~2.0 V, MoS₂/WS₂ heterobilayer ~3.0 V), indicating a stable FE state.
  • Domain writing with ±10 V produces stable “box-in-box” domains with 180° phase contrast visible after one day and one week; corresponding contrast observed in EFM amplitude and phase.
  • Voc-dependent switching behavior aligns with intrinsic ferroelectricity (loops vanish when max bias < coercive voltage), ruling out ion-migration artifacts.
  • AFM thickness of 1L region ~1.6 nm (slightly larger than theoretical ~1.3 nm); Raman modes E (~146.2 cm⁻¹) and A₁ (~237.7 cm⁻¹) observed in thicker flakes but nearly undetectable in 1L.
• STEM mechanism and structure:
  • Cross-sectional HAADF-STEM resolves two FE configurations (P1 and P2) corresponding to distinct interlayer Cu occupation sites (Cu-1 and Cu-2), consistent with a double-well potential and a higher switching barrier stabilizing room-temperature ferroelectricity.
  • EDS confirms chemical homogeneity (Cu, Cr, Se distributions).
• Ferromagnetism:
  • ZFC/FC M–T shows a transition near 120 K at H = 50 Oe; M–H loops below TC with coercive field ~600 Oe at 2 K.
  • Anomalous Hall effect with pronounced hysteresis observed at 2 K in 1L, 2L, and 4L flakes; trilayer shows only linear Hall response, indicating odd–even-layer-dependent ferromagnetism and strong in-plane anisotropy.
• Mechanism of magnetoelectric coupling:
  • Interlayer Cu⁺-induced vertical polarization shifts Cr orbital energies (downward shift of t2g on one Cr and upward shift of eg on another), reducing the eg(Cr1) − t2g(Cr2) energy gap and enhancing ferromagnetic coupling between adjacent CrSe₂ layers in 1L CuCrSe₂.
  • Significant hole doping increases carrier density at the Fermi level, favoring intra- and interlayer ferromagnetic couplings.
• Overall, single-layer CuCrSe₂ is intrinsically multiferroic with ferroelectric polarization intimately linked to and enhancing its ferromagnetism, distinct from conventional type-I and type-II multiferroics.

The study directly confirms intrinsic multiferroicity in single-layer CuCrSe₂ through complementary probes (SHG, PFM, STEM, SQUID magnetometry, and Hall transport). The out-of-plane ferroelectricity persists at room temperature with a relatively large coercive voltage, indicating a robust FE state. Magnetic measurements reveal a ferromagnetic phase with TC ~120 K that survives to the monolayer, corroborated by AHE and hysteresis in transport, and showing odd–even-layer dependence. Mechanistically, interlayer Cu-induced polarization modifies Cr orbital energies, thereby enhancing ferromagnetic exchange—an intertwined magnetoelectric coupling mechanism that is distinct from canonical type-I (independent FE and FM) or type-II (magnetism-driven FE) scenarios. These results address the longstanding challenge of realizing intrinsic 2D multiferroicity at elevated temperatures and underscore CuCrSe₂ as a platform for exploring magnetoelectric interactions and for potential spintronic and magnetoelectric device applications.

The work provides compelling evidence that single-layer CuCrSe₂ is an intrinsic 2D multiferroic, exhibiting room-temperature out-of-plane ferroelectricity and ferromagnetism with TC around 120 K. Ferroelectricity is validated by SHG and PFM (including stable domain writing), and its structural origin is visualized by STEM through two switchable Cu occupation configurations (P1 and P2) consistent with a double-well potential. Magnetic order and its layer dependence are established via SQUID and Hall measurements, with a mechanistic link between ferroelectric polarization and enhanced ferromagnetic coupling through Cr orbital shifts. These findings mark significant progress toward single-layer multiferroics operating at comparatively high temperatures and pave the way for developing multiterminal spintronic and magnetoelectric devices based on 2D materials.

• The ferromagnetic Curie temperature (~120 K) is below room temperature, which may limit immediate applications requiring room-temperature magnetism.
• Trilayer CuCrSe₂ shows only linear Hall effect without clear AHE hysteresis, indicating odd–even-layer-dependent ferromagnetism that may complicate layer-by-layer device design.
• Raman modes are nearly undetectable in the single-layer limit, reducing the utility of Raman as a nondestructive thickness/phase probe for 1L samples.
• Magnetic measurements were performed on films prepared from nanosheet dispersions; while AHE confirms ferromagnetism down to 1L in devices, differences between ensemble films and individual flakes could affect quantitative comparisons.