Magnetic properties of molecular beam epitaxy-grown ultrathin Cr2Ge2Te6 films down to monolayer limit on Si substrates

Cr2Ge2Te6, a prototypical van der Waals ferromagnetic semiconductor, have attracted significant interest for its potential applications in high-performance spintronics. However, the magnetic ground state of monolayer Cr2Ge2Te6 remains elusive due to fragile and irregular-shaped thin flake samples with weak magnetic signals. Here, we successfully grow uniform ferromagnetic Cr2Ge2Te6 films down to monolayer by molecular beam epitaxy. By exploiting a self-limiting growth mode, we achieve synthesis of uniform monolayer Cr2Ge2Te6 films across entire millimeter-scale Si substrates. Through a combination of superconducting quantum interference device magnetometry and anomalous Hall effect measurements, we establish that monolayer Cr2Ge2Te6 exhibits intrinsic ferromagnetism with perpendicular magnetic anisotropy below ~10 K, albeit with strong magnetic fluctuations characteristic of its two-dimensional nature. Furthermore, a systematic thickness-dependent study reveals a crossover from this fluctuation-dominated two-dimensional magnetism turns into conventional long-range ferromagnetic order as the film thickness increases. Our work not only definitively establishes the intrinsic ferromagnetic ground state of monolayer Cr2Ge2Te6, but also provides a scalable, silicon-compatible route for preparing the two-dimensional magnet for future spintronic or quantum devices.

💡 Research Summary

This work reports the molecular beam epitaxy (MBE) growth of ultrathin Cr₂Ge₂Te₆ (CGT) films on Si(111) substrates down to the monolayer (1 L) limit, and provides a comprehensive structural and magnetic characterization of these two‑dimensional (2D) ferromagnetic semiconductors. By co‑evaporating elemental Cr, Ge and Te under ultra‑high vacuum, the authors achieve a self‑limiting growth mode: regardless of a ~20 % variation in deposition time, the film stops at a single CGT layer. STEM reveals that the Si surface is first passivated by a Te monolayer, after which a van‑der‑Waals epitaxial CGT layer nucleates. The in‑plane lattice constant of the first two CGT layers (~6.9 Å) matches the Si(111)‑√3×√3 R30° reconstruction, while thicker regions relax to the bulk CGT value (~7.0 Å). RHEED, XRD and cross‑sectional TEM confirm atomically flat, phase‑pure films over millimeter‑scale areas.

Magnetic properties are probed by both superconducting quantum interference device (SQUID) magnetometry and anomalous Hall effect (AHE) measurements, ensuring that the weak signals from few‑layer samples are intrinsic. For a 25 nm (≈35 L) film, rectangular hysteresis loops are observed from 2 K to 60 K with perpendicular magnetic anisotropy (PMA), consistent with prior reports on CGT grown on (Bi,Sb)₂Te₃‑covered InP. In the few‑layer regime, the authors protect the fragile CGT layers with thin capping layers (Bi₂Te₃, Sb₂Te₃, or CdTe) deposited in‑situ. Despite the extremely small magnetic moment (~2 × 10⁻⁷ emu at 1 T), all capped 1 L samples display clear out‑of‑plane hysteresis in both AHE and SQUID, while in‑plane measurements show no hysteresis, confirming robust PMA. The saturation moment corresponds to ~1.35 μ_B per Cr atom, slightly reduced from the ideal Cr³⁺ value, likely due to defects and enhanced 2D fluctuations.

A systematic thickness‑dependent study (1 L to 6 L) reveals a crossover from fluctuation‑dominated 2D magnetism to conventional long‑range ferromagnetism. The 5 L and 6 L films exhibit rectangular AHE loops, high remanence ratios, and upward‑convex M‑T curves characteristic of bulk‑like ordering with Curie temperatures (T_C) approaching 60 K. In contrast, 1 L and 2 L films show canted loops, low remanence, and downward‑convex M‑T curves that never saturate, indicating strong thermal spin fluctuations that suppress long‑range order even below T_C (~10 K for 1 L). The intermediate 3 L and 4 L samples display nearly linear M‑T behavior, marking the crossover regime. The observed T_C versus thickness agrees well with a theoretical model that accounts for the reduced number of nearest‑neighbor layers and the associated exchange coupling.

Importantly, applying a modest perpendicular magnetic field dramatically enhances the monolayer T_C: at 0 T the 1 L CGT orders below ~10 K, while a field of 0.1 T raises T_C to ~40 K. This field‑induced stabilization is consistent with the Mermin‑Wagner theorem, which predicts that 2D Ising‑type order requires sufficient anisotropy; an external field effectively supplies this anisotropy, quenching spin fluctuations.

Overall, the paper establishes (i) a scalable, silicon‑compatible MBE route to produce uniform monolayer CGT over macroscopic areas via a self‑limiting growth mechanism, (ii) definitive evidence of intrinsic ferromagnetism with perpendicular anisotropy in the monolayer limit, and (iii) a clear thickness‑driven evolution from 2D fluctuation‑dominated magnetism to bulk‑like ferromagnetism, supported by both experiment and theory. These findings open pathways for integrating 2D magnetic semiconductors into spintronic and quantum devices, where controllable magnetic order at the atomic‑layer scale and compatibility with existing semiconductor technology are essential.