Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films

Abstract

While the discovery of two-dimensional (2D) magnets opens the door for fundamental physics and next-generation spintronics, it is technically challenging to achieve the room-temperature ferromagnetic (FM) order in a way compatible with potential device applications. Here, we report the growth and properties of single- and few-layer CrTe₂, a van der Waals (vdW) material, on bilayer graphene by molecular beam epitaxy (MBE). Intrinsic ferromagnetism with a Curie temperature (T_C) up to 300 K, an atomic magnetic moment of ~0.21 [Formula: see text]/Cr and perpendicular magnetic anisotropy (PMA) constant (K_u) of 4.89 × 10⁵ erg/cm³ at room temperature in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device and X-ray magnetic circular dichroism. This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with ~0.2 eV at Г point using angle-resolved photoemission spectroscopy. The FM order is preserved with the film thickness down to a monolayer (T_C ~ 200 K), benefiting from the strong PMA and weak interlayer coupling. The successful MBE growth of 2D FM CrTe₂ films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices.

Fig. 1. Crystal structure and STM characterizations of epitaxially grown CrTe₂ thin films.

a Schematic illustration of MBE growth process of CrTe₂ films on graphene. b The STM topology image (200 × 200 nm²) of a 7 ML CrTe₂ fabricated on graphene/SiC. U = +1 V, I_t = 200 pA. Inset on the left is an optical image. c The line-scan profile taken along the pink line in (b), with an average step height of ~6.14 Å. d XRD spectrum showing Laue fringes around the (001) CrTe₂ reflections. The solid fitting curve indicates the thickness of 39 layers, the roughness of 2 layers, and the lattice constant c = 6.13 Å. e Atomically resolved STM image (4 × 4 nm²) with a hexagonal structure. U = −1.5 mV, I_t = −440 pA. f The line-scan along the green arrow in (e), showing a lattice periodicity of ~3.81 Å.

Fig. 2. SQUID measurements of the CrTe₂ films.

a Temperature dependent magnetization curves of the films with various thicknesses under field-cooled mode. The magnetic field is applied along the out-of-plane direction with a magnitude of 1000 Oe. The high T_C is preserved with thickness decreasing to 3 ML. b, c Magnetic hysteresis loops of 7 ML CrTe₂ at different temperatures with external fields along the perpendicular (b) and parallel orientation (c) with respect to sample plane, indicating a strong out-of-plane magnetic anisotropy. d Enlarged hysteresis loops of 7 ML CrTe₂ at 300 K, where the intrinsic ferromagnetism and PMA still maintains. Top inset: temperature dependence of K_u for 7 ML CrTe₂, where the K_u is preserved at 300 K, despite the lower intensity with the increase of temperature.

Fig. 3. XAS and XMCD characterization of 7 ML CrTe₂ films.

a Schematic geometry of XMCD experimental setup. b Typical pairs of XAS and XMCD spectra of 7 ML CrTe₂ from 5 K to 300 K and the integrals at 5 K, where the dichroism at Cr L₃ edge can be traced to 300 K (spectra at different temperatures are offset for clarify). c The partially enlarged XAS of Cr L₃ edge at 200 K, 250 K, and 300 K. d $m_{\mathrm{s}}$ and $m_{\mathrm{l}}$ versus temperature derived from (b) using sum rules. The error bars reflect the uncertainties in the background estimation for the XMCD sum rules analysis.

Fig. 4. XAS and XMCD characterization of CrTe₂ films with thickness of monolayer.

a Typical pairs of XAS and XMCD spectra of 1 ML CrTe₂ thin film at various temperatures, where the dichroism at Cr L₃ edge is observable up to 200 K. b The partially enlarged XAS spectra near the Cr L₃ edge, where the difference between left- and right-circularly polarized XAS is evident. c XMCD percentage as a function of temperature derived from a. The error bars indicate the uncertainties in the background estimation for the XMCD percentage calculation. d Compiled thickness–temperature phase diagram with the T_C obtained from XMCD and SQUID measurements. The error bars are the uncertainties in determining the T_C.

Fig. 5. Band structure of CrTe₂ ultrathin films.

a, b Plots of valence-band dispersion (a) and the first-principles calculations (b) of 7 ML CrTe₂ with the inclusion of spin polarization along the high symmetry direction M-Г-K. The minority and majority spin bands are plotted in red and blue colors, respectively. The major features seen in the left panel are well reproduced in the right one. c–e Comparison of the valence-band dispersion near the Fermi level taken by He Iα (21.2 eV) (c), He IIα photons (40.8 eV) (d) with theoretical bands (e) along the high symmetry direction M-Г-M. The blue and red dashed lines indicate the position of hole pockets measured by He Iα and He IIα photons, respectively. The light blue/red markers represent the positions of MDC peaks. The error bars represent uncertainties in locating peak positions. f ARPES intensity maps of 1 ML, 2 ML, 3 ML, 5 ML, 7 ML, and 15 ML, respectively. The spectra of various thicknesses were taken along the high symmetry direction M-Г-M.