54 55 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 The discovery of inch-scale room-temperature 2Dmagnets Related publication title and DOI: Zhang, X., Lu, Q., Liu,W., Niu,W., Sun, J., Cook, J., Vaninger, M., Miceli, P. F., Singh, D. J., Lian, S.-W., Chang, T.-R., He, X., Du, J., He, L., Zhang, R., Bian, G., & Xu, Y. Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films. Nature Communications 12 , 2492 (2021). DOI: 10.1038/s41467-021-22777-x Publication keywords: 2Dmagnets; Ultrahigh vacuum (UHV); Molecular beamepitaxy (MBE); X-rayMagnetic Circular Dichroism (XMCD); Angle- Resolved Photoemission Spectroscopy (ARPES) E lectrical and electronic devices produce and dissipate heat, an undesirable energy loss. The search is therefore on for newmaterials for low-dissipation electronics. 2D magnets have great potential for this application; however, synthesising inch-scale 2D magnetic thin films is challenging. Mechanical exfoliation is a useful technique for producing research samples, but only produces micrometre scale thin films. Therefore, an international team of researchers is investigating the synthesis process (growth recipe) of inch-scale 2D magnets and their microscopic properties. Understanding both of these is a prerequisite for developing devices based on these materials. They successfully synthesised the first inch-scale epitaxial CrTe 2 /graphene 2D magnets with a ferromagnetic order persisting up to room temperature. Their Ultrahigh Vacuum (UHV) basedMolecular BeamEpitaxy (MBE) technique has been shown to be a critical tool for material discovery. The team performed X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) measurements on the Beamline for Advanced Dichroism Experiments (I10) at Diamond Light Source. I10 is equipped with a state-of-the-art XMCD end station that enabled the unambiguous determination of the atomic-scale magnetic properties of the epitaxial CrTe 2 /graphene 2Dmagnets. Devices based around 2D magnets are rapidly emerging. Their advantages include inherently sharp interfaces, ease of fabrication and low dissipation. The successful MBE growth of 2D ferromagnetic CrTe 2 films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2Dmagnet-based spintronics devices. This work offers tremendous potential for future devices, as the films produced can readily reach wafer size. Two-Dimensional (2D) magnets exhibit novel phases of quantum matter with abrupt transition in themagnon density of states in atomically thin layers. While the presence of 2Dmaterials with intrinsic magnetismhas been revealed recently 1-3 , their intrinsic Ferromagnetic (FM) order is typically fragile with a low Curie temperature (T C ) as a result of the large spin fluctuation in 2D. On the other hand, most of the 2Dmagnets synthesised so far are thin flakes exfoliated from their bulk counterparts with a typical size of several micrometres. There is a pressing need for obtaining inch-scale epitaxial 2D magnets with intrinsic FM persisting up to high temperatures so that any devices based around these materials would become possible. Recently we have successfully synthesised the first inch-scale monolayer CrTe 2 thin films on graphene in Ultrahigh Vacuum (UHV) using the Molecular Beam Epitaxy (MBE) technique. Previous work 4 has shown that MBE growth is the key to obtain the highest quality single-crystalline thin films and enable designer heterostructures in a way that is compatible with semiconductor manufacture. With the synchrotron-based X-ray Magnetic Circular Dichroism (XMCD) technique, we have unambiguously obtained a robust FM order of these CrTe 2 /graphene thin films with an atomic magnetic moment of ~0.21 μ B /atom and Perpendicular Magnetic Anisotropy (PMA) constant ( K u ) of 4.89 × 10 5 erg/cm 3 at room temperature. In-house Angle-Resolved Photoemission Spectroscopy (ARPES) studies also reveal a splitting of majority and minority band dispersions with ~0.2 eV at the gamma point. The 2D magnet CrTe 2 has a layered trigonal crystal structure as illustrated in Fig. 1a. Each unit cell consists of a hexagonal Cr layer sandwiched by two Te layers. The CrTe 2 thin films used in this work were grown on a bilayer graphene/ SiC(1111) substrate in an integrated MBE-STM system with base pressure below 2 × 10 −10 mbar. The bilayer graphene acts as a buffer layer to support the high-quality layer-by-layer epitaxial growth of the CrTe 2 films. The in-situ Scanning Tunnelling Microscopy (STM) has enabled an accurate determination of the microscopic topography of a few-layer CrTe 2 in their pristine state as shown in Fig. 1b-c: atomically flat terrace features can be clearly seen and a typical step height of 6.14 Å which is consistent with the thickness of the unit cell of CrTe 2 crystal in 1 T phase. Note the STM characterisation performed for all thicknesses (i.e. 1-15 monolayers or MLs) of the as-grown epitaxial CrTe 2 / graphene thin films are highly consistent, suggesting a well optimised growth process and high homogeneity of the samples. There are various stable stoichiometries for chromium chalcogenides [ e.g. , CrTe, CrTe 2 , Cr 2 Te 3 , and Cr 5 Te 8 ] depending on the Cr vacancies that occur in intercalation. Among them only CrTe 2 compounds are genuinely 2D materials. The layered surface morphology with a uniform step height obtained via STM suggests that the films are in a single phase. This is consistent with the X-ray Diffraction (XRD) 2θ-ω scans (Fig. 1e) where the perpendicular constant c = 6.13 Å agrees with the (001) crystal planes of the 1 T -type hexagonal structure. The reflectivity curves show Laue fringes, attesting to the structural coherence of the films. Crystal structure and lattice parameters are critical information as we now know that they can alter the nature of a magnetic exchange coupling: bulk 1 T -CrSe 2 with lattice constants of a = 3.39 Å and c = 5.92 Å shows an Antiferromagnetic (AFM) order,5 in contrast to the FM phase in CrTe 2 (a = 3.79 Å, c = 6.10 Å), for example. The atomic scale magnetic properties of the CrTe 2 /graphene epitaxial thin films were studied using the synchrotron-based XMCD techniques at beamline I10 at Diamond Light Source. High intensity circularly polarised X-rays were used in normal incidence with respect to the sample plane and parallel to the applied magnetic field. The XMCD was obtained by taking the difference of the XAS spectra by flipping the X-ray helicity at a fixed magnetic field of 10 kOe, under which the sample is fully magnetised with negligible paramagnetic contribution. The observed XAS spectral line shape is in line with that of spinel compounds with trivalent Cr cations on O h sites, providing a further spectroscopic fingerprint of 1 T -type CrTe 2 with predominately Cr 3+ cations. XMCD and XAS measurements were repeated at elevated temperatures, and the dichroism of the thin film at Cr L 3 edge persists up to 300 K. The sum-rules derived spin moments (ms) of Cr exhibit a Curie-like behaviour. A remarkably large value of ms (2.85 ± 0.10 μ B /atom) was obtained at 5 K. It retains a sizable value of 0.82 ± 0.10 μ B /atom at 250 K and drops to 0.21 ± 0.05 μ B /atom at 300 K. The electronic band structure of CrTe 2 thin films has beenmapped via an in- house ARPES measurement with two different photon energies of 21.2 eV and 40.8 eV at 107 K, and the origin of the band dispersions has been investigated by first-principle density function theory (DFT) calculations. The metallic state of the CrTe 2 /graphene has been attributed to the hybridisation of Te-5 p and Cr-3 d orbitals crossing the Fermi level at the centre of the Brillouin zone. We have further studied the thickness dependencies of hole pocket features in the energy dispersion spectra. In general with the increasing film thickness, the Fermi level moves towards the valence band with the band shape invariant. To conclude the authors have successfully synthesised the first inch-scale epitaxial CrTe 2 /graphene 2D magnets with a FM order persisting up to room temperature. The UHV-based MBE technique has been proven to be a critical tool for material discovery and unambiguous determination of the material properties at the atomic-scale as enabled by the synchrotron. The project was an international collaboration between the teams of Royal Holloway (UK), University of Missouri (US), and Nanjing University (China). The very helpful beamline team of I10, Dr Paul Steadman, Dr Peter Bencok, Dr Raymond Fan, andMark Sussmuth, has provided technical support tomake thiswork possible. References: 1. Gong, C. et al. Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals. Nature 546 , 265–269 (2017).DOI: 10.1038/ nature22060 2. Huang, B. et al. Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit. Nature 546 , 270–273 (2017). DOI: 10.1038/nature22391 3. Deng, Y. et al. Gate-tunable room-temperature ferromagnetism in two-dimensional Fe 3 GeTe 2 . Nature 563 , 94–99 (2018). DOI: 10.1038/ s41586-018-0626-9 4. Liu, S. et al. Two-dimensional ferromagnetic superlattices. National Science Review 7 , 745–754 (2020). DOI: 10.1093/nsr/nwz205 5. Freitas, D. C. et al. Antiferromagnetism and ferromagnetism in layered 1 T -CrSe 2 with V and Ti replacements. Physical Review B 87 , 014420 (2013). DOI: 10.1103/PhysRevB.87.014420 Funding acknowledgement: This work is supported by UK EPSRC (EP/S010246/1), UK Leverhulme Trust (LTSRF1819\15\12), UK Royal Society (IEC \NSFC\181680), US National Science Foundation (NSF- DMR#1809160), National Key Research and Development Program of China (No. 2016YFA0300803, No. 2017YFA0206304. Corresponding author : Wenqing Liu, Royal Holloway University of London,
[email protected] MagneticMaterials Group Beamline I10 Figure 1: As-grown material characterisation and XMCD spectra. (a) Illustration of the co-evaporation process in UHV; (b) Typical in-situ STM image; (c) Step profile of (b); (d) Fitted XRD spectrum; (e) Zoom-in STM image with hexagonal features; (f ) Step profile of (e); (g) XAS and XMCD spectra between 5 K and 300 K. Figure 2: Valence-band dispersion near the Fermi level obtained with (a) He Iα (21.2 eV) and (b) He IIα (40.8 eV), fitted with the DFT calculated bands; (c) 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.