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1. Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films

   https://doi.org/10.1038/s41467-021-22777-x  

   Zhang, Xiaoqian; Lu, Qiangsheng; Liu, Wenqing; Niu, Wei; Sun, Jiabao; Cook, Jacob; Vaninger, Mitchel; Miceli, Paul F.; Singh, David J.; Lian, Shang-Wei; et al (December 2021, Nature Communications)

   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 CrTe 2 , 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  $${\mu }_{{\rm{B}}}$$ μ B /Cr and perpendicular magnetic anisotropy (PMA) constant ( K u ) of 4.89 × 10 5  erg/cm 3 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 CrTe 2 films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices. 

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2. Spin stiffness and spin excitation gap of van der Waals ferromagnetic ${\mathrm{Fe}}_{3+\delta }{\mathrm{GeTe}}_2$

   https://doi.org/10.1088/1361-648X/ad581f  

   Shu, Zhixue; Zhang, Shufeng; Kong, Tai (June 2024, Journal of Physics: Condensed Matter)

   Abstract ${\mathrm{Fe}}_{3+\delta }{\mathrm{GeTe}}_2$(FGT) has proved to be an interesting van der Waals (vdW) ferromagnetic compound with a tunable Curie temperature ( $T_{\mathrm{C}}$). However, the underlying mechanism for varying $T_{\mathrm{C}}$remains elusive. Here, we systematically investigate and compare low-temperature magnetic properties of single crystalline FGT samples that exhibit $T_{\mathrm{C}}$s ranging from 160 K to 205 K. Spin stiffness (D) and spin excitation gap (Δ) are extracted using Bloch’s theory for crystals with varying Fe content. Compared to Cr-based vdW ferromagnets, FGT compounds have higher spin stiffness values but lower spin wave excitation gaps. We discuss the implication of these relationships in Fe–Fe ion magnetic interactions in FGT unit cells. The itinerancy of magnetic electrons is measured and discussed under the Rhodes–Wohlfarth ratio (RWR) and the Takahashi theory. 

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3. Emergence of ferrimagnetism in Li-intercalated NiPS ₃

   https://doi.org/10.1088/1361-648X/ac8a81  

   Basnet, Rabindra; Ford, Dawn; TenBarge, Kaylee; Lochala, Joshua; Hu, Jin (September 2022, Journal of Physics: Condensed Matter)

   Abstract Intercalation has become a powerful approach to tune the intrinsic properties and introduce novel phenomena in layered materials. Intercalating van der Waals (vdW) magnetic materials is a promising route to engineer the low-dimensional magnetism. Recently, metal thiophosphates,MPX₃, has been widely studied because their magnetic orders are highly tunable and persist down to the two-dimensional limit. In this work, we used electrochemical technique to intercalate Li into NiPS₃single crystals and found the emergence of ferrimagnetism at low temperature in Li-intercalated NiPS₃. Such tuning of magnetic properties highlights the effectiveness of intercalation, providing a novel strategy to manipulate the magnetism in vdW magnets. 

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4. Revealing room temperature ferromagnetism in exfoliated Fe ₅ GeTe ₂ flakes with quantum magnetic imaging

   https://doi.org/10.1088/2053-1583/ac57a9  

   Chen, Hang; Asif, Shahidul; Whalen, Matthew; Támara-Isaza, Jeyson; Luetke, Brennan; Wang, Yang; Wang, Xinhao; Ayako, Millicent; Lamsal, Saurabh; May, Andrew F; et al (March 2022, 2D Materials)

   Abstract Van der Waals (vdW) material Fe 5 GeTe 2 , with its long-range ferromagnetic ordering near room temperature, has significant potential to become an enabling platform for implementing novel spintronic and quantum devices. To pave the way for applications, it is crucial to determine the magnetic properties when the thickness of Fe 5 GeTe 2 reaches the few-layers regime. However, this is highly challenging due to the need for a characterization technique that is local, highly sensitive, artifact-free, and operational with minimal fabrication. Prior studies have indicated that Curie temperature T C can reach up to close to room temperature for exfoliated Fe 5 GeTe 2 flakes, as measured via electrical transport; there is a need to validate these results with a measurement that reveals magnetism more directly. In this work, we investigate the magnetic properties of exfoliated thin flakes of vdW magnet Fe 5 GeTe 2 via quantum magnetic imaging technique based on nitrogen vacancy centers in diamond. Through imaging the stray fields, we confirm room-temperature magnetic order in Fe 5 GeTe 2 thin flakes with thickness down to 7 units cell. The stray field patterns and their response to magnetizing fields with different polarities is consistent with previously reported perpendicular easy-axis anisotropy. Furthermore, we perform imaging at different temperatures and determine the Curie temperature of the flakes at ≈300 K. These results provide the basis for realizing a room-temperature monolayer ferromagnet with Fe 5 GeTe 2 . This work also demonstrates that the imaging technique enables rapid screening of multiple flakes simultaneously as well as time-resolved imaging for monitoring time-dependent magnetic behaviors, thereby paving the way towards high throughput characterization of potential two-dimensional (2D) magnets near room temperature and providing critical insights into the evolution of domain behaviors in 2D magnets due to degradation. 

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5. Room‐Temperature Ferromagnetism in MoTe ₂ by Post‐Growth Incorporation of Vanadium Impurities

   https://doi.org/10.1002/aelm.201900044  

   Coelho, Paula_Mariel; Komsa, Hannu‐Pekka; Lasek, Kinga; Kalappattil, Vijaysankar; Karthikeyan, Jeyakumar; Phan, Manh‐Huong; Krasheninnikov, Arkady_V; Batzill, Matthias (April 2019, Advanced Electronic Materials)

   Abstract Post‐synthesis doping of 2D materials is demonstrated by incorporation of vapor‐deposited transition metals into a MoTe₂lattice. Using this approach, vanadium doping of 2H‐MoTe₂produces a 2D ferromagnetic semiconductor with a Curie temperature above room temperature (RT). Surprisingly, ferromagnetic properties can be induced with very low vanadium concentrations, down to ≈0.2%. The vanadium species introduced at RT are metastable, and annealing to above ≈500 K results in the formation of a thermodynamically favored impurity configuration that, however, exhibits reduced ferromagnetic properties. Doping with titanium, instead of vanadium, shows a similar incorporation behavior, but no ferromagnetism is induced in MoTe₂. This indicates that the type of impurities in addition to their atomic configuration is responsible for the induced magnetism. The interpretation of the experimental results is consistent with ab initio calculations, which confirm that the proposed vanadium impurity configurations exhibit magnetic moments, in contrast to the same configurations with titanium impurities. This study illustrates the possibility to induce ferromagnetic properties in layered van der Waals semiconductors by controlled magnetic impurity doping and thus to add magnetic functionalities to 2D materials. 

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