⇦Chinese Physics Letters, 2020, Vol. 37, No. 8, Article code 080101Views & Comments Search for 2D Ferromagnets: Molecular Beam Epitaxy is a Critical Tool Matthias Batzill* Affiliations Department of Physics, University of South Florida, Tampa, FL 33620, USA Received 28 June 2020; accepted 29 June 2020; published online 03 July 2020 *Corresponding author. Email: mbatzill@usf.edu Citation Text: Batzill M 2020 Chin. Phys. Lett. 37 080101    Abstract DOI:10.1088/0256-307X/37/8/080101 PACS:01.10.-m, 75.70.Ak, 75.70.-i, 81.15.Hi © 2020 Chinese Physics Society Article Text Van der Waals materials with ferromagnetic properties have attracted recent interest because of their promise to enable combination of materials with atomically sharp interfaces, facilitating spin transport and spin injection across these interfaces. The discovery of ferromagnetic ordering to persist to the two-dimensional (2D) limit for Cr$_{2}$Ge$_{2}$Te$_{6}$[1] and CrI$_{3}$[2] has sparked interest of 2D ferromagnetism and their potential for fundamentally new magnetic van der Waals heterostructures and spintronics applications. However, these materials have been exfoliated from bulk and are not easily processible for applications. A bottom up synthesis by direct growth is required to integrate different materials for practical applications and to enable detailed characterization of magnetic interfaces. The growth of VSe$_{2} $[3] and MnSe$_{2}$[4] by molecular beam epitaxy (MBE) have created hope that such materials can exhibit strong magnetism with high Curie temperatures. However, subsequent detailed work on the properties of VSe$_{2}$ have indicated that it is not an itinerant magnetic material,[5,6,7] and the observed magnetism in these materials remains a controversial topic.[8] Other potential ferromagnetic materials that can be reduced to the 2D limit and grown by MBE are currently investigated. Such materials include chromium telluride-based compounds.[9] More generally, direct growth methods, like MBE, allow the development of new 2D materials that are not inherently layered materials and thus would expand the possible materials systems for investigating 2D magnetism. The paper by Yuan et al.[10] neatly demonstrates the potential of MBE growth for the synthesis of novel ferromagnetic 2D materials. In this work, a single layer MnSn alloy on a Si(111) substrate is demonstrated to exhibit ferromagnetic properties in the 2D limit. MnSn has been predicted to be a half-metallic compound with a zinc-blende structure in the bulk. The zinc-blende structure has a 3D covalent bonding and thus may not be thinned to a monolayer by exfoliation. MnSn was directly grown on a Sn-buffer layer on Si(111), detailed STM characterization combined with crystal structure search allowed them to identify the new material they synthesized and showed that it can be grown in a structure with alternating Sn and Mn layers. Importantly, they show that a single molecular layer that is structurally the same as multilayer films can be obtained. Convincing agreement between the simulated STM images of the proposed structure with the experiments verified their obtained compound. Interestingly, this single monolayer exhibits large magnetic moments of $\sim 2.3 \mu_{\rm B}$ per Mn-atom and a $T_{\rm c}$ of $\sim 54$ K is measured. The $T_{\rm c}$ for the single layer is strongly reduced compared to the multilayer films that have a $T_{\rm c} \sim$ 235–245 K. A strong decrease of $T_{\rm c}$ with the reduction of layers is expected due to thermal fluctuations destroying the magnetic ordering in single layer materials and thus this effect supports the notion of a monolayer ferromagnet. The fact that this material can be grown on silicon also shows potential of integrating it with the mature silicon technology. While the initial characterization of the MnSn single layer material is very promising, additional work will be needed to fully understand the magnetic properties of this material. For instance, the in-plane magnetic anisotropy and loss of anisotropy for the multilayers are surprising. Magnetic anisotropy is required to counteract the destruction of spin ordering by thermal fluctuation and usually an out-of plane easy axis is observed for 2D magnets. This may raise the question of the importance of the interface with the substrate to stabilize the magnetic ordering in this system. Very few layered materials that can be exfoliated exhibit ferromagnetic ordering. Therefore, the development of new 2D materials that have no layered bulk counterparts are required. Such materials can only be synthesized as 2D materials by direct growth methods like MBE. Thus, these materials science methods are critically important to advance this field. The study by Yuan et al. showed that new exciting materials can be obtained, but for exploring more such systems the challenge remains in reliable theoretical predictions. And while theoretical guidance is necessary to motivate the synthesis of certain materials systems, another lesson from the work by Yuan et al. is that initial expectations for a compound may have to be adjusted for understanding the structure of the real material. References Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystalsLayer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limitStrong room-temperature ferromagnetism in VSe2 monolayers on van der Waals substratesRoom Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer LimitCharge Density Wave State Suppresses Ferromagnetic Ordering in VSe 2 MonolayersEmergence of a Metal–Insulator Transition and High-Temperature Charge-Density Waves in VSe 2 at the Monolayer LimitElectronic Structure and Enhanced Charge-Density Wave Order of Monolayer VSe 2Can Reconstructed Se‐Deficient Line Defects in Monolayer VSe 2 Induce Magnetism?Molecular Beam Epitaxy of Transition Metal (Ti-, V-, and Cr-) Tellurides: From Monolayer Ditellurides to Multilayer Self-Intercalation CompoundsFerromagnetic MnSn Monolayer Epitaxially Grown on Silicon Substrate
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