The common structural units of Fe-Fe dumbbells form multiple-layer Fe-rich slabs, which are stacked by vdW coupling.
#CRYSTALMAKER EMORY SERIES#
( B) Three stable vdW structures in the series of Fe nGeTe 2 identified for n = 3, 4, and 5 by ab initio calculations. ( A) Schematic illustration of combining two structural motifs of transition metal dichalcognenides MCh 2 ( M, transition metal Ch, chalcogen) and body-centered cubic iron for Fe-rich vdW ferromagnets Fe xMCh 2. This strongly contrasts to the cases of 3D ferromagnets, in which the spin-exchange interaction J alone determines T c, as found in, e.g., a bulk iron with T c ∼ 1000 K ( 30), much higher than found in vdW ferromagnets. Usually, the magnetic anisotropy is set by the magnetocrystalline anisotropy, due to the spin-orbit coupling, and thus is much smaller in energy than exchange interaction, suppressing T c in vdW ferromagnets. The ferromagnetic (FM) order is therefore stabilized against thermal fluctuations by the uniaxial magnetic anisotropy, and its energy scale K, together with spin-exchange interaction J, determines T c as T c ∼ J/ln(3π J/4 K) in the 2D limit ( 29). Furthermore, in the 2D limit, the Mermin-Wagner theorem ( 28) states that any continuous symmetry breaking similar to ferromagnetism cannot be spontaneous at a finite temperature. The pair-exchange interaction is mostly between the neighboring magnetic atoms in the 2D plane and is much weaker than in the 3D magnets. 1A) and forms a 2D magnetic system with a weak magnetic interlayer coupling across the vdW gap. In the typical TMDC-like structure, each layer of transition metal atoms is encapsulated by chalcogen or halogen atoms ( Fig. The relatively low T c of vdW ferromagnets is a consequence of their 2D nature (see Fig. If a stable vdW ferromagnet exists and also has a high conductivity and a large magnetization at room temperature, it can be used as a key component in all-vdW-material–based spintronic applications. Very recently, a vdW ferromagnet, Fe 5– xGeTe 2, has been synthesized, showing T c ∼ 300 K, while its crystal structure and the underlying mechanism of high- T c ferromagnetism have remained as an open question ( 26, 27). This may not be suitable for realistic spintronic applications but suggests that with proper material design, the room temperature ferromagnetism can be stabilized in a bulk vdW ferromagnet. Recently, the nearly room temperature ferromagnetism was induced in a few-layers-thick Fe 3GeTe 2 crystal by the sophisticated electrolyte gating ( 21). Despite recent developments on magnetic vdW materials ( 14– 25), most of the bulk vdW ferromagnets such as Cr(Si,Ge)Te 3 ( 17, 18), Cr(Br,I) 3 ( 19, 20), and Fe 3GeTe 2 ( 22, 23) have relatively low critical temperatures ( T c), well below the room temperature, which poses a challenge for using magnetic vdW materials in spintronic applications. One missing component is a metallic vdW ferromagnet for spin source, which can generate highly spin-polarized current at room temperature.
For spin manipulation, spin-valley locking in transition metal dichalcogenides (TMDCs) ( 5, 6) or spin-momentum locking in the surface state of bismuth chalcogenide–based topological insulators ( 7– 10) provides unprecedented electrical/optical spin control by their strong spin-orbit coupling ( 5, 6, 11– 13). For example, graphene, the first 2D vdW crystal, turns out to be the best-performing spin channel material with the longest spin diffusion length at room temperature due to its high mobility and low spin-orbit coupling ( 1– 4). Particularly in spintronics, 2D vdW crystals often surpass many known 3D materials because of their exceptional transport-, optical-, and spin-related properties. The isolation of various two-dimensional (2D) van der Waals (vdW) crystals has triggered intensive research on exploring new phenomena in the 2D limit and designing novel devices with tailored functionalities in their heterostructures. Junho Seo, Duck Young Kim, Eun Su An, Kyoo Kim, Gi-Yeop Kim, Soo-Yoon Hwang, Dong Wook Kim, … Show All …, Bo Gyu Jang, Heejung Kim, Gyeongsik Eom, Seung Young Seo, Roland Stania, Matthias Muntwiler, Jinwon Lee, Kenji Watanabe, Takashi Taniguchi, Youn Jung Jo, Jieun Lee, Byung Il Min, Moon Ho Jo, Han Woong Yeom, Si-Young Choi, Ji Hoon Shim, and Jun Sung Kim Show Fewer
0 kommentar(er)
