Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals

• Published in: Nature (London) Vol. 546; no. 7657; pp. 265 - 269
• Main Authors: Gong, Cheng, Li, Lin, Li, Zhenglu, Ji, Huiwen, Stern, Alex, Xia, Yang, Cao, Ting, Bao, Wei, Wang, Chenzhe, Wang, Yuan, Qiu, Z. Q., Cava, R. J., Louie, Steven G., Xia, Jing, Zhang, Xiang
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 08.06.2017; Nature Publishing Group
• Subjects: 132/124; 639/301/119/2793; 639/925/357/1018; Anisotropy; Composition effects; Crystals; Discovery and exploration; Electric properties; Electron spin; Excitation; Ferromagnetism; Fluctuations; Humanities and Social Sciences; Iron; letter; Magnetic anisotropy; Magnetic fields; Magnetic properties; Magnetism; multidisciplinary; Optical properties; Physics research; Proximity; Proximity effect (electricity); Scanning; Science; Spintronics; Temperature effects; Transition temperature; Transition temperatures; Van der Waals forces
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature22060

Intrinsic long-range ferromagnetic order is observed in few-layer Cr 2 Ge 2 Te 6 crystals, with a transition temperature that can be controlled using small magnetic fields. Magnetism in flatland The question of what happens to the properties of a material when it is thinned down to atomic-scale thickness has for a long time been a largely hypothetical one. In the past decade, new experimental methods have made it possible to isolate and measure a range of two-dimensional structures, enabling many theoretical predictions to be tested. But it has been a particular challenge to observe intrinsic magnetic effects, which could shed light on the longstanding fundamental question of whether intrinsic long-range magnetic order can robustly exist in two dimensions. In this issue of Nature , two groups address this challenge and report ferromagnetism in atomically thin crystals. Xiang Zhang and colleagues measured atomic layers of Cr 2 Ge 2 Te 6 and observed ferromagnetic ordering with a transition temperature that, unusually, can be controlled using small magnetic fields. Xiaodong Xu and colleagues measured atomic layers of CrI 3 and observed ferromagnetic ordering that, remarkably, was suppressed in double layers of CrI 3 , but restored in triple layers. The two studies demonstrate a platform with which to test fundamental properties of purely two-dimensional magnets. The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications 1 , 2 , 3 , 4 . In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin–Wagner theorem 5 ; however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering 6 , 7 , 8 , 9 , 10 , or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine Cr 2 Ge 2 Te 6 atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. Cr 2 Ge 2 Te 6 is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.