Publication in the Science Magazine
Volume 374/ Issue 6567 / 29 Oct. 2021
Taking the measure of a magnet: The recent discovery of magnetism in two-dimensional (2D) materials has inspired efforts to understand its nature. Whereas the magnetism of monolayers of chromium iodide (CrI3) can be understood in terms of out-of-plane magnetic anisotropy, the related material chromium chloride (CrCl3) has spins that lie in the plane. Bedoya-Pinto et al. used molecular beam epitaxy to grow monolayers of CrCl3 on graphene and studied its magnetic properties. Using x-ray magnetic circular dichroism measurements, the authors found that monolayer CrCl3 is a ferromagnet, unlike bulk CrCl3, which is antiferromagnetic. The scaling of the signal in the critical region indicated that the material belongs to the 2D-XY universality class, distinct from Ising magnetism, which some other 2D magnets exhibit. —JS
The physics and universality scaling of phase transitions in low-dimensional systems has historically been a topic of great interest. Recently, two-dimensional (2D) materials exhibiting intriguing long-range magnetic order have been in the spotlight. Although an out-of-plane anisotropy has been shown to stabilize 2D magnetic order, the demonstration of a 2D magnet with in-plane rotational symmetry has remained elusive. We constructed a nearly ideal easy-plane system, a single CrCl3 monolayer on graphene/6H-SiC(0001), and observed robust ferromagnetic ordering with critical scaling characteristic of a 2D-XY system. These observations indicate the realization of a finite-size Berezinskii-Kosterlitz-Thouless phase transition in a large-area, quasi–free-standing van der Waals monolayer magnet with an XY universality class. This offers a material platform to host 2D superfluid spin transport and topological magnetic textures.
A.B.-P. thanks the HZB and CELLS-ALBA for the allocation of synchrotron radiation beamtime under proposals 192-08773-ST and 202-09917-ST (HZB) and 2019-093862 (CELLS-ALBA). We thank F. Küster for assistance with the LT-STM measurements and C. Luo, K. Chen, S. Thakur, and S. Rudorff for technical support at BESSY.
Fundings: F.R. and A.B.-P. acknowledge financial support for the VEKMAG project and for the PM2-VEKMAG beamline by the German Federal Ministry for Education and Research (BMBF 05K10PC2, 05K10WR1, 05K10KE1) and by HZB. M.V. and P.G. acknowledge additional beamtime through ALBA IHR and proposal ID 2019023487-2, and funding via grants FIS2016-78591- C3-2-R (AEI/FEDER, UE) and FlagEra Sograph MEM PCI2019- 111908-2. K.C. was funded by National Natural Science Foundation of China (grant 12074038)
Author contributions: A.B.-P., K.C., and S.S.P.P. conceived of the study, and A.B.-P. was the lead researcher. A.B.-P. carried out the magnetic characterization by XAS/XMCD, analyzed the data, and wrote the manuscript. J.-R.J. grew the samples for the beamtimes and performed the in situ RHEED and STM characterization. K.C. initiated and optimized the substrate preparation and epitaxial growth. P.S. performed the low-temperature STM characterization. J.-R.J., A.K.P., J.M.T., and F.R. assisted with the XMCD measurements in BESSY; P.G. and M.V. assisted and performed part of the XAS/XMCD experiments in ALBA. All authors discussed the data and commented on the manuscript. S.S.P.P. supervised the entire project.