Magnetic Carbon

It is generally accepted that carbon is the most versatile element of the periodic table, and it offers a plethora of compounds ranging from biology to materials science. While the list of fascinating properties carbon materials offer is long, they are not yet famous for magnetism.<br/>Indeed, most carbon materials are diamagnetic. Defects, dopants and dangling bonds can introduce paramagnetic centers without the potential to generate magnetic ordering. Recently reported magic-angle twisted bilayer graphene may become ferromagnetic due to a half-filled flat band at the fermi level and spin-orbit coupling [1]. A spectacular early report on magnetic carbon in pressurized fullerenes [2] was found to be caused by defects and the paper has been retracted five years later.<br/>We propose an alternative concept to generate carbon materials with strongly coupled magnetic centers. Our materials are based on molecular triangulene and its derivatives, aromatic molecules intrinsically carrying one or two unpaired electrons. Using covalent linkages that preserve electron conjugation, we construct two-dimensional polymers with honeycomb-kagome lattice. The magnetic coupling between the monomers is facilitated by the linker groups. This has been examined in detail for the dimers [3]. When extending this concept to 2D polymers, we predict magnetic carbon materials with intriguing electronic structure that includes Stoner ferromagnetism with Weyl points at the Fermi level, and Mott-Hubbard insulator antiferromagnetism, which would be the first metal-free ferro- and antiferromagnetic materials with strong magnetic coupling with a Curie/Néel temperature above 250 K [4]. If we use two different building blocks in a heterostructure, we are even able to construct materials where a Curie temperature exceeding 500K.<br/><br/><br/><b>References</b><br/><br/>A. A. Sharpe E. J. Fox, A. W. Barnard, J. Finney, K. Watanabe, T. Taniguchi, M. A. Kastner, D. Goldhaber-Gordon, ACS Nano <b>21</b>, 4299 (2021).<br/>T. L. Makarova, B. Sundqvist, R. Höhne, P. Esquinazi, Y. Kopelevich, P. Scharff, V. A. Davydov, L. S. Kashevarova, A. V. Rakhmanina, Nature <b>413,</b> 716, (2001) (retracted).<br/>H. Yu, T. Heine, J. Am. Chem. Soc. <b>145,</b> 19303 (2023).<br/>H. Yu, T. Heine, submitted, arxiv: 2311.09026