Strong Magnetocrystalline Anisotropy Arising from Metal-Ligand Covalency in a Metal-Organic Candidate for 2D Magnetic Order

Layered metal-organic frameworks are promising candidates for new two-dimensional (2D) magnets, as the synthetic programmability of these materials can provide a route to diverse structural and electronic properties. However, such framework materials typically lack the heavy elements that engender magnetocrystalline anisotropy in the monolayer ferromagnets reported to date. Alternative sources of magnetic anisotropy are therefore needed in these materials. Here, we report the synthesis of single crystals of the framework material (NMe4)(2)[Fe2L3] (H-2 L = 3,6-dichloro-2,5-dihydroxybenzoquinone) and evaluate the angular dependence of its magnetic properties. Oriented-crystal magnetization measurements reveal strong uniaxial anisotropy, where the easy axis is aligned with the crystallographic c axis. While the spin carriers of this structure are isotropic S = 5/2 Fe-III metal centers and S = 1/2 organic linkers, the anisotropy energy of the framework material is comparable to that of reported 2D ferromagnets. Density functional theory calculations indicate that the observed magnetocrystalline anisotropy arises from ligand-tometal charge transfer that enhances the magnetic anisotropy of the otherwise-isotropic Fe centers, suggesting that metal-ligand covalency can be utilized as a general additive for the development of 2D magnets. These results show the possibility for (NMe4)(2)[Fe2L3] to retain magnetic order down to the 2D monolayer limit. In addition, the combination of large magnetic anisotropy and semiconducting character in (NMe4)(2)[Fe2L3] highlights its potential as a new 2D magnetic semiconductor.