Superexchange-Mediated Ferromagnetic Coupling in Two-Dimensional Ni-TCNQ Networks on Metal Surfaces

Abstract

We investigate the magnetic coupling of Ni centers embedded in two-dimensional metal-coordination networks self-assembled from 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules on Ag(100) and Au(111) surfaces. X-ray magnetic circular dichroism measurements show that single Ni adatom impurities assume a spin-quenched configuration on both surfaces, while Ni atoms coordinating to TCNQ ligands recover their magnetic moment and exhibit ferromagnetic coupling. The valence state and the ferromagnetic coupling strength of the Ni coordination centers depend crucially on the underlying substrate due to the different charge state of the TCNQ ligands on the two surfaces. The results suggest a superexchange coupling mechanism via the TCNQ ligands.

Breaking time-reversal symmetry at the topological insulator surface by metal-organic coordination networks

“…The latter is provided by the strong chemical bonding to the cyano groups of the organic ligands which prevents adatoms from diffusion on the surface or below it. Importantly, the local spins of the metallic centers don't quench upon contact with the substrate and the FM exchange interactions mediated by the organic ligands have been shown to exist in Fe-and Ni-based MOCNs deposited on the noble metal surfaces [58,60,64].…”

“…Namely, to achieve the long-range FM order in the system of adatoms at the TI surface one has to organize their ordered arrays [14,34,35] and prevent them from diffusion [44,47] or intercalation inside the bulk or the van der Waals gaps [34,[51][52][53][54]. Lately, a type of system which may provide these conditions were successfully grown on metallic substrates-selfassembled metal-organic coordination networks (MOCNs) [55][56][57][58][59][60][61][62][63][64]. In such experiments, strong electron acceptors like tetracyanoethylene (TCNE, C 6 H 4 ), tetracyanobenzene (TCNB, C 10 H 2 N 4 ), and tetracyanoquinodimethane (TCNQ, C 12 H 4 N 4 ) are the most frequently used molecules.…”

Site-specific bonding of copper adatoms to pyridine end groups mediating the formation of two-dimensional coordination networks on metal surfaces

“…Intriguing further aspects arise if the transition-metal core exhibits a magnetic moment [10]. The organic linker may then mediate a superexchange coupling leading to metal-organic ferromagnetism [11,12].…”

“…Self-assembled metal-organic networks consisting of molecular linkers and metal species provide longrange order [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] and a programmable network architecture based on the interplay of the functionalized parts of the organic molecule and the metal species. With the choice of the symmetry and size of the organic linker, the geometry of the metal-organic network can be tuned [5,6].…”

Enhanced Cooperativity in Supported Spin-Crossover Metal–Organic Frameworks

“…26 With this goal in mind, a promising route might come from the use of self-assembled metalorganic framework (MOF). 27 Indeed, magnetic couplings 28 were reported in several 2D MOF based on 3d ions such as Fe, 29,30 Ni, 31 Mn 32,33 or Cu. 34 Recently, Umbach et al have investigated a Au(111)-supported MOF based on Fe(II) ions and 2,4,6-tris(4-pyridyl)-1,3,5-triazine (T4PT) ligand.…”