Competing Intramolecular Superexchange Interactions in the CuFe4 Metallacrown on Au(111)─An Inelastic Tunneling Spectroscopy Study

Abstract

Spin-flip inelastic tunneling spectroscopy (SF-IETS) at low temperatures allows the electrical characterization of surface magnetism, particularly for molecule/ substrate systems which can be interesting for molecular spintronics. Here, SF-IETS was used to explore the competing character of intramolecular superexchange interactions on a device-compatible metallic substrate. For this purpose, the multinuclear metallacrown system CuFe4, [Cu(ii)(DMF) 2 Cl 2 [12-MC Fe(III)N(Shi) -4](DMF) 4 •2DMF], on Au(111) and its resulting "surface" spin ground state were studied. After the deposition of CuFe4 molecules by a solution-based technique onto a Au(111) surface, the CuFe4 system exhibits an evident hallmark of inelastic tunneling, that is, step-like features on the differential conductance spectra, which we attribute to spin-flip excitations. By analyzing the experimental curve with a second-order electron transport model simulation, we determined the exchange coupling constants and the spin ground state. Our results are compared with those spin ground states published in the literature for this compound based on the broken-symmetry density functional theory calculations and temperature-dependent magnetic susceptibility measurements.

Spin-Flip Inelastic Electron Tunneling Spectroscopy on a CuCu4 Metallacrown Complex on Au(111)

“…While CuFe4 is decoupled from the metallic substrate by additional co-ligands orthogonal to the molecular plane, CuCu4 shows clear spin-spin scattering processes with the electron bath of the gold substrate. The J 1 / J 2 values found from the SF-IETS measurements for CuFe4 showed very good agreement with the J 1 / J 2 values determined from χ M T ( T ) measurements , In particular, the steric interaction of the additional ligands and their feedback on the actual CuFe4 core system was shown to be of crucial importance for the observed J 1 / J 2 coupling. This is different for the CuCu4 shown here.…”

“…The investigations of possible spin-spin interactions between metal–organic multispin systems (metallacrowns) and a metallic gold surface using SF-IETS clearly show the potential of this measurement method for the characterization of magnetic properties of such hybrid systems. Especially the comparison between the two metallacrown/Au (111) systems with the CuCu4 investigated here and the recently published CuFe4 (Cu­(II)­(DMF) 2 Cl 2 [12-MC Fe(III)N(Shi) -4]­(DMF) 4 ·2DMF) impressively shows the different behavior of these two systems. While CuFe4 is decoupled from the metallic substrate by additional co-ligands orthogonal to the molecular plane, CuCu4 shows clear spin-spin scattering processes with the electron bath of the gold substrate.…”

“…To avoid this and to ensure the molecular magnetic states’ stability, protective interlayers such as graphene, CuN, or h-BN are usually used between the metallic substrate and the molecule. − However, charge transfer and structural changes can occur as well with a protective interlayer. , Nevertheless, SF-IETS signatures on a molecule adsorbed directly on a metallic surface have been observed. ,− We recently successfully studied spin-flip processes on a heterometallic metallacrown molecule (CuFe4) on Au(111) using SF-IETS . We were able to determine the changes in the strength of intramolecular exchange coupling constants upon adsorption on a Au(111) substrate . However, the CuFe4 molecule bears bulky ligands, which protect the spin centers from close contact with the metal surface, in contrast to the here-studied CuCu4 complex.…”

“…In this context, the use of SF-IETS at low temperatures is an excellent possibility for the detection of spin excitations and correlations of metal–organic complexes adsorbed on surfaces. Only a few SF-IETS studies on such molecular systems have been shown so far. − For most metal complexes, the high molecular weight combined with the thermodynamic instability of metal–organic compounds at high temperatures makes it impossible to use the standard evaporation techniques, while new innovative solvent-based techniques are still scarce. At the same time, it is difficult to observe experimental spin-flip excitations for complex metal–organic spin systems deposited directly on a metallic surface.…”

“…Even without structural changes, the interaction with the electrons of a metallic substrate can suppress molecular conductivity in the observed low-bias region at all. To avoid this and to ensure the molecular magnetic states’ stability, protective interlayers such as graphene, CuN, or h-BN are usually used between the metallic substrate and the molecule. − However, charge transfer and structural changes can occur as well with a protective interlayer. , Nevertheless, SF-IETS signatures on a molecule adsorbed directly on a metallic surface have been observed. ,− We recently successfully studied spin-flip processes on a heterometallic metallacrown molecule (CuFe4) on Au(111) using SF-IETS . We were able to determine the changes in the strength of intramolecular exchange coupling constants upon adsorption on a Au(111) substrate .…”