Multi-technique investigation of magnetic and electronic properties of isolated Cr7Ni rings grafted on Au(111) surface

Heterometallic Cr7Ni rings are a new class of Single Molecule Magnets (SMMs) recently proposed as candidates qubits. They are cyclic systems disposed in an octagonal geometry containing seven Cr(III) ions and one Ni(II) ion antiferromagnetically coupled providing an extra s=1/2 spin. By liquid phase deposition, we demonstrate the possibility to obtain a submonolayer distribution of Cr7Ni rings grafted on Au(111) exploiting the strong S-Au bound. The Cr7Ni-3tpc functionalization consists in sixteen isotropic thiophenecarboxylate ligands. From STM investigation we derived that about 15-25% of the total surface is occupied by well-isolated clusters and by XPS, we verified the ring integrity and confirmed the coverage derived from STM. A submonolayer of Cr7Ni-3tpc rings deposited on gold and a thick film, have been investigated by X-ray Absorption Spectroscopy (XAS), Magnetic Circular Dichroism (XMCD) in order to identify the spin and orbital magnetic moments arising from different specific atoms and Resonant Photoemission Spectroscopy (ResPES) to enlighten the different contributions of Cr and Ni derived states to the valence band electronic structure of the system. For the first time we were able to derive separately, by exploiting the XMCD sum rules, the Cr and Ni contributions to the total spin moment of the ring, at different temperatures (10K÷25K) and magnetic fields (1T÷5T), for a submonolayer distribution of isolated Cr7Ni-3tpc rings on gold. The behaviours experimentally derived are in excellent agreement with theoretical calculation performed by S. Carretta, where the microscopic parameters of the spin Hamiltonian have been directly determined by low-temperature specific heat magnetometry measurements on Cr7Ni-3tpc powders. It is crucial to emphasize, as derived from the analysis of the XAS spectra, the stability of the submonolayer distribution of Cr7Ni-3tpc if compared with the redox instability of the Mn12 systems.