Stability of the Cr7Ni rings grafted on Au(111) surface

Heterometallic Cr7Ni rings are a new class of molecular nanomagnet (MNM) recently proposed as hardware for the qubit encoding. They are cyclic spin systems disposed in an octagonal geometry containing seven Cr(III) ions and one Ni(II) ion antiferromagnetically coupled providing a molecular ground state s=1/2 spin. We report here the design of new Cr7Ni derivatives and procedures to graft them onto suitable surfaces. The aim of this work is to understand if this MNM preserves its properties when grafted on surface, or, better to say, to which extent we are able to control its interaction with the surface. By combining STM and XPS investigations we demonstrate the possibility to obtain, by liquid phase deposition, a submonolayer (sub-ML) distribution of Cr7Ni rings grafted on Au(111) exploiting new strategies and different functionalizations. In particular we designed and tested a new derivative (CoPc-Cr7Ni), where the Cr7Ni ring is attached, through pyridine group, with a single Co ion embedded in a phthalocyanine (CoPc) matrix. The Co(II) sites are five-coordinate and the Co ion has S=1/2 (low-spin configuration). All the Cr7Ni sub-ML distributions obtained for the different derivatives and the corresponding thick films (TF), 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. From the analysis of the XAS and XPS spectra it turns out the chemical and structural stability of the Cr7Ni ring, for all the sub-ML distributions studied. A special code, based on Wien2k simulation package, has been recently developed by V. Bellini at our institute and it is currently used to interpret XMCD spectra from the L23 edges. Preliminary results show good fitting of the main features of the spectra. By exploiting the XMCD sum rules, the Cr and Ni contributions to the total spin moment of the Cr7Ni ring, at different temperatures and magnetic fields (up to 5T), for the sub-ML on gold. The sub-ML and the TF behaviours are compared with that with spin-Hamiltonian simulations, where the microscopic parameters of the spin Hamiltonian have been directly determined by low-temperature specific heat magnetometry measurements on Cr7Ni powders.