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Bisphthalocyaninato terbium complexes show a long magnetization relaxation time at relatively high temperatures-which makes them very interesting as magnets at single-molecule level. Their technological exploitation, however, requires the addressing of the individual molecules, therefore the deposition of single-molecule magnets on surfaces is a topic of great interest as the interaction with the substrate can play a crucial role in the definition of the molecule properties. In this work we investigate the electronic and magnetic properties of anionic and neutral forms of a bis(phthalocyaninato)terbium derivative deposited on graphite by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism, performed at low temperature and high magnetic field at the M4,5edge of Tb. We were able to reproduce the experimental spectra by means of multiplet calculations and to validate the applicability of sum rules to the present case. Sum rules were then used for determining the orbital and spin moments of thick (several monolayers) and of thin films (submonolayer range). Calculations of spectra as a function of the molecule orientation with respect to the impinging x-ray beam, allowed us to ascertain the adsorption geometry of molecules. For both compounds, molecules stay essentially flat when adsorbing as thin film on graphite. This result is also confirmed by scanning probe microscopy, which also finds a very interesting ordered arrangement for the molecules of the neutral form. In the thick film of the neutral compound the molecules keep the same orientational order, arranging almost flat as well. On the contrary, in the thick film of the anionic compound their orientation appears to be random. The origin of this different behavior can be related to the hindrance of the counterion moiety and/or to the different solvent used for each compound. Finally, the comparison of the magnetization values and their dependence on the external magnetic field and temperat

Antiferromagnetic spin segments are quantum objects with discrete spectrum of excitations. Here we report the characterization of two molecular derivatives, namely, Cr6 InNi and Cr7 InNi, which are odd- and even-membered spin cyclic systems magnetically broken by an In3+ ion. Analysis of low-temperature specific heat, susceptibility, and magnetization allows us to determine the microscopic parameters of the spin Hamiltonian and the pattern of the spin excitations. It turns out that the exchange coupling and the anisotropy constants in Cr6 InNi and Cr7 InNi are quite close to those in Cr7 Ni-piv. X-ray absorption experiments evidence that the oxidation state of Cr3+ and Ni2+ and their local symmetries are essentially the same in the Cr6 InNi, Cr7 InNi, and Cr7 Ni-piv clusters. Thus the main difference resides in the number and topology of spins in the different derivatives. Based on these grounds, we show how the spin of Ni2+ can be used as a marker to probe the local magnetization of spin segments by directly comparing the sign and the intensity of the dichroic signal measured in the three derivatives. Quantitative analysis of x-ray absorption spectra allows the evaluation of the spin and orbital magnetic moment in the different derivatives, whose dependence on the magnetic field fits well the results of the microscopic spin-Hamiltonian calculations. © 2009 The American Physical Society

We deeply investigated the properties of submonolayer distributions of isolated molecular Cr7 Ni rings deposited on Au(111) by liquid phase. X-ray absorption spectra measured at the Cr and Ni L2,3 edges show that the grafting of the Cr7 Ni rings onto the gold surface does not affect the oxidation state and the local symmetry of the Cr and Ni sites. The circular dichroism shows a change in sign of the Ni magnetic moment. This is due to a reduction in the exchange coupling constants that, however, preserves the structure of the low-energy levels of the grafted rings, as corroborated by spin-Hamiltonian simulations and comparison with measurements on bulk sample. Density-functional theory calculations show that the Ni-Cr bond gets weaker with slight ring distortion suggesting possible explanation for the observed magnetic behavior. These results show that complex magnetic molecules can be grafted onto surfaces, and that changes in their magnetic behavior must be examined in individual cases. © 2009 The American Physical Society

Single molecule magnets (SMMs) form a new class of magnetic materials consisting of identical nanoscale particles that can show magnetization in the absence of a magnetic field. We have experimentally and theoretically investigated the low-spin (S=4) member of the Mn6 SMM family, properly functionalized with two 3-thiophenecarboxylate (3tpc) ligands in order to graft it on to a Au(111) surface. We report the theoretical density of states calculated within the local density approximation (LDA) scheme accounting for the on-site Coulomb repulsion (LDA+U) for U values ranging from 0 to 8 eV. On the experimental side, by exploiting resonant photoemission at the Mn 2p edge, we were able to single out the Mn 3d derived states in the valence band energy region for a submonolayer distribution of Mn6-3tpc deposited on Au(111). From the comparison between the experimentally derived 3d density of states and the theoretical one, we found that the best agreement occurs for a U value of 4 eV. From the binding energy of Mn 2 p3/2 core line, measured in situ, we also derived a value for the 2p-3d correlation energy of about 5 eV -in agreement with previous determination. © 2008 The American Physical Society

We investigated the electronic and magnetic properties of thick films of molecular Cr8 and Cr7 Ni antiferromagnetic rings by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD). We determined the local symmetries, the electronic configuration, and the values of orbital and spin moments at the Cr and Ni sites of the molecular rings. XMCD measurements show that the correlation between the Cr and Ni spins in the Cr7 Ni molecular ring switches from antiferromagnetic to ferromagnetic with increasing temperature. Experimental data are interpreted using XMCD sum rules that allow the separate evaluation of the spin and the orbital contributions to the total magnetic moment of the ring as a function of temperature and magnetic field. The magnetic behaviors experimentally observed are compared with the results of spin-Hamiltonian calculations, based on microscopic parameters derived by inelastic-neutron scattering and low-temperature specific-heat measurements. The very good agreement between experimental data and calculations is a clear indication of the integrity of molecules. The temperature dependence of the ion magnetic moments results from the interplay between Zeeman and isotropic-exchange contributions, and is well captured by the theoretical model. © 2008 The American Physical Society

We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn6 family (general formula [Mn III6O2(R-sao)6(O2C-th) 2L4-6, where R = H (1) or Et (2), HO2C-th = 3-thiophene carboxylic acid, L = EtOH, H2O and saoH2 is salicylaldoxime) and their grafting on the Au(111) surface. Complex 1 exhibits spin ground-state S = 4, as the result of ferromagnetic coupling between the two antiferromagnetic MnIII3 triangles, while slight structural changes in complex 2, switch the dominant magnetic exchange interactions from anti- to ferromagnetic, enhancing the spin ground-state to S = 12 and, consequently, the effective energy barrier for the relaxation of magnetization. Direct-current and alternating-current magnetic susceptibility measurements show that the functionalized complexes preserve the main magnetic properties of the corresponding not-functionalized Mn6 clusters (i.e., total spin value and magnetic behavior as a function of temperature), though a reduction of the anisotropy barrier is observed in complex 2. For both complexes, the -O2C-th functionalization allows the direct grafting on Au(111) surface by liquid-phase deposition. X-ray photoemission spectroscopy demonstrates that the stoichiometry of the molecular cores is preserved after grafting. Scanning tunneling microscopy (STM) reveals a sub-monolayer distribution of isolated clusters with a slightly higher coverage for complex 1. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both derivatives, the strength of the grafting with the gold surface. © 2008 American Chemical Society

Learning the art of exploiting the interplay between different units at the atomic scale is a fundamental step in the realization of functional nano-architectures and interfaces. In this context, understanding and controlling the magnetic coupling between molecular centers and their environment is still a challenging task. Here we present a combined experimental-theoretical work on the prototypical case of the bis(phthalocyaninato)-lanthanide(III) (LnPc2) molecular nanomagnets magnetically coupled to a Ni substrate. By means of X-ray magnetic circular dichroism we show how the coupling strength can be tuned by changing the Ln ion. The microscopic parameters of the system are determined by ab-initio calculations and then used in a spin Hamiltonian approach to interpret the experimental data. By this combined approach we identify the features of the spin communication channel: the spin path is first realized by the mediation of the external (5d) electrons of the Ln ion, keeping the characteristic features of the inner 4 f orbitals unaffected, then through the organic ligand, acting as a bridge to the external world.