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RESEARCH PRODUCT

Magneto-optical Investigations of Nanostructured Materials Based on Single Molecule Magnets Monitor Strong Environmental Effects

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The determination of the magnetic properties of molecular magnets in environments similar to those used in spintronic devices is fundamental for the development of applications. Single-molecule magnets (SMMs) are molecular cluster systems that display magnetic hysteresis of dynamical origin at low temperature. As they behave like perfectly monodisperse nanomagnets and show clear macroscopic quantum effects in their magnetic properties, they are extremely appealing candidates for the forthcoming generation of molecular devices: they have been proposed as efficient systems for quantum computation, ultra-high-density magnetic recording media, and molecular spintronic systems. These attractive possibilities have stimulated the creativity of chemists and materials scientists in developing several different ways of organizing such systems into addressable nanostructured materials. In particular inclusion into Langmuir-Blodgett (LB) films, mesoporous silica, polymeric matrices, and ultrathin films have been devised and studied. The most appealing approaches, both from the applicative and speculative points of view, are the functionalization and binding of such clusters on conducting surfaces as well as their incorporation into break-junctions, where a single molecule is directly accessible. These results have led to the creation of the first SMM-based molecular spintronic devices, in which the electronic transport properties are modulated by the magnetic state of a single SMM cluster. The considerable difficulties linked to the interpretation of such results have recently stimulated much theoretical work, and a number of predictions have been made. Both topological and quantum tunneling effects on the transport in the Kondo regime have been predicted, and several peculiar fingerprints of the SMM behavior should be apparent in transport measurements. Interesting effects are also predicted when addressing a SMM on a surface with a tunneling current. Although the influence of the surroundings on the magnetic properties of SMMs has been pointed out in several theoretical and experimental works, our understanding of SMM behavior almost totally relies on measurements performed on crystalline samples. Magnetic measurements on SMMs that lie in environments similar to those of spintronic devices have not been reported up to now, mainly because of the very high sensitivity required. In this Communication we try to fill this void by using high-sensitivity instrumentation, based on magneto-optical (MO) techniques on a variety of materials. We inserted three Mn12-based clusters derived from the archetypal SMM, [Mn12O12(CH3COO)16(H2O)4] (1), into different non-crystalline environments by substituting the acetate groups of the molecular clusters with appropriate peripheral ligands, but maintaining the same magnetic core. The first derivative, [Mn12O12(t-BuCOO)16(H2O)4] [18] (2) was embedded into a 3D amorphous environment. The pivalate substituents ease the inclusion and allow to obtain a wide range of concentrations. This permits to compare results at high and low dilutions on the same compound. The second derivative [Mn12O12(PhCOO)16(H2O)4] [2] (3) was included into LB thin films, while the third derivative [Mn12O12(pCH3SC6H4COO)16(H2O)4] [14] (4) was deposited onto Au surfaces. While it is not chemically possible to insert 2, 3 and 4 in the three environments (3 and 4, in particular, are chosen explicitly because they form LB films and SAMs, respectively) it is possible to compare the properties in the amorphous environment, so as to make sure that the ligand substitution itself C O M M U N IC A TI O N