Anisotropic magnetic properties and relaxation dynamics in molecular magnets

Single Molecule Magnets (SMMs) are molecules that show, at cryogenics temperatures, slow relaxation of magnetization and the opening of a hysteresis loop. The origin of such a behavior is rooted at the molecular level, at variance with ferromagnets whose magnetic properties derive from long-range ordering. Indeed, the peculiar features of SMMs arise from their electronic energy structure that exhibits a property known as bistability. Since the discovery of the first SMM, the interest for these systems has been dual: from the fundamental point of view SMMs present very interesting aspect, like the coexistence of classical and quantum phenomena, while from the technological point of view, they have been proposed ad candidates for application in high density storage, molecular spintronic and quantum information processing. In view of a possible technological exploitation of these systems, the efficiency of SMMs, i.e. the temperature at which bistability is observed, has to be improved. The research activity presented in this dissertation concerns the investigation of the properties of SMMs at a more fundamental level. In particular, since any improvement in the SMM efficiency would require a detailed comprehension of the mechanisms involved in the slow relaxation process, we focused on the relation between this and the electronic structure of the involved magnetic ion. Indeed, while the relaxation involving the overcome of a magnetic anisotropy barrier (Orbach process) is quite invariably the dominant one for polynuclear, transition metal based clusters, this is not the case for single metal molecules. Other processes, well known but too often neglected, may indeed be dominant in such molecules. In this respect, an appropriate study of a SMM should include an experimental part that correlates results from different techniques, both magnetic and spectroscopic, and a theoretical part that nowadays is embodied by ab initio calculations. Indeed, only a virtuous interplay between several experimental approaches and a theoretical modeling of these systems will allow us to obtain a detailed understanding of the relation between the electronic structure and the rich low-temperature magnetization dynamics often observed in these systems. The leading idea that spans this Phd dissertation is to report possible protocols to obtain an as full as possible magnetic characterization, taking into account that the goal of any study aimed at investigating the correlation between electronic structure and magnetization dynamics has to be adapted according to the means at our disposal, both in terms of experimental setup and of the specificities of the investigated chemical system. The thesis is organized in six chapters. In chapter 1 we will present a simple theoretical approach that allows to describe most of the SMMs features: indeed to understand and model the dynamic behavior of a SMM it is important to obtain a sketch of its energy level structure and of its state composition. In chapter 2 a summary of the investigation techniques is reported. We will illustrate the most common experiments of AC and DC magnetometry, that allow to study the static and dynamic bulk magnetic properties, then, a digression of Electron Paramagnetic Resonance (EPR) investigation, both in continuous wave and pulsed methods, will be presented. Moreover, we will introduce the basic principles of Muon Spin Relaxation technique (µSR), that is not frequently used in the magnetic characterization of SMMs. In Chapter 3, we will present some results of a µSR study on two lanthanide SMMs containing Er(III) and Dy(III). After a brief recall of the structural features of the samples and a summary of the results of previous magnetic investigations we will discuss the outcomes of the µ SR research. In chapter 4 we will report a comprehensive study of a new Dy(III)-based SMM, on which we performed a complete experimental characterization, obtained by X-ray diffractometry, EPR spectroscopy, single crystal Cantilever Torque Magnetometry (CTM), AC and DC susceptibility flanked by theoretical analysis based on ab initio methods. In chapter 5 a new series of compounds, the Ln(trenovan) family is reported. They are similar to the complexes presented in chapter 3, but at variance with these, no single crystals nor luminescent data were available, making an accurate determination of their electronic structure more demanding. Indeed, their molecular structure was resolved from X-ray powder diffraction (XRPD) and, after the experimental characterization, their magnetic properties were simulated by means of a phenomenological approach. Finally, in chapter 6 the magnetic characterization of a vanadyl-based compound is reported. This system turned out to be characterized by a very long, and relatively field and temperature independent, magnetization relaxation time in applied field. Despite this, being an S=1/2 system, it can not be considered a SMM. However, its relatively long spin-spin relaxation time makes this kind of molecules suitable to work as potential qubits.