AC susceptibility is an important characterisation technique measuring the time dependent magnetisation and dynamics of a magnetic system. It is capable of yielding information on thermodynamic phase transitions, relaxation processes and losses in a variety of interesting magnetic and superconducting materials. In particular it is a powerful probe of the mixed state of superconductivity providing insight into the ux dynamics at play and determination of a number of physical properties such as the critical temperature Tc, field Hc and characteristic length scales. Application of pressure can tune materials through multiple phases and interesting phenomena. The thesis describes the design of a calibratable susceptometer in a piston cylinder pressure cell, achieving AC susceptibility measurements of the same accuracy as a SQUID magnetometer but under pressure. This is used to make measurements on an electrostatically doped capacitance device, a single chain magnet and a heavy fermion superconductor. These studies are summarised below. Electric double layer (EDL) devices provide a means of continuous tuning through a materials phase diagram by applying an electric field, including inducing superconductivity. Application of pressure in tandem with electrostatic doping could improve the efficiency of these devices and provide a second tuning parameter. An EDL capacitor was constructed and measured with the above susceptometer aiming to shift the Tc of a doped high temperature superconducting cuprate La1:9Sr0:1CuO4. The Tc shifts proved irreproducible already at ambient conditions. Indeed during the course of this research further experimental evidence emerged in the literature indicating EDL devices may very well work due to electrochemical doping rather than electrostatic, possibly accounting for the lack of repeatability. Work therefore focused on mapping the ionic liquid DEME-TFSI's glass-liquid phase diagram over the 1 GPa pressure range, rather than extending the study of the EDLC device to high pressure. Single chain magnets (SCM) are an interesting class of material consisting of a one-dimensional molecular magnet chain manifesting magnetic hysteresis and slow relaxation best characterised by AC susceptibility. The susceptometer was used to study the SCM [Co(NCS)2(pyridine)2]n to investigate the effect of pressure on its characteristic magnetic relaxation time and energy barrier. A secondary signal appears at ~0.44 GPa which is attributed to the development of an additional structural phase that has been independently observed in X-ray crystallographic measurements. The heavy fermion superconductor U6Fe has the highest Tc ~4 K of all the U-based compounds and large critical fields of ~10-12.5 T, depending on direction, which increase on initial application of pressure. It exhibits a coexisting charge density wave (CDW) below 10 K making it a promising candidate for the modulated superconductivity of the theorised Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) state. A feature at 110 K is also evident in Mossbauer, resistivity and specific heat measurements, the origin of which has not yet been clearly identified. Evidence for the FFLO state was sought by mapping the upper critical field Hc2 along with the peak effect through AC susceptibility measurements up to pressures of 1 GPa. The data is accounted for by an evolution of collective pinning and superconducting parameters, with no clear evidence for an FFLO state although an enhancement of the reduced field is observed.

Molecular magnets with different dimensionality, whether they are zero-dimensional singlemolecule magnets (SMM) or one-dimensional single-chain magnets (SCM) are very interesting, since they allow probing the fundamental aspects bordering quantum and classical physics at the nanoscale level. This dissertation covers experimental studies of two Mn-based exchangecoupled molecule-based magnets and two Co-based single-chain magnets, using both dc Halleffect magnetometry and electron paramagnet resonance (EPR) techniques. In these multidimensional systems, the spin of the molecule exhibits quantum mechanical behavior at low temperature. It is quite interesting to observe the effect of magnetic exchange interactions on the magnetic properties of various complexes; hence they strongly affect the magnetic behavior. In this dissertation, the research is initiated with the study of low-magnetic-nuclearity molecules, starting with a spectroscopic study of a significantly anisotropic Mn(IV) monomer. At low temperature the molecule possesses easy-plane type anisotropy of a remarkable magnitude. Although the molecule is not a single-molecule magnet, the remarkable anisotropy can initiate synthesis of newer and better molecular magnets with Mn(IV) as the main building block. Furthermore, the interplay between the magnetic anisotropy and the inter-ion exchange interactions (J) within the molecule are probed for a dimer and a trimer where the magnetic core is comprised of two and three ions respectively. In the Mn-based case of the dimer, the low coupling between the atoms leads to significant state mixing, thus making it impossible to assign the individual spin states to the dimer or to the respective individual Mn(II) ions. In the case of iv the trimer, lowering of the symmetry achieved by fine tuning of the inter-ion exchange interactions leads to relieving of frustration in the antiferromagnetic (AF) triangular Mn(III) system, resulting in a well defined ground state and significant zero field splitting. Also a clear hysteretic behavior observed in this system demonstrates its SMM nature at low temperature. Finally, high-field high-frequency magnetic and spectroscopic studies performed on two cobalt-based SCMs reveal that formation of magnetic domains by exchange interactions within the chain are strongly influenced by thermal fluctuations. The chain possesses a uniaxial anisotropy with the quantization axis lying along the length of the chain. Moreover it is shown that modulation of the magnitude of inter- and intra-chain interactions results in a threedimensional dynamics in one of the samples. Interestingly, detailed dc magnetic studies show a tunable crossover between one- and three-dimensional magnetic dynamics as a function of temperature and/or magnetic field sweep rate. Our voyage through several molecular systems of different dimensionality have allowed us to expand our understanding of the role of exchange interactions on the magnetic behavior in molecular magnetism

Dans cette thèse, nous avons synthétisé de nouveaux matériaux moléculaires hétérométalliques pontés par des groupements cyanures en utilisant la chimie de coordination donneur-accepteur. Les précurseurs moléculaires basés sur un ion métallique 3d (MnII/III, FeII, NiII, CuII) agissent en tant qu’unités acceptrices, alors que les précurseurs 4d (RuIII) et 5d (ReIV) de type trans-cyanido ont été utilisés comme groupements donneurs. Le chapitre I présente une approche théorique avec quelques exemples des matériaux magnétiques de basse dimensionnalité : molécule et chaîne aimant qui illustrent le rôle du ligand cyanido pour concevoir les matériaux magnétiques. En outre, nous avons sélectionné des précurseurs cyanido à base de métaux 4d/5d pour leur anisotropie magnétique plus élevée par rapport aux ions de métaux 3d. Le choix des précurseurs moléculaires, leur synthèse et leur caractérisation ont été largement décrits dans le chapitre II. Au cours de ce travail de recherche, nous avons obtenu de nombreux systèmes moléculaires hétérométalliques mais également des systèmes unidimensionnels. Les chapitres III, IV et V décrivent la synthèse, la structure cristallographique et la caractérisation magnétique de tous les complexes synthétisés. L’empilement cristallin compact des molécules conduit à un état fondamental antiferromagnétique pour la plupart des complexes ce qui perturbe souvent les phénomènes de relaxation magnétique. Nous avons donc établi les diagrammes de phases magnétiques pour la plupart de ces systèmes. De plus, certaines des chaînes ReIV/MnIII et une chaîne ReIV/FeII présentent des comportements magnétiques de type "chaîne aimant" et donc une bistabilité magnétique. / In this thesis, we have synthesised new cyanido bridged heterometallic molecule-based magnetic materials with the tool of simple donor-acceptor coordination chemistry. The 3d metal ion (MnII/III,FeII, NiII, CuII) based molecular precursors acted as acceptor building blocks, whereas the 4d (RuIII)and 5d (ReIV) trans-cyanido type molecular precursors were used as donor moieties.Chapter I contains a theoretical approach with examples of low dimensional magnetic materials:Single Molecule and Single Chain Magnets that illustrate the role of the cyanido ligand to design such magnetic materials. Furthermore, we emphasized the 4d/5d metal based cyanido precursors for their higher magnetic anisotropy over the 3d metal ions. The choice of molecular precursors, their synthesis and characterization were extensively described in chapter II. In this research work, we have obtained many heterometallic molecular complexes and also one dimensional systems. Chapter III, IV and V contain the synthesis, X-ray crystallographic and magnetic characterization of all the newly synthesized complexes. The close crystal packing of the molecules lead to an antiferromagnetic ground state for most of the complexes and this often perturbed the magnetic relaxation phenomena. As expected, we have found metamagnetic phase diagrams for most of these systems. Some of the ReIV/MnIII and a ReIV/FeII one dimensional compounds exhibited Single-Chain magnet like properties and thus magnetic bistability.