Spin spectroscopy of materials for quantum information processing, data storage and energy conversion

Magnetic materials are of fundamental importance not only as a playground for a wide class of physical phenomena but also for future technological advances. Lanthanide-based molecular nanomagnets are particularly promising for quantum computing and magnetic data storage applications, while Fe2P based MnFePSi alloys are being regarded as the top candidates to replace conventional refrigerators based on gas compression technology. The diverse applications of these materials require a deep understanding of their underlying physics. This involves, for instance, a detailed modeling of the intra-molecular interactions and the spin relaxation dynamics for molecular nanomagnets, and a precise characterization of the magnetic phase transition in MnFePSi compounds. Only after an insight into the material physics has been achieved, the free parameters left to the material synthesis can be tuned in order to meet the requirements of real-case applications. However, there is a dearth of experimental methods capable of handling the entangled electron-nuclear mixed states in lanthanide molecular nanomagnets of interest for quantum information processing, while a sound model spin Hamiltonian for the coupled lanthanide ion and nucleus is also lacking in the literature. In addition, the spin relaxation dynamics of the newly synthesized dysprosocenium single ion magnet demands for urgent investigation, in view of its possible application to high-density data storage at the highest temperatures ever attained in this class of magnets. Finally, the nature of the first order magneto-structural transitions and the so-called mixed magnetism in MnFePSi compounds are not completely understood as well. In order to tackle all these issues, in this thesis work we employed two different spectroscopies suitable to investigate magnetic systems. On one hand we used nuclear magnetic resonance (NMR), which is a probe of electronic spins in direct space, capable to detect low-energy spin excitations of the system and magnetic order on a local scale, as well as possible inhomogeneities in the latter. On the other hand, we applied inelastic neutron scattering (INS) as a versatile technique sensitive to a higher energy scale than NMR, which can access directly the excited levels and the internal exchange couplings of molecular nanomagnets. By means of NMR we succeeded to completely characterize the lowest-energy levels of the coupled nuclear and electronic spins and study their coherent dynamics in a [Yb(trensal)] molecular nanomagnet. We detected nuclear transitions of 173Yb, studied their decoherence processes, drove several cycles of Rabi oscillations and demonstrated the possibility to implement a quantum error-protected qubit by its multi-level pattern of nuclear transitions, exploiting its electron-nuclear coupling. We studied the proton spin-lattice relaxations in dysprosocenium, which probe the spin-flip dynamics of the electronic moments through their dipolar coupling to 1H nuclei, thus providing valuable knowledge about the relaxation pathways of the out-of-equilibrium magnetization. Moreover, a family of MnFePS alloys was investigated by both zero- and applied-field NMR. Through our experiments we demonstrated the first order nature of the magnetic transitions even in cases where the transition was believed second order from macroscopic measurements, and we helped elucidating the nature of the so-called mixed magnetism in these compounds. The power of the inelastic neutron scattering spectroscopy, on the other hand, provided a unique insight into the crystal field (CF) splitting and exchange interactions of a family of Kramers lanthanide dimers, enabling us to determine a sound model for the CF interactions and exchange couplings.