This thesis explores the electronic and structural properties of rare-earth nickelates, a family of transition metal oxides that exhibit complex and rich phase transitions. When synthesized as epitaxial thin films, these materials provide an ideal platform to study emergent physical properties, such as metal-insulator transitions, coupled magnetic orders, and, more recently, superconductivity in the so-called "infinite-layer" phases.
The work begins with an introduction to transition metal oxides, their perovskite structures, and the oxygen octahedral distortions that impact the physical properties of these systems. Special attention is given to the physics of transition metal d-orbitals, which drive unconventional electronic behaviors. The potential of thin films and heterostructures to access novel quantum states is also discussed. A significant part of this thesis is dedicated to the growth and characterization of thin films obtained by RF magnetron sputtering, using topotactic solid-state reduction to access reduced phases. Structural characterizations are carried out by X-ray diffraction and reciprocal space mapping, while electronic properties are investigated by transport measurements and electron microscopy (both scanning and transmission). The studied systems include solid solutions such as Nd₁₋ₓSmₓNiO₃, which allow investigation of the effect of the ionic radius on the separation between electronic and magnetic transitions. Artificial superlattices combining NdNiO₃, SmNiO₃, and LaAlO₃ are also fabricated to tune interfacial couplings, with original observations on the decoupling of order parameters. Special emphasis is placed on artificial superlattices combining Nd₁₋ₓLaₓNiO₃ and SmNiO₃ layers, where careful control of periodicity and interface nature allows direct manipulation of the competition between metallic and insulating phases. It is demonstrated that a configuration usually metallic can become insulating due to the combined effect of interfaces and superlattice architecture, revealing the crucial role of phase boundaries in stabilizing new electronic states. The second part of the thesis focuses on infinite-layer phases, with the synthesis of NdNiO₂ via reduction of NdNiO₃, and the study of Eu doping effects. These investigations follow the direction first envisaged in the early days of high-temperature superconductivity research. In their 1987 Nobel lecture, Bednorz and Müller already mentioned LaNiO₃ as their starting point in searching for high-Tc superconductivity in oxides, owing to its metallic character and absence of Jahn-Teller distortion. In our infinite-layer nickelate samples, we observe superconductivity. Furthermore, our measurements reveal a spectacular re-entrant superconducting behavior under perpendicular magnetic fields, reminiscent of a mechanism predicted in 1962 by Jaccarino and Peter. The results presented here highlight the central role of structural and chemical engineering in controlling electronic phases. They also showcase the richness of nickelates as a platform for the development of "by design" functional materials, opening avenues for future research in correlated oxides and unconventional superconductivity.
