Local probing spinel and perovskite complex magnetic systems

Materials with multifunctional physical properties are crucial for the modern society, especially those which display a strong coupling between magnetic, lattice and polar degrees of freedom. This by far unexploited capability promises new paradigm-shift technologies for cooling technologies, magnetic data storage, high-frequency magnetic devices, spintronics, and micro-electromechanical systems. Alongside with the understanding of the properties of these materials, the need to improve them and to make them smaller and more efficient is a current goal. Device miniaturization towards very high-density data storage stands also as a trend in modern science and technology. Here, the integration of several functions into one material system has become highly desirable. Research in this area has already highlighted complex magnetic materials with po- tential for multifunctional applications based on spinel type structures like CdMn2O4 or multiferroic CdCr2S4 or even RCrO3 with orthorhombically distorted perovskite struc- ture. Nevertheless, the high sensitivity of complex oxides to crystal chemistry and lattice distortions offers many paths to tailor and engineer new functional materials. Following these ideas, this thesis extends the knowledge about the bulk properties of some potential multifunctional materials, aiming to enhance multiferroic properties and to contribute to the fundamental understanding of magnetoelectric coupling (its origin and how to enhance it). In this way, it opens a pathway for the down scaling of these materials namely for the incorporation into nanostructures. The material’s properties are analyzed across phase transitions induced by different external stimuli such as tem- perature, pressure, magnetic or electric field. To achieve these goals a three way com- plementary strategic research was followed: Use of adequate experimental synthesis tools for the production of materials of desired compositions; Study the magnetoelectric effect and ferroic orders in synthesized compounds using traditional magnetic/dielectric and polarization studies; Combining local scale information (analysis of the magnetic hyperfine field, electric field gradient and local structural information) with macroscopic data to obtain local signatures of the magnetoelectric effect and to better understand, from a fundamental point of view, the origin of the macroscopic properties they exhibit. In particular, this thesis focuses on the development of materials synthesis, general characterization, extended physical studies combined local scale ones. We investigated in detail the properties of chalcogenide sulphur spinel CdCr2S4 and unveiled the off- centering of the magnetic Cr – ion which gives rise to a peculiar entanglement between the polar and magnetic degrees of freedom, stabilizing, in the paramagnetic phase, short range magnetic clusters. Our DC magnetization results and x-ray diffraction mea- surements under hydrostatic pressure show that a competition between direct antiferro- magnetic and superexchange ferromagnetic interactions exists in this compound. The short-range magnetic clusters promoted by the Cr local distortions are strongly influ- enced by pressure increase, thus influencing the CdCr2S4 magnetic properties. Manganites and their characteristic Jahn-Teller distortion were also studied in this thesis. Alongside with their interesting applicability, these materials provide a fertile playground for fundamental physics, where (the potential) frustration, local distortions, local range order, and dynamic mechanisms are yet to be fully understood. Here, the tetragonally distorted CdMn2O4 spinel manganite was studied trough atomic scale prob- ing of the electric field gradient. We observed a dynamic lattice distortion, evidenced by time dependent EFG fluctuations, suggesting a structural instability in this material. Although no macroscopic tetragonal to cubic phase transition was observed at high temperature, local MnO6 octahedra with relaxed Jahn-Teller distortions emerged in non- distorted matrix and their number grew with raising temperature. Finally, we studied rare earth orthochromites with the formula RCrO3 (R – Yb, Er, Y or Sm) which show great potential application as multifunctional materials. We present the results of structural, DC magnetization measurements and magnetocaloric properties for YCrO3 hard magnet. Magnetic refrigeration based on the magnetocaloric effect has given giant steps in recent years and promises, nowadays, to become a cooling tool for both cryogenic and room temperature applications. Thus, in order maximize such magnetocaloric properties in these materials, we studied the effect of chemical pressure (by changing R–Yb, Er, Y, Sm) on the magnetocaloric effect and refrigerant capacity properties of these orthorhombic perovskite-like structures. The influence of using R- magnetic atoms (R – Yb, Er, Sm) in these properties is also analyzed. Finally, we present an atomic scale study of the electric field gradient and magnetic hyperfine field in these structures and correlate our findings with the appearance of ferroelectricity.