Synthesis, macroscopic and local probe characterization of AgCrO$_2$ and CdCr$_2$S$_4$

The search of new materials with new and/or enhanced physical properties has, nowadays, a strict relation with the evolution of technology. The hunt for systems exhibiting simultaneous (anti)ferroelectric and (anti)ferromagnetic orders has re-started in the last years, and remains as one of the hot topics for Physic, Chemistry and Materials Science scientific communities. This is triggered by the possibility to apply these materials into, ${e.g.}$, new multiferroic memories that could be written electrically and read magnetically (or vice-versa). This thesis presents an experimental study on two of these so-called multiferroic materials, the AgCrO$_2$ delafossite and CdCr$_2$S$_4$ spinel type compounds. Different methods have been tested for the synthesis of delafossite silver chromium oxide AgCrO$_2$ namely, solid state reaction (ssr) and sol-gel combustion. The processes were optimized regarding complexing/combusting agents, temperature and duration of the heat treatments, as well as the processing atmospheres. The solid state reaction method yielded single phase samples of AgCrO$_2$ structure whereas in the sol-gel method almost single phase samples were produced but traces of secondary phases were found. The formation of the AgCrO$_2$ phase was followed at each synthesis step through X-ray powder diffraction analysis. Magnetic and electric characterization allowed to show that the produced samples have the expected behavior near the ordering temperature (ferroelectric and antiferromagnetic). The magnetic susceptibility above T$_{N}$ (21 K) has shown a peculiar behavior, generally attributed to the development of 2D short-range magnetic correlations due to strong frustration coming from antiferromagnetic exchange interactions in a triangular lattice. However, correlating our magnetization results with our pac data, a slightly different picture emerges. In fact, we have evidence that a second local environment emerges below 100 K, possibly due to Ag lattice displacements. Our results suggest that these are precursor effects of the ferroelectric/antiferromagnetic phase transition. In parallel we have developed a system to perform the synthesis of sulfide compounds. In particular, in this work we prepared the CdCr$_2$S$_4$ compound with spinel structure and performed a thorough study of it`s physical properties. High quality polycrystalline samples were prepared using a double evacuated quartz ampoule, which is a branch of the solid state reaction method. Powder X-ray diffraction analysis allowed to follow the formation of the phase at each step of the synthesis. At the end we were able to produce a high quality single phase sample. We have performed the structural refining using Rietveld analysis which allowed verify the excellent agreement with those reported in the literature. The magnetic characterization of the CdCr$_2$S$_4$ sample was performed showing that besides the good agreement of the T$_c$ and $\mu_{eff}$ , short-range magnetic correlations (srmc) (or spin-cluster) in the PM phase were observed and it was possible to define a new temperature scale T$_{G} \sim$ 116 K where these clusters start to form. Local probe studies were performed using the $^{111}$In probe, and a systematic study allowed to verify the evolution in temperature of the efg’s detected by the probe. In fact we show that below 120 K time dependent interactions set in which can be connected with the onset of local distortions trigged by the magnetic short-range correlations observed in the magnetization data.