UNIVERSIDAD DE BUENOS AIRES

____________________________________ In this thesis we present an experimental study of the electrical and magnetic properties of mixed valent manganese oxides, also known as manganites. Our work is focused on the analysis of the intrinsic coexistence of different phases presented by several manganites, a state called Phase Separation (PS). This state is usually formed by ferromagnetic (FM) metallic and antiferromagnetic (AFM) charge ordered (CO) insulating regions. We have performed our measurements on ceramic samples of La0.5Ca0.5MnO3, and on samples of the La5/8-yPryCa3/8MnO3 and La5/8-yNdyCa3/8MnO3 systems, all of which are prototypical compounds in the problem of PS. We have studied the influence of the magnetic field on La0.5Ca0.5MnO3 through electrical resistivity (r) and magnetization (M) measurements, as a function of temperature and magnetic field. We have developed a model to explain the growth of the FM fraction that can be induced by magnetic field application, main ingredient of the low field (< 1 Tesla) magnetoresistance presented by phase separated manganites. We have performed different experiments in which we could induce the growth of the FM and non FM regions in the temperature range charaterized by PS, and we have observed a memory effect of the applied magnetic field, associated with that growth. This memory effect is directly related with the dynamical behaviour of the phase separated state and with the characteristics of the compound under study. The dynamical character of the phase separated state was also studied on La0.5Ca0.5MnO3 by repeatedly cycling the samples at low temperature (30 – 300 K) and through measurements of the physical properties as a function of time at fixed temperature. Both experiments have evidenced changes that can be associated with a reduction of the FM phase. We have developed a model to explain the thermal cycling effect according to which it has its origin in a mechanism that occurs at the FM/CO interface. The slow relaxations observed suggests that the system is evolving hierarchically through a distribution of energy barriers which separates the FM and CO states. We have also studied the role played by chemical subtitution on the La5/8yNdyCa3/8MnO3 system. We have observed that the homogeneous FM state, characteristic of the y = 0 compound, is destabilized when replacing La ions by Nd ones, while PS is induced for intermediate doping (y = 0.3 – 0.4). For higher doping (y = 0.5 – 0.625) the phase separated state dissapears and an homogeneous AFM – CO develops. This is related with the fact that the introduction of smaller Nd ions, produces distortions on the Mn – O – Mn bond angles, which favour the CO state. In the doping range characterized by PS, we have obtained evidences of a dynamical behaviour typical of this state, previously reported on the La5/8-yPryCa3/8MnO3 (y = 0.4) compound. The dynamical behaviour of manganites with PS is also responsible for the particular thermal hysteresis displayed on its physical properties. We have performed r vs. T measurements for different electrical current values on La5/8-yPryCa3/8MnO3 (y = 0.34) which show a reduction of r while increasing current in the PS range. We show strong evidence that points that this reduction, commonly attributed to melting of the CO state, is a consequence of an artifact which results from the combination of Joule heating and the irreversible r vs. T dependence. In the last part of this work, we present a thermodynamic model of the PS state accounting for its static and dynamic properties. Through calorimetric measurements on La5/8-yPryCa3/8MnO3 con y = 0.4, the low temperature free energies of the coexisting phases are evaluated. The phase separated state is modeled by free energy densities uniformly spread over the sample volume. The calculations contemplate the out of equilibrium features of the coexisting phase regime, to allow a comparison between magnetic measurements and the predictions of the model. A phase diagram including the static and dynamic properties of the systems is constructed, showing the existence of blocked and unblocked regimes which are characteristics of the phase separated state in manganites.