SPONTANEOUS BEHAVIOR AND MAGNETIC-FIELD AND PRESSURE EFFECTS ON LA2 3CA1/3MNO3 PEROVSKITE/

The effects of magnetic field and pressure on the unusual spontaneous behavior of La2/3Ca1/3MnO3 have been thoroughly investigated. Resistiv ity and volume thermal expansion, both under magnetic field and pressu re, ac susceptibility under pressure, magnetostriction, magnetoresista nce, and neutron diffraction measurements, have allowed us to determin e the relevant underlying mechanisms in this system. Above T-c the neu tron measurements reveal short-range ferromagnetic correlations and th e anomalous volume thermal expansion indicates that local distortions are present. Both experiments support the formation of magnetic polaro ns above T-c. At T-c the compound undergoes a paramagnetic-ferromagnet ic transition accompanied by an insulator-metal-like transition with a nomalies in the electrical and volume properties. Above T-c the magnet ic field and the pressure favor electrical conduction by enhancing the double-exchange interaction. Below T-c the metallic state is favored by the magnetic field and the pressure in a different way.