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Room temperature x-ray diffraction, magnetization and neutron diffraction measurements up to 500 K have been carried out on polycrystalline bulk double perovskites AA'FeMoO6 (AA' = Ba2, BaSr, Sr2, Ca2) in order to determine and correlate their structural and magnetic properties. As the average ionic radius is diminished, the crystallographic structure evolves from cubic (for AA' = Ba2, BaSr) to tetragonal (Sr2) and finally to monoclinic (Ca2). In the case of AA' = Sr2, a novel crossover from a high temperature paramagnetic cubic state to a low temperature ferrimagnetic tetragonal state has been observed. For all the studied compounds, neutron diffraction patterns and magnetization measurements are consistent with a ferrimagnetic ordering of the Fe and Mo sublattices. A remarkable correlation is found between the Curie temperature and the electronic bandwidth, which is controlled by structural parameters.

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The effect of the substitution for Mn with Al in the magnetoresistive perovskite La 2/3 Ca 1/3 MnO 3 has been studied by preparing the series La 2/3 Ca 1/3 Mn 1Ϫx Al x O 3 (xр0.2). A careful study of the magnetic, structural, and transport properties has been carried out by means of electrical resistance, magnetoresistance, ac magnetic susceptibility, x-ray-diffraction, and neutron-diffraction techniques. Up to xϭ0.05 the Curie temperature ͑and the associated metal-insulator transition͒ decreases drastically with Al doping and the magnetoresistive properties do not change very much. For xу0.1 the lattice spontaneously begins to lose oxygen atoms and for xϭ0.2, 3% of oxygen vacancies are present. This fact along with the random distribution of the Al atoms makes these compounds rather disordered from a structural and magnetic point of view. However, the magnetoresistance is enhanced, reaching colossal values of 10 7 % at Hϭ12 T at low temperatures for xϭ0.2.

The effects of magnetic field and pressure on the unusual spontaneous behavior of La 2/3 Ca 1/3 MnO 3 have been thoroughly investigated. Resistivity and volume thermal expansion, both under magnetic field and pressure, ac susceptibility under pressure, magnetostriction, magnetoresistance, and neutron diffraction measurements, have allowed us to determine the relevant underlying mechanisms in this system. Above T c the neutron measurements reveal short-range ferromagnetic correlations and the anomalous volume thermal expansion indicates that local distortions are present. Both experiments support the formation of magnetic polarons above T c . At T c the compound undergoes a paramagnetic-ferromagnetic transition accompanied by an insulatormetal-like transition with anomalies in the electrical and volume properties. Above T c the magnetic 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.

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