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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.

The onset of a Griffiths-like phase has been observed in Tb5Si2Ge2 (TC=110 K) by means of magnetic susceptibility and small-angle neutron scattering experiments. We show the growth of a ferromagnetic cluster system characterized by an inverse susceptibility exponent lower than unity at TC

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Zero-field electrical resistivity over the temperature range of 4–300 K and magnetoresistance in magnetic fields of up to 12 T have been measured in Gd5(Si1.8Ge2.2). This system undergoes a first-order magnetostructural transition at TC≅240 K, from a high-temperature paramagnetic to a low-temperature ferromagnetic phase, accompanied by a large drop in the resistivity. The application of an external magnetic field above TC can induce this transition, and a giant negative magnetoresistance effect (Δρ/ρ≅−20%) is observed associated with this first-order field-induced transition.