The dependence of magnetotransport behaviors on A-site disorder induced by A-site cational size mismatch in ferromagnetic manganites is investigated by characterizing a series of samples with the same A-site cational mean radius ͗r A ͘ = 1.20 Å but different A-site ionic radii variance 2 = ͚ i x i r i 2 − ͗r A ͘ 2 , where x i and r i are the atomic fraction and ionic radii of i-type ions at A-site, respectively. It is revealed that the ground state transits from ferromagnetic metal to cluster-glass insulator upon increasing the variance 2 from 0.0003 for La 0.55 Ca 0.45 MnO 3 to 0.015 for Sm 0.55 ͑Ca 0.6 Ba 0.4 ͒ 0.45 MnO 3 , while the crystallographic structure and lattice constants of these manganites remain unchanged. Nevertheless, the increasing A-site disorder is believed to enhance the random local radial distortion of MnO 6 octahedra and suppress the ferromagnetic long-range order. In the manganites of high A-site disorder, the long-range ferromagnetic ordering is completely melted into the short-range magnetically ordered clusters and then the stepwise magnetization. With decreasing temperature, the short-range ordered clusters become frustrated at the frustrating point, below which a cluster-glass transition occurs due to the weak intercluster interaction.

Perovskite-type polycrystalline La1∕3Sr2∕3FeO3 particles with different sizes (80–2000nm) were prepared using a simple sol-gel technique. In samples of nanoparticles with a diameter of less than 300nm, weak ferromagnetism was observed at room temperature, which was attributed to the lattice distortion. The magnetic and specific heat measurements suggest that the charge ordering state was largely suppressed due to the lowering of the particle size, but the charge ordering temperature remained unaffected.

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