Cluster-glass state in manganites induced by a -site cation-size disorder

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 rA =1.20 but different A -site ionic radii variance σ2 = i xi ri2 - rA 2, where xi and ri 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 σ2 from 0.0003 for La0.55 Ca0.45 Mn O3 to 0.015 for Sm0.55 (Ca0.6 Ba0.4) 0.45 Mn O3, 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 Mn O6 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.