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 < r(A)>=1.20 angstrom but different A-site ionic radii variance sigma(2)=Sigma(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 sigma(2) from 0.0003 for La0.55Ca0.45MnO3 to 0.015 for Sm-0.55(Ca0.6Ba0.4)(0.45)MnO3, 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 MnO6 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.