The enhanced magnetoresistant effects and cluster-glass state induced by A-site cational size disorder

In pervoskite-structure oxides, not only the A-site cational mean radii, but also the variance of the A-site cational radii, controls the physical properties of the sample. The disorder effects induced by A-site cational size mismatch in large band-width manganite La$_{0.55}$Sr$_{0.45}$MnO$_{3}$ have been carefully studied by preparing a series of samples which have the same $$=1.25{\AA} with different variance of the A-site ionic radii. The ground state of the system changed from a ferromagnetic metal to a cluster-glass insulator with increasing of the variance of the A-site ionic radii. Moreover, the magnetoresistant effects is enhanced significantly in the sample with intermediate disorder. We argued that the A-site disorder enhanced the fluctuation between the competing ordered states and then induced the cluster-glass state and the enhanced magentoresistant effects.