Cluster-glass state in manganites induced by<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>-site cation-size disorder

Abstract

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 $⟨{r}_{A}⟩=1.20\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ but different $A$-site ionic radii variance ${\ensuremath{\sigma}}^{2}={\ensuremath{\sum}}_{i}{x}_{i}{r}_{i}^{2}\ensuremath{-}{⟨{r}_{A}⟩}^{2}$, 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 ${\ensuremath{\sigma}}^{2}$ from 0.0003 for ${\mathrm{La}}_{0.55}{\mathrm{Ca}}_{0.45}\mathrm{Mn}{\mathrm{O}}_{3}$ to 0.015 for ${\mathrm{Sm}}_{0.55}{({\mathrm{Ca}}_{0.6}{\mathrm{Ba}}_{0.4})}_{0.45}\mathrm{Mn}{\mathrm{O}}_{3}$, 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 $\mathrm{Mn}{\mathrm{O}}_{6}$ 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.