Isotope effect and characteristic features of the phase diagram for cobaltites with spin-state transitions

摘要

The effect of ~(16)O → ~(18)O oxygen isotope substitution has been studied for (Pr_(1-y) Eu _y) _(0.7)Ca_(0.3)CoO_3 cobaltites (0.12 < y < 0.26). A pronounced isotope shift has been found for the spinstate transition temperature, which increases with the oxygen isotope mass. In contrast, the ferromagnetic transition temperature has slightly lower values for the samples with heavier oxygen. The observed phenomena and constructed phase diagram confirm the results reported previously for (Pr_(1-y) Sm _y)_(0.7)Ca_(0.3)CoO_3 in [G. Y. Wang, X. H. Chen, T. Wu, et al., Phys. Rev. B 74, 165113 (2006)]. The measurements of the specific heat have been performed for (Pr_(1-y) Eu _y) _(0.7)Ca_(0.3)CoO_3 with the main emphasis on the analysis of the isotope effect. The contributions to the isotope effect coming from the lattice and magnetic components of the specific heat have been separated. The mechanisms underlying the large isotope effect are discussed.