Magnetic, electronic, and thermal transport properties ofEu0.55Sr0.45MnO3

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Magnetic, electronic, and thermal transport properties of ${\mathrm{Eu}}_{0.55}{\mathrm{Sr}}_{0.45}\mathrm{Mn}{\mathrm{O}}_{3}$ have been experimentally studied. The compound is found to exhibit a complex magnetic behavior with the change of temperature and magnetic field. Without magnetic field, it stays in a Griffiths-like state in a wide temperature range above $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, characterized by the presence of ferromagnetic (FM) clusters of the size of $\ensuremath{\sim}8$ Mn ions, and an antiferromagnetic (AFM) state below $\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, evidenced by thermopower and heat conductivity. FM phase emerges and grows in the AFM matrix with applied field, resulting in a series of phase transitions from the paramagnetic (PM) state first to the AFM state, then to the FM and the AFM states upon cooling $(\ensuremath{\sim}0.8\phantom{\rule{0.3em}{0ex}}\mathrm{T}lHl\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, or a simple PM-FM transition $(Hg\ensuremath{\sim}2.3\phantom{\rule{0.3em}{0ex}}\mathrm{T})$. The AFM state is unstable under high fields, and the high- and low-temperature AFM transitions are depressed by the fields above $\ensuremath{\sim}1$ and $\ensuremath{\sim}2.3\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, respectively. The FM transition is incomplete when the field is below $\ensuremath{\sim}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, leading to a coexistence of the FM and PM (or AFM) phases in the intermediate temperature range. A spin-glass-like behavior is observed in the AFM background below $\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Significant response of resistance, thermopower, and heat conduction to magnetic transition, either FM or AFM transition, has been observed. Unlike the typical AFM manganites, for which usually a depression of thermal conduction occurs at the AFM state, the AFM transition in ${\mathrm{Eu}}_{0.55}{\mathrm{Sr}}_{0.45}\mathrm{Mn}{\mathrm{O}}_{3}$ enhances the thermal conduction. From the PM phase to the AFM phase and to the FM phase, thermal conductivity increases monotonically. A remarkable result of the present work is the different behaviors of thermopower and resistivity. The former displays a metallic behavior below a distinct temperature that is significantly lower than the metal-to-insulator transition temperature determined by resistivity. Furthermore, thermopower remains metallic while the resistivity shows up an upturn due to the AFM transition in the low-temperature range under the field of $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Based on these data, a magnetic phase diagram is proposed.