Structural and physical properties of SrMn1−xRuxO3SrMn_{1-x}Ru_xO_3 perovskites

We combine the results of magnetic and transport measurements with neutron diffraction data to construct the structural and magnetic phase diagram of the entire family of SrMn$_{1-x}$Ru$_{x}$O$_3$ ($0 \leqslant x \leqslant 1$) perovskites. We have found antiferromagnetic ordering of the C type for lightly Ru-substituted materials ($0.06 \leqslant x \leqslant 0.5$) in a similar manner to $R_{y}$Sr$_{1-y}$MnO$_3$ ($R$=La, Pr), due to the generation of Mn$^{3+}$ in both families of manganite perovskites by either $B$-site substitution of Ru$^{5+}$ for Mn$^{4+}$ or $A$-site substitution of $R^{3+}$ for Sr$^{2+}$. This similarity is driven by the same ratio of $d^4$ / $d^3$ ions in both classes of materials for equivalent substitution level. In both cases, a tetragonal lattice distortion is observed, which for some compositions ($0.06 \leqslant x \leqslant 0.2$) is coupled to a C-type AF transition and results in a first order magnetic and resistive transition. Heavily substituted SrMn$_{1-x}$Ru$_{x}$O$_3$ materials are ferromagnetic due to dominating exchange interactions between the Ru$^{4+}$ ions. Intermediate substitution ($0.6 \leqslant x \leqslant 0.7$) leads to a spin-glass behavior instead of a quantum critical point reported previously in single crystals, due to enhanced disorder.Comment: 9 pages, 10 figures, accepted for publication in Physical Review

Influence of Nonstoichiometry on Magnetocaloric Effect in (La0.7Ca0.3)1−xMn1+xO3(La_{0.7}Ca_{0.3})_{1 - x}Mn_{1+x}O_3

Magnetocaloric effect in $(La_{0.7}Ca_{0.3})_{1 - x}Mn_{1 + x}O_3$ manganites with x = 0, 0.1, 0.2 has been investigated. It is found a strong influence of nonstoichiometry caused by excessive manganese on the magnetic entropy change. The magnetocaloric effect was evaluated from the isothermal curves of spontaneous magnetization versus the applied magnetic field by using the well-known thermodynamical Maxwell relation. The maximum entropy value, $| \Delta S_{M}^\text{max} |,$ near the ferromagnetic-paramagnetic phase transition is shown to increase with the manganese content. The $La_{0.56}Ca_{0.24}Mn_{1.2}O_3$ manganite exhibits the largest $| \Delta S_{M}^\text{max} |$ value equal to $3.09 J kg^{-1} K^{-1}$ at 15 kOe near $T_{C}$ = 267 K. The magnetocaloric effect values obtained allow to propose that the studied manganites are promising materials for future cooling application