Effect of Ru-Doping on Magnetocaloric Effect in Pr Based Charge Ordered Manganites

It is shown that charge-orbital ordered antiferromagnetic state of the parent compound $({\rm x}=0)$ in ${\rm Pr}_{0.5}{\rm Ca}_{0.5}{\rm Mn}_{1-{\rm x}}{\rm Ru}_{\rm x}{\rm O}_{3}$ series is destabilized and ferromagnetism is induced by Ru doping, as small as 3%, at the Mn site. The ferromagnetic transition temperature $({\rm T}_{\rm C})$ increases with Ru content from ${\rm T}_{\rm C}=213\ {\rm K}$ for ${\rm x}=0.03$ to 239 K for ${\rm x}=0.1$ but the field dependence of magnetization does not show metamagnetic transition above or below ${\rm T}_{\rm C}$ unlike ${\rm Pr}_{0.5}{\rm Ca}_{0.5}{\rm Mn}_{1-{\rm x}}{\rm Cr}_{\rm x}{\rm O}_{3}$. The change in magnetic entropy $(\Delta{\rm S}_{\rm M})$ is the largest for ${\rm x}=0.03$ ($\Delta{\rm S}_{\rm M}=-4.2\ {\rm J/kg}\ {\rm K}$ at 213 K for $\Delta{\rm H}=5\ {\rm T}$ ) and decreases with increasing Ru content ( $\Delta{\rm S}_{\rm M}=-3.8\ {\rm J/kg}\ {\rm K}$ at 221 K and $-$ 3.4 J/kg K at 239 K for $\Delta{\rm H}=5\ {\rm T}$, for ${\rm x}=0.05$ and 0.10, respectively). However, the relative cooling power increases with Ru content from 284.9 J/kg for ${\rm x}=0.03$ to 303.6 J/kg for ${\rm x}=0.1$. The large magnetic entropy change found in ${\rm x}=0.03$ and the absence of high field hysteresis in M-H curves make the Ru-doped samples suitable candidates for magnetic refrigeration.