Frustration-Induced Inversion of the Magnetocaloric Effect and Metamagnetic Transition in Substituted Pyrochlore Iridates

We investigate the effect of partial replacement of extended 5d Ir4+ sites by localized 3d Cr3+ moments on the magnetocaloric properties of Y2Ir2O7 (YIO) pyrochlore iridates. We find that Y2Ir2–xCrxO7 (YICO) undergoes a cluster glass transition, possibly due to a Ruderman–Kittel–Kasuya–Yosida (RKKY)-like interaction between localized Cr3+ moments occupying random sites in the pyrochlore network, mediated by 5d Ir conduction electrons. The coexistence of ferromagnetic and antiferromagnetic clusters gives rise to the conventional and inverse magnetocaloric effect (MCE). We observe significant enhancement of conventional as well as inverse MCE with substitution. Although the values of the conventional MCE and inverse MCE in substituted iridates are not large, the effect spans a giant working temperature window, thus leading to orders of magnitude enhancement of cooling power, the value being comparable to that of standard magnetocaloric materials.