Enhanced ferromagnetism and Mott variable-range hopping behavior in Cu doped pyrochlore iridate Y2Ir2O7

The electric and magnetic properties of doped iridates Y2Ir2-xCuxO7 have been studied in the present paper. It is found that the magnetic properties show an enhanced ferromagnetism with doping, which comes from Cu spins and doping-broken Ir networks. The M-T data of all the doped compounds show a magnetic order around 150 K, which should be rooted in antiferromagnetic (AFM) order of Ir spins. However, there is also another transition below 20 K for x >= 0.2 samples, which should be originated from AFM Cu-Ir interactions. The result of bulk magnetization can be further proved by Electron spin resonance (ESR) measurements which suggest ferromagnetic (FM) interactions of Cu ions and the occurrence of magnetic order below 20 K. From the data of M-H curves, we found that there is a metamagnetic-like transition at 20 kOe for all the doped samples, which is considered as reconstruction of the Ir electrons. While another metamagnetic-like transition at 60 kOe for x = 0.4 sample is observed and considered as a spin-reorientation Cu spins. Resistivity data indicate that all the doped samples can be fitted by Mott's variable-range hopping (VRH) model at low temperatures. This is the consequence of localization due to disorder on the Ir/Cu site.