The effect of Ni doping on the magnetic and transport properties in Pr _0.5 Ca_0.5 Mn_1-x Ni_x O₃ manganites

Structural, magnetic, and transport properties in Pr_0.5 Ca _0.5 Mn_1-x Ni_x O₃ (x=0, 0.04, 0.07, 0.1) were investigated. It is remarkable that low Ni-doping levels at Mn sites induce drastic changes in the physical properties of Pr0.5 Ca0.5 MnO₃ due to melting of the charge ordered state and the consequent capability of Ni ions to create ferromagnetic (FM) clusters. It was found that oxygen deficient samples (3-δ=2.84±0.03) exhibit resistivities higher by four to five orders than that of their stoichiometric counterparts and do not exhibit metal-insulator transition. Only a stoichiometric x=0.04 sample with higher content of the FM phase shows metal-insulator transition at T≈80 K. A change in slope in the zero field cooling magnetization curve observed for x=0.04 and 0.07 (may be slightly oxygen deficient samples) are indicative of spin-glass-like state. Applied hydrostatic pressure of about 10 kbars reduces the temperature of charge ordering in x=0 sample by about 10 K indicating on pressure induced suppression of the Jahn-Teller distortions. In distinct contrast to the behavior of Pr_0.5 Ca_0.5 Mn_1-x Cr_x O₃ samples, an applied pressure only slightly affects magnetization of x=0.04 and 0.07 samples possibly due to the difference in magnetic states obtained by substitution of Cr3+ and Ni2+. Electron magnetic resonance (EMR) unambiguously evidences on appearance of a FM phase on Ni doping. Temperature dependence of EMR spectra allow distinguishing between the magnetic states of two 4% Ni-doped samples and to speculate on the effect of magnetic inhomogeneities on the formation of the observed magnetic properties.