Electrical, magnetic, and magnetoresistance studies in chromium-doped Pr-based manganites

The effect of Cr³⁺ on the electrical, magnetic and magnetoresistance effect in Pr_0.75Na_0.05K_0.2Mn_1−xCr_xO₃ (x = 0.0, 0.01, 0.02, 0.03, 0.04, 0.05) synthesised using the solid-state method are reported. An analysis at room temperature of X-ray diffraction data using the Rietveld refinement method showed all samples crystallised in the orthorhombic phase with Pnma space group. It is found that unit cell volume increased from 230.60 ų (x = 0) to 231.25 ų (x = 0.01) suggesting that substitution with a low concentration of Cr³⁺ may predominately substitute for Mn⁴⁺ site. However, further substituting Cr³⁺ caused a decrease in unit cell volume from 231.15 ų (x = 0.02) to 227.74 ų (x = 0.05), indicating at high concentration of Cr³⁺ dominantly substituted the Mn³⁺. Cr³⁺ substitution slightly decreased the Curie temperature \({(T}_{C})\) ${(T}_C)$ value from 140.7 K (x = 0.00) to 139.7 K (x = 0.01) and 139.3 K (x = 0.02) indicates the substitution of Cr³⁺ slightly disturb ferromagnetic (FM) interaction between Mn³⁺–O²⁻–Mn⁴⁺. Interestingly, further substitution of Cr³⁺ increased the \({T}_{C}\) $T_C$ to 140 K (x = 0.03), 140.7 K (x = 0.04) and 143.4 K (x = 0.05) which revealed the possibility of the existence of ferromagnetic interaction between Cr³⁺ and Mn³⁺ ions due to the similar electronic configuration of Cr³⁺ with that of Mn⁴⁺ ions. The observed large increased in resistivity with Cr³⁺ substitution and the decrease in metal insulator transition temperature (T_MI) from 122 K (x = 0) to 90 K (x = 0.05) from resistivity versus temperature curve indicates the Cr substitution weakened mechanism and induce more competition between double-exchange (DE) and super-exchange (SE) mechanism involving Mn³⁺–O²⁻–Mn⁴⁺, Cr³⁺–O²⁻–Mn³⁺, Cr³⁺–O²⁻–Cr³⁺, Cr³⁺–O²⁻–Mn⁴⁺. The observation large magnetoresistance (MR) effect in Cr-substituted sample at low temperature region indicates enhancement of extrinsic MR effect, which likely arises from spin polarized tunnelling.