K doping in the compound of Nd0.75Na0.25-xKxMn1O3 (x = 0, 0.05 and 0.10) manganites have been investigated to study its effect on crystalline phase and surface morphology as well as electrical transport and magnetic properties. The structure properties of the Nd0.75Na0.25-xKxMnO3 manganite have been characterized using X-ray diffraction measurement and it proved that the crystalline phase of samples were essentially single phased and indexed as orthorhombic structure with space group of Pnma. The morphological study from scanning electron microscope showed there was an improvement on the grains boundaries and sizes as well as the compactness with K doping suggestively due to the difference of ionic radius. On the other hand, DC electrical resistivity measurement showed all samples exhibit insulating behavior. However, analysis of dlnρ/dT-1 vs. T revealed the clearly peaks could be observed at temperature 210K for x = 0 and the peaks were shifted to the lower temperature around 190 K and 165 K for x = 0.05 and x = 0.1 respectively, indicate the existence of charge ordering (CO) state in the compound. Meanwhile, the investigation on magnetic behavior showed all samples exhibit transition from paramagnetic phase to anti-ferromagnetic phase with decreasing temperature and the TN was observed to shift to lower temperature suggestively due to weakening of CO state

This paper reports influences of cobalt (Co) substituted at Mn-site of Pr­0.75­Na­0.25Mn1-xCo­x­O­3 and Nd0.75Na0.25Mn1-yCoyO3 on structure, electrical transport and magnetic properties. All of the samples were prepared via standard solid state reaction method. X-ray diffraction measurement indicates that all samples were crystallized in an orthorhombic structure (space group Pnma). Resistivity measurement displays the x = 0 sample manifests an insulator behavior while metal-insulator transition was found at 108 K and 84 K for x = 0.02 and 0.05 respectively for Pr­0.75­Na­0.25Mn1-xCo­x­O­3. On the other hand, all of the samples for Nd0.75Na0.25Mn1-yCoyO3 showed insulator behavior down to low temperature and analysis of the resistivity change with respect to temperature, dlnρ/dT-1 versus T reveals a slope changes of resistivity have been recorded. Two obvious peaks were recorded from the analysis for y = 0.02 and 0.05 which can be suggested to the existence of charge order transition at the vicinity. For magnetic properties, x = 0 sample showed a paramagnetic-antiferromagnetic transition and further substitution of Co, x = 0.02 and 0.05, induce the paramagnetic-ferromagnetic transition and antiferromagnetic arrangement respectively. Meanwhile, further substitution of Co, y = 0.02 and 0.05 indicate antiferromagnetic transition with increasing T­N­­­ as Co increased.