Magnetotransport Behaviour of Nanocrystalline ()

The nanocrystalline samples of (PSMO) ( , 0.50, 0.55, and 0.60) were synthesized by wet-chemical sol-gel route. Structural, magnetic, and magnetotransport properties have been studied systematically. It is found that samples with Sr content and 0.50 show paramagnetic to ferromagnetic (PM-FM) transition at ？K with no trace of FM-AFM transition within the temperature range of 77–350？K. However, interestingly a second transition is observed at 273 and 255？K, respectively, for samples and 0.60 correspond to an A-AFM magnetic structure. This indicates that samples and 0.50 are ferromagnetic below , while other samples ( and 0.60) have a mixed phase consisting of FM and A-type AFM phases. Resistivity versus temperature (ρ-T) curve shows that the resistivity of all the samples is much larger than the single crystals of corresponding compositions due to large contribution of grain boundaries in the present nanocrystalline samples. Moreover, the decrease in metallic component at higher Sr concentration is also evidenced by the successive reduction in magnetoresistance (MR) with increasing Sr content from to 0.60. 1. Introduction Several experimental and theoretical studies have focused on the exploration of grain size effect on the structural, magnetic, and electrical transport properties of alkaline-earth doped rare earth perovskite manganites chemically represented by (RE = rare earth cation and AE = alkaline earth cation) because of their unusual magnetic and electronic properties like colossal magnetoresistance (CMR), charge ordering, orbital ordering, and phase separation [1–10]. These studies focus on and clearly highlight the significance of broken Mn–O–Mn exchange bonds at the grain surface and their likely impact on the magnetic and electrical transport properties. Since size reduction leads to increased contribution from the surface regions, the broken Mn–O–Mn bonds are indeed expected to have a definite impact on magnetoelectrical properties in manganites. However, in view of the fact that manganites exhibit a strong competition and correlation between various structural and electronic degrees of freedom even more intriguing and complex phenomena are expected. It has been shown that in nanomanganites, size reduction below ~100？nm renders the charge and orbitally ordered (CO–OO) ground state with unstable antiferromagnetic (AFM) spin order, giving rise to a ferromagnetic (FM) ground state [3, 11]. Size induced transition from the AFM/CO to the weak ferromagnetic (WFM) state was observed in both nanowires [3] and nanoparticles [4]. It has been shown by