XAFS investigation of the role of orientational disorder in the stabilization of the ferromagnetic metallic phase in nanoparticles of La_0.5Ca_0.5MnO₃

Abstract

The inclusion of the contribution of Jahn–Teller distortion of MnO6 units, in addition to double-exchange, has been largely successful in explaining the magneto-transport behavior of manganites. However, our recent experiments on La0.5Ca0.5MnO3 demonstrated the limitation of these factors in explaining the radical difference between the magneto-transport properties of bulk and nanocrystalline forms. While bulk La_0.5Ca_0.5MnO₃ exhibits insulator character (4–300 K) and an anti-ferromagnetic–ferromagnetic transition at 200 K, the nanocrystalline form stabilizes in a metallic ferromagnetic phase (4–300 K). This is counter-intuitive since large Jahn–Teller distortion, which promotes anti-ferromagnetism or insulator character, exists in the nanocrystals too (as indicated by x-ray diffraction results). In this work, we resolve this paradox by considering the role of structural disorder. Employing x-ray absorption spectroscopy, we establish that the disorder in inter-octahedral coupling is enhanced by 57% in the nanocrystals, as the octahedral units are randomly oriented with respect to each other. This orientational disorder promotes metallic ferromagnetism by destroying the stringent orbital ordering that is needed for anti-ferromagnetism and the co-operative nature of the orbital order.