Crystal structure and physical properties of half-doped manganite nanocrystals of less than 100-nm size

Abstract

In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped La_0.5Ca_0.5MnO₃(LCMO) synthesized by chemical route, having particle size down to an average diameter of 15 nm. It was observed that the size reduction leads to change in crystal structure, and the room temperature structure is arrested so that the structure does not evolve on cooling unlike bulk samples. The structural change mainly affects the orthorhombic distortion of the lattice. By making comparison to observed crystal structure data under hydrostatic pressure, it is suggested that the change in the crystal structure of the nanocrystals occurs due to an effective hydrostatic pressure created by the surface pressure on size reduction. This not only changes the structure but also causes the room temperature structure to freeze. The size reduction also does not allow the long supercell modulation needed for the charge ordering, characteristic of this half-doped manganite, to set in. The magnetic and transport measurements also show that the charge ordering (CO) does not occur when the size is reduced below a critical size. Instead, the nanocrystals show ferromagnetic ordering down to the lowest temperatures along with metallic-type conductivity. Our investigation establishes a structural basis for the destabilization of CO state observed in half-doped manganite nanocrystals.