Crystal structure and physical properties of half-doped manganite nanocrystals with size < 100nm

TL;DR

This study investigates how reducing the size of half-doped manganite nanocrystals to below 100nm alters their crystal structure and suppresses charge ordering, leading to ferromagnetism and metallic conductivity at low temperatures.

Contribution

It reveals that size reduction induces a structural change similar to hydrostatic pressure, preventing charge ordering and stabilizing ferromagnetic metallic states in nanocrystals.

Findings

Size reduction causes structural arrest and prevents structural evolution on cooling.

Charge ordering is suppressed below a critical nanocrystal size.

Nanocrystals exhibit ferromagnetic order and metallic conductivity at low temperatures.

Abstract

In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped $\mathrm{La_{0.5}Ca_{0.5}MnO_3}$(LCMO) synthesized by chemical route, having particle size down to an average diameter of 15nm. 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 with 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-in. 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.