Effect of size reduction on the structural and magnetic order in LaMnO3+δ (δ ≈ 0.03) nanocrystals: a neutron diffraction study

We report a structural transition from the orthorhombic to the rhombohedral phase upon size reduction in nanocrystalline LaMnO3+δ (δ ≈ 0.03) as revealed through neutron diffraction studies. The transition occurs when the average particle (crystallite) size is taken below ∼50 nm without change of δ, which is fixed at around 0.03 as measured by a number of characterization tools. The change in the crystallographic structure is accompanied by a change in the magnetic order, where the canted antiferromagnetic order with moments in the basal (ab) plane for the bulk changes to collinear ferromagnetic order with spins along the c-axis for the nanocrystals. The spontaneous ferromagnetic moment ∼3 μB and the transition temperature of 260 K in LaMnO3+δ nanocrystals are similar to those found in La0.67Ca0.33MnO3 which has a much higher Mn4+ content. The likely origin is traced to change in magnetic exchange interactions due to change in Mn–O bond lengths which become almost identical in the MnO6 octahedron in the rhombohedral structure in the absence of Jahn–Teller distortion. The study provides an example of structural and magnetic phase transition driven purely by size reduction and with no change in the chemical constituents. (paper)