Effect of size reduction on the structural and magnetic order in LaMnO(3+δ) (δ ≈ 0:03) nanocrystals: a neutron...

We report a structural transition from the orthorhombic to the rhombohedral phase upon size reduction in nanocrystalline LaMnO(3+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 μ(B) and the transition temperature of 260 K in LaMnO(3+δ) nanocrystals are similar to those found in La0:67Ca0:33MnO3 which has a much higher Mn(4+) 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.