Role of Surface on Magnetic Properties of La1-xSrxMnO Nanocrystallites

The magnetic La_1-xSr_xMnO₃ particles of ≈50 nm size have been prepared in the flux of sodium nitrite at ≈500°C. A detailed magnetic study, including the neutron diffraction at zero and high pressures, has been performed on compositions x = 0.28 and 0.37, as well as on comparative samples prepared via sol-gel route. Based on these experiments, we conclude that the molten salt prepared particles show significant oxygen excess δ > 0 that can be ascribed mainly to the surface oxygen chemisorption. As a consequence, the valence of Mn ions is shifted toward a higher oxidation state, and this shift probably exhibits a pronounced radial distribution in the particles, reaching possibly pure Mn⁴⁺ at the uppermost surface layer. We suggest that such overdoping is an additional reason for magnetically dead shell in manganite nanoparticles and is also the source of surface stress that compresses the particle core and modifies its physical properties. Such compressive stress drives the magnetic ground state of the composition x = 0.37 toward a mixture of ferromagnetic (FM) and A-type anti-FM (AFM) ordering. Although the surface effects should also occur in the particles with x = 0.28, the AFM phase is practically absent in this sample but can be induced by high pressure.