Role of surface on magnetic properties of La1−xSrxΜn03+δ nanocrystallites

The ferromagnetic La_1-xSr_xMnO_3+δ particles have recently been prepared from the flux of sodium nitrite at ≈500°C [1,2]. The advantage of this facile method is the high yield of cubic-shape nanocrystals without a very fine matter that is common for an alternative preparation via sol-gel route. The size of the as-prepared nanoparticles is ≈50 nm. A detailed magnetic study, including the neutron diffraction at zero and high pressures, has been performed on as-prepared particles of compositions x=0 .28 and 0 .37. Based on these experiments we conclude that as-prepared manganite particles show certain oxygen excess δ>0 that can be ascribed mainly to the surface oxygen chemisorption [3]. As a consequence the valence of Mn ions is shifted toward a higher oxidation state and this shift has probably a pronounced radial distribution in the particles, reaching possibly pure Mn⁴⁺ at the uppermost surface layers. We suggest that such overdoping is the true reason for “magnetically dead” shell in the 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 x=0 .37 particles towards a mixture of FM and A-type AFM ordering. In the x=0 .28 particles, only the FM phase is observed (M_1000kA/m(4.5 K)=59.7 Am²/kg, T_C=319 K), but an A-type AFM component can be induced by application of high external pressures. Our observations are interpreted based on so-called t-J hamiltonian that takes into account the superexchange between localized t_2g spins (S=3/2) and the double exchange mediated by itinerant eg carriers (s=1/2) of Mn³⁺/Mn⁴⁺ perovskite lattice [4].