Disorder-induced phase coexistence in bulk doped manganites and its suppression in nanometer-sized crystals:: The case of La0.9Ca0.1MnO3

X-band electron magnetic resonance together with dc and ac magnetic measurements were employed for comparative study of magnetic ordering in bulk and nanometer-sized La0.9Ca0.1MnO3 single crystals. A dramatic difference between bulk crystal showing mixed magnetic state, constituted by coexisting canted antiferromagnetic A-type matrix and nanometer sized ferromagnetic clusters, and the nanocrystalline form of the same compound, showing mainly ferromagnetic ordering, has been revealed. The complementary study of the structural state and analysis of the electron paramagnetic resonance data in terms of the proposed theoretical model have enlightened the reasons for the observed difference in the magnetic order. The results suggest that the change in magnetic order has an intrinsic nature and is not induced by nonstoichiometry. The cation composition and the oxygen stoichiometry of bulk and nanosized crystals were determined to be the same within the experimental accuracy. Nanometer-sized crystals of La0.9Ca0.1MnO3 are characterized by better chemical and crystalline homogeneity arising both from different fabrication techniques and reduced crystal-lites size. This effect induces a transition from an inhomogeneous confined state of charge carriers in chemically disordered bulk crystal to a more mobile one in an impuritylike band in homogeneous nanocrystals, resulting in the change of magnetic ordering. A realistic model describing size assisted change of magnetic order in doped manganites has been proposed to interpret the data. The experimental results and their analysis indicate that a chemical/magnetic disorder has a strong impact on the magnetic state and the phase diagram of doped manganites.