Formation and magnetic behaviour of manganese oxide nanoparticles

The formation and magnetic behaviour of nanoparticles of various manganese oxides have been studied. For synthesis, manganese oxalate dihydrate is first produced via sol–gel process and then decomposed in oxygen, air, nitrogen, or argon ambient at different temperatures. The decomposition process leads to emergence of Mn3O4 which undergoes oxidation, oxidation–reduction, or reduction process subsequently depending upon the availability of oxygen to yield Mn2O3 in air/O2 and MnO in argon/nitrogen ambient. A partial reaction causes retention of some Mn3O4 alone or Mn3O4 and Mn5O8 together as secondary phase(s). The magnetic susceptibility (χ) data reveal (i) antiferro-magnetic nature for Mn2O3, Mn5O8 and MnO with Néel temperature 75 K, 131 K and 115 K, respectively, (ii) orthorhombic to cubic crystallographic transition in Mn2O3 at 310 K for the first time, and (iii) ferri-magnetic character for Mn3O4 with Curie temperature 46 K. Further, it is demonstrated that a small quantity of Mn3O4 always remains in the products and can be detected by susceptibility data below 50 K only with bifurcation in χZFC and χFC curves and blocking temperature in the range 30–40 K. Also, the decomposition of manganese oxalate at 1100 °C for 4 h in air leads to formation of pure Mn3O4 with saturation magnetization of 1.85μB per molecule and coercivity of 4750 Oe at 10 K.