Influence of Annealing Temperature on Structural and Magnetic Properties of LaMnO3 Nanoparticles

It is known that stoichiometric LaMnO₃ is an A-type antiferromagnet with the Néel temperature TN=135 ~ 140 K. Recently, the thin films and nanoparticles based this material have attracted intensive interest because its magnetic and electrical properties can be controlled in a large temperature range from 60 to 300 K upon tuning the vacancies of cation and/or oxygen in non-stoichiometric compounds of La_1-x..Mn_1-yO_3±δ, leading the mixed valence of Mn²⁺, Mn³⁺ and/or Mn⁴+ ions [1], [2]. Particularly, with non-stoichiometric compounds, one can easily fabricate transparent ferromagnetic conducting thin films [1]. To get more insight into this problem, we fabricated LaMnO₃ nanoparticles (NPs), and then studied the influence of annealing temperature on structural and magnetic properties of NPs. Here, LaMnO₃ NPs were synthesized from precursors of lanthanum(III) acetate sesquihydrate, and manganese(II) acetate tetrahydrate by using a sol-gel method. As-prepared NPs were then annealed at different temperatures (T_an) of 600-1550 °C for 4 h in air. Crystal-structural analyses based on X-ray diffraction patterns revealed the samples annealed at 600-1400 °C crystallized in a rhombohedral structure with R-3c space group, with the ratio of the cation La/Mn varying from ~0.916 for T_an=600 ^°C to 0.887 for T_an=1400 ^°C. Meanwhile the sample annealed at 1550 °C crystallized in an orthorhombic structure. By changing T_an, we fabricated LaMnO₃ particles with average sizes of 16-3000 nm, which were estimated by using the Scherrer equation and scanning electron microscope. Magnetization measurements indicated their ferromagnetic-paramagnetic phase transition temperature (T_C) can be tuned in the range between 43 and 260 K. At 15 K, the saturation magnetization (M_s) and coercivity (Hc) values are tunable in the ranges of 4~84 emu/g and 25~1300 Oe, respectively. We think that these changes in magnetism of NPs versus Tan are due to the changes in the concentration of Mn³+ and Mn⁴⁺ ions, lattice parameters, local geometrical structures, and grain sizes.