Thermally activated processes and superparamagnetism in Bi12MnO20 nanoparticles: A comparative study

Abstract Manganese sillenite (Bi 12 MnO 20 ) nanoparticles having average particle size between 22 and 43 nm were synthesized by a low temperature soft chemical route under refluxing conditions. A careful structural and microstructural characterization by means of high resolution X-ray diffraction experiments and transmission electron microscopy is presented. The as-cast powder displayed an isotropic superparamagnetic (SPM) behavior with a blocked state for temperatures below T B ∼ 13.0 K . We used three different measurement techniques to extract and compare the Bi 12 MnO 20 blocking temperatures. First, we extracted T B with the modified Bean–Livingstone model from the coercive field temperature dependence obtained from hysteresis curves measured as a function of temperature. Then, the blocking temperature distribution function, f ( T B ) , was obtained by deriving the zero field-cooled/field-cooled curves difference. For each applied field, the maximum of the distribution function gave us the mean blocking temperature value. Finally, the maximum of the magnetic susceptibility imaginary part as a function of frequency was used, combined with the Neel–Brown equation, to extract the blocking temperature. All measurement techniques yield an equivalent dependence of T B with H of the Bi 12 MnO 20 superparamagnetic nanoparticles.