Structure and magnetic properties of rf thermally plasma synthesized Mn and Mn-Zn ferrite nanoparticles

Plasma synthesis has previously been shown to be a viable route to producing nanocrystalline magnetite and Ni ferrite nanoparticles. In this work nanocrystalline powders of Mn and Mn-Zn ferrites have been synthesized using a 50 kW-3 MHz rf (radio frequency) induction plasma torch. We investigate these materials for soft magnetic applications. High-energy ball milled Mn + Fe powders and (Mn+Zn) +Fe powders (<10 mum) in the stoichiometric ratio of 1:2 were used as precursors for the ferrite synthesis. Compressed air was used in the oxygen source for oxidation of metal species in the plasma. X-ray diffraction patterns for the plasma-torched Mn ferrite and MnZn ferrite powders were indexed to the spinel ferrite crystal structure. An average grain size of similar to20 nm was determined from Scherrer analysis confirmed by transmission electron microscopy studies. The particles also exhibited faceted polygonal growth forms with the associated truncated cuboctahedral shapes. Room-temperature vibrating sample magnetometer measurements of the hysteretic response revealed saturation magnetization M-s and coercivity H-c of Mn ferrite are 23.65 emu/g and 20 Oe, respectively. The Neel temperatures of Mn ferrite powders before and after annealing (500degreesC, 30 min) were determined to be 200 and 360degreesC, respectively. Inductively coupled plasma chemical analysis and energy dispersive x-ray analysis data on the plasma-torched powders indicated deviations in the Mn or Zn contents than the ideal stoichiometry. MnZn ferrite was observed to have a Neel temperature increased by almost 400degreesC as compared with as-synthesized Mn ferrite but with a larger coercivity of similar to35 Oe. (C) 2003 American Institute of Physics.