Synthesis, morphology, thermal stability and magnetic properties of α"-Fe16N2 nanoparticles obtained by hydrogen reduction of γ-Fe2O3 and subsequent nitrogenation

Typical synthesis of alpha ''-Fe16N2 nanoparticles involves reduction of iron oxides by hydrogen at elevated temperatures which is disadvantageous due to the particle coalescence. Here we report on a process for reduction of iron oxides at elevated pressures and show that by increasing hydrogen pressure from atmospheric to 53 MPa, it is possible to reduce the reaction temperature from 663 K down to 483 K, resulting in phase-pure alpha-Fe nanoparticles without noticeable particle growth. By subsequent nitrogenation in an ammonia flow, fine, 99% phase-pure alpha ''-Fe16N2 nanoparticles could be synthesized. The reduction temperature and the respective particle size has a significant influence on the nitrogenation step. alpha ''-Fe16N2 nanoparticles exhibit semi-hard magnetic properties with M-s(0) = 215 Am-2 kg(-1), mu H-0(c) = 0.22 T, T-C = 634 K and exchange stiffness A(c) = 6.84 pJ m(-1), A(a,b) = 7.53 pJ m(-1). Synthesis conditions, microstructure, chemical composition and thermal stability of the nanoparticles are systematically studied and correlated with the observed magnetic properties. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.