Challenge to the synthesis of α′′-(Fe, Co)16N2 nanoparticles obtained by hydrogen reduction and subsequent nitrogenation starting from α-(Fe, Co)OOH

The α′′-Fe16N2 has attracted much interest as a candidate for semi-hard magnetic materials. As reported coercivity of α′′-Fe16N2 nanoparticles so far were not high, it is necessary to improve magnetocrystalline anisotropy of α′′-phase to be used practically. Since theoretical calculation suggests a possibility of the improvement of magnetocrystalline anisotropy by substituting of Fe with Co, we challenged to make α′′-(Fe1-xCox)16N2 nanoparticles by hydrogen reduction of α-(Fe1-xCox)OOH as a starting material changing X from 0 to 0.1. We were able to produce α-(Fe1-xCox) nanoparticles through (Fe0.98Co0.02)OOH, and (Fe0.95Co0.05)OOH and (Fe0.95Co0.05)2O3. Sintering occurred during reduction and crystallite diameter D increased with increasing reduction temperature. α′′-(Fe1-xCox)16N2 phase containing α-(Fe,Co) nanoparticles were obtained only at reduction of 340 °C for 4 h and nitridation of 170 °C for 5 h for X = 0.02. It has been revealed that α′′-(Fe1-xCox)16N2 or α′′-Fe16N2 could be formed below D = 50 nm of α-(Fe,Co) or α-Fe. And containing Co make it more difficult to produce α′′-phase. It can be said that the formation of α′′-(Fe1-xCox)16N2 is subject to a limit of D and Co content. Long time nitridation is only way to increase the formation yield of α′′-(Fe1-xCox)16N. High Hc = 2000 Oe was obtained for α′′-(Fe0.98Co0.02)16N2 by changing nitridation time 10 hours from 5 hours. Further long nitradation is expected to increase in Hc of α′′-(Fe0.98Co0.02)16N2.