Challenge to the Synthesis of α-(Fe,M)16N2 Nanoparticles Obtained by Hydrogen Reduction and Subsequent Nitridation Starting From α-(Fe,M)OOH (M = Al, V, Cr, Mn, Ni)

Metastable alpha ''-Fe16N2 is considered to be a good candidate as a semi-hard magnetic material with high saturation magnetization. To obtain a higher coercivity H-c, it is necessary to improve the magnetocrystalline anisotropy of alpha '' Fe16N2. For this purpose, many theoretical calculations based on substitution of Fe with other elements (i.e., Co, Ni, Mn, Al) have been performed. In this study, to investigate the effect of substitution of Fe by other elements M (M = Al, V, Cr, Mn, Ni), the synthesis of the alpha ''-(Fe0.95M0.05)(16)N-2 nanoparticles by reduction of alpha-(Fe0.95M0.05)(X)H as the starting material and subsequent nitridation was undertaken. The alpha-(Fe0.95Al0.05)OOH nanoparticles used as the starting material were spherical in shape and the particle size was significantly smaller than that for alpha-FeOOH and that for alpha-Fe0.95Al0.05 obtained by reduction, which was about 25 nm. The alpha ''-(Fe0.95M0.05)(16)N-2 nanoparticles appeared only for M = Al and V. The result suggested that the experimental conditions whereby alpha ''-(Fe0.95M0.05)(16)N-2 was produced were controlled by the crystallite diameter (D-c) for the alpha-Fe0.95M0.05 nanoparticles which was <50 nm before nitridation irrespective of the nature of element M. This finding is critical for preparing materials like alpha ''-(Fe,M)(16)N-2 where the nitrogen atoms are introduced at the interstitial site and where the distorted crystal structure is controlled by the limitation of the crystallite diameter for alpha-(Fe,M) as the precursor. Al was effective in preventing sintering during reduction, which led to a small D-c value being obtained. The highest H-c value for the alpha ''-(Fe0.95Al0.05)(16)N-2 nanoparticles was 2200 Oe for reduction at 450 degrees C for 4 h and subsequent nitridation at 160 degrees C for 20 h.

Automated summary

Metastable alpha ''-Fe16N2 is considered a good candidate as a semi-hard magnetic material, and in this study, it was found that only when M=Al and V were used as substitution elements for Fe, the desired nanoparticles could be obtained. This suggests the importance of the substitution elements in the synthesis process.