Synthesis of α′′-Fe16 N2 Compound Anisotropic Magnet by the Strained-Wire Method

α′′-Fe16N2 is considered as one of the most promising candidates for future rare-earth-free magnets, showing the highest saturation magnetization reported so far. We propose and demonstrate a "strained-wire method" to synthesize α′′-Fe16N2 compound anisotropic magnets with an enhanced hard magnetic property, with a direct experimental observation of the intercoupling between tensile strain and the martensitic phase transition. The principle is helpful for the generation of another martensitic phase. In this paper, the method is demonstrated on an α′′-Fe16N2 compound permanent magnet preparation by starting from pure bulk iron, with urea as the nitrogen provider. A uniaxial tensile stress is applied on the wire-shaped sample during the postannealing stage, producing a promising permanent magnet with a hard magnet property which lacks any rare-earth elements. The sample synthesized in the lab exhibits a coercivity of 1220 Oe and an energy product of up to 9 MGOe. The mechanism of the strained-wire method is analyzed based on scanning-transmission-electron-microscopy characterizations of samples with different strains. We observe a strain-induced recrystallization of α′′-Fe16N2 samples at a low annealing temperature (150 °C). We demonstrate that this strained-wire method can be used on α′′-Fe16N2 samples to increase the α′′-Fe16N2 phase-volume ratio and to fine-tune its microstructure at a low temperature. Some further characterization results are also included in this paper. The physics of the influence of tensile stress on the martensitic phase transition is discussed.