Crystallographic and Magnetic Domains in Heat-Treated Fe–N Alloy Containing α″-Fe₁₆N₂ Phase

Heat treatment of martensite in Fe–N alloy produces α″-Fe₁₆N₂ phase, which appears to show a giant magnetic flux density that exceeds the upper limit predicted by the Slater–Pauling curve. Here, we reveal the relationship between the crystallographic microstructure and the magnetic domain structure in a heat-treated Fe–N alloy. Transmission electron microscopy observations revealed a complex crystallographic microstructure in the heat-treated specimen: lamellae composed of α″-Fe₁₆N₂, α-Fe, and γ′-Fe₄N phases. Electron holography was used to map the magnetic flux lines in the mixed-phase state and revealed a substantial magnetocrystalline anisotropy of the α″-Fe₁₆N₂ phase. The observations provide useful information for engineering of the α″-Fe₁₆N₂ phase.