Structure and magnetic moment of α″-Fe₁₆N₂ compound films: Effect of Co and H on phase formation (invited)

In order to determine the intrinsic value of magnetic moment, σ_s, of α″-Fe₁₆N₂ compound, the magnetic moment of (α″+α′)-Fe₁₆N₂ films is discussed in connection with the degree of N site ordering in nitrogen-martensite. To establish the more general relation between Fe-N martensite structure and σ_s, the Fe-N system is expanded to Fe-Co-N and Fe-H-N systems. As a result, it is found that (1) The existence of completely the same structure as bulk α″ phase was reconfirmed even in the film form by the precise structural analysis using x-ray diffraction, transmission electron microscopy and Mössbauer analysis, (2) Fe-H martensite was synthesized by using sputtering under Ar+H₂ atmosphere, (3) Stable formation of α″-(Fe_100-XCo_X)₁₆N₂ (X=10-30) phase through N site ordering by postannealing is proposed to be fairly difficult due to the sudden decrement of the phase decomposition temperature, T_p.d., of α′-(Fe_100-XCo_X)-N (X=0-30). T_p.d. of α′-(Fe_100-XCO_X)-N (X=0-30) phase decreases from 200 °C to RT with increasing Co and N contents, and (4) The intrinsic value of saturation magnetization of the α″-Fe₁₆N₂ phase is convinced to be no more than 240 emu/g (≈2.4 μ_B per Fe atom on average) at 300 K.