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In order to determine the synthesis conditions of α″-(Fe100−xCox)16N2, FeCo nitrided films were fabricated on MgO single-crystal substrates using reactive sputtering. The thermal stability of the α′FeCo phase was evaluated by measuring the temperature dependence of σs. It was found that: (1) for the as-deposited (Fe100−xCox)-N films, α′FeCo martensite phase with a stoichiometric N content of 11 at% can be formed up to x = 10, and with increasing x, α′FeCo phase with stoichiometric N content is not formed. (2) The phase decomposition temperature, Tpd, of α′-(Fe100−xCox)-N (x = 0–30) phase depends strongly on the Co and N contents of the α′FeCo phase. The Tpd of α′ phase decreases from 200°C (x = 0) to RT (x = 30) with increasing Co and N contents. (3) The formation at RT of stable α″-(Fe100−xCox)16N2 (x = 10–30) phase in the FeCo alloy system is concluded to be fairly difficult.

In order to determine the value of the intrinsic magnetic moment of the α ‘ phase, the films of nitrogen‐martensite with various N content were fabricated under various reactive sputtering conditions. The magnetic moment of (α ‘+α’)‐Fe16N2 films is discussed in connection with the change of the unit‐cell volume of the bct structure and the degree of N site ordering in nitrogen‐martensite. As a result, it is found that (1) the same structure as bulk α ‘‐Fe16N2 is realized in the present films, (2) the saturation magnetization σs of the α’ phase increases about 4% with increasing unit‐cell volume of the α’ phase, (3) the degree of N site ordering from α’ to α ‘‐Fe16N2 does not much affect σs, and (4) the experimentally obtained maximum value of σs for the (α ‘+α’)‐Fe16N2 film was 232 emu/g. The intrinsic value of σs in the α ‘ phase (in the perfectly ordered state) is proposed to be no more than 240 emu/g at 300 K.

In order to determine the intrinsic value of magnetic moment, σs, 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 σs, 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 Mossbauer analysis, (2) Fe–H martensite was synthesized by using sputtering under Ar+H2 atmosphere, (3) Stable formation of α″‐(Fe100−XCoX)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, Tp.d., of α′‐(Fe100−XCoX)–N (X=0–30). Tp.d. of α′‐(Fe100−XCoX)–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 μB per Fe atom on average) at 300 K.