Structure and magnetism of single-phase epitaxial ${\gamma }^{\prime }-{\mathrm{Fe}}_4\mathrm{N}$

Single phase epitaxial pure ${\gamma }^{\prime }-{\mathrm{Fe}}_4\mathrm{N}$ films are grown on MgO (001) by molecular beam epitaxy of iron in the presence of nitrogen obtained from a radio frequency atomic source. The epitaxial, single phase nature of the films is revealed by x-ray diffraction and by the local magnetic environment investigated by Mössbauer spectroscopy. The macroscopic magnetic properties of the ${\gamma }^{\prime }-{\mathrm{Fe}}_4\mathrm{N}$ films are studied in detail by means of transverse Kerr effect measurements. The hysteresis loops are consistent with the cubic atomic structure, displaying easy [100] magnetization directions. The films are single domain at remanence, and the reversal is dominated by 180° or 90° domain wall nucleation and propagation, depending on the applied field direction. When 90° domain walls are responsible for the magnetization reversal, this proceeds in two stages, and the measured coercive fields vary accordingly. Magnetic domain observations reveal the two distinct reversal —driven by 180° or 90° domain walls— modes displaying large domains, of the order of mm. From magnetometer techniques, the saturation magnetization, ${\mu }_0{\mathrm{M}}_s,$ is measured to be 1.8 T. A magneto-optical torque technique is used to obtain a value of the anisotropy constant of $2.9\times {10}^4{\mathrm{J}/\mathrm{m}}^3.$