Magnetic and physical microstructure of Fe16N2 films grown epitaxially on Si(001)

Epitaxial Fe16N2 films were grown on Si(001) substrates with an Ag underlayer by reactive sputtering in nitrogen. Pure alpha'-Fe8N films were obtained which on subsequent annealing resulted in mixtures of alpha'-Fe8N (54%) and alpha ''-Fe16N2 (46%). An average moment of 1780 emu/cc, considerably larger than that of pure alpha-Fe (1714 emu/cc), was measured for both samples. Plan-view transmission electron microscopy of the films confirms the orientation relationship Fe16N2(001)parallel to Ag(001)parallel to Si(001) and Fe16N2[100]parallel to Ag[110]parallel to Si[100], and a small grain size (similar to 100 Angstrom), while electron energy-loss spectroscopy confirms an average composition of Fe8N for both samples. Electron diffraction patterns indicate that the as-deposited alpha' films already contain very small regions of ordered alpha '' which were not previously detected by x-ray diffraction measurements. Mossbauer spectroscopy performed at both 300 and 16 K gave three hyperfine fields corresponding to three different iron sites for both the unannealed alpha' and the annealed alpha'/alpha '' mixtures. Lorentzian fitting of the three iron components for the alpha'/alpha '' spectrum obtained at room temperature gave an intensity ratio of 1:2:1 (FeI:FeII:FeIII) corresponding to the expected occupancy for the three Fe sites in the Fe16N2 structure. Moreover, the pure alpha' film at 300 K and both samples at 16 K showed deviation from this distribution. The three components show notable differences in the temperature dependence of their occupancies; however, all three magnetic components deviate similarly from the surface normal. (C) 1997 American Institute of Physics.