Magnetic and Physical Microstructure of Fe16 N2 Films Grown Epitaxially on Si(001)

Epitaxial Fe 16 N 2 films were grown on Si(001) substrates with an Ag underlayer by reactive sputtering in nitrogen. Pure α′-Fe 8 N films were obtained which on subsequent annealing resulted in mixtures of α′-Fe 8 N (54%) and α″-Fe 16 N 2 (46%). An average moment of 1780 emu/cc, considerably larger than that of pure α-Fe (1714 emu/cc), was measured for both samples. Plan-view transmission electron microscopy of the films confirms the orientation relationship Fe 16 N 2 (001)‖Ag(001)‖Si(001) and Fe 16 N 2 [100]‖Ag[110]‖Si[100], and a small grain size (~100 Å), while electron energy-loss spectroscopy confirms an average composition of Fe 8 N for both samples. Electron diffraction patterns indicate that the as-deposited α′ films already contain very small regions of ordered α″ 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 α′ and the annealed α′/α″ mixtures. Lorentzian fitting of the three iron components for the α′/α″ 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 Fe 16 N 2 structure. Moreover, the pure α′ 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.