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

Abstract

Epitaxial Fe16N2 films were grown on Si(001) substrates with an Ag underlayer by reactive sputtering in nitrogen. Pure α′-Fe8N films were obtained which on subsequent annealing resulted in mixtures of α′-Fe8N (54%) and α′′-Fe16N2 (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 Fe16N2(001)‖Ag(001)‖Si(001) and Fe16N2[100]‖Ag[110]‖Si[100], and a small grain size (∼100 Å), while electron energy-loss spectroscopy confirms an average composition of Fe8N 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. Mössbauer 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 Fe16N2 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.