Nitrogen-induced local magnetic and structural properties of sputtered FeAlN thin films

Abstract

Conversion electron Mössbauer spectroscopy (CEMS) and x-ray diffraction were used to study a series of rf-reactively sputtered FeAlN thin films with the goal of gaining insight into the origins of their uniaxial magnetic anisotropy (UMA) and the effect of sputtering conditions on its thermal stability. At N/Fe=4.6%(±0.5%), N goes into interstitial vacancies without significant reductions in the hyperfine field (Bhf) of the Fe but causing lattice expansions, leading to an increased linewidth in the CEMS spectra of the film while the sample has an in-plane UMA. Reducing the target-to-substrate spacing used in the deposition process does not affect the nitrogen content of the films but yields an increased CEMS linewidth reflecting larger disorder in the local Fe environment. Heavy N doping causes the phase transform into γ′-Fe4N, ε-Fe3N, and ζ-Fe2N phases. A well defined UMA exists in mildly doped FeAlN films when doped N atoms mostly occupy the interstitial sites while FeAl and heavily doped FeAlN films are either isotropic or weakly anisotropic. UMA and its thermal stability are therefore concluded to have a correlation with the local environment of the Fe structure induced by N doping.

Effect of Al doping on phase formation and thermal stability of iron nitride thin films

“…Iron nitride compounds are known to exist in varieties of phases having distinct crystal structure and magnetic properties [1][2][3][4][5][6][7][8][9][10][11][12][13] . Different Fe-N phases can be formed just by increasing the nitrogen concentration in Fe.…”

“…To improve the thermal stability of Fe-N system, it was proposed that if a few atomic percentage of a third element X (X=Al, Ti, Zr, Ta etc. ), which has low ∆H • f for its nitrides and high affinity nitrogen, is added in the Fe-N system, then it can thermally stabilize the iron nitride compounds 3,20,[23][24][25][26][27][28][29][30][31][32][33] . Very recent study elucidates that enhancement of thermal stability actually results from the suppression of Fe self-diffusion 34,35 .…”

Formation of iron nitride thin films with Al and Ti additives

“…Since early 1990s to recent years, various Fe-N thin films were studied by adding a tiny amount of a third element say X, it was observed that using such additives the limitations of binary Fe-N can be reduced to a great extent. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] The basic idea behind adding such element X was to chose an element which has more affinity to nitrogen than iron. When a third element X = Al, Ti, Ta, Zr etc.…”

Effect of dopants on thermal stability and self-diffusion in iron-nitride thin films

“…In attempts to make interstitial Fe-N more stable, addition or doping of impurity element(s) was proposed in such a way * mgupta@csr.res.in/dr.mukul.gupta@gmail.com that the amount of impurity must be low enough to affect the structural or the magnetic properties adversely [9,[21][22][23][24][25][26][27][28][29][30][31][32][33][34]. Such impurity doped systems were termed as Fe-X-N with X = few at.…”