Metastable α-Fe16N2 has attracted much interest as a candidate for rare-earth-free hard magnetic materials. We demonstrate that Fe16N2 thin films were grown epitaxially on Cr seed layers with MgO (001) substrates by facing-target sputtering. Good crystallinity with the epitaxial relation MgO(001)[110]Cr(001)[100]Fe16N2(001)[100] was obtained. The chemical order parameter, which quantifies the degree of N ordering in the Fe16N2 (the N-disordered phase is α-Fe8N martensite), reaches 0.75 for Cr-seeded samples. Cr has a perfect lattice constant match with Fe16N2, and no noticeable strain can be assigned to Fe16N2. The intrinsic saturation magnetization of this non-strained Fe16N2 thin film at room temperature is determined to be 2.31 T by polarized neutron reflectometry and confirmed with vibrating sample magnetometry. Our work provides a platform to directly study the magnetic properties of high purity Fe16N2 films with a high order parameter.

Fe16N2 is an interesting material in that it has both high saturation magnetization (Ms) and large magnetic anisotropy.1–5 So, this compound is one of the potential candidates for permanent magnets.6,7 Efforts have also been made to lower its anisotropy through doping and to use Fe16N2 as a write head material in hard disk drives,8,9 because large Ms is the key to improving write-ability of magnetic writers, thus breaking the magnetic recording trilemma and further increasing the data storage density.10 However, accurate determination of the saturation magnetization of Fe16N2 and theoretical explanation of its large value are still challenging. An important part of the experimental difficulty is material synthesis. Fe16N2 can hardly be obtained in pure phase, whether the sample is thin film, fine particle,11 or bulk.12,13 The coexisting α-Fe and other iron nitrides affect the determination of Ms of Fe16N2. In thin film studies, a layer of Fe was mostly used as a seed layer to support the growth of Fe16N2 (e.g., Refs. 3 and 14). This layer of magnetically soft Fe not only interferes with the determination of Ms of Fe16N2 but also limits its application because it couples with the Fe16N2 layer.3 Although some effort was made to decouple the Fe and Fe16N2 layers by inserting a nonmagnetic Ag15–17 or Cr18 layer between them, the Fe sublayer was still present in the heterostructures. The difficulty of a theoretical explanation lies in the fact that Density Functional Theory (DFT) calculations have failed to reproduce the very high Ms of Fe16N2 as in some experimental reports.19 This situation, together with the fact that giant Ms was mostly found in epitaxial films, inspired Ji et al. to propose that in-plane tensile strain is the cause of high Ms in Fe16N2.20 In this letter, we demonstrated that a nonmagnetic Cr seed layer can epitaxially stabilize high quality Fe16N2 thin films without a Fe sublayer. Cr, like Fe, has a body-centered cubic (bcc) structure and a perfect lattice constant match with Fe16N2. The as-obtained strain-free Fe16N2 film has high phase purity. The order parameter, which quantifies the degree of interstitial N ordering in the mixture of the α-Fe8N martensite phase and α-Fe16N2, is among the highest ever reported.

The Fe16N2 thin films were grown by a facing-target sputtering (FTS) technique.2 A Cr seed layer was deposited at elevated temperature on a single crystal MgO (001) substrate. Fe16N2 was then deposited on this Cr layer by sputtering Fe targets in the N2 and Ar gas mixture at room temperature. A Cr capping layer was then deposited on top of the Fe-N layer for protection from oxidation. The structure of samples used in this study is MgO/Cr (10.2)/Fe-N (26.7)/Cr (10.2) (thickness in nm is indicated in parentheses). The whole structure was deposited in a single chamber, which accommodates multiple pairs of targets, and then annealed in vacuum at 160 °C for 6–36 h. This annealing condition was optimized to give the highest N ordering.

X-ray diffraction (XRD) and X-ray reflectometry (XRR) were carried out on a PANalytical X'Pert diffractometer with Cu K-α radiation. To accurately calculate the order parameter, a Siemens D5005 diffractometer with Cu K-α radiation was used to obtain a higher signal at Fe16N2 (00L) reflections. Polarized neutron reflectometry (PNR) as a means to obtain the depth-resolved magnetization profile21 was carried out at the Magnetism Reflectometer at the Spallation Neutron Source (SNS), Oak Ridge National Lab (ORNL).22 A 10 kOe external field was applied to saturate Fe16N2 in the film plane. Room temperature M-H loops were measured on a Quantum Design PPMS vibrating sample magnetometer (VSM).

Figure 1(a) shows a typical out-of-plane XRD pattern of the Cr-seeded Fe16N2 thin film. Only (00L) reflections of Fe16N2 can be observed, indicating that Fe16N2 is well textured. Based on the integrated intensities of (002) and (004) reflections, the order parameter of Fe16N2 can be calculated as follows:

D=I002exp./I004exp.I002theo./I004theo.,
(1)

where I002exp.(I004exp.) and I002theo.(I004theo.) are the experimental and theoretical intensities of the (002) ((004)) peak, respectively. The theoretical intensity ratio of (002) and (004) reflections is calculated by considering structure factors and Lorentz-polarization factors and is equal to 0.074.23 In calculating the integrated intensities, the peaks were fitted to pseudo-Voigt functions first. The samples in this study generally have an order parameter above 0.6, the highest being 0.75, suggesting that Cr serves as a good seed layer for Fe16N2 thin films. The annealing conditions and order parameters of 5 samples used in this study are summarized in Table I.

FIG. 1.

XRD patterns of MgO/Cr(10.2 nm)/Fe16N2(26.7)/Cr(10.2) samples. Reflections from the substrate and Cr are labeled by MgO and Cr, respectively. Non-labeled peaks are from Fe16N2. (a) 2θ-ω scan with scattering vector along the film normal. The integrated intensities of (002) and (004) are used to calculate the order parameter. The peak labeled “*” is the substrate peak by Cu K-β. (b) Grazing incidence XRD 2θ-ω scan, with an incident angle of 1.4° and in-plane scattering vector along MgO [220]. (c) and (d) 360°-ϕ scan of Fe16N2 (220) [and Cr (110)] and MgO (200) reflections, respectively. The incident angles are 1.35° and 1.4° in (c) and (d), respectively. The zero point is chosen so that 4 main peaks can be clearly seen and are consistent in both (c) and (d), thus confirming the epitaxial relation MgO[100]Fe16N2[110]. Besides the 4 main peaks related by crystal symmetry, there is an additional peak 5.3° to the right of the second main peak (labeled by “°”). It is seen in both (c) and (d). (e) After establishing the orientation of the Fe16N2 crystal, the sample was rotated so that the scattering vector is along Fe16N2 [101], and (202) reflection of Fe16N2 was recorded. Its rocking curve (inset) has a FWHM of 1.25°. (f) The fingerprint peak of Fe16N2 (103) was observed in the expected sample orientation. The rocking curve (inset) has a FWHM of 1.62°.

FIG. 1.

XRD patterns of MgO/Cr(10.2 nm)/Fe16N2(26.7)/Cr(10.2) samples. Reflections from the substrate and Cr are labeled by MgO and Cr, respectively. Non-labeled peaks are from Fe16N2. (a) 2θ-ω scan with scattering vector along the film normal. The integrated intensities of (002) and (004) are used to calculate the order parameter. The peak labeled “*” is the substrate peak by Cu K-β. (b) Grazing incidence XRD 2θ-ω scan, with an incident angle of 1.4° and in-plane scattering vector along MgO [220]. (c) and (d) 360°-ϕ scan of Fe16N2 (220) [and Cr (110)] and MgO (200) reflections, respectively. The incident angles are 1.35° and 1.4° in (c) and (d), respectively. The zero point is chosen so that 4 main peaks can be clearly seen and are consistent in both (c) and (d), thus confirming the epitaxial relation MgO[100]Fe16N2[110]. Besides the 4 main peaks related by crystal symmetry, there is an additional peak 5.3° to the right of the second main peak (labeled by “°”). It is seen in both (c) and (d). (e) After establishing the orientation of the Fe16N2 crystal, the sample was rotated so that the scattering vector is along Fe16N2 [101], and (202) reflection of Fe16N2 was recorded. Its rocking curve (inset) has a FWHM of 1.25°. (f) The fingerprint peak of Fe16N2 (103) was observed in the expected sample orientation. The rocking curve (inset) has a FWHM of 1.62°.

Close modal
TABLE I.

Data of 5 MgO/Cr(10.2 nm)/Fe16N2(26.7)/Cr(10.2) samples used in this study. The samples were annealed at 160 °C. Hswitch is the switching field in the out-of-plane loop, and Δswitch is the amplitude of the switch, shown as a fraction of Ms (see text).

SampleAnnealing time (h)Order parameterHswitch (kOe)Δswitch2Ms
0.66 4.7 0.026 
10.75 0.75 4.3 0.039 
12 0.62 5.2 0.019 
15 0.72 3.7 0.037 
36.25 0.66 5.2 0.020 
SampleAnnealing time (h)Order parameterHswitch (kOe)Δswitch2Ms
0.66 4.7 0.026 
10.75 0.75 4.3 0.039 
12 0.62 5.2 0.019 
15 0.72 3.7 0.037 
36.25 0.66 5.2 0.020 

Grazing incidence XRD 2θω scan shows clearly Fe16N2 (400) and Cr (200) peaks [Fig. 1(b)], together with the substrate (220). Fe16N2 (220) and Cr (110) reflections were also observed along with MgO (200). Because the a lattice constant of Fe16N2 (bulk value 5.72 Å)24 is very close to twice that of Cr (bulk value 2.87–2.89 Å), it is not possible to separate Fe16N2 (2H, 2H, 0) ((2H, 0, 0)) and Cr (HH0) ((H00)) reflections. We did 360°-ϕ scan with the rotation axis along the film normal at Fe16N2 (220) and Cr (110) reflections, and the result is shown in Fig. 1(c). The fourfold symmetry is clearly seen, and thus, the good crystallinity of the epitaxial Fe16N2 thin film is confirmed. In addition to the 4 main peaks 90° apart from each other, there is a peak 5.3° to the right of the second main peak, labeled “°” in Fig. 1(c). This additional peak is also found in the ϕ scan of substrate (200) reflection, also 5.3° to the right of the second main peak [Fig. 1(d)]. So, this effect is due to substrate defects and shows good epitaxy of the Fe16N2 film. Now, the epitaxial relation can be summarized as follows: MgO(001)[110]Cr(001)[100]Fe16N2(001)[100]. By a rotating crystal approach, the (202) and (103) reflections of Fe16N2 were observed, as shown in Figs. 1(e) and 1(f). (103) is a fingerprint peak of Fe16N2, which differentiates it from the N-disordered α-Fe8N martensite phase. Rocking curves [inset Figs. 1(e) and 1(f)] around (202) and (103) have a FWHM of 1.25° and 1.62°, respectively. The lattice constants of Fe16N2 were calculated by a least squares approach to minimize the deviation from d-spacings of (004), (400), (220), (202), and (103) reflections. The obtained values are a = 5.722 Å and c = 6.272 Å, very close to the well-accepted bulk values of a = 5.72 Å and c = 6.29 Å.24 

In the course of our experiments, the authors noticed a similar work.18 Conventional magnetron sputtering was used to grow Fe16N2 thin films on Cr seed layers, confirming that Cr is a reasonable choice to stabilize this metastable compound. However, a layer of Fe was still used in that work. In addition, Fe was deposited directly on MgO, with a 4% lattice mismatch. So, it became difficult to accurately account for the contribution of Fe in the total magnetic moment. Also in that work, a high order parameter, 0.6 as reported, can only be obtained for 10 nm-thick Fe16N2. For films thicker than 15 nm, the order parameter is below 0.3. In our samples, however, order parameters over 0.6 can be routinely obtained for 26.7 nm thick Fe16N2. This shows the superiority of FTS in fabricating Fe16N2.

XRR and PNR were performed to study the chemical and magnetic depth profiles of sample 5 as listed in Table I. PNR, for a not-so-thin film, is necessary due to the inability of magnetometry to account for nitrogen diffusion. The formation of Fe16N2 generally requires annealing at 125–180 °C, while at this temperature, N diffusion has become significant. For example, in the Fe/Fe16N2 structure, a 6.6–14.7 nm-thick interface was caused solely by diffusion when annealed at 152 °C for 6 h based on the diffusion coefficients measured by different methods.25 Within the interface, the local environment of Fe atoms changes and the stoichiometry will deviate from Fe16N2. This diffusion length is not at all negligible when compared to the usual film thickness, i.e., 10–100 nm.

The GenX software was used to fit both XRR and PNR experimental curves.26 First, the XRR curve was fitted to give the chemical depth profile. This was then used as the initial structure model to optimize the magnetization depth profile by fitting PNR curves. The experimental and fitted XRR curves are shown in Fig. 2(a). As can be seen, the fitting adequately describes the true structure. The corresponding x-ray electron scattering length density (ESLD) profile is shown in Fig. 2(b). N diffusion into both bottom and top Cr layers can be identified as dips in ESLD, as N has a lower ESLD than Cr and may cause lattice expansion. Neutron reflectivity for both spin up and spin down are plotted in Fig. 2(c), along with the fitted curves. The corresponding nuclear and magnetic scattering length density (SLD) profiles (NSLD and MSLD, respectively) are shown in Fig. 2(d). MSLD is directly proportional to magnetization (saturated in this experiment),27 and Ms of Fe16N2 was calculated to be 1840 emu/cm3 (2.31 T). With 5% error in the fitting figure of merit (FOM), the Ms range of 1820 emu/cm3 (2.29 T) to 1870 emu/cm3 (2.35 T) was obtained.28 A 3.2 nm-thick interface layer with reduced Ms at the top Fe16N2/Cr interface is likely due to roughnesss. The integrated MSLD (after converting to Ms) gives the total magnetic moment of the sample. This value is 4.1% larger than the VSM result. As the two techniques give consistent results, we believe that the Ms value determined from PNR reflects the intrinsic property of Fe16N2. This saturation magnetization value does not support the giant Ms results and may indicate that seed layers can affect the magnetization of Fe16N2. It has been shown by PNR depth-resolved magnetization that under tensile strain (from MgO epitaxy, with a very thin Fe buffer layer), Fe16N2 has a giant Ms of 3.1 T, while as the strain releases, the Ms decreases to 2.1 T.20 In this work, the Cr seed layer is thick enough to overwrite the strain information of the MgO substrate and our sample contains a strain-free Fe16N2 layer, as can also be seen from XRD. This observation, together with the low Ms Fe16N2 particle results,5,29 points to the possibility that strain is the reason for the observed giant Ms in Fe16N2 thin films.

FIG. 2.

(a) Experimental (open circle) and fitted (solid line) XRR curve of sample MgO/Cr(10.2 nm)/Fe16N2(26.7)/Cr(10.2). (b) The x-ray SLD (in units of the classical electron radius (re = 2.818 fm)/Å3) depth profile corresponding to (a). Imaginary part describes absorption. (c) Polarized neutron reflectivity as a function of wave vector transfer for spin up (up triangle) and spin down (down triangle). Fitted curves for spin up and down are shown with solid and dashed lines, respectively. The corresponding neutron SLD depth profiles are shown in (d). The increased NSLD on the bottom Fe16N2/Cr interface is enriched with N, and the top interface of ∼3.2 nm corresponds to a mixed Cr/Fe16N2 layer with a slightly reduced magnetization.

FIG. 2.

(a) Experimental (open circle) and fitted (solid line) XRR curve of sample MgO/Cr(10.2 nm)/Fe16N2(26.7)/Cr(10.2). (b) The x-ray SLD (in units of the classical electron radius (re = 2.818 fm)/Å3) depth profile corresponding to (a). Imaginary part describes absorption. (c) Polarized neutron reflectivity as a function of wave vector transfer for spin up (up triangle) and spin down (down triangle). Fitted curves for spin up and down are shown with solid and dashed lines, respectively. The corresponding neutron SLD depth profiles are shown in (d). The increased NSLD on the bottom Fe16N2/Cr interface is enriched with N, and the top interface of ∼3.2 nm corresponds to a mixed Cr/Fe16N2 layer with a slightly reduced magnetization.

Close modal

A magnetic M-H loop of the Cr-seeded Fe16N2 thin film is shown in Fig. 3. An interesting feature of the loops, which is found in all Cr-seeded samples, is the clear switching in the out-of-plane loop. The field at which this switching happens and the magnitude of the switching as a fraction of Ms for the 5 samples are listed in Table I. The out-of-plane loop clearly has a nonzero remanence. A similar but less prominent phenomenon was previously found in Fe/Fe16N2 thin films, and this switching was attributed to the small amount of Fe16N2 that is perpendicularly magnetized.3 In those Fe/Fe16N2 samples, a soft Fe seed layer was used and N ordering was low in the iron nitride layer, i.e., the main phase is α-Fe8N martensite. A model was proposed that the Fe layer is magnetized in plane and couples with the Fe16N2 layer. So, the magnetization vector gradually goes to the film normal because of the perpendicular magnetic anisotropy (PMA) of Fe16N2.3 However, in our samples, there is no soft Fe layer and the only ferromagnetic layer is Fe16N2. Also, the degree of N disorder is small. Thus, a new model is needed for an explanation. One possibility is that some Fe16N2 grains are isolated from others so that there is no dominating shape anisotropy that favors in-plane magnetization. Magnetocrystalline anisotropy (MCA) determines the easy axis to be out-of-plane. In this case, the MCA calculated from the switching field, however, is only one third to half that of previous reports (∼1.0 × 107 ergs/cc).3–5 Another possible explanation is that some interface layer in the MgO/Cr/Fe16N2/Cr system has PMA, and its magnetic moment takes up about 2%–4% of the total moment (Table I). In this case, the MCA of Fe16N2 can be calculated (assuming coherent rotation) as 2πMs2MsHK/2, where 2πMs2 is the shape anisotropy and HK is the anisotropy field labeled in Fig. 3 and is 13.4 kOe. The calculated MCA, 8.9 × 106 ergs/cc, agrees well with literature values, and this indicates that the interface layer model is more likely to be true. Ongoing work on thickness and seed layer dependence of the magnetic behavior of Fe16N2 will help clarify this situation.

FIG. 3.

Typical M-H loops of the MgO/Cr(10.2 nm)/Fe16N2 (26.7)/Cr(10.2) sample, with the field applied both in-plane (dashed) and along film normal (solid). The switching in the out-of-plane loop is indicated by red arrows.

FIG. 3.

Typical M-H loops of the MgO/Cr(10.2 nm)/Fe16N2 (26.7)/Cr(10.2) sample, with the field applied both in-plane (dashed) and along film normal (solid). The switching in the out-of-plane loop is indicated by red arrows.

Close modal

In conclusion, we grew Fe16N2 thin films on the MgO/Cr template using facing-target sputtering. The advantages of the Cr seed layer are as follows: (1) Cr has a bcc structure and a perfect lattice constant match with Fe16N2; (2) the only ferromagnetic material in the system is Fe16N2, making it easier to study its intrinsic magnetism; (3) the as-obtained Fe16N2 film has high phase purity, i.e., high order parameter. The XRD results show that our Fe16N2 film is textured in all three directions and has good crystallinity. By (H, K, L ≠ 0) reflections, Fe16N2 is identified from Cr, and by its fingerprint peaks (002) and (103), interstitial N ordering is confirmed. We believe that this work provides a platform for further study of Fe16N2. The intrinsic saturation magnetization of the strain-free Fe16N2 thin film was determined to be 2.31 T by PNR and confirmed with VSM. The out-of-plane M-H loop of the Cr-seeded Fe16N2 film shows an interesting easy-axis switching behavior, the magnitude being 2%–4% of the total magnetic moment. Further study is being carried out to find the origin of this perpendicular component.

Research at ORNL's Spallation Neutron Source was sponsored by the Scientific User Facilities Division and by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC Program.

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MSLD can be expressed as ρM=mn2π2μn·4πM, where mn is neutron mass, μn is neutron magnetic moment equal to 1.913 μN, where μN is nuclear magneton, and is reduced Planck constant. A numerical conversion between MSLD and magnetization is: 4πM = 4.318 ρM, in which 4πM in unit of T and MSLD in unit of fm/Å3.
28.
The errors of the fitting parameters were calculated using a standard procedure (described at http://genx.sourceforge.net/doc/tutorials/xrr_fitting.html) with a default 5% increase in the optimal Figure of Merit (FOM), that results in the asymmetric error bars.
29.
H.
Hiraka
,
K.
Ohoyama
,
Y.
Ogata
,
T.
Ogawa
,
R.
Gallage
,
N.
Kobayashi
,
M.
Takahashi
,
B.
Gillon
,
A.
Gukasov
, and
K.
Yamada
,
Phys. Rev. B
90
,
134427
(
2014
).