J. Magn. Soc. Jpn., 41, 58-62 (2017)<Paper>Morphology and magnetic properties of α”-Fe16N2 nanoparticles
synthesized from iron hydroxide and iron oxides
M. Tobise, T. Ogawa, and S. Saito

Department of Electronic Engineering, Graduate School of Engineering, Tohoku University,

6605, AzaAoba, Aramaki, Aobaku, Sendai 9808579, Japan

Metastable α”-Fe16N2 have attracted much interest as a candidate for rare-earth-free hard magnetic materials. To
realize high coercivity, it is necessary to utilize not only the magnetocrystalline anisotropy but also the shapeanisotropy of α”-Fe16N2 nanoparticles assemblies. An increase in magnetostatic couplings and intergranularexchange couplings among particles typically reduces the coercivity. Therefore, it is very important to evaluate theanisotropy and magnetic interactions among α”-Fe16N2 nanoparticles. We have examined the changes in morphology,
structure and magnetic properties through the synthesis of α”-Fe16N2 nanoparticles from various materials such as
α-FeOOH, ɤ-Fe2O3, and Fe3O4. The magnetic interactions were also estimated based on experimental results
obtained by analysis of the rotational hysteresis loss of randomly oriented nanoparticles. Hc and Hkptc for theα”-Fe16N2 nanoparticle assemblies for different starting materials ranged from 2.2 to 1.1 kOe, and from 11 to 12 kOe
respectively. Experimental results of the normalized coercive force and normalized switching field suggests that theexistence of large magnetic interactions among α”-Fe16N2 nanoparticles.
Key words: nanomagnetics, hard magnetic materials, iron nitride, magnetic anisotropy, rare earth element-free
1. Introduction
The metastable α”-Fe16N2 iron nitride phase withbody-centered tetragonal (b.c.t.) structure has received
much attention as a candidate for rare-earth-free hardmagnetic materials. After a surprising first report
where the α”-Fe16N2 phase exhibited giant saturation
magnetization as a thin film1), many researches haveattempted to produce single phase α”-Fe16N22,3). In 1993,
a thin film method was established to achieve a
relatively high volume fraction of α”-Fe16N2 using
reactive sputtering, where a saturation magnetizationof Ms ≃ 240 emu/g and uniaxial magnetocrystalline
anisotropy constant of Ku ≃ 1×107 erg/cm3 weredetermined4). These values are very attractive withrespect to the realization of a permanent magnet basedon this material. Since it is still very difficult to produce
metastable α”-Fe16N2 in thin films, the synthesis ofsingle phase α”-Fe16N2 in bulk form is considered to be
extremely challenging5,6,7). In 2013, a procedure for thesynthesis of single phase α”-Fe16N2 nanoparticles wasestablished, and α”-Fe16N2 was reported8) to exhibit Ms≃ 226 emu/g and Ku ≃ 9.6×106 erg/cm3 at 300 K.
To produce a hard magnetic material using
magnetic particle, it is necessary to control theanisotropy of particle assemblies and the magnetic
interaction among these particles. Magnetostaticcouplings and intergranular exchange couplings among
particles typically reduce the coercivity. Therefore it isvery important to evaluate these magnetic interactions
among α”-Fe16N2 nanoparticles. In this work, we
examine the changes in the morphology, structure andmagnetic properties of α”-Fe16N2 nanoparticles
synthesized using different starting materials, such as
α-FeOOH, ɤ-Fe2O3, and Fe3O4. We also discuss the58magnetic anisotropy and magnetic interactions basedon the experimental results obtained by the rotational
hysteresis loss analysis9) of randomly orientednanoparticles.
2. Experimental Procedure
The synthesis of α”-Fe16N2 nanoparticles wasconducted using commercially available α-FeOOH(Wako Pure Chemical Industries, Ltd.), ɤ-Fe2O3 (CIKNanoTek Corporation) and Fe3O4 (Mitsui Mining &Smelting Co., Ltd.). The diameters of the iron hydroxide
and iron oxide particles ranged from several tens toseveral hundreds of nanometers. The temperature andduration
forreductionandnitriding
weresystematically examined to obtain almost single phaseα”-Fe16N2, according to the following procedure.
α-FeOOH, ɤ-Fe2O3, and Fe3O4 nanoparticles were first
reduced at 250－600 °C for 1－5 h in a H2 gas flow.
Nitriding was subsequently performed at 100－200 °C
for 2 － 72 h in an NH3 gas flow. Crystal structure
analysis was conducted using powder X-ray diffraction(XRD) with CoKα radiation. Morphological changes
were observed using transmission electron microscopy(TEM). For magnetic properties measurements, α-Feand α”-Fe16N2 nanoparticles were packed into a disk
shape (6 mm diameter, 1 mm thick) with resin. The
magnetization σ, and coercivity Hc, were measured
using vibrating sample magnetometer (VSM) with amaximum applied magnetic field of 14.5 kOe at 300 K.
3. Results and Discussions
3.1 Morphological andnanoparticles preparedmaterials

Journal of the Magnetics Society of Japan Vol.41, No.3, 2017

structural changes offrom various startingFig. 1 shows TEM images of α-FeOOH as a starting
material, α-Fe produced by the reduction of α-FeOOH,and α”-Fe16N2 formed by the nitriding of α-Fe. The
shape of the Fe hydroxide particles is spindle-like.
Reduction was performed in an atmosphere of flowing
H2 at 380 °C for 4 h, and produced aggregates ofnanoparticles by sintering. Subsequent nitriding wasperformed in an atmosphere of flowing NH3 at 160 °C
for 8 h. No morphological differences were observed
between the reduced α-Fe and nitrided α”-Fe16N2
nanoparticle assemblies.
Fig. 2 shows TEM images of the ɤ-Fe2O3 starting
material, α-Fe reduced from ɤ-Fe2O3 and α”-Fe16N2 asthe nitriding product from α-Fe. The shape of theɤ-Fe2O3 particles is spherical. Reduction was conductedin an atmosphere of flowing H2 at 300 °C for 4 h. The
reduction process caused the nanoparticles to form
aggregates by sintering, as with the α-FeOOH starting

material. Nitriding was performed in an atmosphere offlowing NH3 at 145 °C for 8 h. The nitriding process did
not result in any morphological changes, in the same
way that no changes were observed with the α-FeOOH
starting material.
Fig. 3 shows TEM images of the Fe3O4 starting
material, α-Fe reduced from Fe3O4, and α”-Fe16N2 by thenitriding of α-Fe. The shape of the Fe3O4 particles iscubic. Reduction was performed in an atmosphere offlowing H2 at 340 °C for 4 h. The reduction andnitriding processes did not generate any obvious
macroscopic morphological changes, in contrast to thatobserved with the α-FeOOH and ɤ-Fe2O3 starting
materials.
Fig. 4 shows representative XRD patterns of theα”-Fe16N2 nanoparticles produced from various starting
materials. In the case of α”-Fe16N2 nanoparticles
produced from α-FeOOH, ɤ-Fe2O3, all X-ray diffracted

Fig.1

TEM images of α-Fe and α”-Fe16N2 produced from α-FeOOH as a starting material.

Fig. 2

TEM images of α-Fe and α”-Fe16N2 produced from ɤ-Fe2O3 as a starting material.

Fig. 3

TEM images of α-Fe and α”-Fe16N2 produced from Fe3O4 as a starting material.

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Fig. 5 Torque loss curves of α”-Fe16N2 randomly
oriented particle assemblies produced from α-FeOOH
changing applying field. CW indicates clockwise
rotation and CCW indicates counter-clockwise
rotation.

Fig. 4 XRD patterns of the α”-Fe16N2 nanoparticles
produced from various starting materials. Asterisks
show super-lattice diffraction of N ordered site.

Fig. 6 Rotational hysteresis loss curves of α-Fe andα”-Fe16N2 nanoparticles assemblies produced from
α-FeOOH.

line were identified as α”-Fe16N2. On the other hand,very small diffracted line besides diffractions ofα”-Fe16N2 observed at diffraction angles around 48 and99 degrees for α”-Fe16N2 nanoparticles produced fromFe3O4. It is supposed that these additional diffracted
line correspond Fe4N and α-Fe, respectively.

linearly extrapolating to zero hysteresis loss. Hp is themagnetic field where Wr reaches its maximum whichcorresponds to the switching field of assemblies ofnanoparticles. Hkptc and Hp were evaluated with
applying magnetic field up to 15 kOe. In the case of
assemblies of particles which were magneticallyisolated ideally, the value of magnetic field where Wr
vanishes is independent of particle alignment(crystallographic orientation) and corresponds to Hkptc.
Similar rotational hysteresis loss analysis had been
already applied to the thin film recording media10,11).
Table 1 shows the magnetic properties of α-Fe andα”-Fe16N2 nanoparticle assemblies produced from thedifferent starting materials, as measured with the VSM
and torque magnetometer. Ms for the α-Fe nanoparticle
assemblies ranged from 205 to 198 emu/g according to
the starting material. Compared to the literature valueof the magnetization 218 emu/g for α-Fe, these values
are lower from 6 to 9 % than the literature value. It is
assumed that oxidation of surface of the α-Fenanoparticles may cause this decrease in magnetization.
Hc for the α-Fe nanoparticle assemblies ranged from 0.8
to 0.3 kOe according to the starting material, and Hkptc

3.2 Shape anisotropy with respect to theagglomeration of nanoparticles
Fig. 5 shows the torque loss curves of α”-Fe16N2
randomly oriented particle assemblies produced fromα-FeOOH changing applying field from 3 to 10 kOe.
Rotational hysteresis loss Wr is measured as the region
ofenclosed
area
bytheseclockwise
andcounterclockwise torque curves. In the case of applying
magnetic field H is larger than the half of anisotropy
field Hk and smaller than Hk for the randomly orientedparticles, rotational hysteresis loss Wr is observed andWr changes as a function of magnetic field9). Fig. 6
shows rotational hysteresis loss curves of α-Fe andα”-Fe16N2 nanoparticles assemblies produced fromα-FeOOH. Hkptc was defined by plotting Wr versus
inverse applied field and finding the intercept by60

Journal of the Magnetics Society of Japan Vol.41, No.3, 2017

Table 1. Magnetic properties of α-Fe and α”-Fe16N2 nanoparticle assemblies
produced from various starting materials.

for the α-Fe nanoparticle assemblies also ranged from
5.4 to 7.2 kOe. For α-Fe nanoparticle assemblies, theanisotropy field Hkptc is considered to be due to theshape anisotropy of the nanoparticle assemblies
because the magnetocrystalline anisotropy constant K1for α-Fe with cubic symmetry is too small (ca. 1×105
erg/cm3). It is considered that the differences of Hkptc arederived from the difference of shape anisotropy in theassemblies of α-Fe nanoparticles that are formed during
the reduction process. Hc for the α-Fe nanoparticle
assemblies decreased from 0.8 to 0.3 kOe even thoughHkptc of them increased from 5.4 to 7.2 kOe. It is
assumed that Hc is not determined by only shapeanisotropy. Further detailed investigation is necessary
to reveal the factor which affects Hc of α-Fe.
Shape anisotropy is expressed as Kushape = N·Ms2/2(or Hkshape=N·Ms).N is the difference of thedemagnetization coefficients between the long and shortaxes of a particle. The experimentally determined Hkptc
was used to estimate ΔNptc as an equivalentdemagnetization coefficients as nanoparticle assemblies.
Fig. 7 shows an example of the calculated result12) forFig. 7 Shape anisotropy field as a function of theaspect ratio for the ellipsoid form.

Hkshape as a function of the aspect ratio, assuming anellipsoid form and that the Ms of α-Fe is 218 emu/g. In
this way, the values of ΔNptc were determined to range
from 3.3 to 4.6 for the corresponding aspect ratios ofα-Fe nanoparticle assemblies estimated with thisscheme, from 2.2 to 3.5. As shown in Fig. 7, themaximum shape anisotropy is expected to be 8×106
erg/cm3 with a high aspect ratio of more than 50 whichis considered to be very difficult to realize.
On the other hand, Hc for the α”-Fe16N2 nanoparticle
assemblies ranged from 2.2 to 1.1 kOe for different
starting materials. The reason why that Hc of α”-Fe16N2
nanoparticles which were produced from Fe3O4 is lower
than those of α”-Fe16N2 nanoparticles which wereproduced from α-FeOOH and ɤ-Fe2O3 is considered dueto the additional α-Fe which was observed in X-ray
diffraction(Fig. 4). Hkptc for the α”-Fe16N2 nanoparticle
assemblies were not significantly different, ranging
from 11 to 12 kOe. The magnetocrystalline anisotropy
field Hkint of α”-Fe16N2 determined for a pseudo single-crystalline film was 11.2 kOe 13). This Hkptc for theα”-Fe16N2 nanoparticle assemblies are almost equal toHkint. If we assume α”-Fe16N2 nanoparticle assemblies
have the same magnetocrystalline anisotropy field Hkintof a pseudo single-crystalline film, the shape anisotropy
is considered to be very small in α”-Fe16N2 nanoparticle
assemblies.
3.3 Evaluation of magnetic interaction amongα”-Fe16N2 nanoparticle assemblies
Fig. 8 shows plots of the normalized coercive force(Hc/Hkptc) versus normalized switching field (Hp/Hkptc)
for α”-Fe16N2 nanoparticle assemblies produced fromthe various starting materials. Hc/Hkptc varied from 0.10
to 0.26 and Hp/Hkptc varied from 0.26 to 0.35. In theideal magnetization reversal mode due to the coherent
rotation of the magnetic moment with nonmagnetic
interaction (Stoner-Wohlfarth type)14) , Hc/Hkptc is equal
to 0.48 and Hp/Hkptc is equal to 0.51. According to themicromagnetic simulation, and it was demonstrated

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Fig. 8 Plots of Hc/Hkptc and Hp/Hkptc for α”-Fe16N2nanoparticles prepared from various starting materials.

the aspect ratio of α-Fe nanoparticle assemblies were
estimated to be in the range of 2.2－3.5.
2. Hc for the α”-Fe16N2 nanoparticle assemblies
ranged 2.2 to 1.1 kOe for different starting materials.
Hkptc for the α”-Fe16N2 nanoparticle assemblies were notsignificantly different, ranging from 11 to 12 kOe.
3. The normalized coercive force (Hc/Hkptc) andnormalized switching field (Hp/Hkptc) are 0.10－0.26 and0.26－0.35 respectively, which indicates large magnetic
interactions.
4. As the expected shape anisotropy is in the orderof 106 erg/cm3 for α”-Fe16N2 nanoparticles even though ahigh aspect ratio is realized, magnetocrystalline
anisotropy on the order of 107 erg/cm3 is still required,in addition to improvement of the magnetic isolation
among nanoparticles to realize a permanent magnet.

that both Hc and Hp decrease with increasing magnetic
interaction among grains in thin film media11).
Compared to the ideal values of Hc/Hkptc and Hp/Hkptc,
these values of α”-Fe16N2 nanoparticles prepared fromvarious starting materials are relatively small. To
increase the coercivity of α”-Fe16N2 nanoparticles, it isnecessary to decrease magnetic interaction.

Acknowledgements This work was partially supportedby the New Energy and Industrial Technology
Development Organization (NEDO), the Matching
Planner Program from the Japan Science andTechnology Agency (JST) and the Technology Research
Association of Magnetic Materials for High-Efficiency
Motors (MagHEM).

3.4 Toward an increase in the coercivity of α”-Fe16N2
nanoparticle assemblies
The empirical figure known as the magnetic
hardness parameter = (K1/μ0Ms2)1/2 is a usefulparameter to consider how to approach the constructionof modern permanent magnet15). The  value for
Nd2Fe14B as a typical modern magnet, is 1.54, whilethat for α”-Fe16N2 is 0.53. It is recommended that  begreater than 1, which corresponds to a value of K1≃
3×107 erg/cm3, assuming the same magnetization of
Nd2Fe14B, 165 emu/g. The expected shape anisotropy isin the order of 106 erg/cm3 for α”-Fe16N2 nanoparticles
even though a high aspect ratio is realized (Fig. 7);
therefor, magnetocrystalline anisotropy in the order of
107 erg/cm3 is still required, in addition to improvementof the magnetic isolation among nanoparticles, forapplication as a permanent magnets.

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4. Conclusion
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Received Sep. 28, 2016; Revised Dec. 19, 2016; Accepted Mar.
08, 2017

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