The formation mechanism of α″-Fe16N2 phase was investigated in the form of nanoparticles. Both α-Fe and γ-Fe2O3 nanoparticles were used to prepare α″-Fe16N2 by using a low-temperature nitriding process (≤180 °C). The synthesized α″-Fe16N2 nanoparticles have a high α″-Fe16N2 volume ratio up to 93%, with a specific saturation magnetization of 178 emu/g (room temperature) and coercivity of 2.0 kOe. The formation of α″-Fe16N2 phase includes three stages: (1) the heterogenous nucleation of α″-Fe16N2 with simultaneous chemical reaction, (2) the growth of α″-Fe16N2 with a local electric field in the Fe16N2 layer, and (3) the termination of Fe16N2 growth due to the nucleation of other Fe–N phases (ε-Fe3N or γ′-Fe4N). In low-temperature nitriding, NH3 was used as the nitrogen source. The adsorbed NH3 molecules on the Fe surface decompose into N and H atoms, and then N atoms react with Fe and nucleation of α″-Fe16N2 simultaneously occurs at the high-energy surface sites of reduced Fe nanoparticles. The growth of α″-Fe16N2 phase can be explained by the electric field modified diffusion theory, where the electric field is established by the migration of Fe and N ions and electrons. Finally, the nucleation of Fe–N stable phases (ε-Fe3N or γ′-Fe4N) ceases the further growth of α′′-Fe16N2 layer. Then, there is critical thickness for the α″-Fe16N2 layer, which is estimated to be 10–15 nm from the surface. Therefore, single-phase α″-Fe16N2 nanoparticles are expected in fine particles with less than 30 nm in diameter.
REFERENCES
1.
J.-P.
Wang
, N.
Ji
, X. Q.
Liu
, Y. H.
Xu
, C.
Sanchez-Hanke
, Y. M.
Wu
, F. M. F.
de Groot
, L. F.
Allard
, and E.
Lara-Cruzio
, “Fabrication of Fe16N2 films by sputtering process and experimental investigation of origin of giant saturation magnetization in Fe16N2
,” IEEE Trans. Magn.
48
(5
), 1710
–1717
(2012
). 2.
J.
Wang
, “Environment-friendly bulk Fe16N2 permanent magnet: Review and prospective
,” J. Magn. Magn. Mater.
497
, 165962
(2020
). 3.
A.
Nagatomi
, S.
Kikkawa
, T.
Hinomura
, S.
Nasu
, and F.
Kanamaru
, “Synthesis of iron nitrides FexN (x: 2, 2-3, 4, 16/2) by nitrogenizing α-Fe in ammonia gas, and magnetic properties of the bulk sample of Fe16N2
,” J. Jpn. Soc. Powder Powder Metall.
46
(2
), 151
–155
(1999
). 4.
T.
Ogawa
, Y.
Ogata
, R.
Gallage
, N.
Kobayashi
, N.
Hayashi
, Y.
Kusano
, S.
Yamamoto
, K.
Kohara
, M.
Doi
, and M.
Takano
, “Challenge to the synthesis of α"-Fe16N2 compound nanoparticle with high saturation magnetization for rare earth free new permanent magnetic material
,” Appl. Phys. Express
6
(7
), 3
–6
(2013
). 5.
I.
Dirba
, C. A.
Schwobel
, L. V. B.
Diop
, M.
Duerrschnabel
, L.
Molina-Luna
, K.
Hofmann
, P.
Komissinskiy
, H.-J.
Kleebe
, and O.
Gutgleisch
, “Synthesis, morphology, thermal stability and magnetic properties of α″-Fe16N2 nanoparticles obtained by hydrogen reduction of γ-Fe2O3 and subsequent nitrogenation
,” Acta Mater.
123
, 214
–222
(2017
). 6.
K. H.
Jack
, “The iron-nitrogen system: The preparation and the crystal structures of nitrogen-austenite(γ) and nitrogen-martensite(α′)
,” Proc. R. Soc. A
208
, 200
–215
(1951
).7.
K. H.
Jack
, “The occurrence and the crystal structure of α″-iron nitride; a new type of interstitial alloy formed during the tempering of nitrogen-martensite
,” Proc. R. Soc. A
208
(1093
), 216
–224
(1951
).8.
K. H.
Jack
, “The synthesis, structure, and characterization of α″-Fe16N2 (invited)
,” J. Appl. Phys.
76
(10
), 6620
–6625
(1994
). 9.
K. H.
Jack
, “The synthesis and characterization of bulk α″-Fe16N2
,” J. Alloys Compd.
222
, 160
(1995
). 10.
T. K.
Kim
and M.
Takahashi
, “New magnetic material having ultrahigh magnetic moment
,” Appl. Phys. Lett.
20
(12
), 492
–494
(1972
). 11.
Y.
Sugita
, K.
Mitsuoka
, M.
Komuro
, H.
Hoshiya
, Y.
Kozono
, and M.
Hanazono
, “Giant magnetic moment and other magnetic properties of epitaxially grown Fe16N2 single-crystal films (invited)
,” J. Appl. Phys.
70
(10
), 5977
–5982
(1991
). 12.
N.
Ji
, L. F.
Allard
, E.
Lara-Curzio
, and J.-P.
Wang
, “N site ordering effect on partially ordered Fe16N2
,” Appl. Phys. Lett.
98
(9
), 092506
(2011
). 13.
N.
Ji
, Y.
Wu
, and J.-P.
Wang
, “Epitaxial high saturation magnetization FeN thin films on Fe(001) seeded GaAs(001) single crystal wafer using facing target sputterings
,” J. Appl. Phys.
109
, 07B767
(2011
). 14.
N.
Ji
, V.
Lauter
, X.
Zhang
, H.
Ambaye
, and J.-P.
Wang
, “Strain induced giant magnetism in epitaxial Fe16N2 thin film
,” Appl. Phys. Lett.
102
(7
), 072411
(2013
). 15.
X.
Zhang
, N.
Ji
, V.
Lauter
, H.
Ambaye
, and J.-P.
Wang
, “Strain effect of multilayer FeN structure on GaAs substrate
,” J. Appl. Phys.
113
(17
), 17E149
(2013
). 16.
M.
Yang
, L. F.
Allard
, N.
Ji
, X.
Zhang
, G.-H.
Yu
, and J.-P.
Wang
, “The effect of strain induced by Ag underlayer on saturation magnetization of partially ordered Fe16N2 thin films
,” Appl. Phys. Lett.
103
(24
), 242412
(2013
). 17.
X.
Hang
, X.
Zhang
, B.
Ma
, V.
Lauter
, and J.-P.
Wang
, “Epitaxial Fe16N2 thin film on nonmagnetic seed layer
,” Appl. Phys. Lett.
112
(19
), 192402
(2018
). 18.
X.
Zhang
, K.
Nomura
, and J. P.
Wang
, “New insight on the mössbauer spectra for Fe16N2 thin films with high saturation magnetization
,” Jpn. J. Appl. Phys.
58
(12
), 120907 (2019
).19.
H.
Takahashi
, M.
Igarashi
, A.
Kaneko
, H.
Miyajima
, and Y.
Sugita
, “Perpendicular uniaxial magnetic anisotropy of Fe16N2 [001] single crystal films grown by molecular beam epitaxy
,” IEEE Trans. Magn.
35
(5
), 2982
–2984
(1999
). 20.
X.
Li
, M. Y.
Yang
, M.
Jamali
, F. Y.
Shi
, S. S.
Kang
, Y. F.
Jiang
, X. W.
Zhang
, H. S.
Li
, S.
Okatov
, S.
Faleev
, A.
Kalitsov
, G. H.
Yu
, P. M.
Voyles
, O. N.
Mryasov
, and J. P.
Wang
, “Heavy-metal-free, low-damping, and non-interface perpendicular Fe16N2 thin film and magnetoresistance device
,” Phys. Status Solidi Rapid Res. Lett.
13
(7
), 1900089
(2019
). 21.
Y.
Jiang
, V.
Dabade
, L. F.
Allard
, E.
Lara-Curzio
, R.
James
, and J. P.
Wang
, “Synthesis of α′′-Fe16N2 compound anisotropic magnet by the strained-wire method
,” Phys. Rev. Appl.
6
(2
), 024013
(2016
). 22.
Y.
Jiang
, M.
Al Mehedi
, E.
Fu
, Y.
Wang
, L. F.
Allard
, and J. P.
Wang
, “Synthesis of Fe16N2 compound free-standing foils with 20 MGOe magnetic energy product by nitrogen ion-implantation
,” Sci. Rep.
6
, 24536
(2016
).23.
J.
Liu
, G.
Guo
, F.
Zhang
, Y.
Wu
, B.
Ma
, and J. P.
Wang
, “Synthesis of α′′- Fe16N2 ribbons with a porous structure
,” Nanoscale Adv.
1
(4
), 1337
–1342
(2019
). 24.
J.
Liu
, G.
Guo
, X.
Zhang
, F.
Zhang
, B.
Ma
, and J.-P.
Wang
, “Synthesis of α″- Fe16N2 foils with an ultralow temperature coefficient of coercivity for rare-earth-free magnets
,” Acta Mater.
184
, 143
–150
(2020
). 25.
M. Q.
Huang
, W. E.
Wallace
, S.
Simizu
, A. T.
Pedziwiatr
, R. T.
Obermyer
, and S. G.
Sankar
, “Synthesis and characterization of Fe16N2 in bulk form
,” J. Appl. Phys.
75
(10
), 6574
–6576
(1994
). 26.
X.
Bao
, R. M.
Metzger
, and M.
Carbucicchio
, “Synthesis and properties of α″- Fe16N2 in magnetic particles
,” J. Appl. Phys.
75
(10
), 5870
–5872
(1994
). 27.
S.
Kikkawa
, K.
Kubota
, and T.
Takeda
, “Particle size dependence in low temperature nitridation reaction for Fe16N2
,” J. Alloys Compd.
449
(1–2
), 7
–10
(2008
). 28.
See https://www-s.nist.gov/srmors/certificates/660c.pdf for “Certificate of standard reference materials 660C.”
29.
Certificate of Standard Reference Materials 1979.
30.
V. G.
Gavriljuk
and H.
Berns
, High Nitrogen Steels—Structure, Properties, Manufacture, Application
(Springer
, 1999
).31.
N.
Cabrera
and N. F.
Mott
, “Theory of the oxidation of metals
,” Rep. Prog. Phys.
12
, 163
(1949
). 32.
A. T.
Fromhold
, “Metal oxidation kinetics from the viewpoint of a physicist: The microscopic motion of charged defects through oxides
,” Langmuir
3
(6
), 886
–896
(1987
). 33.
A. T.
Fromhold
, “Fundamental theory of the growth of thick oxide films on metals
,” J. Phys. Soc. Jpn.
48
(6
), 2022
–2030
(1980
). 34.
H.
Takahashi
, H.
Shoji
, and M.
Takahashi
, “Structure and magnetic moment of Fe16N2 sputtered film
,” J. Magn. Magn. Mater.
174
(1–2
), 57
–69
(1997
). 35.
C. M.
Wang
, D. R. B. L. E.
Thomas
, J. E.
Amonette
, J.
Antony
, Y.
Qiang
, and G.
Duscher
, “Void formation during early stages of passivation: Initial oxidation of iron nanoparticles at room temperature
,” J. Appl. Phys.
98
(9
), 094308 (2005
).36.
A.
Shavel
, B.
Rodríguez-González
, M.
Spasova
, M.
Farle
, and L. M.
Liz-Marzán
, “Synthesis and characterization of iron/iron oxide core/shell nanocubes
,” Adv. Funct. Mater.
17
(18
), 3870
–3876
(2007
). 37.
Y.
Sun
, X.
Zuo
, S. K. R. S.
Sankaranarayanan
, S.
Peng
, B.
Narayanan
, and G.
Kamath
, “Quantitative 3D evolution of colloidal nanoparticle oxidation in solution
,” Science (80-.)
356
(6335
), 303
–307
(2017
). 38.
R.
Zulhijah
, A. B. D.
Nandiyanto
, T.
Ogi
, T.
Iwaki
, K.
Nakamura
, and K.
Okuyama
, “Effect of oxidation on α″- Fe16N2 phase formation from plasma-synthesized spherical core-shell α-Fe/Al2O3 nanoparticles
,” J. Magn. Magn. Mater.
381
, 89
–98
(2015
). 39.
T.
Bell
, “Martensite transformation start temperature in iron-nitrogen alloys
,” J. Iron Steel INST
206
, 1017
–1021
(1968
).40.
S. J.
Roosendaal
, A. M.
Vredenberg
, and F. H. P. M.
Habraken
, “Oxidation of iron: The relation between oxidation kinetics and oxide electronic structure
,” Phys. Rev. Lett.
84
(15
), 3366
–3369
(2000
). 41.
K. K.
Fung
, B.
Qin
, and X. X.
Zhang
, “Passivation of a-Fe nanoparticle by epitaxial γ-Fe2O3 shell
,” Mater. Sci. Eng. A
286
, 135
–138
(2000
). 42.
C. K. S. R.
Logan
and R. L.
Moss
, “The catalytic decomposition of ammonia on evaporated iron films
,” Trans. Faraday Soc.
54
, 922
(1958
). 43.
S.
Brunauer
, K. S.
Love
, and R. G.
Keenan
, “Adsorption of nitrogen and the mechanism of ammonia decomposition over iron catalysts*
,” J. Am. Chem. Soc.
64
(4
), 751
–758
(1942
). 44.
N.
Tsubouchi
, H.
Hashimoto
, and Y.
Ohtsuka
, “High catalytic performance of fine particles of metallic iron formed from limonite in the decomposition of a low concentration of ammonia
,” Catal. Lett.
105
(3–4
), 203
–208
(2005
). 45.
F.
Tessier
, A.
Navrosky
, R.
Niewa
, A.
Leineweber
, H.
Jacobs
, S.
Kikkawa
, M.
Takahashi
, F.
Kanamarau
, and F. J.
DiSalvo
, “Energetics of binary iron nitrides
,” Solid State Sci.
2
(4
), 457
–462
(2000
). 46.
C. N. R.
Rao
, G. U.
Kulkarni
, P. J.
Thomas
, and P. P.
Edwards
, “Size-dependent chemistry: Properties of nanocrystals
,” Chem. A Eur. J.
8
(1
), 28
–35
(2002
). 47.
K. K.
Nanda
, A.
Maisels
, F. E.
Kruis
, H.
Fissan
, and S.
Stappert
, “Higher surface energy of free nanoparticles
,” Phys. Rev. Lett.
91
(10
), 106102
(2003
). 48.
C.-B.
Rong
, D.
Li
, V.
Nandwana
, N.
Poudyal
, Y.
Ding
, Z. L.
Wang
, H.
Zeng
, and J. P.
Liu
, “Size-dependent chemical and magnetic ordering in L10-FePt nanoparticles
,” Adv. Mater.
18
(22
), 2984
–2988
(2006
). 49.
A.
Kovács
, K.
Sato
, V. K.
Lazarov
, P. L.
Galindo
, T. J.
Konno
, and Y.
Hirotsu
, “Direct observation of a surface induced disordering process in magnetic nanoparticles
,” Phys. Rev. Lett.
103
(11
), 115703
(2009
). 50.
G.
Amsel
and D.
Samuel
, “The mechanism of anodic oxidation
,” J. Phys. Chem. Solids
23
(12
), 1707
–1718
(1962
). 51.
G. W. R.
Leibbrandt
, G.
Hoogers
, and F. H. P. M.
Habraken
, “Thin oxide film growth on Fe(100)
,” Phys. Rev. Lett.
68
(12
), 1947
–1950
(1992
). 52.
R.
Zulhijah
, A. B.
Dani Nandiyanto
, T.
Ogi
, T.
Iwaki
, K.
Nakamura
, and K.
Okuyama
, “Gas phase preparation of spherical core-shell α″- Fe16N2/SiO2 magnetic nanoparticles
,” Nanoscale
6
(12
), 6487
–6491
(2014
). 53.
D.
Li
, M.
Freitag
, J.
Pearson
, Z. Q.
Qiu
, and S. D.
Bader
, “Magnetic phases of ultrathin Fe grown on Cu(100) as epitaxial wedges
,” Phys. Rev. Lett.
72
(19
), 3112
–3115
(1994
). 54.
P.
Graat
and M. A. J.
Somers
, “Quantitative analysis of overlapping XPS peaks by spectrum reconstruction: Determination of the thickness and composition of thin iron oxide films
,” Surf. Interface Anal.
26
(11
), 773
–782
(1998
). 55.
E. A.
Shafranovsky
and Y. I.
Petrov
, “Aerosol Fe nanoparticles with the passivating oxide shell
,” J. Nanoparticle Res.
6
(1
), 71
–90
(2004
). 56.
B.
Yu
, L.
Lin
, B.
Ma
, Z. Z.
Zhang
, Q. Y.
Jin
, and J. P.
Wang
, “Fabrication and physical properties of [Fe/Fe4N]N multilayers with high saturation magnetization
,” AIP Adv.
6
(5
), 056108
(2016
). 57.
W.
Arabczyk
and R.
Pelka
, “Studies of the kinetics of two parallel reactions: Ammonia decomposition and nitriding of iron catalyst
,” J. Phys. Chem. A
113
(2
), 411
–416
(2009
). © 2020 Author(s).
2020
Author(s)
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