A Moessbauer investigation of the dissociation of the Nd2Fe14B phase

Article abstract of DOI:10.1088/0953-8984/11/25/313

The dissociation process of the Nd₂Fe₁₄B phase in a sintered Nd₁₆Fe₇₆B₈ powdered magnet was investigated between 300 and 650°C. The oxidized powders were characterized by Moessbauer spectrometry and x-ray diffraction. The contributions of the dissociation products to the Moessbauer spectra were identified and the results interpreted consistently with previous transmission electron microscopy observations. The micro structure of the dissociated phase was characterized and found to depend on the oxidation temperature. For temperatures less than 400°C, the microstructure consists in α-Fe nanograins (less than 10 nm in size, with a distorted structure) and Fe₃O₄ superparamagnetic particles (size less than 10 nm), mixed with amorphous or poorly crystallized Nd₂O₃ regions. For temperatures higher than 400°C the micro structure consists in bigger α-Fe grains (their size being 100 nm or more), Fe₃O₄ precipitates (size less than 10 nm) and crystallized Nd₂O₃ precipitates. It appears that the presence of additives in sintered Nd-Fe-B magnets does not change the dissociation process. Apart from some contributions in very weak proportions in Co containing samples, the same contributions are detected, owing to the same reactions.

Full text of DOI:10.1088/0953-8984/11/25/313

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J. Phys.: Condens. Matter 11 (1999) 4941–4959. Printed in the UK
PII: S0953-8984(99)99830-2
A M o¨ ssbauer investigation of the dissociation of the Nd Fe B
2
14
phase
J-M Le Breton and S Steyaert
Magn e´ tisme et Applications, GMP UMR 6634 CNRS-Universit e´ de Rouen, Facult e´ des Sciences,
6821 Mont-Saint-Aignan Cedex, France
7
Received 1 December 1998, in final form 17 March 1999
Abstract. The dissociation process of the Nd2Fe14B phase in a sintered Nd16Fe76B8 powdered
◦
magnet was investigated between 300 and 650 C. The oxidized powders were characterized by
M o¨ ssbauer spectrometry and x-ray diffraction. The contributions of the dissociation products to
the M o¨ ssbauer spectra were identified and the results interpreted consistently with previous trans-
mission electron microscopy observations. The microstructure of the dissociated phase was char-
◦
acterized and found to depend on the oxidation temperature. For temperatures less than 400 C, the
microstructure consists in α-Fe nanograins (less than 10 nm in size, with a distorted structure) and
Fe3O4 superparamagnetic particles (size less than 10 nm), mixed with amorphous or poorly crys-
◦
tallized Nd2O3 regions. For temperatures higher than 400 C the microstructure consists in bigger
α-Fe grains (their size being 100 nm or more), Fe3O4 precipitates (size less than 10 nm) and crys-
tallized Nd2O3 precipitates. It appears that the presence of additives in sintered Nd–Fe–B magnets
does not change the dissociation process. Apart from some contributions in very weak proportions
in Co containing samples, the same contributions are detected, owing to the same reactions.
1. Introduction
During the last 12 years, the corrosion behaviour of Nd–Fe–B permanent magnets has been
extensively studied either in warm humid air/steam [1–3] or at high temperature [4–6].
Sintered Nd–Fe–B magnets have a three phase microstructure (Nd2Fe14B, Nd1.1Fe4B4 and
an intergranular Nd-rich phase). The Nd-rich phase is the less corrosion resistant, being
responsible for the poor corrosion resistance of Nd–Fe–B magnets under humid conditions
[
7]. The Nd2Fe14B hard magnetic phase, responsible for the good intrinsic magnetic properties
◦
of the magnet, oxidizes at temperatures above 100 C [8, 9]. The Nd1.1Fe4B4 phase is good
corrosion resistant, as its oxidation is not observed below 400 C [5, 6].
◦
The oxidation mechanism of the Nd2Fe14B hard magnetic phase was first characterized
◦
by transmission electron microscopy (TEM) on an Nd16Fe76B8 magnet heated at 423 C in
ambient air [10]: the Nd is oxidized and the phase dissociates into α-Fe crystals, which grow
epitaxially from the Nd2Fe14B substrate. The orientation of the α-Fe crystals is intimately
linked to the Nd2Fe14B one. Electron diffraction analyses revealed that the oxidized Nd2Fe14B
grains are composed mainly of monocrystals of α-Fe having a common [111] crystallographic
◦
direction, most adjacent monocrystals being rotated 30 from each other. It was stated that
the α-Fe monocrystals are typically several hundred nanometres in diameter, and contain fine
precipitates, with a dispersion of size around 5 nm. According to electron diffraction analyses,
these precipitates were considered to be hexagonal Nd2O3.
◦
Another TEM study made on a sintered Nd–Dy–Fe–Co–V–Al–B magnet heated at 110 C
under 100% relative humidity revealed that the microstructure of the dissociated Nd2Fe14B
0
953-8984/99/254941+19$19.50 © 1999 IOP Publishing Ltd
4941

4942
J-M Le Breton and S Steyaert
phase is constituted of α-Fe nanocrystallites (mean size: 2 nm), organized in domains with the
same crystallographic orientation [9]. The size of the domains is about 100 nm. It was stated
that the α-Fe nanocrystallites are embedded in an amorphous neodymium oxide (Nd2O3).
As the microstructure of the dissociated Nd2Fe14B phase is not clearly established, a
complementary study is needed in order to characterize it accurately. Because M o¨ ssbauer
spectrometryisalocalprobeoftheironatomicenvironment, theidentificationoftheM o¨ ssbauer
contributions to the spectrum of an oxidized magnet can help to characterize the microstructure
of the dissociated phase.
In this study, the dissociation of the Nd2Fe14B phase in sintered Nd–Fe–B magnets
◦
57
oxidized at high temperature (300–650 C in ambient atmosphere) is investigated by Fe
M o¨ ssbauer spectrometry and x-ray diffraction. The microstructure is discussed as a function
of both temperature and treatment time. An investigation of oxidized magnets containing
additives (Co, Al, Nb, V, Mo) is also reported.
2. Experimental procedure
Several sintered Nd–Fe–B magnets with different compositions (table 1) were powdered by
mechanical grinding and sieved (the diameter of the particles being less than 20 µm). The
◦
powders were heated in a conventional air furnace, in the 300–650 C range for times up to
7
days. The oxidized powders were analysed by x-ray diffraction (XRD) and transmission
Fe M o¨ ssbauer spectrometry (TMS) at room temperature. Some M o¨ ssbauer spectra were
5
7
recorded at 77 K.
Table 1. Nominal atomic compositions of the samples investigated.
Name
Composition
Nd16Fe76B8
DCVA
CVA
CMA
DNA
Nd13.5Dy1.5Fe67Co5V4B8Al
Nd14Fe69.6Co5V3B7Al1.4
Nd14.4Pr0.2Fe69.7Co5Mo2.5B7.2Al
Nd13.9Pr0.2Dy0.4Fe77.9Nb0.7B6.2Al0.7
XRD analyses were performed by reflexion, in an evacuated chamber using a fast curved
detector INEL CPS 120. The x-ray generator was equipped with a Co anticathode, using Co
(
Kα) radiation (λ = 0.179 09 nm).
TMS analyses were performed using a conventional 57Co source in a rhodium matrix. The
−2
samples contain about 10 mg cm of natural iron. The isomer shift (relative to metallic α-Fe
at room temperature), quadrupolar splitting, quadrupolar shift and hyperfine field are denoted
δ, 1, ε and B respectively. Estimated errors for the hyperfine parameters originate from the
statistical errors σ given by the fitting program [11], taking 3σ.
◦
3
. Powders oxidized in the 300–650 C temperature range
◦
3
.1. The dissociation products at 300 C
Infigure1areshowntheM o¨ ssbauerspectraofapowderedNd16Fe76B8 magnetbeforeoxidation
◦
and after oxidation at 300 C for 12 hours. The M o¨ ssbauer spectrum of the unoxidized powder
is fitted with the contributions of Nd2Fe14B [12] and Nd1.1Fe4B4 [13] phases. No more
Nd2Fe14B is detected in the spectrum of the oxidized powder, 12 hours being the time necessary
◦
for the complete dissociation of the Nd2Fe14B phase at 300 C [14]. The contributions used
to fit this spectrum are displayed in figure 2, and the corresponding hyperfine parameters and

M o¨ ssbauer investigation of the dissociation of Nd2Fe14B
4943
Figure 1. Room temperature M o¨ ssbauer spectra of the Nd16Fe76B8 powder before (a) and after
◦
oxidation at 300 C for 12 hours (b). The contribution of the Nd1.1Fe4B4 phase is displayed.
Figure 2. The contributions used to fit the room temperature M o¨ ssbauer spectrum of the
◦
Nd16Fe76B8 powder oxidized at 300 C for 12 hours.

4944
J-M Le Breton and S Steyaert
Table 2. Hyperfine parameters and relative absorption areas (±1%) of the contributions to the
◦
M o¨ ssbauer spectra of the Nd16Fe76B8 powder oxidized at 300 C (for 12 hours) and in the
◦
∗
3
50–650 C temperature range (for 2 hours).
means that the contribution was fitted using a
†
distribution of hyperfine fields (B is the mean hyperfine field). means that the contribution was
fitted using a sextet (B = 33.0 ± 0.1 T). The hyperfine parameters for Fe3O4 were fixed to the
values obtained from the literature [15].
Hyperfine parameters
Relative absorption area (%)
◦ ◦ ◦ ◦ ◦ ◦ ◦ ◦
300 C 350 C 400 C 450 C 500 C 550 C 600 C 650 C
δ
2ε, 1
(mm s
B
Contribution (mm s 1
−
)
−1
)
(T) 12 h
2 h
2 h
2 h
2 h
2 h
2 h
2 h
∗
∗
∗
26†
15†
3†
α-Fe
0.00
0.02
0.34
0.02
0.27
0.04
0.26
0.00
± 0.01
33.3 67
38.0 14
58
14
9
42
—
—
±
±
±
∗
∗
∗
mag
−0.10
± 0.02
−0.17
± 0.05
−0.16
−0.20
0.58
8
—
34
9
—
40
12
4
—
53
10
1
—
61
0
—
67
—
—
5
α-Fe2O3
51.4
3
20
5
± 0.2
Fe3O4
49.0
46.0
—
—
3
—
4
0
.67
Nd1.1Fe4B4 0.03
4
5
0
±
±
0.02
0.01
0.04
0.09
0.04
± 0.04
Fe3B
0.00
27.8
—
—
—
—
—
5
5
fixed
0.00
fixed
0.00
fixed
0.07
± 0.2
26.2
± 0.2
22.6
−
±
±
±
±
±
±
0
.11
0.04
0.36
0.02
0.38
0.05
1.33
0.03
0.30
0.02
± 0.2
∗
∗
∗
NdFeO3
para1
43.2
—
13
—
—
—
12
3
—
18
3
—
21
5
7
7
14
—
—
20
16*
—
± 0.02
0.73
—
7
—
—
21
± 0.07
0.54
para2
—
2
—
± 0.07
para3
1.05
—
—
—
—
13
12
± 0.05
relative absorption areas are reported in table 2. It is worth mentioning that these contributions
◦
were used to fit the M o¨ ssbauer spectra of the same powder oxidized between 150 and 300 C
[14]. This indicates that the dissociation process of the Nd2Fe14B phase is the same in this
temperature range.
The α-Fe contribution is fitted with a discrete distribution of hyperfine fields, the mean
hyperfine field value being 33.3 T. The width of the distribution is related to the poor crystallized
nature of the α-Fe phase. The fact that the mean hyperfine field of α-Fe in the oxidized powders
is slightly higher than that of pure α-Fe (33.0 T) can be related to the presence of strains,
probably due to the presence of interstitial B atoms in the structure.
An unidentified magnetic contribution (namely mag) is fitted with a discrete distribution
of hyperfine field, distinct from that of α-Fe, to account for the non-symmetric broadening at
the basis of the α-Fe lines. Another fit with one large discrete distribution shows two maxima
at 33 and 38 T (indicating two different kinds of environment around the Fe atoms) which
correspond to the mean hyperfine fields of the two distinct distributions (figure 3). However,
because such a fit does not account for the asymmetry of the broadening at the basis of the
α-Fe lines, all the M o¨ ssbauer spectra presented here were fitted with two distinct distributions.
The α-Fe2O3 contribution is fitted, evidencing the oxidation of some α-Fe. The paramag-
netic contribution of the Nd1.1Fe4B4 phase is detected, showing that this phase is not oxidized

M o¨ ssbauer investigation of the dissociation of Nd2Fe14B
4945
60
40
P(B)
20
0
30
35
40
45
Hyperfine Field B (T)
Figure 3. The two hyperfine field distributions (solid lines) used to fit the α-Fe and mag
contributions. The single hyperfine field distribution (dotted line) is shown for comparison.
◦
up to at least 300 C, in agreement with previous work [5, 6]. One paramagnetic unidentified
contribution (namely para1) is fitted to account for the broadening at the centre of the spectra.
Apart from the Nd1.1Fe4B4 contribution, all the contributions correspond to the
dissociation products of the Nd2Fe14B phase.
The XRD pattern of the oxidized Nd16Fe76B8 powder is shown in figure 4. No more
Nd2Fe14B peak is observed, a very broad α-Fe(110) peak being present instead. Some α-Fe2O3
peaks are observed as well. These observations are in agreement with the M o¨ ssbauer results.
The main peak of the cubic Nd2O3 phase (c-Nd2O3) is detected, its presence being attributed
to the oxidation of the Nd-rich phase [6].
◦
3
.2. Structural evolution of the dissociation products in the 350–650 C range
With the aim of following the behaviour of the dissociation products during oxidation, and thus
of obtaining more information on the structure of the dissociated phase, Nd16Fe76B8 powders
◦
were oxidized for 2 hours in the 350–650 C range.
The corresponding XRD patterns are shown in figure 4. As the temperature increases,
several remarks can be made. First, the intensity of the broad α-Fe peak decreases as the
intensities of the α-Fe2O3 peaks increase, in agreement with the oxidation of the α-Fe phase.
◦
At 500 C, the α-Fe peak is sharp, indicating that a change in the structure of the α-Fe phase
◦
has occurred. At 550 C, the intensity of the α-Fe peak is weak: the α-Fe phase is almost
fully oxidized. Second, the Nd1.1Fe4B4 peaks are no longer observed at 500 C, indicating
◦
that this phase starts to oxidize at this temperature. Third, the main peaks of hexagonal Nd2O3
◦
(
h-Nd2O3) can be observed from about 500 C. As this temperature corresponds to a change in
the structure of the α-Fe phase, the appearance of the h-Nd2O3 peaks can be related to a change
in the structure of the Nd2O3 amorphous regions (that is to the crystallization of amorphous
Nd2O3), the Nd2O3 amorphous regions resulting from the dissociation of the Nd2Fe14B phase
◦
[
9]. Fourth, from 500 C, the peaks of NdFeO3 are observed, indicating that some α-Fe2O3
phase reacts with c-Nd2O3 or h-Nd2O3 to form NdFeO3.
The room temperature M o¨ ssbauer spectra of the oxidized Nd16Fe76B8 powders are shown
in figure 5. The hyperfine parameters and relative absorption areas of the different contributions
used to fit the spectra are reported in table 2. Except for the Fe3O4 magnetic contribution, the

4946
J-M Le Breton and S Steyaert
α-Fe
h-Nd
c-Nd
2 3
O
O
2 3
h:
c:
2 3
O
4 4
η: Nd1.1Fe B
6
50°C
6
5
00°C
50°C
5
00°C
4
4
50°C
00°C
α-Fe
α-Fe
3
50°C
α-Fe
3
00°C
1
2
17
22
27
32
37
θ(°)
◦
Figure 4. XRD patterns of the Nd16Fe76B8 powder oxidized in the 300–650 C range.
hyperfine parameters of the different phases were allowed to vary only in the spectra where
their corresponding relative absorption area is maximum (errors for the hyperfine parameters
are deduced from these fittings). The spectra were fitted by keeping constant the hyperfine
parameters of the same phase in the different samples and allowing the relative areas to vary.
The hyperfine parameters used for the Fe3O4 phase come from [15].
◦
The M o¨ ssbauer spectrum of the powder oxidized at 350 C was fitted with the same
◦
contributions as those used to fit the spectrum of the powder oxidized at 300 C, and in addition
a paramagnetic doublet (para2) with a weak relative area. With increasing temperature in the

M o¨ ssbauer investigation of the dissociation of Nd2Fe14B
4947
Figure 5. Room temperature M o¨ ssbauer spectra of the Nd16Fe76B8 powder oxidized for 2 hours
◦
in the 350–650 C temperature range. The contribution of the α-Fe phase is displayed.
◦
50–500 C range, the relative areas of both α-Fe and mag contributions decrease, as those
3
of α-Fe2O3 and Fe3O4 increase, according to the oxidation of α-Fe. The relative area of the
para1 doublet increases as well, indicating that this contribution could correspond to an iron
oxide. The relative areas of the Nd1.1Fe4B4 and para2 contributions remain constant, within
◦
the accuracy of the measurement. Between 500 and 550 C, new contributions appear in the
◦
spectra, while some others disappear. The Fe3O4 relative area decreases from 500 C and
Fe3O4 disappears at 600 C. The NdFeO3 contribution appears and its relative area increases
up to 650 C. The Nd1.1Fe4B4 contribution disappears at 550 C, and this corresponds to the
◦
◦
◦
appearance of the Fe3B contribution. This seems to indicate that Nd1.1Fe4B4 oxidizes at
◦
5
50 C, resulting in the formation of Fe3B, in agreement with the fact that the relative area of
◦
Fe3B is equal to that of Nd1.1Fe4B4. At 500 C, the paramagnetic contributions para1 and para2
disappear and another paramagnetic contribution (namely para3) appears instead, suggesting
that the para1 and para2 compounds transform into para3 due to an oxidation reaction.

4948
J-M Le Breton and S Steyaert
4
. Characterization of the M o¨ ssbauer contributions
As the hyperfine parameters of the unidentified contributions (mag, para1, para2 and para3)
do not correspond to any Fe–B–O or Fe–B phase, further heat treatments were performed to
allow their identification.
4.1. The magnetic contribution
As previous investigations showed that α-Fe nanograins are formed during the dissociation
process of the Nd2Fe14B phase [9], the mag contribution is attributed to Fe atoms located at the
surface of the α-Fe nanograins while the α-Fe contribution corresponds to Fe atoms within the
nanograins. The fact that both the isomer shift and the hyperfine field of the surface contribution
are higher than those of the bulk contribution could be related to a reduced interfacial density
at the surface of the nanograins, in agreement with theoretical calculations [16, 17], and to the
fact that in the dissociated structure the Fe surface atoms can have Nd and O neighbours.
Assuming that, first, the hyperfine field enhancement is observed for Fe atoms in the first
surface layer (thickness: e = 0.25 nm) [18, 19] and, second, that the nanograins are spherical,
one can give an estimation of the size of the α-Fe nanograins from the M o¨ ssbauer data. The
diameter D of a spherical nanograin is related to the volume fraction pα of bulk α-Fe by:
2
e
D =
.
/3
1
1
− pα
The volume fraction of bulk α-Fe is obtained from the M o¨ ssbauer relative absorption
areas of the ‘α-Fe’ and ‘mag’ components (%α and %mag respectively), assuming that the
Lamb–M o¨ ssbauer factors of the two contributions (bulk and surface) are equal:
%
α
pα =
.
%
α + %mag
From the M o¨ ssbauer spectra of samples oxidized in this work and in [5, 6, 8, 14, 20], the
◦
◦
estimated values of D are 6 ± 2 nm in the 150–300 C range, and 9 ± 2 nm in the 300–400 C
◦
◦
range. At temperatures higher than 450 C, or at 400 C for longer times, the mag contribution
no longer appears in the M o¨ ssbauer spectra of the oxidized powders, indicating that the α-Fe
grain size is higher than 100 nm. In figure 6 is shown a high resolution scanning electron
◦
micrograph of the oxidized layer of an Nd16Fe76B8 sintered magnet heated at 423 C for 12 days
[20]. Nanograins of 60–100 nm in size can be clearly seen, in agreement with our estimation.
These results indicate that the size of the nanograins increases with the oxidation temperature.
The broadening of the α-Fe peak observed in XRD patterns of powders oxidized at
◦
temperatures less than 400 C cannot be explained in terms of grain size effects only, and
this confirms the presence of strains, in relation with the presence of interstitial B in the α-Fe
grains, which could be responsible for the distorted structure of the α-Fe lattice.
The disappearance of the mag contribution in the M o¨ ssbauer spectrum of powders oxidized
◦
in the 350–650 C range can thus be related to the structural change of the α-Fe phase that
◦
occurs simultaneously. At 450 C, the mag contribution disappeared and the α-Fe contribution
is fitted with one sextet in agreement with an increase of the α-Fe grain size. The linewidth
−
1
of the fine sextet is equal to the experimental linewidth (0.26 mm s ) in agreement with a
structural refinement of the grains. This structural refinement can also be evidenced from
the evolution of the α-Fe lattice parameter. The lattice parameter deduced from the XRD
◦
measurements in powders oxidized at less than 400 C is higher than that of pure α-Fe in
relation with the presence of interstitial B atoms (figure 7). When the oxidation temperature

M o¨ ssbauer investigation of the dissociation of Nd2Fe14B
4949
3
00 nm
Figure 6. High-resolution scanning electron micrograph of a dissociated grain in an Nd16Fe76B8
◦
sintered magnet heated at 423 C for 12 days. From [19].
0
.29
0
0
0
0
.289
.288
.287
.286
pure α-Fe
2
00
500
800
Temperature (°C)
Figure 7. The lattice parameter of the α-Fe phase as a function of the oxidation temperature.
increases, a reduction of the lattice parameter down to the value of pure α-Fe is observed,
which corresponds to a sharpening of the α-Fe diffraction peak.
These assumptions are supported by the fact that in the M o¨ ssbauer spectrum of an
◦
◦
Nd16Fe76B8 powder oxidized at 300 C for 12 hours and annealed at 600 C under vacuum for
days, the α-Fe contribution is fitted with one fine sextet (figure 8(a)), with a relative absorption
7
area equal to that of the mag and α-Fe contributions together before annealing. It follows that,
upon annealing, the growth of the α-Fe grains leads to both a reduction of the relative fraction
of the surface atoms and a structural refinement of the grains, that occur simultaneously.

4950
J-M Le Breton and S Steyaert
Figure 8. Room temperature M o¨ ssbauer spectrum (a) and XRD pattern (b) of the Nd16Fe76B8
◦
◦
powder oxidized at 300 C for 12 hours and vacuum annealed at 600 C for 7 days. In the M o¨ ssbauer
spectrum, the paramagnetic contribution of FeO (two doublets) is displayed.
4.2. The paramagnetic contributions
◦
The room temperature M o¨ ssbauer spectrum of the powder oxidized at 300 C for 12 hours and
◦
annealed at 600 C for 7 days reveals that the para1 contribution disappeared during annealing,
the paramagnetic contribution of FeO [21] being present instead (figure 8(a)). The presence
of FeO is confirmed by XRD analysis (figure 8(b)). The appearance of FeO is in agreement
with the Fe–O phase diagram if one considers that the Fe3O4 phase is present in the oxidized
◦
powder before annealing: FeO is formed from 570 C owing to a reaction between α-Fe and
Fe3O4 according to the following formula [22]:
α-Fe + Fe3O4 → 4FeO.
(1)
◦
◦
The FeO compound is stable above 570 C, and metastable below about 200 C [22]. The
M o¨ ssbauer relative areas of the contributions detected in the spectrum of the powder before
and after annealing are in complete agreement with reaction (1) if one assumes that the para1
contribution corresponds to Fe3O4. The para1 contribution is thus attributed to Fe3O4.

M o¨ ssbauer investigation of the dissociation of Nd2Fe14B
4951
Figure 9. M o¨ ssbauer spectra recorded at room temperature and at 77 K of the Nd16Fe76B8 powder
◦
◦
oxidized at 400 C for 7 days (a) and at 600 C for 2 hours (b). The contribution of para1 is
displayed in (a). The contribution of para3 is displayed in (b).
◦
In the 77 K M o¨ ssbauer spectrum of the powder oxidized at 400 C for 7 days (figure 9(a)),
no more para1 contribution is observed, and a large magnetic distribution is fitted instead. This
behaviour is comparable to that observed in the case of 6 nm Fe3O4 nanoparticles at 80 K [23].
The relative area of this magnetic distribution is equal to the relative areas of the para1 and
Fe3O4 contributions together in the room temperature M o¨ ssbauer spectrum. This indicates
that the Fe3O4 particles related to the para1 contribution are small, being superparamagnetic
at room temperature. According to the literature, the behaviour of the M o¨ ssbauer spectrum
of 6 nm Fe3O4 particles is similar to that of para1, and this is not the case for 10 nm Fe3O4
particles [23]. The mean size of the Fe3O4 particles related to the para1 contribution is thus
very probably less than 10 nm.
◦
The 77 K M o¨ ssbauer spectrum of the powder oxidized at 600 C for 2 hours (figure 9(b))
was fitted with the following contributions: α-Fe2O3, Fe3B, a magnetic distribution including
the NdFeO3 contribution and para3. The relative area of the para3 contribution is lower
at 77 K than at room temperature. This means that the compound corresponding to
para3 is paramagnetic at room temperature and partially magnetic at 77 K, thus showing
a superparamagnetic behaviour. This compound appears very probably as a distribution of
small particles, which could explain the presence of a remaining para3 doublet at 77 K. The
room temperature hyperfine parameters of the para3 contribution (see table 2) correspond to
−
1
those of superparamagnetic α-Fe2O3 particles with size less than 10 nm (δ = 0.34 mm s
,
−1
−1
−1
1
= 1.02 mm s estimated from [24], and δ = 0.32 mm s , 1 = 1.06 mm s after
[
25]). For this reason, and because the room temperature M o¨ ssbauer spectrum of an α-Fe2O3
powder with a mean particle size less than 10 nm is paramagnetic [26], the mean size of the
superparamagnetic α-Fe2O3 particles is estimated here to be less than 10 nm.

4952
J-M Le Breton and S Steyaert
From the previous observations, it can be concluded that the transformation of para1 into
◦
para3 at 500 C corresponds to the oxidation of Fe3O4 into α-Fe2O3 according to the formula:
4
Fe3O4 + O2 → 6α-Fe2O3.
(2)
The size of these particles does not change significantly upon oxidation, remaining less than
10 nm. The fact that the Fe3O4 and α-Fe2O3 oxides were not detected by XRD is attributed
to the nanometric size of the particles and to the fact that these particles are probably not well
crystallized.
The para2 doublet is not clearly identified. Its isomer shift corresponds to the Fe(II)
oxidation state, indicating that the related phase contains Fe2 ions. On the other hand, the
para2 contribution is still paramagnetic at 77 K, indicating that the related phase is very
probably non-magnetic. We therefore assume that the para2 contribution corresponds to Fe2
ions, dispersed in Nd oxides.
+
+
5. Discussion
5.1. The structure of the dissociated Nd2Fe14B phase
The results of both M o¨ ssbauer and XRD investigations are consistent and show that at tem-
◦
peratures lower than 400 C, the structure of the dissociated Nd2Fe14B phase consists in α-Fe
nanograins (less than 10 nm in size, with a distorted structure) and Fe3O4 superparamagnetic
particles (less than 10 nm in size), mixed with amorphous or poor crystallized Nd2O3 regions.
This structure is outlined in figure 10. The presence of superparamagnetic Fe3O4 is related to
the oxidation of the Nd in the Nd2Fe14B phase: the reaction between the Nd atoms and oxygen
releases enough energy to oxidize the Fe atoms in their close vicinity, forming very small Fe3O4
particles that are embedded in Nd2O3 regions (thus showing a superparamagnetic behaviour).
This interpretation is consistent with TEM observations of an Nd–Dy–Fe–Co–V–Al–B magnet
◦
oxidized at 110 C for 120 hours [9] and indicates that the size of the α-Fe nanograins depends
◦
◦
on the temperature, being 2 nm at 110 C [9], 6 nm in the 150–300 C range and 9 nm in the
00–400 C range. The α-Fe nanograins are organized in domains (about 100 nm in size) with
◦
3
the same crystallographic orientation, as shown by TEM observations [9].
◦
From 400 C, a change in the structure of the α-Fe phase is observed: a structural
◦
refinement and a growth of the α-Fe grains occur simultaneously. Thus at 400 C for long
times (several days) or at temperatures higher than 450 C, the microstructure of the dissociated
◦
phase is constituted of bigger α-Fe grains (their size being 100 nm or more), Fe3O4 precipitates
(size less than 10 nm) and crystallized h-Nd2O3 precipitates. The growth of the α-Fe grains
probably results from the coalescence of the α-Fe nanograins having the same crystallographic
orientation. This structure is outlined in figure 10. This interpretation is consistent with TEM
◦
observations of an Nd–Fe–B magnet oxidized at 423 C for 12 days [10].
Consequently, the TEM observations mentioned in section 1 [9, 10] are not inconsistent,
but reflect the influence of the oxidation temperature on the structure of the dissociated
Nd2Fe14B phase.
5.2. The oxidation reactions
According to the relative absorption areas of α-Fe and Fe3O4 deduced from the M o¨ ssbauer
spectra, the following reaction can be written:
4
Nd2Fe14B + 11O2 → 48.5α-Fe(B0.082) + 2.5Fe3O4 + 4Nd2O3.
(3)

M o¨ ssbauer investigation of the dissociation of Nd2Fe14B
4953
Dissociated layer
Nd
2
O
3
T ≤400°C
α-Fe
3 4
Fe O
≈
10 nm
Nd
2
O
3
4
00°C < T < 500°C
3 4
Fe O
α-Fe
≈
100-200 nm
Figure 10. Schematic microstructure of the dissociated Nd2Fe14B phase as a function of the
◦
oxidation temperature. The common orientation of the α-Fe nanograins for T 6 400 C is
evidenced.
Considering the global atomic composition of a dissociated grain (Nd8.8Fe62.2B4.5O24.5),
the amount of oxygen obtained from this reaction is very consistent with electron probe
microanalysis measurements of the composition of the dissociated grains in bulk samples
◦
oxidized at 423 C (Nd9±1Fe60±3B4±1O27±3) [6].
During further oxidation, several reactions occur that depend on the oxidation temperature.
◦
For temperatures less than 500 C, the α-Fe grains are oxidized first into Fe3O4 and then into
α-Fe2O3 according to:
3
α-Fe + 2O2 → Fe3O4
(4)
and to reaction (2), and their magnetic contributions are detected in the M o¨ ssbauer spectra.
The oxidation proceeds from the surface of the particle to its inner part, as shown in figure 11,
due to the diffusion of oxygen within the oxide layer. The fact that the contribution of
superparamagnetic Fe3O4 is still observed is attributed to a faster diffusion of oxygen through
the α-Fe2O3 grains than through the Nd2O3 amorphous regions (the Nd2O3 amorphous regions
act for the Fe3O4 nanoparticles as a protective layer against oxidation).
◦
For temperatures higher than 500 C, the same reactions occur due to the diffusion of
oxygen into the particle, as shown in figure 12. The superparamagnetic α-Fe2O3 contribution
is observed instead of Fe3O4 due to the oxidation of the nanoparticles of Fe3O4 formed in
the Nd2O3 regions into α-Fe2O3 according to formula (2). This is attributed to the diffusion
of oxygen through the h-Nd2O3 regions at these temperatures, in relation with the structural
◦
change of the Nd2O3 amorphous regions at 500 C (see section 3.2). The NdFeO3 contribution
is detected in the M o¨ ssbauer spectra. As this phase forms owing to the following reaction
between Nd2O3 and α-Fe2O3:
α-Fe2O3 + Nd2O3 → 2NdFeO3
(5)

4954
J-M Le Breton and S Steyaert
Captions
2
Unaffected Nd Fe14B
monograin particle
of a 0-20 µm powder
Dissociated layer
Layer composed of:
≈
10 µm
2 3
Nd O
Nd
2
Fe14B dissociation:
reaction (3)
Fe
3
O
4
3 4
Fe O
≈
100-200 nm
Surface oxidation of α-Fe:
reactions (2) and (4)
Layer composed of:
2 3
Nd O
Fe
3
O
4
T < 500°C
2 3
α-Fe O
≈
100-200 nm
◦
Figure 11. The oxidation process of an Nd2Fe14B particle at temperatures less than 500 C.
it is concluded that NdFeO3 regions form at the Nd2O3/α-Fe2O3 interfaces. The α-Fe2O3
precipitates in the Nd2O3 regions probably transform in NdFeO3 as well.
The reactions (2), (4) and (5) are in agreement with the M o¨ ssbauer relative areas of the
different phases deduced from the M o¨ ssbauer spectra of the oxidized powders. All these
results, obtained for Nd–Fe–B powders, are consistent with the investigation of the oxidation
of bulk Nd–Fe–B magnets at high temperature [6].
The dissociation of the Nd2Fe14B phase being well characterized for the Nd16Fe76B8
powdered magnet, the influence of additives on the dissociation process was investigated for
Nd–Fe–B powders containing additives.
6. Oxidized Nd–Fe–B powders containing additives
The nominal atomic compositions of the samples containing additives (DCVA, CVA, CMA and
DNA) are reported in table 1. The microstructural investigation of these samples is reported
elsewhere [27]. This study revealed the presence of both intergranular and intragranular

M o¨ ssbauer investigation of the dissociation of Nd2Fe14B
4955
Captions
2
Unaffected Nd Fe14B
monograin particle
Dissociated layer
of a 0-20 µm powder
Layer composed of:
2 3
Nd O
≈
10 µm
Nd
2
Fe14B dissociation:
reaction (3)
α-Fe
2
O
3
3 4
Fe O
≈
100-200 nm
Layer composed of:
2 3
Nd O
Surface oxidation of α-Fe:
reactions (2) and (4)
α-Fe
2
O
3
2 3
α-Fe O
≈
100-200 nm
Layer composed of:
Formation of NdFeO
3
:
reaction (5)
2 3
Nd O
NdFeO
3
2 3
α-Fe O
T > 500°C
≈
100-200 nm
◦
Figure 12. The oxidation process of an Nd2Fe14B particle at temperatures higher than 500 C.
X–Fe–B precipitates (with X = V, Nb, Mo). These phases were found to be corrosion
◦
resistant in the 200–300 C temperature range. The M o¨ ssbauer spectra of the fully oxidized
DCVA, CVA and CMA samples are very similar, and were fitted with the same contributions
(in the same proportions, within the experimental accuracy). The spectrum of the CMA sample
is shown in figure 13, and the hyperfine parameters of the contributions used to fit the spectrum
are reported in table 3. On the other hand, the M o¨ ssbauer spectrum of the fully oxidized DNA
sample is very similar to that of the fully oxidized Nd16Fe76B8 sample.

4956
J-M Le Breton and S Steyaert
◦
Figure 13. Room temperature M o¨ ssbauer spectrum of the CMA powder oxidized at 300 C for
days. The contributions used to fit the spectrum are displayed.
3
The Nd2Fe14B phase is fully dissociated as its contribution no longer appears. For each
sample, a paramagnetic contribution attributed to the corrosion-resistant X–Fe–B phase is
fitted. Concerning the dissociation products, the same contributions as in the spectra of
oxidized samples with no additives are fitted (bulk and surface contributions of α-Fe, α-
Fe2O3, magnetic and superparamagnetic Fe3O4 and a paramagnetic contribution attributed to
2+
Fe ions in Nd oxides). Other contributions (Fe3B and para4) are fitted in the Co containing
samples.
◦
AsfortheNd16Fe76B8 powderoxidizedat300 Cfor12hours, themaincontribution(fitted
with a discrete distribution of hyperfine fields) corresponds to α-Fe. However, for the DCVA,
CVA and CMA samples, the α-Fe contribution is characterized by a higher mean hyperfine
field (34.6–34.7 T) than that obtained for the Nd16Fe76B8 and DNA samples (33.3–33.4 T).
This is attributed to the presence of Co in the α-Fe phase, in agreement with the fact that
Co atoms substitute for Fe atoms in the Nd2Fe14B unit cell [27]. By interpolating the results

M o¨ ssbauer investigation of the dissociation of Nd2Fe14B
4957
Table 3. Hyperfine parameters and M o¨ ssbauer relative intensities (±1%) of the contributions to
◦
the M o¨ ssbauer spectrum of the CMA powder oxidized at 300 C for 3 days. * means that the
contribution was fitted using a distribution of hyperfine fields (B is the mean hyperfine field). The
hyperfine parameters for Fe3O4 were fixed to the values obtained from the literature [15]. The
hyperfine parameters for Fe3B were fixed to the values obtained for the Nd16Fe76B8 oxidized
sample.
Relative
δ
2ε, 1
(mm s )
B
absorption
area (%)
−
−1
Contribution
(mm s 1
)
(T)
∗
bulk α-Fe
0.05 ± 0.02
0.01 ± 0.02
34.7
38.7
56
(
distribution)
surface α-Fe
distribution)
∗
0.34 ± 0.02 −0.08 ± 0.02
0.35 ± 0.04 −0.16 ± 0.05
13
(
α-Fe2O3
Fe3O4
51.2 ± 0.2
49.0
46.0
27.8
26.2
22.6
—
—
—
—
7
4
0.26
.67
−0.16
−0.20
0.00
0.00
0.00
0
Fe3B
0.01
0.09
2
−
0
.11
superpara Fe3O4
0.41 ± 0.03
0.84 ± 0.04
11
3
2
0
para2
1.65 ± 0.03
0.02 ± 0.04
0.70 ± 0.04
0.37 ± 0.01
para4
Mo2FeB2
−0.15 ± 0.03
0.09 ± 0.01
2
of a study of the mean hyperfine field in Fe1−xCox compounds (with 0 6 x 6 0.5) [28],
the Co content in α-Fe was estimated to be 8 at.%. A similar effect is observed for the mag
contribution, in agreement with the fact that this contribution corresponds to the surface atoms
in the α-Fe grains.
The fact that the Fe3B and para4 contributions are not observed in the spectrum of
oxidized Nd16Fe76B8 or DNA powders is attributed to the presence of Co in the Nd2Fe14B
phase, which leads to the formation of other dissociation products (in weak proportions)
during the dissociation process. The formation of some Fe3B during the process could
be related to the fact that the Co content in the α-Fe phase (8 at.%) is higher than in the
Nd2Fe14B phase (5 at.% [27]). The para4 contribution, in very weak amount, remains
unidentified.
It appears that the presence of additives in sintered Nd–Fe–B magnets does not change the
dissociation process. Apart from some contributions in very weak proportions in Co containing
samples, the same contributions appear, owing to the same reactions. This is consistent with
the fact that the activation energy of the dissociation reaction is the same for samples with and
without additives [27].
7. Summary and conclusion
The dissociation process of the Nd2Fe14B phase in a sintered Nd16Fe76B8 powdered magnet
◦
was studied between 300 and 650 C. The oxidized powders were analysed by M o¨ ssbauer
◦
spectrometry and x-ray diffraction. In the 300–350 C temperature range, the main dissociation
product is α-Fe, which results from the oxidation of Nd in the Nd2Fe14B phase. It was shown
◦
that the α-Fe phase appears as nanograins. In the 350–650 C temperature range, the Nd2Fe14B
dissociation process going very fast, the oxidation of the α-Fe phase is mainly observed. The

4958
J-M Le Breton and S Steyaert
characterization of the oxidation reactions occurring at high temperature allowed us to identify
the dissociation products accurately. The results were interpreted consistently with previous
TEM observations and the microstructure of the dissociated phase was thus characterized
accurately as a function of the oxidation temperature.
◦
Fortemperatureslessthan400 C, themicrostructureconsistsinα-Fe nanograins(lessthan
10 nm in size, with a distorted structure) and Fe3O4 superparamagnetic particles (about 6 nm
in size), mixed with amorphous or poorly crystallized Nd2O3 regions. For temperatures higher
◦
◦
than 400 C (or at 400 C for several days) the microstructure consists in bigger α-Fe grains
their size being 100 nm or more), Fe3O4 precipitates (size less than 10 nm) and crystallized
(
h-Nd2O3 precipitates.
From the M o¨ ssbauer measurements, a dissociation reaction was proposed for the
Nd2Fe14B phase which is consistent with previous EPMA measurements. The oxidation
reactions occurring at high temperatures are in agreement with the relative intensities deduced
from the M o¨ ssbauer analyses as well.
The dissociation process was investigated for Nd–Fe–B powders containing additives.
It was found that the presence of additives in sintered Nd–Fe–B magnets does not change
significantly the dissociation process.
Acknowledgments
The authors wish to thank I R Harris (University of Birmingham) for providing the Nd–Fe–B
sample, and P Tenaud (Ugimag) and F Vial (Centre de Recherches de Voreppe) for providing
the samples containing additives. They are thankful to G Le Ca e¨ r (Ecole des Mines de Nancy)
and J-C Joubert (ENSPG, Grenoble) for helpful discussions.
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