J. Mater. Sci. Technol., 2011, 27(6), 503-506.
Structure and Magnetic Properties of S-doped Mn3 O4 /S
Composited Nanoparticles and Mn3 O4 Nanoparticles
X. He, Z.H. Wang† , D.Y. Geng and Z.D. Zhang
Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Centre for Materials
Physics, Chinese Academy of Sciences, Shenyang 110016, China
[Manuscript received January 19, 2011, in revised form March 14, 2011]
Composited nanoparticles, consisting of Mn3 O4 , S-doped Mn3 O4 and S, were synthesized by co-precipitation
reaction and Mn3 O4 nanoparticles were then obtained after removing the pure S from the composited nanoparticles. The Mn3 O4 -type phase with larger lattice constant a was formed by doping sulfur. At fixed temperatures below Curie temperature (TC ), the magnetization of the S-doped Mn3 O4 /S composited nanoparticles
was smaller than that of the Mn3 O4 nanoparticles. The blocking temperature was 36.3 and 34.8 K for Sdoped Mn3 O4 /S composite and Mn3 O4 nanoparticles, respectively. The anisotropy field of S-doped Mn3 O4 /S
composite was determined to be about 55.3 kOe.
KEY WORDS: Nanostructured materials; Chemical synthesis; X-ray diffraction;
Magnetic measurements
1. Introduction
Much attention has been devoted to investigate
the magnetic properties[1–4] , and spin canting structure of Mn3 O4 with normal spinel structure. The
stable phase Mn3 O4 at room temperature is tetragonal hausmannite, in which Mn3+ and Mn2+ ions
occupy octahedral and tetrahedral positions of the
spinel structure, respectively. The octahedral symmetry is tetragonally distorted, due to the Jahn–Teller
effect on Mn3+ ions (electronic configuration 3d4 )[5] .
Although Mn3 O4 is tetragonal at normal pressure and
room temperature, it is subjected to the Jahn–Teller
transition (JTT) at 1160◦ C[6] , which is a first-order
phase transition accompanied by a significant lattice
deformation. The role of effectively disordered particles decreases with increasing interparticle interactions in Mn3 O4 nanoparticles. Below the ferromagnetic transition temperature, differences in the temperature dependence of the magnetization and hysteresis loop were observed for the Mn3 O4 particles
† Corresponding author. Assoc. Prof., Ph.D.; Tel.: +86
24 83978846; Fax: +86 24 23971320; E-mail address: [email protected] (Z.H. Wang).
and those dispersed in a polymer. Such differences
are due to different strength of interparticle interactions and different contributions from free spins (and
cluster ones) on the surface of the particles[7] . The
effect of the substitution of other transition metals,
such as Cr and Zn, on its structural and magnetic
properties has been investigated[8,9] . Various methods
such as chemical bath deposition[10] , solvothermal[11]
and co-precipitation[3] have been employed to prepare
nanocrystalline Mn3 O4 . It has been confirmed that
the chemical methods efficiently control the morphology and chemical composition of Mn3 O4 powders.
Here, we synthesize composited nanoparticles, consisting of Mn3 O4 , S-doped Mn3 O4 and S, by a simple
co-precipitation reaction. Mn3 O4 nanoparticles are
then obtained after removing the pure S from the composited nanoparticles. The structure and magnetic
properties of both the S-doped Mn3 O4 /S composited
nanoparticles and the Mn3 O4 nanoparticles are investigated. It is found that the structure of the Mn3 O4
nanoparticles after removing sulfur is distorted, due
to the substitution of sulfur ions for oxygen ions.
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X. He et al.: J. Mater. Sci. Technol., 2011, 27(6), 503–506
(a)
Intensity / a.u.
S1
S2
Mn O
3
4
Mn O
3
4
S
20
30
40
50
2
60
70
80
/ deg.
were synthesized by a deposition reaction of manganese nitrate and sodium sulfide at room temperature in distilled water[12] .
Na2 S·9H2 O and
Mn(NO3 )2 ·6H2 O with chemical purity as starting materials were dissolved in distilled water according to
nominal composition, respectively. Then Na2 S·9H2 O
solution was added to Mn(NO3 )2 ·6H2 O aqueous solution with stirring. The collected precipitate was
washed by distilled water several times and dried at
100◦ C in air for about 2 h. The final product was Sdoped Mn3 O4 /S composited nanoparticles denoted as
S1. The appropriate liquid carbon bisulfide (CS2 ) was
added to S1, then the mixture was stirred in order to
remove the sulfur. After several times, the sulfur in
S1 was removed effectually by carbon bisulfide (CS2 )
to obtain Mn3 O4 nanoparticles. Mn3 O4 nanoparticles
were dried at 100◦ C in air for about 2 h that were denoted as S2 (although trace pure sulfur still existed according to the analysis below). The structure of both
the S-doped Mn3 O4 /S composited nanoparticles and
the Mn3 O4 nanoparticles was investigated by means
of X-ray diffraction (XRD, D/max-γA). Transmission
electron microphotographs (TEM, JEOL 1200EXII)
were obtained. X-ray photon spectroscope was used
with AlKα=1486.6 eV. The magnetic properties were
investigated by a superconducting quantum interference device (SQUID).
3. Results and Discussion
Fig. 1 (a) XRD patterns of Mn3 O4 /S-doped Mn3 O4 /S
composited nanoparticles (S1), and Mn3 O4
nanoparticles (S2) after removing sulfur by carbon bisulfide. (b) and (c) TEM images of typical
morphology of S1 and S2, respectively
2. Experimental
The S-doped Mn3 O4 /S composited nanoparticles
Figure 1 represents XRD patterns of S-doped
Mn3 O4 /S composited nanoparticles S1 and Mn3 O4
nanoparticles S2. The XRD pattern of S1 is matched
well to that of Mn3 O4 . The lattice constants were determined to be a=0.576 nm and c=0.943 nm (PDF
file No. 24-0734). Moreover, some weak peaks,
according to Mn3 O4 phase with a=0.816 nm and
c=0.943 nm (such as 2θ=30.99 deg.) (PDF file
No. 65-2776), were observed, indicating that some
S-doped Mn3 O4 nanoparticles may be formed. The
characteristic peaks of pure sulfur were detected.
The relative content of pure sulfur in samples S1
and S2 was estimated from XRD using the highest peak of Mn3 O4 (2θ=36.2 deg.) and S (2θ=23.1
deg.), the value of I(S)/I(Mn3 O4 ) is 41.9% and 3.6%
for S1 and S2, respectively. The pure sulfur (and
maybe some substitutional sulfur) can be removed
effectually by CS2 . However, the trace of pure sulfur was still observed in the S2 sample (still denoted as Mn3 O4 nanoparticles for convenience). Nevertheless, the peaks of Mn3 O4 phase with tetragonal crystal structure (a=0.816 nm and c=0.943 nm)
(PDF file No. 65-2776) were clearly observed after removing sulfur. The formation of the Mn3 O4
phase with a larger lattice constant a can be interpreted as follows: When starting materials reacted,
the mono-sulfide phase β-MnS should be synthesized
at first[13] . However, the β-MnS was unstable, which
505
X. He et al.: J. Mater. Sci. Technol., 2011, 27(6), 503–506
(a)
12
10
8
2
FC
M / emu
g
-
M
1
S
/ emu
Intensity / a.u.
g
-
1
10
S1
160
162
164
166
168
170
172
2
6
0
10
2
T
30
40
50
/ K
1
S
ZFC
2
60
90
T
120
150
180
/ K
Fig. 3 Temperature dependence of ZFC and FC magnetizations between 5 and 200 K for the S1 and
S2 nanoparticles measured at a magnetic field of
100 Oe. Inset is M -T curves between 5 and 50 K
2
S
Intensity / a.u.
20
4
30
S1
168
4
0
(b)
164
6
S
Binding energy / eV
160
8
172
Binding energy / eV
Fig. 2 XPS images of the S1 and S2 nanoparticles:
(a) as-prepared surface, (b) the surface after
cleaned by Ar+ sputtering for the S 2p3/2
was oxidized to Mn3 O4 in air quickly; part of sulfur ions existed in the Mn3 O4 phase would substitute
oxygen ions. According to the XRD result, the lattice constant a is increased and c is unchanged. It is
thought that the sulfur ions substituted the oxygen
ions which located in four equatorial distances of the
MnO6 octahedron structure. Because the radius of a
sulfur ion (0.184 nm) is much longer than that of an
oxygen ion (0.14 nm), even if one oxygen ion was substituted by a sulfur ion, the MnO6 octahedron structure would be distorted seriously. This would lead to
a large Jahn-Teller distortion[14] and result in the formation of a metastable state with the increased lattice
constant. After almost all the sulfur atoms (including
the pure sulfur and the substitutional one) were removed by CS2 , part of the tetragonal structure with
the metastable state could be transformed to another
stable tetragonal one with the much larger lattice constant. Thus, XRD peaks for another kind of tetragonal Mn3 O4 as the secondary phase were observed.
The morphology of S1 and S2 was investigated by
transmission electron microscopy (TEM). The shape
of the particles in the two samples is nearly spherical. The spherical shape of the particles is clearer
after removing the sulfur. A typical particle with the
size of about 50 nm is shown in Fig. 1(b) and (c).
The experimental evidence for sulfur states in
Mn3 O4 comes from the analysis of S 2p3/2 X-ray photon spectroscopy (XPS) lines (shown in Fig. 2(a) and
(b)). Two experiments of surface analysis were conducted respectively for the S1 and S2 samples. The
XPS spectra obtained from the surface of S1 and
S2 contain two characteristics peaks near the sulfur
2p3/2 peaks. These located at 162.5 eV in S2 and
163.7 eV in S1 are attributed to the features of pure
sulfur, while those at 167.7 eV in S2 and 168.5 eV in
S1 are attributed to MnSO4 . Although a little pure
S was still left, most part of the pure S in S2 was removed by CS2 as XRD analysis indicated. MnS can
be oxidized to MnSO4 directly in air. The different
values are due to the complex situation on the surfaces. After Ar+ sputtering was performed to remove
oxides on the surface and other contaminations, as
shown in Fig. 2(b), the intensities of the peak 167.7
eV attributed to MnSO4 are decreased obviously, in
comparison with those on the surface. The binding
energies near 162.8 and 162.3 eV are attributed to
pure S for S1 and S2, respectively. The binding energy near 160.8 eV was observed only in S1. This
peak may be attributed to Mn-S bond.
In Fig. 3, the temperature dependences of magnetization recorded at H=100 Oe are reported for the
S1 and S2 samples. In field-cooling (FC) magnetization curves of both samples, a rapid increase with
decreasing temperature is observed at TC =42 K, corresponding to the Curie temperature of bulk Mn3 O4 .
Meanwhile, a magnetic irreversibility appears between FC and Zero-field-cooling (ZFC) curves, indicating an induced preferential orientation of the
particles0 moments along the applied magnetic field[7] .
With decreasing temperature from TC , ZFC magnetization, MZFC , increases gradually, reaching a maximum at T =36.3 and 34.8 K for S1 and S2, respec-
506
/ K
T
10
M / emu
g
-1
12
8
36
33
30
27
24
21
0
55
T 50
H 45
40
35
30
8 10
B
k
H
14
k / kOe
X. He et al.: J. Mater. Sci. Technol., 2011, 27(6), 503–506
2
6
4
H
6
/ kOe
00 Oe
000 Oe
3000 Oe
6000 Oe
9000 Oe
1
1
4
2
0
10
20
30
40
50
60
T
70
80
90
100
/ K
Fig. 4 Temperature dependence of ZFC magnetization
between 5 and 100 K for S1 at different external magnetic fields. Magnetic field dependence of
blocking temperatures and anisotropy fields of S1
nanoparticles (the inset)
tively. The temperature at which the thermal activation overcomes all the energy barriers is known as the
blocking temperature TB (Fig. 3)[15,16] . The values of
MZFC and MFC of S1 are smaller than those of S2 at
a fixed temperature below TC , mainly because of the
mass reduction of the magnetic phase due to the exis[17]
tence of pure S. From the equation TB = K(D)3 /kB ,
K is the magnetic anisotropy, kB is the Boltzmann
constant and D is the particle size. The blocking temperature is sensitive not only to nanoparticle size, but
also to the magnetic anisotropy. The particle size is
almost the same in S1 and S2 (although the particle
size of S1 is slightly smaller than that of S2), which
could be found in TEM images. From the facts that
the TB of S1 is higher than that of S2 and the particle size is almost the same, we could reach a conclusion that the magnetic anisotropy of S1 is bigger
than that of S2. The blocking temperature decreases
with increasing external field. TB = T0 (1−H/Hk )3/2 ,
with T0 representing the blocking temperature as the
external field approaches 0 and Hk representing the
anisotropy field. So T0 and Hk of S1 sample were calculated from the M -T curves with different external
field, as shown in Fig. 4. This can be analyzed by
considering this formula for TB within the framework
of the random anisotropy model[16] . The fitting of the
experimental data by TB = T0 (1−H/Hk )3/2 is shown
by the solid line in Fig. 4. By the fitting, the two
parameters T0 and Hk are determined to be about
36.9 K and 55.3 kOe, respectively.
4. Conclusion
In conclusion, the S-doped Mn3 O4 /S composited
nanoparticles and the Mn3 O4 nanoparticles were synthesized by a simple deposition reaction of manganese
nitrate and sodium sulfide at room temperature in
distilled water. Mn3 O4 with bigger lattice constant
was formed after removing sulfur because sulfur ions
substituted oxygen ions. The XRD result showed
that only the lattice constant a was increased and
c was unchanged, because the sulfur ions substituted
the oxygen ions that located in four equatorial distances of the MnO6 octahedron structure. The blocking temperature was 36.3 and 34.8 K for the S-doped
Mn3 O4 /S composited nanoparticles and the Mn3 O4 ,
respectively. T0 and Hk of S1 sample were determined
to be about 36.9 K and 55.3 kOe, respectively. The
Mn-S bond at about 160.8 eV was observed from the
XPS results in S-doped Mn3 O4 . This work offers some
insights into the magnetic properties of Mn3 O4 by using S substitution to modify the bonding and lattice
constants.
Acknowledgements
This work was financially supported by the National
Basic Program of China (No. 2010CB934603), Ministry
of Science and Technology of China and by the National
Natural Science Foundation of China under Grant Nos.
50831006 and 50703046.
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