Room-temperature ferromagnetism observed in Fe-doped NiO

Article abstract of DOI:10.1063/1.2130532

Here we report a kind of diluted magnetic semiconductor (DMS) system, i.e., Fe-doped NiO. Monophase Ni1-x Fex O (x=0-0.02) was synthesized by using a chemical coprecipitation method and post-thermal decomposition processing. Pure NiO without Fe doped is antiferromagnetic. The results demonstrated the samples with Fe doped were ferromagnetic at room temperature, which is stronger than most DMS oxide systems, e.g., Mn or Ni-doped ZnO, and Co-doped Ti O2 reported recently. The effect of Fe doping was discussed and relevant mechanism was proposed, by comparing with previous studies of finite size effect in NiO nanoparticles and other DMS systems.

Full text of DOI:10.1063/1.2130532

Room-temperature ferromagnetism observed in Fe-doped NiO
Jianfei Wang, Jingnan Cai, Yuan-Hua Lin, and Ce-Wen Nan
Citation: Applied Physics Letters 87, 202501 (2005); doi: 10.1063/1.2130532
View online: http://dx.doi.org/10.1063/1.2130532
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APPLIED PHYSICS LETTERS 87, 202501 ͑2005͒
Room-temperature ferromagnetism observed in Fe-doped NiO
a͒
Jianfei Wang, Jingnan Cai, Yuan-Hua Lin, and Ce-Wen Nan
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and
Engineering, Tsinghua University, Beijing 100084, People’s Republic of China
͑
Received 3 June 2005; accepted 20 September 2005; published online 8 November 2005͒
Here we report a kind of diluted magnetic semiconductor ͑DMS͒ system, i.e., Fe-doped NiO.
Monophase Ni1−xFe O ͑x=0–0.02͒ was synthesized by using a chemical coprecipitation method
x
and post-thermal decomposition processing. Pure NiO without Fe doped is antiferromagnetic. The
results demonstrated the samples with Fe doped were ferromagnetic at room temperature, which is
stronger than most DMS oxide systems, e.g., Mn or Ni-doped ZnO, and Co-doped TiO reported
2
recently. The effect of Fe doping was discussed and relevant mechanism was proposed, by
comparing with previous studies of finite size effect in NiO nanoparticles and other DMS
systems. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2130532͔
Diluted magnetic semiconductors ͑DMS͒ are a new kind
position. Step mode around high angle peaks was also used
1
of materials for spintronic application, especially those
to reveal the effect of Fe doping on the NiO grain size.
Scanning electron microscopy ͑SEM͒ equipped with energy
dispersive x-ray spectrometer ͑EDX͒ was employed to study
the microstructure and composition of the samples. The mag-
netic behavior was measured by a vibrating sample magne-
tometer equipped with a dewar for low temperature testing.
Figure 1 shows the XRD patterns of the Ni1−xFexO
2
whose curie temperatures are above room temperature.
Since the discovery of room-temperature ferromagnetism in
3
cobalt-doped TiO thin films, more reports on high-Curie-
2
temperature ferromagnetic DMS have appeared, including
4
–8
3,9,10
11
12
13
14
ZnO,
TiO2,
SnO2, ZnTe, In O , Cu O based
2 3 2
DMS. Among these, ZnO-based n-type DMS have attracted
2
+
more and more interest. Several kinds of elements, such as
samples. Because the radius difference between Ni ion
4
5,6
7
8
3+
2+
Mn, Co, Ni, and V, have been doped into ZnO, in which
magnetism has been observed beyond room temperature.
However, few p-type DMS have been reported.
͑0.69 Å͒ and Fe ion ͑0.64 Å͒ or Fe ͑0.74 Å͒ is not re-
2
+
markable, Ni ions in lattice structure can be replaced by Fe
ions. The XRD analysis shows that these samples with x
ഛ0.02 remain a single phase, i.e., NiO face-center-cubic
Stoichiometric NiO, which is known as an antiferromag-
netic Mott-Hubbard insulator, could be semiconducting after
the introduction of Ni2 vacancies or doping with other cat-
1
9
͑fcc͒ phase. However, the solubility of Fe in NiO is limited.
+
As x=0.05, a trace of Fe2O3 appears in the XRD pattern of
the sample. As x increases further, the samples become a
mixture phase constituted by NiO, Fe2O3, and NiFe2O4 fer-
rite. Next we will focus on the Fe-doped NiO samples with a
single fcc phase.
Table I shows the effect of Fe dopant on the crystal size
of NiO particles, which is calculated from three high angle
peaks, i.e., ͑331͒, ͑420͒, and ͑422͒, using the common Scher-
rer formula after taking out the instrumental broadening. It
can be seen from the results that the crystal grains are around
1
5,16
ions and thus become a p-type semiconductor.
The mag-
netic properties of NiO nanoparticles ͑31.5 nm in diameter͒
1
7
have recently been studied by Kodama et al. They found
that due to finite size effect the NiO nanoparticles could
exhibit anomalous magnetic properties, such as large mo-
ments and coercivities and loop shifts at low temperatures.
These features are absent in NiO of larger size at
1
8
room-temperature.
In this letter, we report the magnetic properties of Fe-
doped NiO, which are different from nanosize effect induced
3
5–60 nm in diameter and become a little smaller as Fe
1
7,18
dopant content increases, indicating that Fe ions could dis-
turb the NiO crystal lattice and obstruct the crystal growth.
magnetism observed in nanosized NiO.
Remarkable
room-temperature magnetism, comparable with most DMS
systems reported so far, has been observed in Fe-doped NiO.
Pure Ni͑NO ͒ ·6H O, Fe͑NO ͒ ·9H O and NH HCO
3
2
2
3 3
2
4
3
were employed as starting raw materials. Nominal amounts
of Ni͑NO ͒ ·6H O and Fe͑NO ͒ ·9H O were weighed and
3
2
2
3 3
2
dissolved into distilled water. The hydroxycarbonate precur-
sor was chemically precipitated by slowly adding an aqueous
ammonium bicarbonate NH HCO solution whose pH was
4
3
ϳ8 at 295 K, accompanying magnetic stirring. The resultant
gels were washed several times with distilled water and eth-
anol until free of nitrate ions, filtrated and dried in air at
353 K. Afterwards, dried gels were calcined at 873 K for 4 h
in air to obtain the resultant Fe-doped NiO samples.
X-ray diffraction ͑XRD͒ patterns were obtained at room
temperature using Cu K␣ radiation to study the phase com-
a͒Electronic mail: cwnan@tsinghua.edu.cn
FIG. 1. XRD patterns of the Ni1−xFe O samples.
x
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003-6951/2005/87͑20͒/202501/3/$22.50 87, 202501-1 © 2005 American Institute of Physics
55.33.16.124 On: Wed, 26 Nov 2014 11:58:08
0
1

202501-2
Wang et al.
Appl. Phys. Lett. 87, 202501 ͑2005͒
TABLE I. Fe dopant dependence of the grain size of Ni1−xFe O ͑unit: nm͒.
x
͑
hkl͒
NiO
Ni0.995Fe0.005
O
Ni0.99Fe0.01
O
Ni0.98Fe0.02O
͑331͒
͑420͒
͑422͒
58.6
53.1
60.8
48.6
44.6
45.0
48.7
44.9
47.4
43.1
36.6
38.1
SEM analysis ͑not presented here͒ indicates the particle sizes
of all samples are around 100–200 nm. Thus the aggrega-
tions of grains and particles occur. The distribution of Fe
element in NiO is uniform at by EDX map analysis.
Figure 2 illustrates the magnetic properties of Ni1−xFe O
x
measured at room temperature. The inset is an enlarged part.
It can be clearly seen that the moment of pure NiO shows a
linear dependence on the extrinsic magnetic field, which is a
characteristic of common antiferromagnetic NiO. When the
Fe dopant content increases, the curve exhibits a distortion
towards ferromagnetism. For example, the sample containing
1
mol% Fe becomes magnetic, while for 2 mol% Fe, a dis-
tinct ferromagnetic hysteresis loop can be observed, with
maximum magnetization of about 0.575 emu/g
ϳ0.38 ␮ /Fe͒ at the maximum applied field of 10 kOe.
a
͑
B
This value is comparable with that reported for Mn-doped
1
4
Cu O, and much larger than those for other DMS systems
such as Ni-doped ZnO aggregates, Mn, Co, and V
doped ZnO films.
2
7
4
5,6
8
FIG. 3. ͑a͒ Temperature dependence of Ms and Hc of Ni0.98Fe0.02O sample,
and ͑b͒ the loop shift as function of temperature for the sample
Ni0.98Fe0.02O. The inset shows the hysteresis loops obtained at different
temperatures.
Such ferromagnetic behavior has been observed in nano-
1
7,18
18
sized NiO.
According to Makhlouf et al., small NiO
particles of Ͻ8 nm in diameter would show less linear and
larger magnetizations at 296 K, but for NiO particles of
larger than 30 nm, no finite-size-effect induced magnetism
could be observed. Although the grain size in our samples is
several tens of nanometers ͑see Table I͒, it is still too large to
result in ferromagnetism at room temperature. Kodama et
ture over the testing range from 78 K to room temperature,
which indicates that the Curie temperature is over the room
temperature. However, the coercivity increases linearly when
the temperature decreases. Of interest to note is the hyster-
esis loop shift observed in the sample. The loop shift could
be enhanced greatly through decreasing temperature, reach-
ing a large amount of 307 Oe at 78 K as shown in Fig. 3͑b͒.
For the undoped NiO sample, it is antiferromagnetic be-
low the Neel temperature ͑573 K͒, which can be seen in Fig.
1
7
al. studied the hysteresis loops of NiO nanoparticles
31.5 nm in diameter͒ at 5 K. They found that even up to
0 kOe, the moment was unsaturated and the loop was open,
͑
7
which showed very different characteristics from our Fe-
doped NiO particles having the similar grain size. Moreover,
for all samples the average particle size obtained via SEM is
about 100–200 nm, suggesting that the grain aggregation ex-
ists, which might strongly weaken the finite size effect in our
samples.
2
and understood through the ordinary two sublattice model.
As doped by magnetic Fe ions, defects could be introduced
into NiO, i.e.,
NiO
•
Ni
Shown in Fig. 3͑a͒ is temperature dependence of the
Fe O ——→ 2Fe + VЉ + 3O
2
3
Ni
o
magnetization and coercivity of Ni0.98Fe O. The magneti-
0.02
zation of the sample is nearly independence of the tempera-
•
ϫ
Ni
Fe ↔ Fe + h.
Ni
Thus the magnetic order in NiO grains would be interrupted,
which causes relatively weak coupling between the sublat-
tices and gives rise to ferromagnetism by double exchange
through the introduced magnetic Fe ions and free charge
carriers.
The ferromagnetism observed in the Fe-doped NiO
could be associated with the ferromagnetic clusters in the
samples. The ideal situation is that the doped Fe ions dis-
perse uniformly in the NiO matrix. But for most cases there
exists some composition inhomogeneity due to the limitation
of the sample preparation process. Thus regions of both rich
and lacking of Fe ions would co-exist, corresponding to the
ferromagnetic and antiferromagnetic regions, respectively.
2
0,21
Meiklejohn and Bean
had discovered the exchange an-
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FIG. 2. Hysteresis loops of Ni Fe O samples measured at 298 K.
isotropy and referred it to the properties of exchange coupled
1
−x
x
1
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202501-3
Wang et al.
Appl. Phys. Lett. 87, 202501 ͑2005͒
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magnetic and anti ferromagnetic regions is weakened by
thermal disturbance and thus the hysteresis loop is symmetri-
cal. As the temperature decreases, the exchange interaction
between the above two types of regions becomes stronger,
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1