Magnetic crossover of NiO nanocrystals at room temperature

Article abstract of DOI:10.1063/1.2357562

The nature of room-temperature ferromagnetism in diluted magnetic semiconductors, ZnO:M (M is Ni, Co, Fe, etc.), remains in debate, most likely because previous theoretical and experimental studies have excluded the magnetic contribution from secondary phases of transition metal oxide nanocrystals including NiO and CoO. In this work, the authors initiated a study on NiO nanocrystals that demonstrated room-temperature ferromagnetic behavior with relatively large coercive forces, in apparent contradiction to the previous conjectures in the literature. With size reduction, NiO nanocrystals showed an abnormal magnetic crossover, which is closely related to weakened superexchange interactions in their multisublattices.

Full text of DOI:10.1063/1.2357562

Magnetic crossover of NiO nanocrystals at room temperature
Liping Li, Lijuan Chen, Rima Qihe, and Guangshe Li
Citation: Applied Physics Letters 89, 134102 (2006); doi: 10.1063/1.2357562
View online: http://dx.doi.org/10.1063/1.2357562
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APPLIED PHYSICS LETTERS 89, 134102 ͑2006͒
Magnetic crossover of NiO nanocrystals at room temperature
Liping Li, Lijuan Chen, Rima Qihe, and Guangshe Lia͒
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China and Graduate School
of the Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China
͑Received 2 July 2006; accepted 3 August 2006; published online 25 September 2006͒
The nature of room-temperature ferromagnetism in diluted magnetic semiconductors, ZnO:M ͑M is
Ni, Co, Fe, etc.͒, remains in debate, most likely because previous theoretical and experimental
studies have excluded the magnetic contribution from secondary phases of transition metal oxide
nanocrystals including NiO and CoO. In this work, the authors initiated a study on NiO nanocrystals
that demonstrated room-temperature ferromagnetic behavior with relatively large coercive forces, in
apparent contradiction to the previous conjectures in the literature. With size reduction, NiO
nanocrystals showed an abnormal magnetic crossover, which is closely related to weakened
superexchange interactions in their multisublattices. © 2006 American Institute of Physics.
͓DOI: 10.1063/1.2357562͔
Spintronics based on diluted magnetic semiconductors
͑DMSs͒ provides a technology that transforms reading and
writing information for many uses by spin rather than by
electron charge.1,2 The practical applications of spintronic
devices are, however, strongly challenged by low reproduc-
ibility of room-temperature ferromagnetism of less than 10%
for typical DMSs.3 For example, although ZnO doped with
transition metals ͓e.g., Ni2+,4,5 Co2+,6,7 and Fe2+ ͑Ref. 8͔͒ is
viewed as one of the promising DMSs because of its room-
temperature ferromagnetism, Ueda et al.3 did not observe
ferromagnetic behavior in Ni2+ doped ZnO, which is in ap-
parent contradiction to the theoretical prediction for
3d-transition-metal-doped ZnO.9 The low reproducibility and
controversial observations raise concerns over the origins of
room-temperature ferromagnetism in DMSs.
To understand the origin of room-temperature ferromag-
netism in DMSs, extensive investigations have been con-
duced on the fabrication of transition metal doped ZnO nano-
crystals and thin films using solution chemistries,4 vapor
phase depositions,10͑a͒ and ion implantation.10͑b͒ The ferro-
magnetism observed in some of these materials was ex-
plained by researchers in terms of the carrier-induced ferro-
magnetism, double exchange mechanism, or micro-metallic-
clusters, while the magnetic contributions from the
secondary phases of component transition metal oxide nano-
crystals were always excluded as a likely source under the
consideration that ͑1͒ the component oxides ͑e.g., NiO, CoO͒
were generally antiferromagnetic and that ͑2͒ most of the
nanoscale counterparts would only exhibit ferromagneticlike
behavior at very low temperatures11 such as at 4.2 K, due to
the low Curie temperature ͑e.g., TCϽ5 K for 3.1 nm NiO͒.12
These explanations lack evidence and should be critically
examined, because the simple addition of transition metal
ions to a reaction system does not necessarily result in incor-
poration of dopants into the ZnO lattice. Instead, the driving
force for the phase segregation into NiO and ZnO is very
large13 during the preparation of Ni2+:ZnO. Since phase seg-
regation is thermodynamically favored to most likely yield
nanoscale secondary phases at different physical dimensions
of the component oxides,14 fundamental changes in the mag-
netic order are reasonable to occur throughout the particles
owing to the uncompensated spins on the surfaces of
nanocrystals.15 On the other hand, magnetic data for transi-
tion metal oxide nanocrystals ͑e.g., NiO͒ are strikingly simi-
lar to those for thin films.15͑b͒ As a consequence, experimen-
tal verification of room-temperature magnetic properties of
the transition metal oxide nanocrystals as a function of their
physical dimension can be expected to have many implica-
tions in developing an understanding of the origins of room-
temperature ferromagnetism in DMS nanostructures.
In this letter, we initiated a systematic study of particle
size dependence of room-temperature magnetic properties of
NiO nanocrystals. NiO was chosen as the target material
because it has lattice and magnetic structural similarities to
that of transition metal oxides such as CoO, FeO, and MnO
͑Ref. 16͒ that could be possible component oxides in DMSs
of doped ZnO. We found that with a size reduction of NiO
nanocrystals, a maximum coercivity force of 933 Oe was
obtained, which is strikingly much larger than 100–126 Oe
reported in literature17 for Ni2+:ZnO nanocrystalline thin
films and aggregates. This discovery unambiguously demon-
strates a pronounced magnetic contribution from the phase
segregation of room-temperature ferromagnetism of DMSs.
High quality NiO nanocrystals were prepared by a
chemical method. A 300 ml ammonia solution and a 200 ml
NiSO4·6H2O solution with fixed concentration of 0.5 mol/l
were fully mixed at ambient temperature, into which 50 ml
NaOH solution of 5 mol/l was slowly added to give a green
precipitate of Ni͑OH͒2. This precipitate was kept at ambient
condition for 10 h and washed sufficiently with distilled wa-
ter. NiO nanocrystals with different sizes were obtained by
carefully annealing the precipitate in air at given tempera-
tures for 3 h. The structures of all samples were character-
ized by powder x-ray diffraction with a Cu K␣ radiation. The
average grain size was calculated by the Scherrer formula.
The magnetization data of the samples were collected using a
Quantum Design PPMS-7 apparatus.
All samples exhibited a highly crystalline fcc NaCl-type
structure with x-ray diffraction ͑XRD͒ patterns that were es-
sentially identical to the standard data for bulk NiO.18 As
indicated in Fig. 1, no other crystalline phases were detected.
a͒
Author to whom correspondence should be addressed; electronic mail:
guangshe@fjirsm.ac.cn
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134102-2
Li et al.
Appl. Phys. Lett. 89, 134102 ͑2006͒
FIG. 3. Magnetic field dependence of magnetization for 7.1 and 24 nm NiO
measured at room temperature. The inset is the enlarged M-H curves show-
ing open hysteresis loops.
FIG. 1. XRD pattern for 41.3 nm NiO obtained after annealing treatment at
800 °C for 3 h. The inset shows the enlarged diffraction line ͑200͒ for NiO
nanocrystals obtained at given temperatures.
Magnetic field dependence of magnetization for 7.1 and
24 nm NiO measured at room temperature is illustrated in
Fig. 3. All samples exhibited open hysteresis loops. The satu-
ration magnetization was not reached even at a maximum
applied field of 40 kOe. These observations are consistent
with the presence of an antiferromagnetic particle core that is
superimposed to a net moment due to the intrinsic weak
ferromagnetism and to the nanoscale uncompensated mo-
ment at room temperature. The magnetic parameters were
found to be strongly dependent on the particle sizes. For NiO
nanocrystals at DϽ50 nm, the coercivity Hc deviated from
zero, which compares to the antiferromagnetic phase of bulk
NiO. When the particle size decreased from 50 to 24 nm, the
coercivity force increased readily to reach a maximum of
Hc=933 Oe. Further size reduction below D=24 nm led to a
sharp decrease in coercivity force down to 337 Oe for D
=7.1 nm. Even so, all of these room-temperature coercivity
forces are much larger than Hc=193.6 Oe reported for
2–3 nm NiO,20 or than 100–126 Oe observed for Ni2+
doped ZnO nanocrystalline films and aggregates.17 These
findings are fundamentally important in dealing with the
controversies about the contributions of secondary phases of
component transition metal oxides ͑e.g., NiO͒ to the room-
temperature ferromagnetism of DMSs that still remain. It is
also noted that this seems to be the only example of a room-
temperature magnetic crossover in 3d-transition-metal-oxide
nanocrystals.
The inset of Fig. 1 shows enlarged diffraction peaks ͑200͒ of
the samples. It is clear that for increasing the annealing tem-
perature from 350 to 800 °C, the linewidth for peak ͑200͒
became narrower. The XRD linewidth for the sample after
thermal treatment at 350°C corresponded to an average
nanocrystal diameter of about 7.1 nm. The particle size in-
creased up to 41.3 nm after annealing at 800 °C for 3 h.
From the inset of Fig. 1, it is also noted that the diffraction
peaks showed a systematic shift, which is associated with the
occurrence of lattice variations at small sizes.
The relationship between lattice volume and particle size
is shown in Fig. 2. The lattice volume remained almost con-
stant at Vb=72.85ϫ10−3 nm3 when the particle size D was
larger than 30 nm. A size reduction to DϽ30 nm led to a
significant lattice expansion that could be precisely described
by the following relationship:
C
⌬V = V − V0 =
,
D
where V0 ͓=72.71͑1͒ϫ10−3 nm3͔ denotes the lattice volume
of an imaginary lattice at infinitely large particle size and C
͓=4.2͑1͒ϫ10−3 nm4͔ is a constant. This relationship gives a
critical size Dc of about 30 nm when assuming that at this
point, the lattice volume becomes constant as that for bulk
NiO. The lattice expansion indicated in Fig. 2 corresponds to
a negative pressure that was probably produced by the strong
repulsive interactions of the parallel surface defect dipoles at
small particle sizes. Similar nanoscale lattice modifications
have been observed by Li et al. in rutile TiO2 nanocrystals.19
The surface-driven spin rearrangements associated with these
surface defect dipoles are thus expected to yield fundamental
changes in the magnetic properties of NiO nanocrystals.
What, then, is the cause of the crossover in magnetic
properties of NiO nanocrystals at room temperature? The
magnetic moments of nanocrystals have already been ex-
plained in terms of the uncompensated surface spins,21,22
which are, however, not fully responsible for the present
magnetic crossover ͑Fig. 4͒. We suppose that there are three
possible mechanisms of the abnormal magnetic crossover in
FIG. 2. Size dependence of lattice volume for NiO nanocrystals. As indi-
cated by the solid line, all data points were fitted in terms of ⌬V=V−V0
=C/D, where V0 ͓=72.71͑1͒ϫ10−3 nm3͔ is the imaginary bulk lattice infi-
FIG. 4. Size dependence of the room-temperature coercivity for NiO nano-
crystals. Reference data ͑Refs. 20 and 23͒ were also given for comparison.
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nitely large. Dc and Vc denote the critical size and lattice volume.
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134102-3
Li et al.
Appl. Phys. Lett. 89, 134102 ͑2006͒
NiO nanocrystals. The first mechanism of the abnormal mag-
netic crossover is the transition of double sublattice to mul-
tisublattice states. The second mechanism is related to the
numbers of uncompensated spins from surface and particle
core. The third mechanism is associated with the lattice ex-
pansion and the enhanced surface anisotropy that might play
a key role in the interactions between the uncompensated
surfaces and particle core spins. The first mechanism could
explain the results for Hcꢀ0 at physical dimension smaller
than 50 nm, since the double-sublattice magnetic structure in
bulk NiO corresponds to antiparallel moments ͑as indicated
by zero Hc for 59 nm NiO͒.23 When the particle size of ma-
terials in this work was reduced to the nanoscale regime, this
double-sublattice magnetic structure probably partially trans-
forms into multisublattice magnetic ones with uncompen-
sated spins to show ferromagnetism. The intrinsic magnetic
moments for NiO nanocrystals experimentally estimated by
extrapolation of linear parts of magnetization curves at ex-
tremely low temperatures are generally much larger than the
values predicted by a double-sublattice model,15 which pro-
vides evidence for these assumptions.
On the other hand, the multisublattice states could be-
come prevalent at high temperatures above 300 K.24 There-
fore, the presence of multisublattice magnetic structure in
these NiO nanostructures with the number of the magnetic
sublattice being increased with size reduction15 is the domi-
nant cause for the room-temperature ferromagnetism for D
Ͻ50 nm. Furthermore, according to theoretical work on
equilibrium spin configurations in NiO nanostructures,15 the
multisublattice spin configuration directly follows from the
exchange interactions between the magnetic ions and the low
coordination at the surface sites of nanostructures. The in-
creased numbers of the surface spins and magnetic multi-
sublattices as size reduces could give rise to enhanced net
moments of uncompensated core spins for a significant in-
crease in coercivity force. However, further size reduction
below 30 nm led to a pronounced lattice expansion ͑Fig. 2͒
in the presence of many broken bonds and defects as well as
disordered distribution of uncompensated spins. Such a lat-
tice expansion is most likely to give an increased bond
length of Ni–O by significantly weakening the strength of
superexchange interactions of magnetic ions in the multisu-
blattice. Consequently, the superexchange interactions could
have been partially balanced by the surface anisotropy in
yielding a significant decrease in the net moments of the
surface spins. All these mechanisms help explain the cross-
over of magnetic properties of NiO nanocrystals at room
temperature.
demonstrated to occur in NiO nanocrystals at room tempera-
ture, which is closely related to the lattice expansion and the
resulting weakened strength of superexchange interactions in
multisublattices. These results show a new aspect of room-
temperature ferromagnetism in nanoscale DMSs and have
the exciting possibility of structurally controlling magnetic
properties for broad technological applications.
This work was financially supported by a science and
technology program from Fujian Province ͑Nos.
2004HZ01-1 and 2005HZ01-1͒ and a grant from Hundreds
Youth Talents Program of CAS ͑G. L.͒. The authors thank
Richard Lee Smith, Jr. ͑Tohoku University, Japan͒ for read-
ing this letter and for many discussions.
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behavior at room temperature at particle sizes smaller than
50 nm, which is in contradiction to the antiferromagnetic
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