The enhanced magnetic properties of Ni nanocrystallites induced by the coupling between Ni/NiO interfaces

Article abstract of DOI:10.1016/j.jallcom.2006.01.012

A composite Ni/NiO ultrafine and homogeneous nanocrystallites (NCs) system has been prepared by vacuum thermolysis of coprecipitated product from NiCl₂ and NaOH aqueous solution in the presence of polyvinylpyrrolidone (PVP). It is interesting to note that high magnetic properties have been achieved in this investigated system, including an exchange bias field H_E of 180.4 kA/m and a high coercivity H_C of 477.9 kA/m at 4.2 K which is one order larger than the bulk Ni or 10 nm pure Ni crystallite systems. These results can be attributed to the antiferromagnetic (AFM) pining effect from AFM NiO to ferromagnetic (FM) Ni.

Full text of DOI:10.1016/j.jallcom.2006.01.012

Journal of Alloys and Compounds 425 (2006) 1–3
The enhanced magnetic properties of Ni nanocrystallites induced by the
coupling between Ni/NiO interfaces
Shandong Lia,∗, Meimei Liu , Hong Bi , Liya Lu ,
a
b
c
¨
Wenqin Zouc, Zhigao Huanga, Youwei Duc
a Department of Physics, Fujian Normal University, Shangsan Road 8#, Fuzhou 350007, China
b College of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, China
c National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University, Nanjing 210093, China
Received 27 November 2005; received in revised form 4 January 2006; accepted 5 January 2006
Available online 10 February 2006
Abstract
A composite Ni/NiO ultrafine and homogeneous nanocrystallites (NCs) system has been prepared by vacuum thermolysis of coprecipitated
product from NiCl2 and NaOH aqueous solution in the presence of polyvinylpyrrolidone (PVP). It is interesting to note that high magnetic properties
have been achieved in this investigated system, including an exchange bias field HE of 180.4 kA/m and a high coercivity HC of 477.9 kA/m at 4.2 K
which is one order larger than the bulk Ni or 10 nm pure Ni crystallite systems. These results can be attributed to the antiferromagnetic (AFM)
pining effect from AFM NiO to ferromagnetic (FM) Ni.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Exchange coupling; Nanocrystallites; Exchange bias; Magnetic recording media
1. Introduction
TB [9], and loop squareness in FM embedded in or grown on
an AFM film [10,11]. With the right choice of FM and AFM
components, exchange anisotropy coupling could ultimately
allow magnetically stable crystallites only few nanometers in
size: such crystallites would surpass the storage-density goal of
1 Tbit/in.2, as set by the magnetic storage industry.
In this paper, a nanocomposite Ni/NiO system with a dra-
matically high coercivity and exchange bias at 4.2 K has been
prepared. The mechanism of those results was discussed in terms
of exchange coupling between Ni and NiO interfaces.
Interest in magnetic nanocrystallites has increased in the past
few years by virtue of their potential for applications in fields
such as ultrahigh-density magnetic storage media, medicine and
nanoscale building blocks for building advanced materials by
self-assembly [1–6]. In the field of magnetic recording, decreas-
ing the crystal size is an effective way to increase recording
density. However, when the crystal size of NCs is smaller than
a critical size, NCs become superparamagnetic (i.e. superpara-
magnetic limit, SPL). At this time, the magnetic storage media
composed of NCs is not a reliable material for recording any
more.
2. Experimental procedure
A typical synthesis procedure of nanocomposite Ni/NiO system is as
follows: 10 g NiCl2·6H2O was dissolved in 200 ml H2O in the presence
of polyvinylpyrrolidone (PVP, Aldrich Chemicals, average molecular weight
40,000–70,000) at room temperature, then 3.4 g NaOH was added into the solu-
tion under stirring, green precipitate Ni(OH)2 rapidly produced, then it was
filtered, washed by distilled water twice and then dried in air. Then the green
Ni(OH)2 powderwasannealedat573 Kfor2.5 hinairand3.0 hinvacuum(back-
ground pressure less than 2 × 10−3 Pa), respectively. The magnetic properties
of nanocomposite Ni/NiO system were measured by use of vibrating sample
magnetization (VSM) from 300 to 4.2 K. The microstructure is characterized
by X-ray diffraction (XRD), using Cu K␣ radiation, and transmission electron
microscope (TEM).
Recently, much attention has been focused on a novel method
to beat the SPL with exchange bias. It was reported that magnetic
exchange coupling induced at the interface between FM and
AFM systems can provide an extra source of anisotropy, leading
to magnetization stability [7], an enhancement of coercivity [8],
as well as a considerable increase of NCs blocking temperature
∗
Corresponding author. Tel.: +86 591 28198956; fax: +86 591 83165313.
E-mail address: dylsd007@yahoo.com.cn (S. Li).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.01.012

2
S. Li et al. / Journal of Alloys and Compounds 425 (2006) 1–3
It is should be mentioned here that carbon, some carbides or polymer decom-
position residues may be present in the NCs. In order to rule out the effect of
these impurities on the magnetic properties, clean crystallites are required. Thus,
the sample was repeatedly washed by acid and alkali aqueous solution for more
than six times until no impurities were detected by XRD, TEM and the Fourier
transition infrared spectroscopy (FTIR). Thus, it can be thought that clean NCs
are obtained.
3. Results and discussion
It is well known that PVP is widely used as a protective
reagent in preparing well-defined inorganic NCs, and in most
cases, the use of PVP has promoted an increase in surface area
and a decrease in crystal size [12]. It has been reported that the
synthesis of well-dispersed composite Ni/NiO NCs with con-
trolled size has been achieved in the presence of PVP, using
biscycloocta-1,5-diene nickel as a precursor, upon controlling
the water and dioxygen content of the reacting medium [13]. In
this paper, two kinds of annealing circumstances (i.e. in air and
vacuum) have been employed. Fig. 1 shows the XRD patterns
for the precursor annealed in air and vacuum, respectively. It has
been found that the sample annealed in air is composed of pure
NiO phase, while the one annealed in vacuum is composed of
NiO and two types of Ni crystallites (i.e. fcc-Ni and hcp-Ni).
It should be mentioned here that hcp-Ni is stable at low tem-
perature, undergoing a transition to a fcc-Ni phase above 653 K
[13].
Egger et al. have studied the thermal degradation of hybrid
PVP-YSZ (yttria-stabilized zirconia) in Ar and in air, respec-
tively [14]. They found that the combustion and the organics
removal start at 513 K during the air treatment, while the burning
out of the organics will be accomplished at higher tempera-
tures by annealing in Ar. Moreover, newly derived aromatic
compounds (␯C C), which are reductive, are visible starting
at 573 K. Similarly, in our case, when annealing at 573 K for
2.5 h in air, the protective PVP layer has burnt out and NiO
NCs were certainly obtained. However, when annealing in vac-
uum circumstance for 3.0 h, partial Ni(II) can be reduced to
Ni(0) by C C bonds arising from the decomposition of aro-
matic compounds of PVP, in the absence of oxygen. This can
Fig. 2. TEM image (a) and electron diffraction pattern (b) of the sample NNO.
be inversely demonstrated by our previous work [15], where the
sample was fabricated under the condition of PVP-free. Not any
trace of Ni was observed in that sample although annealing in
vacuum.
In this paper, we emphasize on the composite Ni/NiO system
(referred to as NNO) due to some significant properties exhib-
ited in this system. Using Scherrer equation, the average size of
9.8 nm for NiO and 16.6 nm for fcc-Ni, respectively, has been
evaluated for sample NNO (see Fig. 1(b)). The microstructure
of NNO was also observed by TEM. Shown in Fig. 2 are the
TEM image and its electron diffraction pattern. From Fig. 2(a),
it can be seen that most of the particles are spherical within a nar-
row size range from 10 to 20 nm, except for a few stick-shaped
particles in excess of 50 nm. Fig. 2(b) shows that the sample
is composed of hcp-Ni, fcc-Ni and NiO. Those results agree
satisfactorily with the XRD results.
The field cooling (FC) hysteresis loop of NNO has been
measured by VSM at 4.2 K under a cooling field of 1 T, shown
in Fig. 3. As illustrated in this figure, the sample exhibits a
very large coercivity of 477.9 kA/m and an exchange bias field
of 180.4 kA/m at low temperature. It is interesting to note
that co-existence of an exchange bias HE and a very large
coercivity HC in the investigated system. Moreover, HC is
more than one order larger than that for bulk Ni or pure Ni
crystallites with about 10 nm in size, reported by Sun and Dong
[16]. Exchange bias at FM/AFM interface is characterized
by coercivity enhancement and hysteresis loop shift [17–19].
Fig. 1. XRD patterns of the samples annealed in air (a) and vacuum (b), respec-
tively.

S. Li et al. / Journal of Alloys and Compounds 425 (2006) 1–3
3
increasing the density of magnetic recording media by FM/AFM
coupling regime.
4. Conclusions
The microstructure and magnetic properties of an ultra-
fine nanocomposite Ni/NiO system have been reported. The
enhanced coercivity and exchange bias field are attributed to the
exchange coupling at the interface between FM Ni and AFM
NiO.
Acknowledgements
This work was financially supported by NSFC (Grant No.
10504010) and Science Foundation of Fujian Province (Grant
No. 2005J023).
Fig. 3. The FC hysteresis loop of NNO at 4.2 K under a cooling field of 1 T.
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