APPLIED PHYSICS LETTERS 94, 153105 ͑2009͒
Masataka Hakamada,1,a͒ Masaki Takahashi,2 Toshiyuki Furukawa,2 and Mamoru Mabuchi2
1Materials Research Institute for Sustainable Development, National Institute of Advanced Industrial
Science and Technology (AIST), 2266-98 Anagahora, Shimo-shidami, Moriyama, Nagoya 463-8560, Japan
2Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University,
Yoshidahonmachi, Sakyo, Kyoto 606-8501, Japan
͑Received 27 February 2009; accepted 26 March 2009; published online 14 April 2009͒
Nanoporous Ni specimens with the ligament lengths of 10–210 nm were produced by the dealloying
of Ni0.25Mn0.75 alloy and annealing at 473–873 K. The coercivity ͑Hc͒ increased with increasing
ligament length ͑L͒ up to 50 nm, and decreased with increasing L above 50 nm. At LϽ50 nm, the
size dependence of Hc for nanoporous Ni is lower than that for nanocrystalline Ni. The low size
dependence of Hc is discussed on basis of a random anisotropy model. © 2009 American Institute
Recently, many research studies on the preparation, char-
acterization, properties, and applications of nanosized mate-
rials in various fields have been performed because of the
unique electronic, optical, magnetic, and chemical properties
of nanosized materials that are greatly different from those of
unique magnetic properties, namely, magnetic nanoparticles
crystalline metals have a very low coercivity.9,10
Nanoporous metals have been developed by the selective
Ni exhibits a higher coercivity and a lower saturation mag-
netization than bulk Ni. However, the magnetic properties of
the nanoporous metals have not been understood sufficiently.
The ligament lengths or pore sizes of the nanoporous metals
nanoporous Ni specimens with the ligament lengths of 10–
210 nm are produced by the dealloying of Ni0.25Mn0.75 alloy
and annealing, and the relationship between coercivity and
ligament length is investigated.
Commercially available Ni ͑Ͼ99.9%͒ and Mn
͑Ͼ99.9%͒ ingots were melted together by arc melting under
Ar atmosphere to prepare a precursor Ni0.25Mn0.75 alloy in-
got. Subsequently, the alloy ingot was annealed at 1173 K for
24 h, quenched in water, and ground to remove surface ox-
ides. After cold rolling of the alloy, nanoporous Ni was pro-
duced by the selective electrochemical dealloying. A three-
electrode electrochemical cell controlled by a potentiostat
͑HZ-5000 by Hokuto Denko͒ was used. Dealloying was car-
ried out at room temperature using a platinum electrode as a
counter electrode and 1 mol/l ͑NH4͒2SO4 as an electrolyte.
The alloy was held at an applied electrochemical potential of
Ϫ650 mV ͑versus a saturated calomel electrode͒ for 24 h
until no current was detected.
processed specimen revealed that the specimen is fcc nickel.
XRD patterns of the samples indicated crystal face orienta-
samples were mechanically pulverized to powdery form for
randomization of the orientation before the magnetization
hysteresis measurement. To vary the ligament length, anneal-
ing was carried at 473, 573, 673, 773, and 873 K for 15 min
in an inert gas. The ligament length was determined to be
11 nm for the specimen annealed at 473 K, 18 nm for the
specimen annealed at 573 K, 33 nm for the specimen an-
nealed at 673 K, 79 nm for the specimen annealed at 773 K,
and 210 nm for the specimen annealed at 873 K, where more
than 100 ligaments are measured by scanning electron mi-
croscopy observations.
Magnetization hysteresis loops were recorded at room
temperature using a vibrating sample magnetometer, and the
coercivity of nanoporous Ni was investigated, as exemplified
in Fig. 2͑a͒. Figure 2͑b͒ shows the relationship between the
coercivity and ligament length of the nanoporous Ni. Note
that the coercivity ͑Hc͒ increases with increasing ligament
length ͑L͒ up to 50 nm, and decreases with increasing L
above 50 nm. This size dependence of coercivity for the
nanocrystalline Ni.10 The energy of a magnetic particle is
proportional to its size or volume via the number of magnetic
molecules in a single magnetic domain. When the energy is
comparable to the thermal energy, thermal fluctuations sig-
nificantly reduce the total magnetic moment. This is respon-
sible for the low coercivity of nanoparticles because the di-
50
(a)
(b)
Average ligament length = 10 nm
Standard deviation = 3.0 nm
40
30
20
10
0
50 nm
Figure 1 shows a scanning electron micrograph ͑a͒ and
the ligament length distribution ͑b͒ of an as-processed nano-
porous Ni specimen. The average ligament length and stan-
dard deviation of this specimen were 10 and 3.0 nm, respec-
tively. The x-ray diffraction ͑XRD͒ measurement on the as-
10
20
Ligament length (nm)
FIG. 1. Scanning electron micrograph ͑a͒ and ligament length distribution
͑b͒ of as-processed nanoporous Ni. The nanoporous Ni is produced by the
dealloying of Ni0.25Mn0.75 alloy. The average ligament length of the as-
processed nanoporous Ni is 10 nm.
a͒
Electronic mail: masataka-hakamada@aist.go.jp.
0003-6951/2009/94͑15͒/153105/3/$25.00
94, 153105-1
© 2009 American Institute of Physics
130.217.227.3 On: Wed, 09 Jul 2014 20:33:19