Highly crystalline cobalt nanowires with high coercivity prepared by soft chemistry

Article abstract of DOI:10.1002/pssa.200881260

Cobalt natiorods and wires were prepared by reduction of a cobalt salt in a liquid polyol. These particles crystallize with the hcp structure and the growth axis is parallel to the crystal-lographic c-axis. The kinetic control of the growth allows to vary

Full text of DOI:10.1002/pssa.200881260

Phys. Status Solidi A 206, No. 4, 663–666 (2009) / DOI 10.1002/pssa.200881260
a
p s s
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applications and materials science
Highly crystalline cobalt nanowires
with high coercivity prepared by soft chemistry
G. Viau*, 1, C. García1, T. Maurer2, G. Chaboussant2, F. Ott2, Y. Soumare3, and J.-Y. Piquemal3
1
Université de Toulouse, INSA, LPCNO, UMR CNRS 5215, 135, avenue de Rangueil, 31077 Toulouse Cedex 4, France
2
Laboratoire Léon Brillouin, CEA/CNRS UMR12, Centre d’Etudes de Saclay, 91191 Gif sur Yvette, France
3
ITODYS, Université Paris 7 Denis Diderot – UMR CNRS 7086, 2, place Jussieu, 75251 Paris Cedex 5, France
Received 9 May 2008, revised 20 October 2008, accepted 20 October 2008
Published online 2 February 2009
PACS 75.20.En, 75.30.Gw, 75.50.Tt, 75.50.Vv, 75.75.+a
*
Corresponding author: e-mail guillaume.viau@insa-toulouse.fr, Phone: +33 561 559 663, Fax: +33 561 559 697
Cobalt nanorods and wires were prepared by reduction of a oriented wires present very high coercivity (up to 9 kOe) re-
cobalt salt in a liquid polyol. These particles crystallize with sulting from both a high shape anisotropy and the high mag-
the hcp structure and the growth axis is parallel to the crystal- netocrystalline anisotropy of the hcp cobalt. Micromagnetic
lographic c-axis. The kinetic control of the growth allows to simulations showed that the magnetization reversal is shape
vary the mean diameter of the rods and their aspect ratio. dependent. The conical tips of the dumbbell particles strongly
Dumbbell like shape particles consisting of a central rod with contribute to the coercivity decrease and must be precluded
two conical tips were also obtained. Magnetization curves of for permanent magnet applications.
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Magnetic nanowires and nanorods depending on the growth conditions [1]. The experimental
present several interests in the field of high density mag- protocol is very simple: the cobalt precursor is dissolved in
netic recording [1], spintronics and microwave applications a sodium hydroxide solution of polyol (cobalt concentra-
[2] and can be new building blocks for permanent magnets tion of 0.08 M). A small amount of ruthenium chloride
[3]. Several ways have been developed for the preparation (RuCl ) is added to seed the medium (the Ru/Co molar ra-
3
of such objects: the most studied consists in the electro- tio is fixed equal to 2.5%). The mixture is then heated to
chemical deposition using porous polycarbonate mem- 170 °C with a controlled temperature ramp. The solution
branes or anodic aluminium oxide as template [4]. Liquid turns to black progressively when the reduction occurs. Af-
phase syntheses like organometallic chemistry or reduction ter 30 min at 170 °C the reduction is complete and the so-
in a polar solvent are an alternative way presenting several lution is cooled down. The magnetic powder is recovered
advantages. They provide high yield and can easily be by centrifugation, washed with absolute ethanol and dried
scaled-up to produce large quantities of nanoparticles. under vacuum. In order to preserve the particles from oxi-
More they have the interest to form highly crystalline par- dation they are kept in the polyol solution. The same pro-
ticles. In this paper we present results on the synthesis of cedure is also used to synthesize bimetallic cobalt-nickel
magnetic rods and wires by the polyol process. We show particles. In that case a mixture of cobalt and nickel carb-
that parallel assemblies of these nano-objects present hard oxylates in the desired proportion is reduced in the liquid
magnetic properties. Micromagnetic simulations are also polyol. The cobalt/nickel ratio measured by EDX in the fi-
very useful to optimize the particle shape in order to im- nal metal particles is very close to the ratio in the starting
prove the magnetic properties.
medium.
Various metal carboxylates M(C H CO ) can be
n
2n+1
2 2
2 Co-based nanowires synthesis
used as metal precursor, from the simplest acetate (n = 1)
2.1 Experimental The reduction of a cobalt carboxy- to long chain carboxylate like laurate (n = 11) or palmitate
late in sodium hydroxide solution in 1,2 butanediol leads to (n = 15). The basicity, the carboxylate chain length and the
cobalt metal particles presenting a great variety of shape slope of the temperature ramp have a strong influence on
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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G. Viau et al.: Highly crystalline cobalt nanowires with high coercivity
–1
the reaction rate on which it is possible to act in order to ramp of 5 °C min . Electron microscope images of such
modify the metal particle shape.
cobalt nanorods are presented Fig. 1. High resolution
transmission electron microscopy (HRTEM) showed that
these rods crystallize with the hexagonal structure and that
the growth axis is the c-axis of the hcp phase (inset Fig. 1a).
We proposed recently that the cobalt particle shape was
tuned via the kinetic control of the growth step of the hcp
phase [5, 6]. The aspect ratio (mean length/mean diameter)
can be modified by varying the experimental parameters
that control the growth rate: the basicity of the medium, the
cobalt precursors and the temperature. When the growth
rate is slowed down either by increasing the basicity, by
increasing the carboxylate chain length or by decreasing
the temperature ramp the growth perpendicular to the
c-axis is favored. At the opposite when the rate is high
enough the growth develops along the c-axis. The particle
2.2 Shape and structure characterizations Co-
balt rods were obtained by reduction of cobalt laurate in
basic solution of 1,2 butanediol when the NaOH concentra-
tion was in the range 0.02–0.1 M and with a temperature
(a)
mean diameter (d ) could be varied in the range 8–35 nm,
m
the mean length (L ) in the range 100–350 nm and the re-
m
sulting aspect ratio from 4 to 30. In some cases dumbbell
shape and diabolo like particles are formed (Fig. 1c) result-
ing from a steady decreasing of the growth rate during the
reaction. Reduction of mixture of cobalt and nickel acetate
with the molar ratio 80/20 allowed also to synthesize bi-
metallic Co Ni nanowires with a small mean diameter
c axis
80
20
(Fig. 1d). These wires present generally two hexagonal
heads located at the extremities that are slightly richer in
nickel with respect to the global composition [5].
(b)
3 Magnetic properties
3.1 Magnetization curves The cobalt-based rods
and wires are ferromagnetic at room temperature. The satu-
ration magnetization per gram of the dried powders
reaches generally between 50% and 70% of the bulk value.
These values result from the superficial oxide layer (pres-
ence of CoO was inferred from X-ray diffraction and
100 nm
(d)
(c)
200 nm
200 nm
Figure 1 (a) TEM image of cobalt wires prepared by reduc-
tion of cobalt laurate in a basic solution of 1,2 butanediol
(Lm = 100 nm, dm = 12.5 nm); Inset: high resolution image of a
cobalt wire in the [2110] zone axis; (b) SEM image of cobalt
wire assembly deposited on Si substrate under a magnetic field;
(c) cobalt dumbbell shape particles; (d) Co Ni nanowires pre-
Figure 2 Magnetization curves of cobalt nanorods (Lm = 100 nm;
dm = 12.5 nm), (a) deposited on a flat substrate with an external
magnetic field (T = 300 K); (b) frozen in toluene under an exter-
nal magnetic field (T = 150 K); (c) simulation of the frozen sam-
ple magnetization curve using the Stoner–Wolfarth model for an
assembly of non-interacting wires.
80
20
pared by reduction of a mixture of cobalt and nickel acetates in a
basic solution of 1,2 butanediol (Lm = 250 nm, dm = 7.5 nm).
© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
Paper
Phys. Status Solidi A 206, No. 4 (2009)
665
Table 1 Coercivity and remanence to saturation ratio of Co rods
(Lm = 100 nm; dm = 12.5 nm) and Co Ni wires (Lm = 240 nm;
HRTEM) and from the presence of organic matter remain-
ing in the powders.
80
20
dm = 7 nm).
The coercivity and the remanence of the magnetization
curves depend on the particle shape and on the orientation
of the particles with respect to the magnetic field. Figure
1b illustrates well that the wires can be aligned with an ex-
ternal magnetic field. The magnetization curve of cobalt
rods deposited on a flat substrate with an external magnetic
field applied during the evaporation of the solvent was
pressed powder
(T = 300 K)
frozen suspension
(T = 140 K)
composition
Hc(Oe)
Co Ni
Co
Co
Co Ni
80 20
80 20
3.6
0.64
4.5
8.9
6.5
0.94
Mr/Ms
0.57
0.95
measured at 300 K. A coercivity H = 5.5 kOe and a rema-
c
nence to saturation ratio M /M = 0.81 were obtained
r s
branes [3]. We think that it is the result of a very good
crystallinity of the wires grown by our chemical process.
In order to increase the rod orientation, a suspension in
toluene was cooled under a magnetic field of 50 kOe
and frozen at 140 K. A square-shaped hysteresis with a
(Fig. 2a). Although the orientation of the wires was
only partial as revealed by the M /M ratio, the coercivity
r s
value is significantly higher than those reported in the
literature for iron or cobalt nanowires grown by electro-
deposition in the one dimensional pores of alumina mem-
(a)
(b)
(c)
(d)
Figure 3 (online colour at: www.pss-a.com) (a) A model of the small Co dumbbells of Fig. 1c. (b) The micromagnetic state just be-
fore reversal in a dumbbell like Fig. 1c: a symmetry plane is created breaking the cylindrical symmetry. Inset: view along the rotation
axis. (c) Reversal in a dumbbell with tips 1.5 times bigger: a vortex state forms before reversal and the cylindrical symmetry is pre-
served. (d) The micromagnetic state before reversal for a string of particles (separated by 2 nm). The reduction of the stray field has lit-
tle effect on the reversal process.
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a
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G. Viau et al.: Highly crystalline cobalt nanowires with high coercivity
remanence to saturation ratio close to 0.95 was obtained
The very high values of coercivity and remanence
showing a much better wire alignment (Fig. 2b). The make the cobalt rods good candidates for the elaboration
increase of coercivity is also very important with the of permanent magnets. Dense arrays of these particles
parallel wire orientation reaching 9 kOe. The magnetiza- obtained by compaction may present high (BH) values.
max
tion curve at 140 K was fitted using the Stoner–Wolfarth This study shows that shapes such as those showed in
model for an assembly of elongated particles presenting Fig. 1c must be precluded in order to get high coercivity.
both a shape and a magnetocrystalline anisotropy with Chemical composition with high magnetization must also
a dispersion of the particle orientation with respect to be targeted to get both a high remanence and a high coer-
the applied magnetic field. More details are given in civity.
reference [3]. A standard deviation on the wire orientation
4 Conclusion We have showed that it is now possible
σ = 15° (HWHM),
θ
a
magnetocrystalline anisotropy
to synthesize by a liquid phase process cobalt and cobalt–
nickel nanowires with various diameter and length. The
aspect ratio is varied by acting upon the growth conditions.
These wires present a very high coercivity in comparison
with nanowires prepared by other routes. A diameter in the
nanometer range, a high aspect ratio and a high saturation
magnetization are required to get such coercivity. Magne-
tocrystalline anisotropy of cobalt and cobalt-nickel alloys
with hcp structure also contributes to the hard magnetic
behaviour. Compaction of such wires to shape dense mate-
rials could be a way to get permanent magnets. Some ques-
tions are still open. Higher saturation magnetization is ex-
pected to increase the coercivity of particles presenting
high shape anisotropy. Iron and iron–cobalt alloys would
be best candidates as building blocks for permanent mag-
net application. Nevertheless, the growth of anisotropic
field H = 4.2 kOe and a shape anisotropy contribution
MC
of H
= 4.7 kOe were deduced from the fit presented
shape
Fig. 2c.
The effect of the wire orientation was also measured on
Co Ni nanowires prepared by the polyol process [4].
80
20
Nevertheless, the coercivity measured on cobalt-nickel
wires was lower, reaching 6.5 kOe at 140 K and 3.6 kOe at
300 K on randomly oriented particles (Table 1). It is note-
worthy that the coercivity measured on the bimetallic wires
is lower despite a higher aspect ratio. The lower magneti-
zation of the cobalt–nickel alloys could be responsible of
this phenomenon. Indeed, the magnetic anisotropy related
to the particle shape increases with the saturation magneti-
zation (in the case of long ellipsoids, one would expect an
ideal coercive field of M /2).
s
3.2 Shape optimization The lower value of coer- nanoparticles with body centered cubic structure by a
civity can also be attributed to the particular shape of liquid phase process remains a challenge.
the CoNi particles that present conical heads. We have used
Acknowledgements Alain Mari (LCC Toulouse) is ac-
knowledged for the magnetic measurements. Vincent Collière is
the micromagnetic simulation package “Nsim” [7, 8] to per-
form 3D micromagnetic simulations of dumbbell cobalt par-
thanked for SEM images and Christophe Gatel for the HRTEM
ticles with different size of conical tips and different aspect
images. The authors acknowledge the Agence Nationale de la
ratio. A model of particle is presented in Fig. 3a.
Recherche for their financial support (project P-Nano MA-
In the case of dumbbells with dimensions similar to
GAFIL).
those of Fig. 1c, before reversal, a C-shaped tilt of the
magnetization appears in the tips (Fig. 3b). The coercivity
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© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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