Magnetic field-induced growth and self-assembly of cobalt

nanocrystallites

Helin Niu,a,b Qianwang Chen,*a,b Hongfei Zhu,a,b Yushun Linb and Xing Zhangb

aThe Structure Research Laboratory, University of Science & Technology of China, Hefei230026, China. E-mail: cqw@ustc.edu.cn; Fax: 186-551-3607292; Tel: 186-551-3607292

bDepartment of Materials Science & Engineering, University of Science & Technology ofChina, Hefei 230026, China

Received 18th March 2003, Accepted 19th May 2003

First published as an Advance Article on the web 2nd June 2003

The growth and assembly behavior of cobalt magnetic nanocrystallites under an external magnetic field were

studied. Co polycrystalline wires with an average length of 2 mm and diameter of 13 mm were formed by the

self-assembly of Co nanocrystallites (15 nm on average) under the induction of a 0.25 T external magnetic field.

The wires were nearly parallel because their axes were all parallel to the magnetic line of force. The Ms and Hc

values of the sample, 111 emu g21 and 389 Oe, are higher than those of the sample prepared without an

external magnetic field applied (91 emu g21 and 375 Oe), which might be associated with the special

nanostructure in which Co nanocrystallites were arranged in polycrystalline wires acting as permanent magnetic

dipoles. The process could be used to fabricate large arrays of uniform wires of some magnetic materials and

improve the magnetic properties of nanoscale magnetic materials.

Introduction

Magnetic materials including Fe,1,2 Co3,4 and Ni5,6 magneticmetals have been studied for many years. Over the pastdecade nanoscale magnetic materials have attracted inten-sive interest because of their potential applications in high-density magnetic recording, magnetic sensors and addressingsome basic issues about magnetic phenomena in low dimen-sional systems.7–10 Various approaches have been developedto prepare nanoscale magnetic materials.11–14 For example,cobalt nanoparticle rings15 and nanodisks16 were fabricatedby metal carbonyl pyrolysis. However, little attention hasbeen paid to the effect of a magnetic field on the nucleationand growth process of the magnetic materials and on themovement and self-assembly behavior of magnetic nanocrys-tallites. Indeed, it is found that a magnetic field can significantlyinfluence the movement of magnetic particles. An interest-ing phenomenon is that migratory and homing animals aswell as bacteria seem to possess a built-in compass whichresponds to the earth’s magnetic field for navigation.17,18 Onehypothesis is that a dipolar-interaction-directed self-assemblyprocess could give rise to chains. Mertl19 suggests thatiron-laden cells may provide these creatures with a legend tothe earth’s magnetic road map. These ferromagnetic crystalsare polarized like a bar magnet under geomagnetic inductionand they appear to be usually arranged in one or more linearchains which act as permanent magnetic dipoles, therebyenabling them to impart a permanent moment to so-calledmagnetoreceptors which results in their alignment and motionparallel to geomagnetic field lines and makes it easy for thesecreatures to navigate. These results imply that a magneticfield would affect the growth and directional aggregation ofnanocrystallites, resulting in the formation of self-assemblystructures of magnetic materials. It is, therefore, significantto study the growth and assembly behavior of magneticnanocrystalllites under an external magnetic field. Cobaltis an excellent model system for nanocrystal growth kine-tics studies.20 In this paper the growth and assembly ofcobalt nanocrystallites under an external magnetic field arereported.

Experimental

Analytically pure aqueous cobalt acetate [Co(CH3CO2)2?4H2O, 2.49 g] and ethanol [C2H5OH, 40 ml] were mixedat room temperature, and H2NCH2CH2NH2 [3 ml] wasadded dropwise to the solution. A brown solution of athree-coordinate compound of [Co(en)3]

21 was prepared aftervigorous stirring for several minutes, then 80 wt% hydrazinehydrate solution [H4N2?H2O, 8 ml] was added to the brownsolution. After vigorous stirring for 10 minutes the mixedsolution was transferred into two Teflon-lined stainless steelautoclaves with 60 ml capacity (one without an externalmagnetic field, the other with a permanent rare earth NdFeBmagnet under the Teflon vessel: the magnetic field strength onthe inner surface of the Teflon vessel is 0.25 T at roomtemperature, 0.19 T at the reaction temperature of 110 uC).Both of the autoclaves were closed tightly to performsolvothermal processes at 110 uC for 12 h. After the reactionwas completed, the resulting black solid powder was separatedeasily by using a magnet, washed with ethanol and distilledwater three times respectively, then the product was dried in airat 40 uC. The same reaction was also carried out at 110 uC for24 h and 36 h, respectively. The samples obtained werecharacterized by X-ray powder diffraction (XRD) using an18 KW advance X-ray diffractometer with Cu Ka radiation(l ~ 1.54056 A) and high-resolution TEM (JEOL-2010).The parallel linear array was observed by scanning electronmicroscopy (X-650, HITACHI). Magnetic hysteresis loopswere measured on a Vibrating Sample Magnetometer (VSM,BHV-55), for magnetization measurements the powder sampleswere pressed strongly and fixed in a small cylindrical plastic box.

Results and discussion

A low-temperature controlled solvothermal reduction methodwas developed to prepare Co nanoparticles. The chemical reac-tion for the synthesis of Co nanoparticles can be expressed as

Co(AC)2 1 3en < CO[en]213 1 2AC2 (1)

2Co[en]213 1 N2H4 1 4OH2< 2Co 1 N2 1 6en 1 4H2O (2)

DOI: 10.1039/b303024e J. Mater. Chem., 2003, 13, 1803–1805 1803

This journal is # The Royal Society of Chemistry 2003

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AC and en stand for CH3COOH and NH2CH2CH2NH2

respectively. On the basis of the standard electrode potentialcalculations, the standard Gibbs free energy change DG0 ofreaction (2) was estimated to be about 2181 kJ mol21, whichimplies a very strong tendency for reaction (2) to progresstoward the right side.Fig. 1 depicts X-ray diffraction (XRD) patterns of the

product prepared at 110 uC for 36 hours with (A) and without(B) an external magnetic field applied, which indicates thatboth of the two routes produce pure nanocrystallites of cobalt.The peaks can be well indexed with the reflections of hexagonalmetal cobalt (standard cards, JCPDS 5-0727, space groupP63/mmc) with a unit cell a ~ 2.503 A, c ~ 4.060 A.Furthermore, the broadening of the peaks is obvious, thecrystalline sizes for the two types of samples are about 15 nmcalculated by the Scherrer equation from the full width athalf maximum (FWHM) of the (002), (101), (110) and (112)reflections. These sizes are consistent with the stable singledomain range,21 hence each crystallite might contain a singlemagnetic domain. It is also observed that the intensities ofthese reflections do not agree with those in the PDF card(JCPDS 05-0727). For instance, in the card, the diffractionpeak of (101) is the strongest one, while the (002) peak isweaker. However, the relative intensity of the (002) peak for thesample prepared under a 0.25 T external magnetic fieldincreases significantly, which might result from the orientedaggregation of the nanocrystallites with their easy magneticaxes parallel to the magnetic line of force. This is also con-sistent with the recent result observed in the sample preparedby a c-irradiation route under an external magnetic field.22

The reaction at 110 uC for 36 hours under a 0.25 T externalmagnetic field yielded nearly parallel wires with an averagelength of 2 mm, and the magnified image shows the widthis approximately 13 mm (Fig. 2a, b). Magnified SEM imagesreveal that the wires were composed of individual finerparticles. HRTEM images further reveal the particles, withsizes ranging from 2 nm to 20 nm, to be single crystalline (2 nmin Fig. 2c), which is nearly in agreement with the resultsobtained from the XRD pattern. The lattice planes (Fig. 2c) arevisible, corresponding to the (002) planes of a hexagonal cobaltstructure. It is known that [0001] is the magnetic easy axis ofhexagonal cobalt crystal;23 combined with the XRD results, itis reasonable to suggest that there exists preferred growth andalignment of the nanocrystallites along the easy magnetic axes,and the [0001] magnetic easy axis of the hexagonal cobalt maybe primarily parallel to the linear axes of the wires formed.

To study the effect of an applied external magnetic fieldon the morphology and growth of cobalt nanocrystallites,no surfactant was added to control the size and shape ofthe growing particles. Particles formed without an externalmagnetic field applied have no clear alignment (Fig. 3a), butthe products obtained under a 0.25 T external magnetic fieldhave linear chains consisting of spherical particles with sizesfrom 10 mm to 20 mm as shown in Fig. 3c. Fig. 3a, b reveal thatthese spherical particles consist of finer particles with sizesaround 700 nm. According to the results of HRTEM imagingand XRD, it is suggested that large spherical particles withan average diameter of 700 nm are also aggregates of Conanocrystallites. Regardless of whether or not an externalmagnetic field was applied, the single crystal Co nanocrys-tallites were firstly formed, then these primary nanoparticlesaggregated to form spherical particles with sizes from severalhundred nanometers to micrometer size, as shown in Fig. 3.The formation of a spherical morphology is favorable fordecreasing the surface energy. Fig. 3b also shows interlinksbetween micro-sized spherical particles in arbitrary directionsfor the sample formed without an external magnetic fieldapplied. Ferromagnetic spherical particles magnetize oneanother by dipolar interaction in arbitrary directions. Asdiscussed above, although the nanocrystallites contain singledomains, the orientation of each domain is spontaneouslyrandom when no external magnetic field is applied. However,when an external magnetic field is applied, the sphericalparticles tend to align along the magnetic line of force andfavor the formation of linear chains. Magnetization makes allsingle domain particles orientate along the magnetic line offorce, as a result dipole-directed self-assembly through dipolarinteraction along the magnetic line of force could happen,leading to the formation of linear chains. The single domainstructure of nanocrystallites increases the magnetic moment

Fig. 1 X-Ray diffraction patterns of cobalt nanocrystallites prepared at110 uC for 36 hours with a 0.25 T magnetic field (A), and without amagnetic field (B).

Fig. 2 a) SEMmicrographs of Co wires obtained at 110 uC for 36 hoursunder a 0.25 T external magnetic field, b) higher magnification of a),and c) HRTEM image of a particle in the wire, showing the particle tobe single crystalline. The magnified SEM image shows nearly parallelwires, and the HRTEM suggests that the wire is composed of Conanocrystallites.

Fig. 3 SEM micrographs of products prepared at 110 uC for 24 hoursa, b) without and c) with a 0.25 T external magnetic field. Image b)is the magnified image of a).

1804 J. Mater. Chem., 2003, 13, 1803–1805

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and magnetic interactions. Under the attraction of an externalmagnetic field, wires were formed through spherical particledrum-like connections (Fig. 4d) and fine nanocrystallitesfurther diffuse and fill in the gaps between spherical particles.The wires were nearly parallel, which could be the result ofmagnetic attraction, resulting in the wires aligning parallel tothe magnetic line of force.Fig. 4 shows SEM images of products of 12 h and 24 h under

a 0.25 T external magnetic field. The main difference is that thewires after 24 h are smoother than those after 12 h. Manyspherical particles can be clearly observed from the productsafter 12 h (Fig. 4a), but the transition state from chain to wirecan also be found from the 24 h product (Fig. 4b). An enlargedimage of the boxed region in b) (Fig. 4c) further confirmed thesuggestion that the cobalt spherical particles self assemblealong magnetic lines of force to form smooth micrometer wires.It is possible that the reaction and nanocrystallite formationhave finished within 12 hours, then self-assembly and migrationof the nanocrystallites occurred to form relatively uniform lines.Fig. 5 shows the room temperature M–H hysteresis loops of

the products with (A) and without (B) an external magneticfield applied. The saturation magnetization values Ms for thesamples with 0.25 T and without an external magnetic field are111 and 91 emu g21, respectively. And the coercivity values Hc

for the two samples are 389, 375 Oe, respectively. All these aremuch lower than the corresponding values of the bulk sample(Ms ~ 168 emu g21, Hc ~ 1500 Oe), which is accordancewith the literature results on cobalt nanoparticle.24,25 Surfaceoxidation of the nanocrystallites should not be responsiblefor the phenomenon because no significant oxidation wasobserved by X-ray photoemission spectroscopy analysis. But itis worthwhile to emphasize the fact that the values of Ms andHc for the sample formed with a 0.25 T magnetic field is higherthan that without an external magnetic field applied. This may

be associated with the oriented growth of nanocrystallites andlinear array structure of the wires formed under the externalmagnetic field attraction.

Conclusions

In conclusion, Co nanocrystallites with an average diameter of15 nm were formed at 110 uC by a solvothermal reductionprocess. The nanocrystallites aggregate to form micron-sizedspherical particles for the sake of decreasing surface energy.The spherical particles assemble to form linear chains bydipolar interaction along the magnetic line of force under theattraction of an external magnetic field. Then diffusion ofCo nanocrystallites within the chains leads to the formation ofpolycrystalline wires consisting of nanocrystallites. The wireswere nearly parallel because their axes are all parallel to themagnetic line of force. The Ms and Hc values of the sampleprepared under a 0.25 T external magnetic field are higher thanthose for the sample prepared without an external magneticfield. This might be associated with the special nanostructure inwhich Co nanocrystallites were arranged in polycrystallinewires acting as permanent magnetic dipoles. This suggests thatthe process could be used to fabricate large arrays of uniformwires of magnetic materials and improve the magneticproperties of nanocrystallites.

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Fig. 4 SEM images of cobalt formed under a 0.25 T magnetic field fora) 12 h, b) 24 h. c) An enlargement of the boxed region in b), showingspherical particles self-assembled to form a linear chain. d) Magnifiedimage of c), showing the connection of spherical particles in a wire.

Fig. 5 The room temperature hysteresis loops for the productsobtained with (A) and without (B) an external magnetic field.

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