INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY

Nanotechnology 17 (2006) 5406–5411 doi:10.1088/0957-4484/17/21/020

Synthesis of a new type of GdAl2nanocapsule with a large cryogenicmagnetocaloric effect and novel coral-likeaggregates self-assembled by nanocapsulesSong Ma, Dianyu Geng, Weishan Zhang, Wei Liu, Xiuliang Maand Zhidong Zhang

Shenyang National Laboratory for Material Science, Institute of Metal Research, andInternational Centre for Material Physics, Chinese Academy of Sciences, 72 Wenhua Road,Shenyang 110016, People’s Republic of China

E-mail: Songma@imr.ac.cn

Received 18 July 2006, in final form 20 September 2006Published 13 October 2006Online at stacks.iop.org/Nano/17/5406

AbstractA new type of GdAl2 nanocapsule with single-phase intermetallic compoundGdAl2 as the core and amorphous Al2O3 as the shell has been synthesized bythe arc-discharge technique with modified strategies. Meanwhile, novelthree-dimensional coral-like hierarchical branching macro-aggregates wereself-assembled by disordered nanocapsules synthesized simultaneously in thearc-discharge process. The GdAl2 nanocapsules display superparamagneticproperties between their blocking temperature of 100 K and Curietemperature of 162 K. The magnetocaloric effect of the GdAl2 nanocapsuleswas measured between 5 and 165 K. The absolute value of the change ofmagnetic entropy of the GdAl2 nanocapsules sharply increases withdecreasing temperature and reaches 14.5 J kg−1 K−1 at 5 K in magneticfields varying from 0 to 50 kOe. As a result, this new type of nanocapsule canprospectively be applied in cryogenic magnetic refrigerator devices.

1. Introduction

Recently, interest in research on magnetic nanocapsuleshas been enhanced considerably because their intermediatestates between bulk and atomic materials may presentdifferent magnetic behaviours from their correspondent bulkcounterparts. This difference offers an opportunity forresearchers to investigate the dimensionally confined systemin basic research areas and develop many important technicalapplications, including magnetic refrigerators, magneticrecording, magnetic fluids, catalysts, superconductors andmedicine [1]. As the principal contributor of the novelproperties, various magnetic cores of nanocapsules, includingrare earths and their carbides [2, 3], Fe [4, 5], Co [6, 7],Fe–Co [8, 9], Ni [7, 10, 11], Mn [12], and Co–Cr solidsolution [13], have been researched extensively over the pasttwo decades. Apart from single magnetic elements andtheir carbides, however, cores of compounds need to be

exploited to adapt to high-speed developing nanoscience andnanotechnology. Consequently, cores of magnetic rare-earthintermetallic compounds are turning into one of the principalcandidates in the field of nanotechnology.

Ferromagnetic intermetallic compounds on the nanoscalewill have potential applications in low-temperature magneticrefrigeration because of their superparamagnetic properties.According to theoretical calculation, McMichael concludedthat the nanoparticles may exhibit an effective magneticmoment that is greater than the magnetic moment of theconstituent atoms, so the magnetic entropy of the nanoparticlesmay have a maximum enhancement of the magnetocaloriceffect (MCE) [14]. Yamamoto et al [15, 16] investigated theentropy change in Fe2O3–Ag nanocomposites, confirming theprediction of McMichael. Provenzano et al [17] reported anMCE of R3Ga5−x Fex O12 nanocomposites (R = Gd, Dy andHo), but the largest �SM is −1.7 J kg−1 K−1. Nelson et al[18] attempted to prepare Gd nanoparticles in solution, yet

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they had considerable difficulties in preventing oxidation ofthe particles. It is a common fact that it is extremely difficultto synthesize the nanoparticles/nanocapsules with rare-earthelements or compounds because of the chemical activity of therare earths. Therefore, it is necessary to explore a new typeof nanoparticle to acquire larger entropy change and to preventthe magnetic nanoparticles from being oxidized. A new typeof superparamagnetic GdAl2 nanocapsule with high atomicmoments, low crystalline anisotropy and shell protectionwould have much more advantages than other nanocompositesfor application in cryogenic magnetic refrigerator devices.

During preparation of the nano-products, these nano-units,such as nanoparticles, nanoclusters, nanowires and nanorods,can also self-assemble into the novel structural aggregatesthrough several routes, including electron irradiation deposi-tion [19], chemical vapour deposition [20], laser vaporization–condensation [21], charge transferring [22], an organic reagent-assisted method [23], the solution–liquid–solid method [24]and catalytic vapour–liquid–solid growth [25]. With theseroutes, various nanoscale or microscale aggregates can demon-strate novel architectures, including tree-like, web-like, spheri-cal, nanowire-like, network and fishbone-like aggregates. As awell-known method for producing the nanocapsules, however,arc-discharge has rarely been used to synthesize the aggregatesself-assembled by the nanocapsules prepared simultaneously inarc discharge. Nevertheless, it is possible that arc discharge canbe developed into a new way of synthesizing the aggregates.

In the present work, we utilized an arc-discharge techniquewith modified strategies, involving changing the hydrogenpressure, introducing a gadolinium–aluminium alloy ingot asthe anode, and adjusting the proportions of elements in theanode according to their evaporation pressures, to synthesizea new type of nanocapsule, with the intermetallic compoundGdAl2 as the core and amorphous Al2O3 as the shell, whichenlarges the family of magnetic nanocapsules. At the sametime, regularly aligned three-dimensional macro-aggregatesself-assembled by the nanocapsules without any template andcatalyst were simultaneously synthesized in an arc-dischargeprocess.

2. Experiment details

The GdAl2 nanocapsules, which self-assembled into the coral-like aggregates, were synthesized by the well-known arc-discharge method with modified strategies. In the purifiedargon atmosphere, an anode Gd–Al alloy button was preparedby melting Al and Gd flakes of 99.9 wt% purity at leastfour times by arc discharge so that the components werehomogeneous. The optimum atomic proportions of the buttonwere 80 at.% Gd and 20 at.% Al on the basis of theirevaporation pressures. The cathode was a tungsten needle witha diameter of 3 mm. The anode target was placed into onepit of a water-cooled copper crucible. The angle between theanode and the cathode was about 60◦. After the chamber wasevacuated to 6.6 × 10−3 Pa, argon was introduced into thechamber to reach 0.02 MPa, and then H2 was introduced intothe chamber to reach 0.006 MPa, exceeding the critical value of0.03 MPa, above which the metal atoms can evaporate rapidly.Then the arc was ignited and the current was maintained at80 A for 5 h. After being passivated in 0.02 MPa argon for

Figure 1. X-ray diffraction (XRD) spectrum of the products.

72 h, large-area black macro-aggregates were collected on thecopper crucible in front of the target.

Phase analysis of the products was performed bypowder x-ray diffraction (XRD) on a Rigaku D/max-2000diffractometer at a voltage of 50 kV and a current of 250 mAwith graphite monochromatized Cu Kα (λ = 0.154 056 nm).The architecture of aggregates self-assembled by nanocapsuleswas characterized by a Philips XL-30 scanning electronmicroscope (SEM), with an emission voltage of 20 kV. Thedetailed morphology of the corresponding nanocapsules wasexamined by means of high-resolution transmission electronmicroscopy (HRTEM) using a JEOL 2010 and Tecnai G2

F30 with emission voltages of 200 and 300 kV, respectively.X-ray photoelectron spectroscopy (XPS) measurements wereperformed on an ESCALAB-250 with a monochromaticx-ray source (an aluminium Kα line of 1486.6 eV energyand 150 W) to characterize the valence of surface Alatoms of the nanocapsules at a depth of 1.6 nm. Themagnetic properties of the nanocapsules were measuredon a Quantum Design MPMS-7 superconducting quantuminterference device (SQUID).

3. Result and discussion

The XRD pattern clearly shows the phase composition of theas-prepared products in figure 1. In addition to several peaksindicating the existence of a small amount of Gd2O3, mostsharp reflection peaks could be indexed to the single-phaseintermetallic compound GdAl2. The formation of these phasesis due to the bumping of the metal atoms evaporating fromthe molten pool of the anode [26]. It is noteworthy that onlyGdAl2 binary compound appears in the present products andthat no other intermetallic compounds in the Gd–Al binarysystem are found in the XRD pattern. The facts that Gd andAl atoms are of appropriate proportions and that the GdAl2 isa stable phase with the highest decomposition temperature inthe Gd–Al phase diagram play important roles in the formationof single-phase GdAl2. When the composition of the anodeis changed, Gd3Al2 can be found in the products, indicatingthe importance of appropriate atomic proportions of the twodifferent metals. In the arc-discharge process, a small numberof gadolinium particles formed by excessive gadolinium atomswere oxidized into Gd2O3 when exposed to the air.

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Figure 2. Low-magnification SEM image of the upper part of theaggregate arrays and one of the big branches in an aggregateindicated by arrows.

Figure 3. Typical SEM images of the middle part of the aggregates.(a) The architecture of aggregates built up by rod-like branches.(b) One big cluster in the rod branch indicated by the arrow in (a) andone small cluster indicated by the arrow. (c) Small clusters buildingup the big clusters and flocculent structure indicated by the arrow.

3.1. Structural analysis of the aggregates

The morphology of the upper part of the aggregates wasexamined by scanning electron microscopy (SEM), as isshown in figure 2. A low-magnification SEM image revealsthat every aggregate displays coral-like architecture withhierarchical branching characteristics along the axial andlengthwise directions, and the average diameter ranges from

Figure 4. Different magnification SEM images of the root of aggregates indicating (a) the morphology of the root, (b) a local image of theaggregates in the frame of (a), (c) flocculent structure arrays in the frame of (b), and (d) particles adhering together to form the flocculentstructure in the frame in (c).

100 to 200 μm. Large branches of aggregates with a lengthof about 200 μm and a width of 50 μm closely arrange frominside to outside. The distance between two aggregates is keptto about 50 μm, which indicates that each aggregate grewseparately.

A typical SEM image of the middle part of an aggregate isshown in figure 3 at different magnification. The morphologyof the aggregates indicates their growth characteristics in thearc-discharge process. Obviously, the rod-like big brancheslocate in the space built up by under-floor branches andthe same floor branches are almost of the same height.Accordingly, it can be concluded that the big branchesaround the main stem of aggregates grew separately andconstructed the space for the growth of upside floor branchessimultaneously. It can be observed from figures 3(a) and (b)that a big branch comprises some big clusters and a big clusteralso consists of many small irregular spherical clusters, withdiameter distribution from 5 to 10 μm, stacking together andkeeping a distance of about 2 μm. The SEM image at a highmagnification shows the clear morphology of small clusters(figure 3(c)), which are composed of flocculent structureformed by the small particles adhering together. The existenceof the distance between clusters and big branches can beexplained by the fact that they carried the redundant negativecharge from the plasma to repulse each other [19].

In figure 4, the root of an aggregate is clearly shown. Thediameter of the root is about 80 μm, smaller than the middleand upper parts of the aggregates (figure 4(a)). The localmorphology of the root shows that big clusters with a diameterof about 40 μm congregate together to construct the root(figure 4(b)). A higher-magnification SEM image of the rootshows that the flocculent structures array like a lawn along thesame direction (figure 4(c)). A high-magnification SEM imageof a flocculent structure is shown clearly in figure 4(d), inwhich the particles adhere together to form line-like flocculentstructures. These characteristics indicate that the flocculentstructures directly constructed the roots of the aggregates oradhered together to form big clusters on the roots of aggregates,

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Figure 5. (a) The diameter distribution of the nanocapsules. (b) The SAD pattern of the two particles. (c) HRTEM image of the GdAl2nanocapsules. (d) HRTEM image of Gd2O3 nanoparticles. (e) XPS spectrum and corresponding fitting curve of the Al atoms at a depth of1.6 nm of the nanocapsules.

because the flocculent structures carrying negative charge fromthe plasma are more easily attracted to the surface of the anodeto form roots at the initial stage of the arc-discharge process.

3.2. Structure of the nanocapsules

The morphology of the nanocapsules constructing theaggregates was examined from the low-magnification TEMimages (figure 5(a)). The particles are of irregular sphericalshape or a strip with curvature, and the diameter distributionis 20–50 nm, which makes it easy for them to adheretogether to form a flocculent structure. From the selectedarea electron diffraction (SAED) pattern, GdAl2 and Gd2O3

can be determined from the characteristic diffraction rings,indicating the characteristic lattice planes (111), (220), (311),(400) and (422) of GdAl2 and (222) and (440) of Gd2O3,respectively (figure 5(b)), which is consistent with the resultsof the XRD pattern. In figure 5(c), the HRTEM imageclearly shows the shell/core structure with a crystalline coreand an amorphous shell 4.2 nm thick. In the core, the d-spacing of 0.456 nm corresponds to the lattice fringe {111}of GdAl2. An HRTEM image of the nanoparticle, withouta shell/core structure (figure 5(d)), shows the characteristiclattice fringes’ {222} plane and {013} plane with d-spacingsof 0.311 and 0.341 nm respectively, which corresponds toGd2O3. The shell of the nanocapsules can be determinedas Al2O3 from the binding energy peak of 74.1 eV [27],which is shown in the XPS spectrum with an etching depthof 1.6 nm (figure 5(e)). Another binding energy peak of71.6 eV of Al2p3/2 represents the compound GdAl2 [27],the intensity of which is lower than that of Al2O3, indicatingthe existence of some nanocapsules with a shell thinner than1.6 nm. The formation of an Al2O3 shell was explained bythe fact that Al atoms with higher velocities, due to smalleratomic mass, were more easily absorbed on the outside ofthe GdAl2 nanoparticles and subsequently condensed on thesurface. When the products were exposed to air, Al atoms wereeasily oxidized to amorphous Al2O3 [26].

3.3. Magnetic properties and MCE of the nanocapsules

The zero-field-cooled and field-cooled (ZFC–FC) curves of theGdAl2 nanocapsules were measured from 5 to 300 K at anapplied field of 100 Oe. From figure 6(a), it can be clearlyobserved that a peak of the ZFC curve appears at 100 K,indicating the blocking temperature TB, and the bifurcationbetween ZFC and FC curves appears. Furthermore, the peaksin the temperature dependence of ac susceptibility (in the insetof figure 6(a)) do not shift their positions with an increasein frequency from 10 to 1000 Hz, which is different fromthe character of the spin-glass-like state in which the peakswould shift to lower temperatures with increasing frequency. Apivotal point at 162 K in the two overlapping curves indicatesthe Curie temperature TC of GdAl2 nanocapsules, which isslightly lower than 172 K for the bulk counterpart because ofthe dimensional effect [1]. It can be concluded that GdAl2

nanocapsules are of a superparamagnetic property betweenthe blocking temperature TB and the Curie temperature TC,because their sizes are decreased to the nanoscale. From themagnetic hysteresis loops in figure 6(b), the coercive forceat 5 K is 280 Oe, showing their ferromagnetic behaviourat low temperatures. The magnetization at 180 K increaseslinearly with the increase in the magnetic field, indicatingtheir paramagnetism. Because the only phase components inthe nanocapsules are ferromagnetic GdAl2 and paramagneticGd2O3, this may exclude the contribution of antiferromagneticcomponents to magnetization at low temperatures.

By grouping spins together in superparamagnetic GdAl2nanocapsules, the magnetic moments are more easily alignedthan in the paramagnetic system for certain ranges of fieldat low temperature, and the entropy of the spins is moreeasily changed by the application of a field. According to theexperimental results of Yamamoto et al [15, 16], the isothermalmagnetic entropy change around 250 K in 9% Fe of Fe2O3–Ag nanocomposite reached about 2.1 × 10−3 J mol−1(Fe) ina magnetic field varying from 0 to 70 kOe, which exceededthat of normal paramagnetic Fe3+ in the same temperatureand magnetic field range by about two orders of magnitude.

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Figure 6. (a) Zero-field-cooled and field-cooled (H = 100 Oe)magnetization curve of the nanocapsules; the inset is the temperaturedependence of ac susceptibility with frequencies from 10 to 1000 Hz.(b) Magnetic hysteresis loop at 5 K; the insets are, respectively, themagnetic field dependence of magnetization at 180 K and thelow-field part of the hysteresis loop at 5 K.

However, the extensive measure of the MCE is reduced by afactor proportional to the ratio of the volumes (or the mass) ofthe inactive matrix [28]. In the magnetic cores of the presentnanocapsules, the total angular momentum of the Gd atom isJ = 7/2 and its effective magnetic moment is 7.2 μB. SoGdAl2 nanocapsules will have large magnetization, ensuringthat more spins contribute to the entropy change. On the otherhand, the lower crystalline anisotropy [29] of GdAl2 will makethe moments of particles align easily along the applied field. Itis also worth noting that the Al2O3 shell of nanocapsules canprevent the GdAl2 cores from being oxidized and interactingwith each other. In view of the advantages mentioned above,the change in entropy in the GdAl2 nanocapsules was measuredfrom 5 to 165 K in a magnetic field varying from 0 to 50 kOe.At low temperatures, the absolute �SM value sharply increaseswith a decrease in temperature and reaches 14.5 J kg−1 K−1 at5 K (figure 7). Delightfully, not only the GdAl2 nanocapsuleswith large entropy change at low temperatures will havepotential applications in the future, but also the strategy ofpreparation of GdAl2 nanocapsules will open up a new routefor preparing more compounds of magnetic nanocapsules.

4. Conclusion

Using an arc-discharge technique with a modified strategy,nanocapsules with a single-phase GdAl2 core were fabricatedand three-dimensional coral-like macro-aggregates wereself-assembled by the new type of GdAl2 nanocapsulessimultaneously. The phase compositions of the product were

Figure 7. The change in magnetic entropy of GdAl2 nanocapsulesbetween 5 and 165 K in an applied field varying from 0 to 50 kOe.

determined as single-phase GdAl2 and a small amount ofGd2O3 from XRD and TEM measurements. Using SEM,the architecture of the hierarchical branching aggregateswas shown to be constructed from nanocapsules, flocculentstructure, small clusters, big clusters and big branches stepby step at different scales, which indicates the self-similarattributes of the structure. The GdAl2 nanocapsules havea shell/core structure with crystalline GdAl2 as the coreand amorphous Al2O3 as the shell, respectively. Thenew type of GdAl2 nanocapsules shows superparamagneticproperties between their blocking temperature of 100 K andCurie temperature of 162 K. The GdAl2 nanocapsules showferromagnetic behaviour at low temperatures. The peaks inthe temperature dependence of ac susceptibility do not shifttheir positions with an increase in frequency from 10 to1000 Hz, which excludes the possibility of the existence ofa spin–glass-like state. On the other hand, the only phasecomponents in the nanocapsules are ferromagnetic GdAl2 andparamagnetic Gd2O3, which excludes possible contributions ofthe antiferromagnetic components at low temperature. Theabsolute value of the change in magnetic entropy sharplyincreases with an decrease in temperature at low temperatureand reaches 14.5 J kg−1 K−1 at 5 K. Consequently, GdAl2nanocapsules can be applied as a promising MCE materialfor low-temperature MCE devices. At the same time, thepresent strategy using an arc-discharge technique opens up anew way of synthesizing more magnetic nanocapsules of rare-earth compounds.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China under grant no. 50331030. The authorsthank Dr W F Li for helpful discussions.

References

[1] Zhang Z D 2004 Nanocapsules (Encyclopedia of Nanoscienceand Nanotechnology vol 6) ed H S Nalwa (California:American Scientific) pp 77–160

[2] Saito Y, Okuda M, Yoshikawa T, Kasuya A andNishina Y 1994 J. Phys. Chem. 98 6696

[3] Ruoff R S, Lorents D C, Chan B, Malhotra R andSubramoney S 1993 Science 259 346

[4] Sun X C and Nava N 2002 Nano Lett. 2 765[5] Fernandez-Pacheco R, Arruebo M, Marquina C, Ibarra R,

Arbiol J and Santamaria J 2006 Nanotechnology 17 1188[6] Saito Y 1995 Carbon 33 979

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GdAl2 nanocapsules with large cryogenic magnetocaloric effect and self-assembled coral-like aggregates

[7] Liu Y, Ling J, Wei U and Zhang X G 2004 Nanotechnology15 43

[8] Dong X L, Zhang Z D, Chuang Y C and Jin S R 1999 Phys.Rev. B 60 3017

[9] Pozas R, Ocana M, Morales M P, Tartaj P, Nunez N andSerna C J 2004 Nanotechnology 15 S190

[10] Si P Z, Zhang Z D, Geng D Y, You C Y, Zhao X G andZhang W S 2003 Carbon 41 247

[11] Ang K H, Alexandrou I, Mathur N D, Amaratunga G A J andHaq S 2004 Nanotechnology 15 520

[12] Si P Z, Li D, Lee J W, Choi C J, Zhang Z D, Geng D Y andBruck E 2005 Appl. Phys. Lett. 87 133122

[13] Ma S, Wang Y B, Geng D Y, Li J and Zhang Z D 2005 J. Appl.Phys. 98 094304

[14] McMichael R D, Shull R D, Swartzendruber L J andBennett L H 1992 J. Magn. Magn. Mater. 111 29

[15] Yamamoto T A, Tanaka M, Nakayama T, Nishimaki K,Nakagawa T, Katsura M and Niihara K 2000 Japan. J. Appl.Phys. 39 4761

[16] Yamamoto T A, Tanaka M, Shiomi K, Nakayama T,Nishimaki K, Nakagawa T, Numazawa T, Katsura M andNiihara K 2000 Mater. Res. Soc. Symp. Proc. 581 297

[17] Provenzano V, Li J, King T, Canavan E, Shirron P, Dipirro Mand Shull R D 2003 J. Magn. Magn. Mater. 266 185

[18] Nelson J A, Bennett L H and Wagner M J 2002 J. Am. Chem.Soc. 124 2979

[19] Cho S O, Lee E J, Lee H M, Kim J G and Kim Y J 2006 Adv.Mater. 18 60

[20] Dick K A, Deppert K, Larsson M W, Martensson T, Seifert W,Wallenberg L R and Samuelson L 2004 Nat. Mater. 3 380

[21] ElShall M S, Li S T, Graiver D and Pernisz U 1996 Synthesisof nanostructured materials using a laser vaporization-condensation technique Nanotechnology (ACS SymposiumSeries) vol 622 pp 79–99

[22] Naka K, Roh H and Chujo Y 2003 Langmuir 19 5496[23] Lu Q Y, Gao F and Zhao D Y 2002 Nanotechnology

13 741[24] Boal A K, Ilhan F, Derouchey J E, Thurn-Albrecht T,

Russell T P and Rotello V M 2000 Nature 404 6779[25] Zhu Y Q, Hsu W K, Zhou W Z, Terrones M, Kroto H W and

Walton D R M 2001 Chem. Phys. Lett. 347 337[26] Geng D Y, Zhang Z D, Zhang W S, Si P Z, Zhao X G, Liu W,

Hu K Y, Jin Z X and Song X P 2003 Scr. Mater. 48 593[27] Moulder J F, Stickle W F, Sobol P E and Bomben K D 1992

Physical Electronics Division (Handbook of X-rayPhotoelectron Spectroscopy) ed J Chastain (Minnesota:Perkin-Elmer) p 55

[28] Gschneidner K A Jr, Pecharsky V K and Tsokol A O 2005 Rep.Prog. Phys. 68 1479

[29] Morales M A, Williams D S, Shand P M, Stark C,Pekarek T M, Yue L P, Petkov V andLeslie-Pelecky D L 2004 Phys. Rev. B 70 184407

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