Enhancement of surface spin disorder in hollow NiFe 2 O 4 nanoparticlesG. Hassnain Jaffari, Abdullah Ceylan, C. Ni, and S. Ismat Shah Citation: Journal of Applied Physics 107, 013910 (2010); doi: 10.1063/1.3277041 View online: http://dx.doi.org/10.1063/1.3277041 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/107/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Role of inhomogeneous cation distribution in magnetic enhancement of nanosized Ni0.35Zn0.65Fe2O4: Astructural, magnetic, and hyperfine study J. Appl. Phys. 114, 093901 (2013); 10.1063/1.4819809 Surface spin disorder and exchange-bias in hollow maghemite nanoparticles Appl. Phys. Lett. 101, 022403 (2012); 10.1063/1.4733621 Sol-gel NiFe2O4 nanoparticles: Effect of the silica coating J. Appl. Phys. 111, 103911 (2012); 10.1063/1.4720079 Metal-semiconductor transition in NiFe2O4 nanoparticles due to reverse cationic distribution by impedancespectroscopy J. Appl. Phys. 109, 093704 (2011); 10.1063/1.3582142 Magnetic properties of polydisperse and monodisperse NiZn ferrite nanoparticles interpreted in a surfacestructure model J. Appl. Phys. 97, 10G104 (2005); 10.1063/1.1850334

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Enhancement of surface spin disorder in hollow NiFe2O4 nanoparticlesG. Hassnain Jaffari,1 Abdullah Ceylan,2 C. Ni,3 and S. Ismat Shah1,3,a�

1Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA2Department of Physics Engineering, Hacettepe University, Beytepe, Ankara 06800, Turkey3Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, USA

�Received 30 July 2009; accepted 25 November 2009; published online 7 January 2010�

Hollow NiFe2O4 nanoparticles are synthesized by self-templating process utilizing coupledinterfacial chemical reactions and Kirkendall effect between the core �Ni33Fe67� and the shell�NiFe2O4� of the core/shell structure. Reaction temperature and time dependent structural andmorphogical transformations are presented in detail. The kinetics of the transformation from�Ni33Fe67� / �NiFe2O4� nanoparticles to single phased NiFe2O4 hollow nanoparticles was studied bydifferential scanning calorimetry. Hollow morphology of the particles induces surface effects in themagnetic properties due to the formation of additional inner surfaces. Field cooled hysteresis loopexhibits significantly large shift due to unidirectional anisotropy resulting from the additional innerspin disordered surface along with the existing outer spin disordered surface. The enhancement inthe surface anisotropy is also noticeable which leads to an increase in the blocking temperature ofthe particles with hollow morphology. © 2010 American Institute of Physics.�doi:10.1063/1.3277041�

I. INTRODUCTION

Interest in magnetic nanoparticles and nanogranularstructures has increased by virtue of their potential for appli-cations in fields such as ultrahigh-density recording andmedicine.1–4 Permanent magnetic particles have an aniso-tropy that forces the magnetization to lie in direction along apreferred axis.5 As the particle size decreases, the magneticanisotropy energy responsible for holding the magnetic mo-ment along certain direction becomes comparable to the ther-mal energy. As a result, the nanoparticles lose their stablemagnetic order and become superparamagnetic.5,6 However,several applications require magnetic order of the nanopar-ticles to be stable with time. Miniaturization, therefore, has acost associated with the superparamagnetism caused by thereduction in the anisotropy energy per particle. Because ofsuperparamagnetic limit in recording media, new sources ofanisotropy are required in order to increase anisotropy en-ergy per particle to push the superparamagnetism limit toeven lower dimensions. One of the possible solutions toavoid superparamagnetism is to utilize magnetic exchangecoupling induced at the interface between ferromagnetic andantiferromagnetic systems,7,8 which can provide an extrasource of anisotropy. This idea has already been tested andverified in superparamagnetic particles.9 It is also known thatthe promotion of magnetic coupling between the orderedcore and the disordered surface spins for magnetic nanopar-ticles gives rise to unidirectional anisotropy with leads to aphenomena known as exchange bias.10 In this paper we dem-onstrate increase in the surface area of hollow nanoparticlesdue to the formation of additional inner surfaces. The in-creased surface area is related to the enhanced spin disorderwhich gives rise to higher anisotropy.

During the past decade, there have been several reportson the synthesis of hollow nanoparticles. Most of the reportsare on particles synthesized through chemical routes.11–14

Physical routes, e.g., laser-induction evaporation are also be-ing used.15 Synthesis of magnetic hollow particles have beenreported earlier but detailed magnetic characterizations werelacking.12,13 Formation of voids in the nanoparticles andnanotubes is explained as a consequence of the “Kirkendalleffect.” Kirkendall effect is related to the difference in thediffusivities of atoms at the interface of two different mate-rials which causes supersaturation of lattice vacancies. Thesesupersaturated vacancies can condense to form “Kirkendallvoids” close to the interface.16 Fan et al.16 presented a com-prehensive review of the formation of hollow nanoparticlesand nanotubes induced by the Kirkendall effect. On the mac-roscopic scale, these Kirkendall voids deteriorate the proper-ties of the interface. However, on the nanometer scale theKirkendall effect can be controlled to fabricate hollow nano-scale objects.

We present results of the synthesis of hollow NiFe2O4

particles formed as a result of the Kirkendall effect at theinterface of alloy �Ni33Fe67� core and the oxide �NiFe2O4�shell of the nanoparticles prepared by inert gas condensation�IGC�. In addition to interface effects, a chemical transfor-mation during annealing in air leads eventually to the forma-tion of NiFe2O4 structure. The reaction temperature in air,TR, and reaction time, t, dependent formation and collapse ofnanosize NiFe2O4 hollow spheres are discussed in detail. Thekinetics of transformation of �Ni33Fe67� / �NiFe2O4� core/shellnanoparticles to NiFe2O4 hollow nanoparticles is presentedthrough the discussion of results from differential scanningcalorimetry �DSC�. Magnetic behavior is also reported withspecific discussion of the effects of the additional inner sur-face on the spins structure of hollow nanoparticles. In par-ticular, hysteresis loops are shifted due to the unidirectionalanisotropy resulting from the coupling between the addi-

a�Author to whom correspondence should be addressed. Electronic mail:ismat@udel.edu. Tel.: 302-831-1618. FAX: 302-831-4545.

JOURNAL OF APPLIED PHYSICS 107, 013910 �2010�

0021-8979/2010/107�1�/013910/7/$30.00 © 2010 American Institute of Physics107, 013910-1

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tional inner disordered spin surface along with the outer dis-ordered spin surface. There is also a consistent enhancementin the surface anisotropy that leads to an increase in theblocking temperature of the particles with hollow morphol-ogy.

II. EXPERIMENTAL SECTION

In our previous work, we showed that it is possible tosynthesize alloy nanoparticles with metals of similar meltingpoints by simultaneously evaporating the metals, followedby rapidly cooling the vapors to a solid phase.17 Pure Ni:Fesamples, with atomic ratio 33:67, were resistively evaporatedfrom Al2O3 coated tungsten boats that were heated to1500 °C in the presence of 100 Torr He which was circu-lated by roots blower pumps operating at 1800 rpm. IGC wasused to form Ni33Fe67 nanoparticle as the starting material.After the metal nanoparticles were synthesized by IGC tech-nique, oxygen passivation leads to the formation of metal/metal oxide �core/shell� structures. The pressure of the depo-sition system was raised to the atmospheric pressure in10–15 min after the completion of the deposition of the par-ticles. Gas-solid reaction of “as-prepared” core/shellNi33Fe67 /NiFe2O4 structure were carried out ex situ in a fur-nace by annealing the particles in air at three different tem-peratures: 350, 450, and 550 °C. This gas-solid reactionsunder optimized reaction time and temperature conditionslead to the formation and the subsequent collapse of voids inthe single phase NiFe2O4 structure.

Structural properties and morphology of the samples areinvestigated by x-ray diffraction �XRD� and transmissionelectron microscopy �TEM�. XRD studies are performed ona Rigaku D-Max B horizontal diffractometer using Cu K� xrays. TEM analyses are carried out using a JEM 2010FXfield emission transmission electron microscope operated at200 kV. DSC experiments are done in a Diamond DSC sys-tem by PerkinElmer Co. Magnetic properties of the samplesare measured by using a physical properties measurementsystem dc extraction magnetometer by Quantum Design Cor-poration.

III. RESULTS AND DISCUSSION

A. Structural characterization

Figure 1 shows XRD patterns of as-prepared and heattreated samples. Typical XRD pattern of the as-preparedsample is shown in Fig. 1�a�. It shows two types of phases,Ni33Fe67 and NiFe2O4. The Ni33Fe67 phase is from the nano-particles that were formed in situ by IGC technique and ox-ide phase forms during the oxygen passivation of the alloynanoparticles. The large breadth of the diffraction peaks isattributable to the small crystallite sizes of these phases. As-prepared sample has dissimilar materials in the core�Ni33Fe67� and the shell �NiFe2O4�, as later confirmed byTEM analyses. XRD patterns of the samples after annealingin air at 350, 450, and 550 °C are shown in Fig. 2�b�. Allthese samples are chemically transformed into a single phaseNiFe2O4 structure.

B. Morphology

Figure 2 shows transmission electron micrographs of as-prepared and heat treated samples. Figures 2�a�–2�c� show

20 30 40 50 60 70 80

(620)

(622)

(222)

2� (deg.)

350oC

450oC

550oC

(220)

(311)

(400)

(422)

(511)

(440)

(533)

(b)

20 30 40 50 60 70 80-20

0

20

40

60

80

100

120

NiFe2O4(220)

NiFe2O4(440)

NiFe2O4(422)

2� (deg.)

Intensity(a.u.)

NiFe2O4(533)

NiFe2O4(400)

NiFe2O4(222)

NiFe2O4(511)

Fe67Ni33(200)

Fe67Ni33(220)

Fe67Ni33(111)

NiFe2O4(311)

(a)

FIG. 1. XRD patterns for �a� as-prepared sample and �b� samples obtained at different reaction temperatures TR, 350, 450, and 550 °C. All peaks correspondto NiFe2O4 structure.

(a) (b) (c) (d)

(e) (f) (g) (h)

FIG. 2. �Color online� TEM micrographs of �a� as-prepared particles havingNi33Fe67 as core and NiFe2O4 as shell, �b� and �c� high magnification bright-field images of as-prepared particles revealing core/shell structure, �d� and�e� sample annealed in air at 350 °C which shows hollow NiFe2O4 mor-phology, �f� high magnification bright-field image of sample annealed at350 °C, �e� sample obtained at reaction temperature of 450 °C, and �g�sample obtained at reaction temperature of 550 °C.

013910-2 Jaffari et al. J. Appl. Phys. 107, 013910 �2010�

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electron micrographs of the unannealed as-prepared particlesat different magnifications. The high magnification bright-field images confirm the Ni33Fe67 /NiFe2O4 core/shell struc-ture. These same phases are also observed in the XRD pat-terns, as shown in Fig. 1�a� and discussed above. Figures2�d�–2�f� show TEM micrographs of samples annealed at350 °C. This is the temperature where Kirkendall effectleads to the formation of hollow NiFe2O4 nanoparticles as aresult of the diffusion of metal core atoms toward the shell,as discussed later. TEM micrographs �shown in Fig. 2�g�� ofsample with TR of 450 °C shows collapse of voids inNiFe2O4. At this temperature diffusion mechanism stopssince chemical transformation has already completed and thesamples starts to minimize the surface area. Further increasein TR to 550 °C leads to the completion of the collapse ofvoids leaving holeless particles with noticeably increased ag-glomeration, as shown in Fig. 2�h�. The structure, averageparticle size �dXRD� calculated from XRD and morphology ofthe particles are summarized in Table I.

Figure 3 shows schematic of the formation/collapse pro-cess of NiFe2O4 hollow spheres as a function of TR and time.As-prepared core/shell structure serves as the precursor inthe reaction. Ni33Fe67 metal in the core reacts with the sur-face oxide at the interface by solid-state diffusion at 350 °C

in air. The outward diffusion rate of core elements is highercompared to the inward diffusion rate of oxygen. Since thenet outward matter flux is higher, there is an inward vacancyflux to compensate for the metal diffusion, which leads to theformation of void or cavity at the center of the particles. Thisvoid formation based on the difference of the diffusion ratesat the interface of two different materials is known as Kirk-endall effect as proposed by Kirkendall.18,19 As the reactiontime increases, the NiFe2O4 layer increases in thickness atthe surface of the particles as a result of the reaction betweenthe core �Ni33Fe67� and the shell �NiFe2O4�. Reaction contin-ues until all the metal at the core is consumed and the struc-ture completely transforms chemically to NiFe2O4. Within arange of the experimental variables, time and temperature,hollow NiFe2O4 structures could be obtained throughout thesample. This optimized time is 12 h at 350 °C. Smaller par-ticles have less metal precursor at the core and can transformto hollow morphology faster compared to the larger particles.As a result, size distribution of the particles also leads to asimilar distribution of cavity size. Void formation based onKirkendall effect was proposed in 1940s.17,18 However, tofabricate hollow nanoobject has be implied recently. Yin etal.20 showed that cobalt nanoparticles reacted with sulfurcontaining precursors in the solution to form nanoscale co-

TABLE I. Reaction temperature dependent morphological, structural, and magnetic properties of particles.

Sample Structure �XRD�dXRD

�nm� Morphology �TEM�HC

�Oe�HEB

�Oe�MS@5 K�emu/gm�

TB

�K�

As-prepared �Ni33Fe67� / �NiFe2O4� 8 Core/shell 911 248 39.5 71TR=350 °C NiFe2O4 8 Hollow 786 301 23.7 130TR=450 °C NiFe2O4 10 Hollow 580 165 25.5 123TR=550 °C NiFe2O4 16 Nonhollow 471 59 34.8 88

NiFe2O4

Ni33Fe67

t = 12hr

TR=350oC

VOID

t < 12hr

TR=350oC

NiFe2O4

Ni33Fe67

TR=450oC

t=12hr

TR=550oC

t=12hr

NiFe2O4

NiFe2O4 VOID

NiFe2O4

FIG. 3. �Color online� A Schematics of the reaction temperature TR and reaction time t dependent formation/collapse processes of nanosize NiFe2O4 hollowspheres.

013910-3 Jaffari et al. J. Appl. Phys. 107, 013910 �2010�

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balt sulfide hollow spheres due to the much faster cobaltdiffusion rate compared to that of sulfur. This effect has alsobeen extended to solid-gas interface reactions in which solidcadmium nanowires form CdS cylinder shells by a heat treat-ment in H2S.21 In our case, Kirkendall effect and a simulta-neous chemical transformation lead to the formation of hol-low NiFe2O4 particles from as-prepared samples, which havedissimilar materials in the core �Ni33Fe67� and shell�NiFe2O4�. Formation of hollow nanoparticles leads to alarger surface area compared to conventional nonhollownanoparticles. Increasing the reaction temperature to 450 °Cleads to the collapse of the holes for particles with smallersize and decrease in the size of the cavity for the particleswith larger size, as shown in Fig. 2�g�. As TR increases,cavity collapse and agglomeration cause minimization of theoverall surface area. For samples annealed at higher tempera-ture, TR=550 °C, there is a complete collapse of the holesand higher degree of agglomeration leading to higherNiFe2O4 average grain size, as given in Table I.

It is known that changes in the melting point for nano-scale materials are related to their much higher surface tovolume ratio than bulk materials.22 Likewise, it can be ex-pected that other phase change temperatures are also affectedin materials with nanostructures. According to the equilib-rium binary phase diagram of Ni–Fe system, the Ni33Fe67

alloy is composed of two separate phases, a fcc phase �Ni-rich� and a bcc phase �Fe-rich� between room temperature toapproximately 525 °C.23 However, in Ni33Fe67 nanoparticlesno such phase separation is observed in this temperaturerange. The formation of nanoparticles extends the solubilitylimits and renders a metastable single phase alloy for sys-tems where such solubility is not predicted under equilibriumconditions.17 In our case, it is also important to note thatchemical transformation coupled with Kirkendall effect sup-presses any phase separation effect. Therefore, Kirkendalleffect can be utilized to tailor hollow morphology withouthaving phase separation at nanoscale, unlike the bulk coun-terpart.

C. Calorimetry studies

Since the morphological and chemical transformationsare thermally activated, therefore thermal analysis has been

carried out by DSC. Figure 4 shows DSC data for an as-prepared sample. The rate of energy absorption by thesample is proportional to the specific heat of the samplesince the specific heat at any temperature determines theamount of thermal energy necessary to change the sampletemperature by a given amount. All transitions accompaniedby a change in specific heat result in a discontinuity in thepower signal. The areas of the exothermic or endothermicpeaks are proportional to the enthalpy changes. Figure 4�a�shows two endothermic transitions in the first heating cycleof DSC data which is recorded at the heating rate of10 °C /min. The peaks are positioned at 377 and 533 °C.First transition is related to a combination of chemical trans-formation and the hole formation processes in the nanopar-ticles driven by the Kirkendall effect and consistent withXRD and TEM studies. Second peak is the combination ofenthalpic changes due to the collapse of holes, agglomera-tion, and Curie transition. During the second heating cycle,the first peak completely vanishes while the second peak isonly partially reproducible, as shown in Fig. 4�a�. Irrevers-ibility of the first peak confirms the Kirkendall effect andchemical transformation, both occur during first cycle only.Partial reversibility of second peak confirms the collapse ofhole �which occurs during first cycle� and reversible curietransformation. These results, i.e., hole formation/collapsetemperatures and chemical transformation temperature ob-served by XRD and TEM are consistent with DSC endother-mal peak positions, i.e., 377 and 533 °C.

The kinetics of the transformation of the core/shellNi33Fe67 /NiFe2O4 structure to NiFe2O4 hollow nanoparticlesis studied by recording the DSC traces obtained using vari-able heating rates. In order to do so we employed theoreticalmodel proposed by Kissinger �Fig. 4�b��.24 It is known thathigher the scan rate, the higher is the endothermic peak in-tensity and the transformation temperature. This is due torapid heat absorption and time dragging. The variation in theDSC peak temperatures �calculated from the first peak, �Fig.4�b�� as a function of the scan rates are monitored and usedfor the Kissinger analysis which is the most common noniso-thermal kinetic analysis for a phase transformationreaction.24

100 200 300 400 500 600

0.0

0.5

1.0

1.5

2.0

2.5

SecondCycleEndoUp(mW)

T(oC)

(a)First Cycle

0 100 200 300 400 500 600

0

1

2

3

4

5

6

7

8

9

10

1.48 1.50 1.52 1.54 1.56 1.58 1.60 1.62-12.5

-12.0

-11.5

-11.0

-10.5

-10.0

-9.5

ln(R

0T-2 P)

T-1P(10-3K-1)20oC/min

15oC/min

10oC/min

5oC/min

EndoUp(a.u.)

T(oC)

2.5oC/min

(b)

FIG. 4. �a� DSC traces for two heating cycles at heating rate of 10 °C, �b� DSC traces recorded at various heating rates. Inset shows Kissinger plot.

013910-4 Jaffari et al. J. Appl. Phys. 107, 013910 �2010�

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ln�R0

Tp2� = −

Ea

RTp+ const. �1�

where R0 is the scan rate �°C /min�, Tp is the peak tempera-ture, Ea is the activation energy of phase transformation, andR is the gas constant. Kissinger plots are produced in Fig.4�b�. Activation energy calculated from the slope of thecurve is 160 kJ/mol. Previously, activation energy for theformation of CdS nanotubes is reported to be 36 kJ mole−1,as calculated from XRD results.25 A higher value of activa-tion energy is expected since the diffusion as well as chemi-cal transformations processes are involved in the formationof hollow NiFe2O4 nanoparticles.. Same thermodynamic ar-gument holds for the instability of hollow nanoshells. Vacan-cies flow outward from the inner surface and are absorbed atthe outer surface since the vacancy concentration at the innersurface is higher than at the outer surface. This leads to theshrinkage of the spheres and finally ends up in eliminatingthe empty space completely to minimize the interface energy.Gusak et al.26 showed that shrinkage is even more severe forsingle-element hollow nanospheres than for compounds be-cause of a “reverse Kirkendall effect.” Atoms flow inward tocompensate the out-diffusion of vacancies, which then re-duce the vacancy flux occurring in the reverse Kirkendalleffect. However, in our case we have binary metal precursorNi33Fe67 for the chemical reaction which is why we wereable to see the full formation and collapse of holes as afunction of reaction temperature. Collapse of holes requireshigher thermal energy which requires higher reaction tem-peratures ��400 °C� which appears as a second endothermin DSC traces, as seen in Fig. 4.

D. Magnetic properties

Figure 5�a� shows the magnetic hysteresis curves re-corded at room temperature for all the samples. All samplesare superparamagnetic at 300 K. Hysteresis curves recordedin the cooling field of 5 kOe at 5 K are shown in Fig. 5�b�. Itis noticeable that for both temperatures, 5 and 300 K, satu-ration magnetization MS is at a minimum for sample with TR

of 350 °C and increases as TR increases to 450 °C and the to550 °C. It is well known in nanoparticles that surface spindisorder leads to spin glasslike behavior and to the suppres-

sion of MS.27 For hollow NiFe2O4 particles �TR=350 °C�inner surface spin disorder leads to a lower value of MS. Forsample with TR=450 °C, cavity diameter decreases whichleads to lower the inner surface area and lesser degree of spindisorder compared to sample with TR=350 °C. Hence, MS

increases for sample with TR=450 °C. For sample with TR

=550 °C, cavity in hollow NiFe2O4 collapses completelyleading to further increase of MS.

In order to further establish spin glass behavior, we havecarried out exhange bias measurements. In nanoparticles, ex-change bias arises due to the interaction between the ordercore and disordered surface spins or spin glasslike structureof surface spins.10,27 This is because the core spins arealigned ferromagnetically while spin behavior is quite com-plex at the surface. The atomic coordination number of thesurface spins is different from that of the core which in turncauses perturbations in the crystal field destabilizing themagnetic order at the surface. Therefore, surface spin align-ments can take a multiplicity of forms which leads to severaldifferent ground states resulting in “spin glassstructure.”8,27,28 The core shell interaction at the interfacegives rise to the phenomenon of exchange bias effect. Thepresence of an exchange bias reported for the NiFe2O4 and�-Fe2O3 nanoparticles is an example of two different spinpopulation systems present in the nanoparticle, one that isferromagnetically ordered and another has spin glassstructure.27,28 In small particles, surface spin glass behaviorleads to interesting magnetic properties such as exchangebias, high field irreversibility, and relaxation dynamics.29,30

In order to investigate shift in the M�H� loops, in otherwords the exchange bias HEB, we have carried out 2 T fieldcooled M�H� measurements at 5 K. As expected, all the threemorphologies of NiFe2O4 structure exhibited loop shifts, asshown in Fig. 5. Sample with TR=350 °C with hollowNiFe2O4 morphology has the highest loop shift of 301 Oe.The loop shift decreases to 165 Oe as the cavity size de-creases for sample with TR=450 °C. However, sample withTR=550 °C shows significantly less loop shift of about 59Oe. This decrease in the loop shift is understandable in termsof surface spin glass model for nanoparticles. Sample withhollow morphology �TR=350 °C� has the highest surfacearea due to the presence of inner and outer surfaces. Corre-

FIG. 5. �Color online� �a� Hysteresis loops recorded at 300 K �b� Field cool hysteresis loops recorded at 5 K in 2 T applied field. Inset shows magnificationof low field portion.

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spondingly, this sample has the highest surface spin disorder,exhibiting highest loop shift. For the sample with less con-tribution from the inner surface �TR=450 °C�, there is a de-crease in exchange bias because of the collapse of hollowstructure. As the hollow morphology disappears for samplewith TR=550 °C, there is a significant decrease in the HEB.Both MS and HEB results show strong effects of surface dis-order on magnetic properties and are summarized in Table I.

For small particles exhibiting superparamagnetism, mag-netization, versus temperature measurements gives furtherinsight in to the magnetism of such systems. Superparamag-netism is a form of magnetism which leads to the randomfluctuations of the magnetic moment of the single-domainparticles when the thermal energy becomes comparable withthe anisotropy energy. Decreasing the temperature below acertain value, known as the blocking temperature �TB�, theanisotropy energy will overcome the thermal energy and themagnetization of each particle begins to align along the easyaxes. Under these conditions, the magnetic system behavesas a classical paramagnet.31 According to Stoner–Wohlfarththeory, the blocking temperature �TB= �K /25kB� ·V� increaseswith the volume V of the nanoparticles. Here, K representsthe anisotropy constant and kB is the Boltzmann constant.32

In order to observe effect on the anisotropy with the mor-phology of the particles, magnetization as a function of tem-perature is recorded and is shown in Fig. 6. Blocking tem-perature of hollow NiFe2O4 particles �TR=350 °C� is 130 Kand decreases to 123 K with the collapse of voids for thesample with TR=450 °C. It is noticeable that for samplehaving nonhollow morphology �TR=550 °C� TB further de-creases to 88 K. Therefore, samples with hollow morphologyhave higher anisotropy constant. The average particle size is8, 10, and 16 nm for samples with TR=350, 450, and550 °C, respectively. Even if there is an increase in the par-ticles size, TB �=�K /25kB� ·V� decreases. Therefore, there is asignificant increase in the anisotropy of the particles withhollow morphology. Additonal surface effects lead to an in-verse trend in blocking temperature variation with the par-ticles size. This shows that the inner surface of the hollownanoparticles acts as an extra source of anisotropy. Inner andouter surface spin disorder can be used to tailor the aniso-tropy of the magnetic particles to push superparamagneticlimit to even lower dimensions which is favorable for otherapplications. Skumryev et al.9 showed that antiferromagnetic

matrix can be used as an extra source of anisotropy for su-perparamagnetic nanoparticles to enhance the blocking tem-perature significantly. This study shows that the hollow mag-netic nanoparticles may also be used, with their inner surfacecontributing as an extra source of anisotropy to overcomesuperparamagnetic effects.

IV. CONCLUSION

We have discussed the formation of hollow nanoparticlesbased on Kirkendall effect coupled with interfacial chemicaltransformation to NiFe2O4 structure. Formation and collapseof voids in the nanoparticles has been tracked in detail. Thekinetics of the �Ni33Fe67� / �NiFe2O4� nanoparticles toNiFe2O4 hollow nanoparticles transformation has been dis-cussed and the activation energy for the transformation isfound to be 160 kJ/mol, as measured by DSC. NanosizedNiFe2O4 has been studied extensively since surface spinglass behavior leads to interesting magnetic properties suchas exchange bias, high field irreversibility, and relaxationdynamics.10,27,29,30 We described strong effects of enhancedsurface area of particles with hollow morphology comparedto particles with nonhollow morphology. Significantly highervalue of exchange bias and blocking temperature for par-ticles with hollow morphology was the reason for the en-hancement in anisotropy due to the additional spin glassphase at the inner surface of the hollow particles. Noticeably,we have found inverse trend of blocking temperature as afunction of particle size.

The synthesis technique described is simple and can beextended to the synthesis of various other ferrites such asXFe2O4 �X=Co, Mn, and Cu� from their respective core/shell metal/ferrite structures as precursors. The hollow mor-phology can be used as an extra degree of freedom to controlmagnetic properties. These particles have potential applica-tions in Memristors,33 hyperthermia agent in biomedicine,34

and targeted drug delivery agent.4,35

ACKNOWLEDGMENTS

The authors would like to acknowledge the support ofPakistan Higher Education Commission under the project“Development and Study of Magnetic Nanostructures.”

1R. Y. Hong, B. Feng, L. L. Chen, G. H. Liu, H. Z. Li, Y. Zheng, and D. G.

FIG. 6. Magnetization vs temperature measurement in 5 kOe applied field.

013910-6 Jaffari et al. J. Appl. Phys. 107, 013910 �2010�

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