research article magnetic and structural studies of cofe...

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2013 Article ID 741036 9 pageshttpdxdoiorg1011552013741036

Research ArticleMagnetic and Structural Studies of CoFe2O4 NanoparticlesSuspended in an Organic Liquid

Branka BabiT-StojiT1 Vukoman JokanoviT1 Dušan MilivojeviT1 Zvonko JagliIiT2

Darko Makovec3 Nataša JoviT1 and Milena MarinoviT-CincoviT1

1 Vinca Institute of Nuclear Sciences University of Belgrade PO Box 522 11001 Belgrade Serbia2 Institute of Mathematics Physics and Mechanics Jadranska 19 1000 Ljubljana Slovenia3 Department for Materials Synthesis Jozef Stefan Institute Jamova 39 1000 Ljubljana Serbia

Correspondence should be addressed to Branka Babic-Stojic babicvincars

Received 16 September 2013 Accepted 17 October 2013

Academic Editor Haifeng Chen

Copyright copy 2013 Branka Babic-Stojic et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

We present a study of magnetic and structural properties of CoFe2O4nanoparticles suspended in an organic liquid Transmission

electron microscopy shows that the nanoparticles have a narrow size distribution of average particle size 59 plusmn 10 nm X-raydiffraction shows that the particles are of cubic spinel crystal structure Dynamic light scatteringmeasurements reveal the existenceof an organic shell around the CoFe

2O4nanoparticles with an average hydrodynamic diameter of 144 nm Coercive magnetic field

at 119879 = 5K is found to be 118 kOe Disappearance of the coercive field and remanent magnetization at about 170K suggests thatthe CoFe

2O4nanoparticles are superparamagnetic at higher temperatures which is confirmed by the room temperatureMossbauer

spectrum analysis Saturation magnetization of the nanoparticles of 808 emug(CoFe2O4) at 5 K reaches the value detected in the

bulk material and remains very high also at room temperature The cobalt ferrite nanoparticle system synthesized in this workexhibits magnetic properties which are very suitable for various biomedical applications

1 Introduction

Ferrofluids are stable colloidal suspensions of single domainmagnetic particles in a carrier liquid Stability of themagneticcolloid depends on the thermal contribution and on thebalance between attractive and repulsive interactions Mag-netic dipole and van der Waals interactions have tendency toagglomerate the particles In order to prevent agglomerationthe magnetic particles should be small enough (usually about10 nm in size) and coated with a shell of an appropriatematerialThis coating can be a sufactantmade of long chainedmolecules or ionic if it is an electric shell [1ndash3] The particlesare usually made of maghemite 120574-Fe

2O3 magnetite Fe

3O4

and other ferrites of the type MFe2O4 where M = Mn

Co Ni Cu The carrier liquid is usually an organic solventor water Ferrofluids respond to an external magnetic fieldThis enables the ferrofluidrsquos location to be controlled throughthe application of a magnetic field Ferrofluids have a widerange of applications They can be used to improve the

performance of loud speakers [2] Ferrofluids are also usedin high-speed computer disk drives to eliminate harmfuldust particles or other impurities that can cause the data-reading heads to crash into the disks In recent years a lotof research work has been devoted to biomedical applica-tions of ferrofluids In magnetic resonance imaging (MRI)ferrofluids are used as contrast agents [3ndash5] Ferrofluidsspecially designed can carry drugs to specific locations inthe living body through the use of applied magnetic fields[3 6]Heat treatment of organs or tissues called hyperthermiauses the property of ferrofluids of absorbing electromagneticenergy This allows heating of a localized part of a livingbody such as malignant tumor up to temperature 42ndash46∘Cwhich reduces the viability of cancerous cells [3 4 6] Forvarious applications particularly in biomedicine magneticparticles in ferrofluids besides the requirement for smallsizes must have large enough magnetization Depending onthe relative magnitude of anisotropy energy with respectto thermal energy the particles exhibit superparamagnetic

2 Journal of Nanomaterials

behavior when magnetic moment of the particle overcomesthe anisotropy energy barrier by thermal activation For agiven temperature 119879 and time 120591 characteristic to a specificmeasurement there is a critical particle diameter119863sp (assum-ing spherical shape) that separates smaller particles 119863 lt 119863spexhibiting superparamagnetism from larger particles 119863 gt

119863sp which have ferro(ferri)magnetic properties For cubicmagnetocrystalline anisotropy the energy barrier is 119864 =

1198701198814 where 119881 is the particle volume The critical size ofthe CoFe

2O4particles with anisotropy constant 119870 = 2 times

106 erg cmminus3 at 119879 = 300K [7] and for 120591 = 10

2 s can be esti-mated [8] as 119863sp asymp 16 nm In the case of axial magnetocrys-talline anisotropy the energy barrier is 119864 = 119870119881 and for thesame values of the parameters119870 119879 and 120591 the critical particlesize is119863sp asymp 10 nm

There are two dominant mechanisms of magnetic relax-ation in a ferrofluid the Brownian relaxation and Neel relax-ation When the relaxation is dominated by the Brownianmechanism the magnetic moment is rotated together withthe nanoparticle in the carrier liquid under the influence of anexternal magnetic field [9] Relaxation time for the Brownianrotational motion is

120591119861=3119881ℎ120578

119896119861119879

(1)

where 119881ℎis the hydrodynamic volume of the particle 120578 is

the dynamic viscosity of the carrier liquid and 119879 is thetemperature At temperatures below the freezing point of thecarrier liquid the particles are immobilized and in that caserelaxation of the particle magnetic moment can take placeonly by rotation of themomentwithin the particle Relaxationtime for this rotation known as the Neel relaxation is

120591119873= 1205910exp( 119864

119896119861119879

) (2)

where 1205910is the characteristic time of the system

Up to now the research attention has been focusedmainly on magnetic properties and magnetic relaxation inferrofluids with iron oxide nanoparticles (NPs) [10ndash15]Thesematerials are already in use as contrast agents for MRI andare under investigation for several other applications Onthe other hand the use of materials with larger magneticanisotropy and larger magnetic moment is considered sinceit may allow the reduction of particle size The bulk cobaltferrite is a magnetic material with almost the same saturationmagnetization as the bulk iron oxide Fe

3O4 but with one

order of magnitude higher magnetocrystalline anisotropycompared to magnetite Because of these properties thecobalt ferrite nanoparticle systems are promising materialsfor high-density magnetic recording media and also veryinteresting for various biomedical applications [16ndash20] Inthis work we have synthesized CoFe

2O4NPs in an organic

carrier liquid Microstructure morphology and static anddynamic magnetic properties of the prepared nanomaterialwere studied using X-ray diffraction (XRD) transmissionelectron microscopy (TEM) dynamic light scattering (DLS)Fourier-transform infrared (FTIR) spectroscopy differentialscanning calorimetry (DSC) dc magnetization ac suscepti-bility measurements andMossbauer spectroscopyWe found

that the properties of this NPs system correspond to ferroflu-ids it is stable for months magnetic nanoparticles do notagglomerate and have reproducible magnetic properties Itsmain characteristics are high saturationmagnetization at lowtemperatures close to that of the bulkmaterial and superpara-magnetic properties of most of the CoFe

2O4nanoparticles at

room temperature

2 Experimental Details

21 Synthesis Synthesis of the CoFe2O4nanoparticles was

carried out via the polyol route using Co(II) acetate Fe(III)acetonylacetonate and diethylene glycol (DEG) fromAldrichChemical Co as starting materials These precursors weretaken in stoichiometric ratio for obtaining the CoFe

2O4

phase 223775 g Fe(III) acetonylacetonate and 78870 g Co(II)acetate were mixed as solutions inside of diethylene glycolsolvent which has been previously divided in two parts thefirst volume of 150mL for dissolution of Fe(III) acetonylace-tonate and the second volume of 50mL for dissolution ofCo(II) acetate The mixture of both solutions was magneti-cally stirred under the flow of nitrogen After that the spaceof the vessel filled with nitrogen was closed and the mixturewas heated firstly to 120∘C with a heating rate of 4∘C perminute and then the temperature was increased to 160∘Cand maintained at this level for 3 h Finally the temperaturewas increased to 180∘C and after one hour was abruptlylowered with a high cooling rate by cold water introducedthrough corresponding inlet The water flowed with a highflux around the working volume of the vessel causing afast change of temperature from the top temperature tothe room temperature The particle solution was filtered toexclude aggregates AVivaspin filter (polyethersulfone (PES)Vivascience Sartorius Hannover Germany) with pore size02 120583m was used and the filtering process was performedby centrifuging the sample at 1750 rpm for about 30minuntil all fluid had passed the filter Besides the liquid samplea powder sample was also prepared by drying the suspensionof CoFe

2O4NPs at 200∘C for 10 h

22 Experimental Technique X-ray characterization of thesample was carried out on a Philips PW 1050 powder diffrac-tometer using Cu K120572 radiation The X-ray diffraction (XRD)pattern was taken in the 17∘ndash70∘2120579 range with a step of 005∘at a slow scan rate of 40 s per step For transmission electronmicroscopy (TEM) measurements the sample was preparedby drying a drop of the diluted suspension on a copper-grid-supported perforated transparent carbon foil High-resolution transmission electron microscopy (HRTEM) andenergy dispersive X-ray spectroscopy (EDXS) were carriedout using a field-emission electron-source transmission elec-tron microscope JEOL 2010 equipped with an EDXS micro-analysis system (LINK ISIS EDS 300) operated at 200 kVAverage FeCo ratio in the composition of the nanoparticleswas obtained from quantification of a large number ofEDXS spectra using the Oxford ISIS software The spectrawere collected from areas of the sample containing severalhundreds of the nanoparticles For the quantification of the

Journal of Nanomaterials 3

spectra the CoFe2O4ceramics were used as a standard

The relative standard deviation of the method calculatedfrom 20 measurements of the FeCo ratio on the standardwas found to be inside plusmn26 (FeCostandard = 1992 plusmn

0052) The size distribution of the CoFe2O4nanoparticles

was measured by dynamic light scattering (DLS) usingZetasizer Nano ZS (Malvern UK) equipped with greenlaser (532 nm) Intensity of scattered light was detected atthe angle of 173∘ All measurements were conducted atroom temperature Ten measurements were performed foreach sample All data processing was done by the Zetasizersoftware 620 (Malvern instruments) The size distributionis reported as distribution by number Fourier-transforminfrared (FTIR) transmission spectrum was taken at roomtemperature on a Perkin-Elmer 983G spectrophotometer inthe range 600ndash3900 cmminus1 Differential scanning calorimetry(DSC) measurements were performed using Setaram 151 R(softer SETSOFT 2000) instrument in the temperature range150ndash240K under helium atmosphere Magnetic measure-ments were carried out on a SQUID magnetometer (MPMSXL-5 Quantum Design) equipped with an ac option Zero-field-cooled (ZFC) and field-cooled (FC) measurements ofmagnetization were performed at applied magnetic fieldsfrom 100 to 10000Oe in the temperature range from 2to 300K Magnetic field dependence of magnetization wasmeasured at applied magnetic fields up to 50 kOe in thetemperature range from 5 to 300K The ac magnetizationmeasurements were made under an ac exciting field withamplitude of 65Oe using different frequencies in the range1ndash1500Hz 57Fe Mossbauer spectrum was recorded at roomtemperature using a conventional transmission Mossbauerspectrometer operating in constant acceleration mode Thesource was 57Co in Rh matrix Experimental data were fittedto Lorentzian absorption lines by using a least square basedmethod The spectrometer was calibrated using 120572-Fe foil atroom temperature All the isomer shifts are given relative tothe center of the 120572-Fe spectrum

3 Results and Discussion

31 Structure Analysis Figure 1 shows the XRD pattern ofdry powder sampleThe diffraction peaks are consistent withthose of the cubic spinel phase (space group Fd3m) Thelattice parameter obtained from Rietveld analysis was foundto be 119886 = 8390 A The average crystallite size determinedfrom the width of the X-ray diffraction lines using Scherrerrsquosformula was estimated as 5 nm

The morphology particle size and chemical composi-tion of the CoFe

2O4nanoparticles deposited from organic

liquid dispersion were characterized by TEM HRTEM andEDXS TEM analysis (Figure 2(a)) shows that the samplecontains the nanoparticles with a narrow size distributionThe measurement of approximately 500 nanoparticles fromTEM images gave equivalent diameter of 59 plusmn 10 nmHRTEM imaging showed good crystallinity of the nanopar-ticles (Figure 2(b)) The electron diffraction pattern (insetof Figure 2(a)) corresponds to a spinel structure The EDXSspectra analysis confirmed stoichiometric composition of the

300

350

400

450

500

(220

)(3

11)

(400

) (511

)

(440

)

Inte

nsity

(au

)

(111

)

20 30 40 50 60 70

2120579 (∘)

Figure 1 Powder X-ray diffraction pattern of the CoFe2O4dried

powder sample indexed with the (hkl) reflections of the cubic spinel

cobalt ferrite nanoparticles in the margins of the experimen-tal uncertainty The measured FeCo ratio was 208 plusmn 014

The size distribution of the CoFe2O4nanoparticles in

the organic liquid was studied using dynamic light scattering(DLS) The result is presented in Figure 3 Measured numberof particles as a function of hydrodynamic particle diametercan be well described by a log-normal distribution

119891 (119889) =1

radic2120587120590119889

exp[minusln2 (119889119889

0)

21205902

] (3)

The best fit yields the distribution width 120590 = 022 andmedian of the distribution 119889

0= 141 nm related to the

average hydrodynamic diameter 119889av = 1198890exp(12059022) =

144 nm The hydrodynamic diameters are larger than thosemeasured by TEM This is because DLS measures the sizeof the particles together with organic shell which cannotbe observed in the TEM images due to poor contrast withrespect to the background as a result of low electron densityof the organic structure Difference between the averagehydrodynamic diameter of the particles and equivalent TEMdiameter gives a measure of the thickness of the organic shellThe organic shell thickness thus estimated is about 4 nm

FTIR transmission spectrum of the organic liquid sus-pension of CoFe

2O4NPs was measured at room temperature

in the region from 600 to 3900 cmminus1 (Figure 4) The band at2350 cmminus1 can be assigned to (Co Fe)-OH vibrations causedby the interaction of theCoFe

2O4minus119909

(OH)119909surface layer of the

nanoparticles with polar group chains of the diethylene glycol(DEG) Almost negligible band at 2331 cmminus1 belongs probablyto the symmetric stretching mode of the ]

119904CH2 The band at

1625 cmminus1 belongs to the ]119904(COOminus) asymmetric stretching

mode of carboxyl acids A slightly pronounced band at1316 cmminus1 belongs to the C-O-C stretching vibrations insideof residual DEG chains [17 21]The slightly pronounced bandarising from the residual DEG chains with missing othercharacteristic bands for DEG at 1050 and 1080 cmminus1 shows


(a) (b)

Figure 2 TEM (a) and HRTEM (b) images of the CoFe2O4nanoparticles Electron diffraction pattern in the inset of Figure 2(a) was indexed

according to the spinel structure

0

5

10

15

20

25

30

Num

ber o

f par

ticle

s (

)

5 10 15 20 25 30 35Hydrodynamic diameter (nm)

Figure 3 Hydrodynamic size distribution of the CoFe2O4nanopar-

ticles suspended in the organic liquid measured by dynamic lightscattering The solid curve is the fit of the dependence (3) to theexperimental data

that DEG is mostly changed probably into diethylene glycolacetate (ester compound) and diethylene glycol acetony-lacetonate The band at 2350 cmminus1 proves the existence ofsignificant interaction between the CoFe

2O4minus119909

(OH)119909surface

layer of the nanoparticles and organic shell It can be noticedalso that a very small fraction of DEG remained unreacteddue to excess of DEG compared to carboxyl acid groupsinside of starting mixture for the synthesis of the organicliquid suspension of CoFe

2O4NPs

32 Magnetic Properties

321 dc Magnetization The temperature dependence ofzero-field-cooled and field-cooled magnetization curves forthe organic liquid suspension of CoFe

2O4NPs was mea-

sured at applied magnetic fields from 100 to 10000Oe

80

84

88

92

96Tr

ansm

ittan

ce (

)

2350 20791984

1625 1316

20162331

3500 3000 2500 2000 1500 1000

Wave number (cmminus1)

Figure 4 Fourier-transform infrared transmission spectrum ofthe organic liquid suspension of CoFe

2O4nanoparticles at room

temperature

The concentration of the CoFe2O4phase in the liquid sample

was 0045 gmL which corresponds to the volume fractionof 084 Magnetization values are expressed per unit massof CoFe

2O4taking into account the mass fraction of the

CoFe2O4nanoparticles in the liquid The representative

ZFC-FC curves for several selected magnetic field valuesare shown in Figure 5 To explore possible aging effectswe compared these measurements with the previous runsperformed several months ago Measured magnetizationswere reproduced within a few percent of precision As canbe seen the ZFC and FCmagnetizations of the liquid samplebifurcate at a certain temperature and show a broad peakin the ZFC curve with a maximum at temperature 119879maxThe broad peak in the ZFC magnetization is characteristicof superparamagnetic nanoparticles with size distributionwhere the temperature of the maximum 119879max represents theaverage blocking temperature The temperature 119879max for the


0

10

20

30

40

50

05

1015202530

0 50 100 150 200 250 300

0 50 100 150 200 250 300

1000Oe

500Oe

250Oe100Oe

T (K)

T (K)

Powder

TmaxTp

H = 1000Oe

M(e

mu

g(C

oFe 2

O4))

M(e

mu

g(C

oFe 2

O4))

Figure 5 Temperature dependence of the zero-field-cooled (ZFC)and field-cooled (FC) magnetization curves for the organic liquidsuspension of CoFe

2O4nanoparticles at several selected magnetic

field values The inset shows the temperature dependence of theZFC-FC magnetization of dry powder at 119867 = 1000Oe

liquid sample extends in the range 156ndash164K dependingon the magnetic field values in the range 119867 le 1000OeBesides the broad peak in the ZFC magnetization a cusp atabout 119879

119901asymp 200Kwas also detectedwhich can be clearly seen

for magnetic fields 119867 ge 500Oe (Figure 5)In order to get some insight into the origin of the cusp

in the ZFCmagnetization of the organic liquid suspension ofCoFe2O4NPs at about 200K we have measured the ZFC-

FC magnetization of dry powder at 119867 = 1000Oe whichis shown in the inset of Figure 5 As can be seen there isno cusp in the ZFC magnetization of the powder samplein contrast to the liquid sample where this cusp is clearlyobserved This finding suggests that the origin of the cusp inthe ZFC magnetization of the liquid sample at about 200K isassociated with the carrier liquid We performed differentialscanning calorimetry (DSC) measurements on the liquidsample and the result is presented in Figure 6 Three distinctpeaks in DSC graph were found The first peak at about204K indicates the temperature where a part of the organicliquid enters the frozen or so called mixed state [11] Thesecond peak at about 183 K probably is due to freezing ofthe remaining part of the organic liquid The peak at about164K indicates transition temperature of the liquid from thepartly frozen (mixed) state to the completely frozen (solid)state It can be noticed that the temperature of the cuspobserved in the ZFC magnetization at about 119879

119901asymp 200K is

just below the temperature 204K where the organic liquidenters the frozen state The fact that the magnetization ofthemagnetic nanoparticles dispersed in the liquid is sensitiveto the Brownian motion of the particles in that liquid andthat there is no anomalous behavior of the magnetizationin the dry powder gives evidence that the cusp in the ZFCmagnetization of the organic liquid suspension of CoFe

2O4

NPs is due to the freezing of the organic liquid

0

2

4

6

Temperature (K)

160 180 200 220 240

minus4

minus2

204K

164K

183K

Hea

t flow

(au

)

Figure 6 Differential scanning calorimetry curve for the liquidsample of the CoFe

2O4nanoparticles in the temperature range 150ndash

240K

The magnetic field dependence of magnetization forthe suspension of the CoFe

2O4NPs at 119879 = 5K and

300K is presented in Figure 7The119872(119867) curve at 119879 = 300Kexhibits superparamagnetic behavior the coercive field 119867

119888

and remanent magnetization 119872119903are zero and the satura-

tion magnetization estimated by extrapolation of the 119872

versus 1119867 curve in the limit 1119867 rarr 0 is 501 emug(CoFe

2O4) For comparison saturation magnetization of the

bulk CoFe2O4at 300K is 119872

119904(bulk) asymp 76 emug [22] At 119879 =

5K the liquid sample exhibits hysteresis loop The 119872(119867)

measurements carried out at magnetic fields in the range ofplusmn50 kOe gave 119867

119888= 118 kOe 119872

119903= 391 emug(CoFe

2O4)

and 119872119904= 808 emug(CoFe

2O4) which was estimated also

by the extrapolation of the 119872 versus 1119867 dependence in thelimit 1119867 rarr 0 The saturation magnetization at 119879 = 5Kfalls into the region of the 119872

119904values for the bulk CoFe

2O4

ranging from 80 to 94 emug [23] The saturation magnetiza-tion of the CoFe

2O4NPs in the liquid sample at 5 K as well as

that at room temperature is considerably higher than that forthe CoFe

2O4NPs obtained by either thermal decomposition

[24 25] or microemulsion route [26] or mechanical milling[27] It appears that the method of synthesis applied in thepresent work favors good crystallinity andmagnetic orderingresulting in a high saturation magnetization

Saturationmagnetization of 808 emug(CoFe2O4) for the

CoFe2O4NPs in the liquid sample at 119879 = 5K corresponds

to 120583119904= 34120583

119861per unit chemical formulawhich is only slightly

larger than that for inverse spinel (3120583119861) Utilizing the formula

(Co1minus120575

Fe120575)[Co120575Fe2minus120575

]O4to describe the cation distribution

in the spinel structure of the CoFe2O4nanoparticles where 120575

is the inversion parameter and assuming that Fe3+ and Co2+ions have a magnetic moment of 5120583

119861and 3120583

119861 respectively

we find that 120575 = 091 The value of the inversion parameter120575 indicates that the crystal structure of the CoFe

2O4NPs is

very close to the inverse spinelCoercive field and saturation magnetization of the liquid

sample as a function of temperature are presented in the


0

1

2

3

0

0

0

4

8

12

20 6040

70

80

H (kOe)

(a)

(b)

T (K)

60

minus3

minus20minus40minus60

minus2

minus1

5050 100150200250300

5K300K

M(120583

Bfo

rmul

a uni

t)

Ms

(em

ug(

CoF

e 2O4))

Hc(

kOe)

Figure 7 Magnetic field dependence of the magnetization for theorganic liquid suspension of the CoFe

2O4nanoparticles at 5 K and

300K The inset shows the temperature dependence of (a) coercivefield and (b) saturation magnetization

inset of Figure 7 The coercive field decreases with increasingtemperature and vanishes at about 170K The saturationmagnetization exhibits specific properties First there isa small kink in the 119872

119904versus 119879 plot at 170K where the

coercive field vanishes and second the character of thefunction 119872

119904(119879) is changed at this temperature Remanent

magnetization (not shown) also decreases with increasingtemperature and vanishes at about 170K

The broad peak in the ZFC magnetization at 119879maxextending from 156 to 164K for the magnetic field in therange 119867 le 1000Oe as well as the disappearance of thecoercive field and remanent magnetization at about 170Ksuggests that a large fraction of the CoFe

2O4nanoparticles

in the organic liquid are subject to the blocking process attemperatures below 170 K

322 ac Magnetic Susceptibility Figure 8 shows the tem-perature dependence of (a) the real part 120594

1015840 and (b) theimaginary part 12059410158401015840 of the ac magnetic susceptibility for thesuspension of the CoFe

2O4NPs at different frequencies in

the range 1ndash1500Hz It can be seen that the real component1205941015840 has a maximum at a temperature 119879

1198981in the vicinity

of 200K which shifts towards higher temperatures withincreasing frequency The imaginary part 12059410158401015840 has a maxi-mum at a temperature 119879

1198982around 160K which also shifts

towards higher temperatures with increasing frequency Thisproperty is not only characteristic of most of the spin glasssystems but also of the interacting and noninteracting super-paramagnetic nanoparticles It should be noticed that thetemperature 119879

1198981falls into the temperature region of freezing

of the organic liquid while 1198791198982

falls into the temperatureregion of the broad peak in the ZFC magnetization of theliquid sample In fact the temperature 119879

1198982is significantly

lower than the temperatures around 200K where the organicliquid enters the frozen (mixed) state Therefore we couldexpect that at 119879

1198982the particles are mainly frozen and cannot

move and relaxation of the particle magnetic moments inthat case should occur relative to the particle through theNeelrelaxation

According to Neel [28]magneticmoment of noninteract-ing single domain particleswith uniaxialmagnetic anisotropyrelaxes via an activated process 120591 = 120591

0exp(119864119896

119861119879) that is

the Arrhenius law where 119864 is the barrier due to anisotropyenergy required for reversal of magnetic moment orientationand 120591 is the relaxation time At temperatures where therelaxation time is comparable with the experimental timescale the spins will seem to be frozen Thus the blockingtemperature depends on the relaxation time 120591 sim 1119891where 119891 is the frequency of the experiment We assume thatthe temperature of the maximum in 120594

10158401015840 is the blocking tem-perature of the CoFe

2O4particle magnetic moments Taking

120591 = 12120587119891 the relation between 1198791198982

and frequency 119891 is119891 = 119891

0exp(minus119864119896

1198611198791198982) where 119891

0= 12120587120591

0 We find that

1198791198982

can be fitted to the Arrhenius law However the fittedparameters are large or even have unphysical values 120591

0=

121205871198910= 29 times 10

minus18 s and 119864119896119861= 5360K

In the further analysis we take the interparticle interac-tions into account and describe the relaxation time of theparticles in the vicinity of the temperature 119879

1198982by the Vogel-

Fulcher law

120591 = 1205910exp[ 119864

119896119861(119879 minus 119879

0)

] (4)

where 119864 is the energy barrier and the parameter 1198790depends

on the interparticle interactions [11] Now the relationbetween 119879

1198982and frequency 119891 is 119891 = 119891

0exp[minus119864119896

119861(1198791198982

minus

1198790)] A good fit of the Vogel-Fulcher law (4) to the exper-

imental data ln119891 versus 1198791198982(119891) is obtained with physically

meaningful parameters and the result is shown in Figure 9The best fit yields 119891

0= (16 plusmn 03) times 10

11Hz 1205910= 12120587119891

0=

(10 plusmn 03) times 10minus12 s 119864119896

119861= (2300 plusmn 300)K and 119879

0= (465 plusmn

5)K The parameter 1198790presents the average value of the

interparticle interactions observed in the 12059410158401015840(119879) dependencein the measured frequency range In the case of axial mag-netocrystalline anisotropy the energy barrier of superpara-magnetic particles is related to the anisotropy constant 119864 =

119870119881 For 119864119896119861= 2300K and average volume of the particles

assuming spherical geometry ⟨119881⟩ = 1075 times 10minus19 cm3

119870 = 3 times 106 ergcm3 which is somewhat larger than that

in the bulk material [23] High saturation magnetization ofthe synthesized CoFe

2O4NPs with a narrow size distribution

in combination with superparamagnetic properties observedat room temperature makes these particles with appropriatecoating and functionalization very promising materials forbiomedical applications especially in magnetic targeted drugdelivery and in magnetic fluid hyperthermia

Temperature of the maximum in the real part of theac magnetic susceptibility 119879

1198981appears in the vicinity of

200K Figure 8(a) that is in the region where the organicliquid enters the frozen (mixed) state In the mixed statethe system is not yet a rigid solid and the nanoparticles


000

001

002

003

004

005

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400

(em

uO

e g(C

oFe 2

O4))

(a)

0000

0001

0002

0003

0004

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400998400

(em

uO

e g(C

oFe 2

O4))

(b)

Figure 8 Temperature dependence of (a) the real part 1205941015840 and (b) the imaginary part 12059410158401015840 of the ac magnetic susceptibility for the organicliquid suspension of the CoFe

2O4nanoparticles at different frequencies in the range 1ndash1500Hz

0

2

4

6

8

140 160 180 200 220

lnf

(Hz)

T (K)

Tm1

Tm2

Figure 9 Frequency dependent temperature 1198791198981

of the maximumin the real part 1205941015840 and temperature 119879

1198982of the maximum in the

imaginary part 12059410158401015840 of the acmagnetic susceptibilityThe solid curvesrepresent fit of the Vogel-Fulcher law (4) to the experimental data

are free to move locally Brownian relaxation arises as aresponse of the nanoparticles magnetic moments to theexternal magnetic field through the particle movement TheVogel-Fulcher law was found to describe successfully theBrownian relaxation in the ferrofluids containing magnetitenanoparticles [11 15] The exponential temperature depen-dence of 120591

119861is attributed to divergence of the viscosity of the

liquid at the characteristic temperature 11987910158400 as found in many

glass-forming liquids [29] Using the Brownian relaxation

time in the form 120591119861

= 1205911015840

0exp[1198641015840119896

119861(119879 minus 119879

1015840

0)] the relation

between 1198791198981

and frequency 119891 is 119891 = 1198911015840

0exp[minus1198641015840119896

119861(1198791198981

minus

1198791015840

0)] We find that the Vogel-Fulcher law can be fitted to

the experimental data ln119891 versus 1198791198981(119891) with the following

parameters 12059110158400= 12120587119891

1015840


minus6 s 1198641015840119896119861

=

(20plusmn02)times102 K and 119879

1015840

0= (168plusmn10)KThe result of this fit

is shown also in Figure 9 Difference between the parametersin the Vogel-Fulcher law describing the Neel relaxationand the Brownian relaxation can be seen immediately Thecharacteristic time of the system 120591

1015840

0in the Brownian relax-

ation is much larger than 1205910in the Neel relaxation The

large difference between the parameters 1198641015840 associated withthe viscosity of the liquid [11] and 119864 in the Neel relaxationalso indicates different nature of the two relaxations Theparameter 1198791015840

0should correspond to the temperature of the

divergence of the viscosity that is to the temperature wherethe organic liquid passes from the partly frozen (mixed) stateto the completely frozen (solid) state It is interesting to notethat 1198791015840

0= 168K obtained from the Vogel-Fulcher expression

for the Brownian relaxation time as a fitting parameter isin good agreement with 119879 = 164K identified as transitiontemperature from the mixed state to the solid state of theorganic liquid recorded in the DSC measurements

323Mossbauer Spectroscopy 57FeMossbauermeasurementwas performed on the CoFe

2O4nanoparticles with average

particle size of 59 nm The experimental spectrum collectedat room temperature is presented in Figure 10Themain con-tribution to the spectrum belongs to a quadrupolar doubletcharacteristic of a paramagnetic compound showing that thequadrupole interaction is much stronger than the magnetichyperfine interactionThis quadrupolar doublet is ascribed to

8 Journal of NanomaterialsIn

tens

ity (times

106

au)

334

336

338

340

342

minus12 minus8 minus4 0 4 8 12Velocity (mms)

Sextet 1

Doublet 1Doublet 2

SingletIcalcIobs

Figure 10 Mossbauer spectrum of the CoFe2O4nanoparticles in

the dried powder sample at room temperature

the superparamagnetic CoFe2O4nanoparticles Besides the

quadrupolar doublet a six-line hyperfine pattern of lowerintensity was also recordedThe presence of both the doubletsand sextets in the Mossbauer spectrum is usually detectedin an assembly of ultrafine particles with a size distributionThe Mossbauer spectrum of the CoFe

2O4powder sample

has been fitted as a superposition of a broad singlet twoquadrupolar doublets and one sextet The broad singlet isintroduced to delineate process of magnetic relaxations insome grains due to the particle size distribution The isomershift values of two doublets (034 plusmn 001)mms and (043 plusmn006)mms indicate the presence of the Fe3+ ions in two dif-ferent local environmentsThequadrupolar splitting values ofthese doublets are (055plusmn003)mms and (198plusmn019)mmsrespectively Therefore both doublets are assigned to theFe3+ ions one located in the particle core and the otherlocated at the particle surfaceThe doublet withmore positiveisomer shift (043mms) and higher quadrupolar splitting(198mms) can be ascribed to the iron ions located at thesurface layer of the particles The hyperfine field is obtainedas 119861hyp = (437 plusmn 1) kOe This hyperfine field is somewhatlower than the minimal value of the B-site hyperfine fieldfound in the bulk cobalt ferrite [30] Lower values of thehyperfine fields compared to those usually reported for thecobalt ferrite nanoparticles [31 32] were obtained to be about440 kOe at the room temperature Mossbauer spectrum ofthe CoFe

2O4NPs with average diameter of 96 nm [33] In

a detailed analysis of the Mossbauer spectra of the CoFe2O4

NPs with average particle size of 51 86 and 122 nmacceptable fitting data were obtained only when the B-sitepattern was assumed to be a superposition of more than onesextet including those with lower hyperfine fields [27] TheMossbauer spectrum of the CoFe

2O4powder sample studied

in the present work indicates that most of the CoFe2O4

nanoparticles are superparamagnetic at room temperaturewhile a minor fraction of the nanoparticles have blockedmagnetic moments with blocking temperatures above 300KBecause the contribution of the CoFe

2O4particles with

blocked magnetic moments is minor in the spectrum moredetailed analysis concerning the hyperfine fields is unreliable

4 Conclusion

The synthesis procedure of the organic liquid suspensionof the CoFe

2O4nanoparticles applied in the present work

appears to be very promising from the aspect of the tailoringof these particles with well-pronounced magnetic propertiesThe narrow size distribution of the particles high saturationmagnetization and superparamagnetic properties at roomtemperature make them very attractive and suitable forapplication in biomedicine especially in magnetic targeteddrug delivery and in magnetic fluid hyperthermia

Acknowledgment

Financial support for this study was granted by the Ministryof Education Science and Technological Development ofSerbia (Project no 172026)

References

[1] S Odenbach ldquoMagnetic fluidsmdashsuspensions of magneticdipoles and their magnetic controlrdquo Journal of Physics Con-densed Matter vol 15 no 15 pp S1497ndashS1508 2003

[2] S Odenbach ldquoRecent progress in magnetic fluid researchrdquoJournal of Physics Condensed Matter vol 16 no 32 pp R1135ndashR1150 2004

[3] C Scherer and A M Figueiredo Neto ldquoFerrofluids propertiesand applicationsrdquo Brazilian Journal of Physics vol 35 no 3 pp718ndash727 2005

[4] C C Berry ldquoProgress in functionalization of magneticnanoparticles for applications in biomedicinerdquo Journal ofPhysics D vol 42 no 22 Article ID 224003 9 pages 2009

[5] H Tan J M Xue B Shuter X Li and J Wang ldquoSynthesis ofPEOlated Fe

3O4SiO

2nanoparticles via bioinspired silification

for magnetic resonance imagingrdquo Advanced Functional Materi-als vol 20 no 5 pp 722ndash731 2010

[6] R Rahisuddin P K Sharma M Salim and G Garg ldquoAppli-cation of ferrofluid as a targeted drug delivery system in nan-otechnologyrdquo International Journal of Pharmaceutical SciencesReview and Research vol 5 no 3 pp 115ndash119 2010

[7] R Skomski and J M D Coey Permanent Magetism Institute ofPhysics Publishing Bristol UK 1999

[8] M Walker P I Mayo K OrsquoGrady S W Charles and R WChantrell ldquoThe magnetic properties of single-domain particleswith cubic anisotropy I Hysteresis loopsrdquo Journal of Physicsvol 5 no 17 pp 2779ndash2792 1993

[9] E Blums A Cebers and M M Maiorov Magnetic FluidsWalter de Gruyter Berlin Germany 1997

[10] T Jonsson J Mattsson C Djurberg F A Khan P Nordbladand P Svedlindh ldquoAging in amagnetic particle systemrdquo PhysicalReview Letters vol 75 no 22 pp 4138ndash4141 1995


[11] J Zhang C Boyd andW Luo ldquoTwo mechanisms and a scalingrelation for dynamics in ferrofluidsrdquo Physical Review Lettersvol 77 no 2 pp 390ndash393 1996

[12] A Skumiel A Jozefczak T Hornowski andM Labowski ldquoTheinfluence of the concentration of ferroparticles in a ferrofluidon its magnetic and acoustic propertiesrdquo Journal of Physics Dvol 36 no 24 pp 3120ndash3124 2003

[13] M Blanco-Mantecon and K OrsquoGrady ldquoInteraction and sizeeffects in magnetic nanoparticlesrdquo Journal of Magnetism andMagnetic Materials vol 296 no 2 pp 124ndash133 2006

[14] L-Y Zhang Y-H Dou L Zhang and H-C Gu ldquoMagneticbehaviour and heating effect of Fe

3O4ferrofluids composed of

monodisperse nanoparticlesrdquo Chinese Physics Letters vol 24no 2 pp 483ndash486 2007

[15] M B Morales M H Phan S Pal N A Frey and H SrikanthldquoParticle blocking and carrier fluid freezing effects on themagnetic properties of Fe

3O4-based ferrofluidsrdquo Journal of

Applied Physics vol 105 no 7 Article ID 07B511 3 pages 2009[16] G Baldi D Bonacchi C Innocenti G Lorenzi and C Sangre-

gorio ldquoCobalt ferrite nanoparticles the control of the particlesize and surface state and their effects on magnetic propertiesrdquoJournal of Magnetism and Magnetic Materials vol 311 no 1 pp10ndash16 2007

[17] G Baldi D Bonacchi M C Franchini et al ldquoSynthesis andcoating of cobalt ferrite nanoparticles a first step toward theobtainment of new magnetic nanocarriersrdquo Langmuir vol 23no 7 pp 4026ndash4028 2007

[18] G Baldi G Lorenzi and C Ravagli ldquoHyperthermic effect ofmagnetic nanoparticles under electromagnetic fieldrdquo Processingand Applications of Ceramics vol 3 no 1-2 pp 103ndash109 2009

[19] M C Franchini G Baldi D Bonacchi et al ldquoBovine serumalbumin-based magnetic nanocarrier for MRI diagnosis andhyperthermic therapy a potential theranostic approach againstcancerrdquo Small vol 6 no 3 pp 366ndash370 2010

[20] V Cabuil V Dupuis D Talbot and S Neveu ldquoIonic mag-netic fluid based on cobalt ferrite nanoparticles influence ofhydrothermal treatment on the nanoparticle sizerdquo Journal ofMagnetism and Magnetic Materials vol 323 no 10 pp 1238ndash1241 2011

[21] H Guo X Yang T Xiao W Zhang L Lou and J MugnierldquoStructure and optical properties of sol-gel derived Gd

2O3

waveguide filmsrdquo Applied Surface Science vol 230 no 1ndash4 pp215ndash221 2004

[22] H Shenker ldquoMagnetic anisotropy of cobalt ferrite (Co101

Fe200

O362

) and nickel cobalt ferrite (Ni072

Fe020

Co008

Fe2O4) rdquo

Physical Review vol 107 no 5 pp 1246ndash1249 1957[23] V A Brabers ldquoProgress in spinel ferrite researchrdquo inHandbook

of Magnetic Materials K H J Buschow Ed vol 8 North-Holland Amsterdam The Netherlands 1995

[24] M Grigorova H J Blythe V Blaskov et al ldquoMagnetic prop-erties and Mossbauer spectra of nanosized CoFe

2O4powdersrdquo

Journal of Magnetism and Magnetic Materials vol 183 no 1-2pp 163ndash172 1998

[25] V Blaskov V Petkov V Rusanov et al ldquoMagnetic propertiesof nanophase CoFe

2O4particlesrdquo Journal of Magnetism and

Magnetic Materials vol 162 no 2-3 pp 331ndash337 1996[26] NMoumen P Veillet andM P Pileni ldquoControlled preparation

of nanosize cobalt ferrite magnetic particlesrdquo Journal of Mag-netism and Magnetic Materials vol 149 no 1-2 pp 67ndash71 1995

[27] E Manova B Kunev D Paneva et al ldquoMechano-synthesischaracterization and magnetic properties of nanoparticles of

cobalt ferrite CoFe2O4rdquo Chemistry of Materials vol 16 no 26

pp 5689ndash5696 2004[28] L Neel ldquoThermoremanent magnetization of fine powdersrdquo

Review of Modern Physics vol 25 no 1 pp 293ndash295 1953[29] N Menon and S R Nagel ldquoEvidence for a divergent suscepti-

bility at the glass transitionrdquo Physical Review Letters vol 74 no7 pp 1230ndash1233 1995

[30] G A Sawatzky F van der Woude and A H MorrishldquoMossbauer study of several ferrimagnetic spinelsrdquo PhysicalReview vol 187 no 2 pp 747ndash757 1969

[31] N Moume P Bonville and M P Pileni ldquoControl of the size ofcobalt ferrite magnetic fluidsMossbauer spectroscopyrdquo Journalof Physical Chemistry vol 100 no 34 pp 14410ndash14416 1996

[32] A Hutlova D Niznansky J-L Rehspringer C Estournes andM Kurmoo ldquoHigh coercive field for nanoparticles of CoFe

2O4

in amorphous silica sol-gelrdquo Advanced Materials vol 15 no 19pp 1622ndash1625 2003

[33] S R Ahmed S B Ogale G C Papaefthymiou R Rameshand P Kofinas ldquoMagnetic properties of CoFe

2O4nanoparticles

synthesized through a block copolymer nanoreactor routerdquoApplied Physics Letters vol 80 no 9 pp 1616ndash1618 2002

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


behavior when magnetic moment of the particle overcomesthe anisotropy energy barrier by thermal activation For agiven temperature 119879 and time 120591 characteristic to a specificmeasurement there is a critical particle diameter119863sp (assum-ing spherical shape) that separates smaller particles 119863 lt 119863spexhibiting superparamagnetism from larger particles 119863 gt

119863sp which have ferro(ferri)magnetic properties For cubicmagnetocrystalline anisotropy the energy barrier is 119864 =

1198701198814 where 119881 is the particle volume The critical size ofthe CoFe

2O4particles with anisotropy constant 119870 = 2 times

106 erg cmminus3 at 119879 = 300K [7] and for 120591 = 10

2 s can be esti-mated [8] as 119863sp asymp 16 nm In the case of axial magnetocrys-talline anisotropy the energy barrier is 119864 = 119870119881 and for thesame values of the parameters119870 119879 and 120591 the critical particlesize is119863sp asymp 10 nm

There are two dominant mechanisms of magnetic relax-ation in a ferrofluid the Brownian relaxation and Neel relax-ation When the relaxation is dominated by the Brownianmechanism the magnetic moment is rotated together withthe nanoparticle in the carrier liquid under the influence of anexternal magnetic field [9] Relaxation time for the Brownianrotational motion is

120591119861=3119881ℎ120578

119896119861119879

(1)

where 119881ℎis the hydrodynamic volume of the particle 120578 is

the dynamic viscosity of the carrier liquid and 119879 is thetemperature At temperatures below the freezing point of thecarrier liquid the particles are immobilized and in that caserelaxation of the particle magnetic moment can take placeonly by rotation of themomentwithin the particle Relaxationtime for this rotation known as the Neel relaxation is

120591119873= 1205910exp( 119864

119896119861119879

) (2)

where 1205910is the characteristic time of the system

Up to now the research attention has been focusedmainly on magnetic properties and magnetic relaxation inferrofluids with iron oxide nanoparticles (NPs) [10ndash15]Thesematerials are already in use as contrast agents for MRI andare under investigation for several other applications Onthe other hand the use of materials with larger magneticanisotropy and larger magnetic moment is considered sinceit may allow the reduction of particle size The bulk cobaltferrite is a magnetic material with almost the same saturationmagnetization as the bulk iron oxide Fe

3O4 but with one

order of magnitude higher magnetocrystalline anisotropycompared to magnetite Because of these properties thecobalt ferrite nanoparticle systems are promising materialsfor high-density magnetic recording media and also veryinteresting for various biomedical applications [16ndash20] Inthis work we have synthesized CoFe

2O4NPs in an organic

carrier liquid Microstructure morphology and static anddynamic magnetic properties of the prepared nanomaterialwere studied using X-ray diffraction (XRD) transmissionelectron microscopy (TEM) dynamic light scattering (DLS)Fourier-transform infrared (FTIR) spectroscopy differentialscanning calorimetry (DSC) dc magnetization ac suscepti-bility measurements andMossbauer spectroscopyWe found

that the properties of this NPs system correspond to ferroflu-ids it is stable for months magnetic nanoparticles do notagglomerate and have reproducible magnetic properties Itsmain characteristics are high saturationmagnetization at lowtemperatures close to that of the bulkmaterial and superpara-magnetic properties of most of the CoFe

2O4nanoparticles at

room temperature

2 Experimental Details

21 Synthesis Synthesis of the CoFe2O4nanoparticles was

carried out via the polyol route using Co(II) acetate Fe(III)acetonylacetonate and diethylene glycol (DEG) fromAldrichChemical Co as starting materials These precursors weretaken in stoichiometric ratio for obtaining the CoFe

2O4

phase 223775 g Fe(III) acetonylacetonate and 78870 g Co(II)acetate were mixed as solutions inside of diethylene glycolsolvent which has been previously divided in two parts thefirst volume of 150mL for dissolution of Fe(III) acetonylace-tonate and the second volume of 50mL for dissolution ofCo(II) acetate The mixture of both solutions was magneti-cally stirred under the flow of nitrogen After that the spaceof the vessel filled with nitrogen was closed and the mixturewas heated firstly to 120∘C with a heating rate of 4∘C perminute and then the temperature was increased to 160∘Cand maintained at this level for 3 h Finally the temperaturewas increased to 180∘C and after one hour was abruptlylowered with a high cooling rate by cold water introducedthrough corresponding inlet The water flowed with a highflux around the working volume of the vessel causing afast change of temperature from the top temperature tothe room temperature The particle solution was filtered toexclude aggregates AVivaspin filter (polyethersulfone (PES)Vivascience Sartorius Hannover Germany) with pore size02 120583m was used and the filtering process was performedby centrifuging the sample at 1750 rpm for about 30minuntil all fluid had passed the filter Besides the liquid samplea powder sample was also prepared by drying the suspensionof CoFe

2O4NPs at 200∘C for 10 h

22 Experimental Technique X-ray characterization of thesample was carried out on a Philips PW 1050 powder diffrac-tometer using Cu K120572 radiation The X-ray diffraction (XRD)pattern was taken in the 17∘ndash70∘2120579 range with a step of 005∘at a slow scan rate of 40 s per step For transmission electronmicroscopy (TEM) measurements the sample was preparedby drying a drop of the diluted suspension on a copper-grid-supported perforated transparent carbon foil High-resolution transmission electron microscopy (HRTEM) andenergy dispersive X-ray spectroscopy (EDXS) were carriedout using a field-emission electron-source transmission elec-tron microscope JEOL 2010 equipped with an EDXS micro-analysis system (LINK ISIS EDS 300) operated at 200 kVAverage FeCo ratio in the composition of the nanoparticleswas obtained from quantification of a large number ofEDXS spectra using the Oxford ISIS software The spectrawere collected from areas of the sample containing severalhundreds of the nanoparticles For the quantification of the











300

350

400

450

500

(220

)(3

11)

(400

) (511

)

(440

)

Inte

nsity

(au

)

(111

)

20 30 40 50 60 70

2120579 (∘)






119891 (119889) =1

radic2120587120590119889

exp[minusln2 (119889119889

0)

21205902

] (3)








2O4minus119909







(a) (b)



0

5

10

15

20

25

30

Num

ber o

f par

ticle

s (

)





2O4minus119909

(OH)119909surface


2O4NPs



2O4NPs was mea-


80

84

88

92

96Tr

ansm

ittan

ce (

)

2350 20791984

1625 1316

20162331

3500 3000 2500 2000 1500 1000




temperature







0

10

20

30

40

50

05

1015202530

0 50 100 150 200 250 300

0 50 100 150 200 250 300

1000Oe

500Oe

250Oe100Oe

T (K)

T (K)

Powder

TmaxTp

H = 1000Oe

M(e

mu

g(C

oFe 2

O4))

M(e

mu

g(C

oFe 2

O4))









119901asymp 200K is


2O4


0

2

4

6

Temperature (K)

160 180 200 220 240

minus4

minus2

204K

164K

183K

Hea

t flow

(au

)



240K




119888









119888= 118 kOe 119872

119903= 391 emug(CoFe

2O4)

and 119872119904= 808 emug(CoFe




2O4








to 120583119904= 34120583



(Co1minus120575

Fe120575)[Co120575Fe2minus120575




119861and 3120583

119861 respectively


2O4NPs is




0

1

2

3

0

0

0

4

8

12

20 6040

70

80

H (kOe)

(a)

(b)

T (K)

60

minus3


minus2

minus1

5050 100150200250300

5K300K

M(120583

Bfo

rmul

a uni

t)

Ms

(em

ug(

CoF

e 2O4))

Hc(

kOe)










2O4nanoparticles


















0exp(119864119896

119861119879) that is





and frequency 119891 is119891 = 119891

0exp(minus119864119896

1198611198791198982) where 119891

0= 12120587120591

0 We find that

1198791198982


0=

121205871198910= 29 times 10

minus18 s and 119864119896119861= 5360K



Fulcher law

120591 = 1205910exp[ 119864

119896119861(119879 minus 119879

0)

] (4)



1198982and frequency 119891 is 119891 = 119891

0exp[minus119864119896

119861(1198791198982

minus





11Hz 1205910= 12120587119891

0=


119861= (2300 plusmn 300)K and 119879

0= (465 plusmn













000

001

002

003

004

005

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400

(em

uO

e g(C

oFe 2

O4))

(a)

0000

0001

0002

0003

0004

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400998400

(em

uO

e g(C

oFe 2

O4))

(b)



0

2

4

6

8

140 160 180 200 220

lnf

(Hz)

T (K)

Tm1

Tm2










= 1205911015840

0exp[1198641015840119896

119861(119879 minus 119879

1015840

0)] the relation

between 1198791198981

and frequency 119891 is 119891 = 1198911015840

0exp[minus1198641015840119896

119861(1198791198981

minus

1198791015840



parameters 12059110158400= 12120587119891

1015840


minus6 s 1198641015840119896119861

=


1015840



1015840












tens

ity (times

106

au)

334

336

338

340

342


Sextet 1

Doublet 1Doublet 2

SingletIcalcIobs





2O4powder sample








2O4particles with


4 Conclusion




Acknowledgment


References






3O4SiO
























2O3



Fe200

O362


Fe020

Co008

Fe2O4) rdquo




2O4powdersrdquo














2O4



2O4nanoparticles









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials













300

350

400

450

500

(220

)(3

11)

(400

) (511

)

(440

)

Inte

nsity

(au

)

(111

)

20 30 40 50 60 70

2120579 (∘)






119891 (119889) =1

radic2120587120590119889

exp[minusln2 (119889119889

0)

21205902

] (3)








2O4minus119909







(a) (b)



0

5

10

15

20

25

30

Num

ber o

f par

ticle

s (

)





2O4minus119909

(OH)119909surface


2O4NPs



2O4NPs was mea-


80

84

88

92

96Tr

ansm

ittan

ce (

)

2350 20791984

1625 1316

20162331

3500 3000 2500 2000 1500 1000




temperature







0

10

20

30

40

50

05

1015202530

0 50 100 150 200 250 300

0 50 100 150 200 250 300

1000Oe

500Oe

250Oe100Oe

T (K)

T (K)

Powder

TmaxTp

H = 1000Oe

M(e

mu

g(C

oFe 2

O4))

M(e

mu

g(C

oFe 2

O4))









119901asymp 200K is


2O4


0

2

4

6

Temperature (K)

160 180 200 220 240

minus4

minus2

204K

164K

183K

Hea

t flow

(au

)



240K




119888









119888= 118 kOe 119872

119903= 391 emug(CoFe

2O4)

and 119872119904= 808 emug(CoFe




2O4








to 120583119904= 34120583



(Co1minus120575

Fe120575)[Co120575Fe2minus120575




119861and 3120583

119861 respectively


2O4NPs is




0

1

2

3

0

0

0

4

8

12

20 6040

70

80

H (kOe)

(a)

(b)

T (K)

60

minus3


minus2

minus1

5050 100150200250300

5K300K

M(120583

Bfo

rmul

a uni

t)

Ms

(em

ug(

CoF

e 2O4))

Hc(

kOe)










2O4nanoparticles


















0exp(119864119896

119861119879) that is





and frequency 119891 is119891 = 119891

0exp(minus119864119896

1198611198791198982) where 119891

0= 12120587120591

0 We find that

1198791198982


0=

121205871198910= 29 times 10

minus18 s and 119864119896119861= 5360K



Fulcher law

120591 = 1205910exp[ 119864

119896119861(119879 minus 119879

0)

] (4)



1198982and frequency 119891 is 119891 = 119891

0exp[minus119864119896

119861(1198791198982

minus





11Hz 1205910= 12120587119891

0=


119861= (2300 plusmn 300)K and 119879

0= (465 plusmn













000

001

002

003

004

005

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400

(em

uO

e g(C

oFe 2

O4))

(a)

0000

0001

0002

0003

0004

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400998400

(em

uO

e g(C

oFe 2

O4))

(b)



0

2

4

6

8

140 160 180 200 220

lnf

(Hz)

T (K)

Tm1

Tm2










= 1205911015840

0exp[1198641015840119896

119861(119879 minus 119879

1015840

0)] the relation

between 1198791198981

and frequency 119891 is 119891 = 1198911015840

0exp[minus1198641015840119896

119861(1198791198981

minus

1198791015840



parameters 12059110158400= 12120587119891

1015840


minus6 s 1198641015840119896119861

=


1015840



1015840












tens

ity (times

106

au)

334

336

338

340

342


Sextet 1

Doublet 1Doublet 2

SingletIcalcIobs





2O4powder sample








2O4particles with


4 Conclusion




Acknowledgment


References






3O4SiO
























2O3



Fe200

O362


Fe020

Co008

Fe2O4) rdquo




2O4powdersrdquo














2O4



2O4nanoparticles









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)



0

5

10

15

20

25

30

Num

ber o

f par

ticle

s (

)





2O4minus119909

(OH)119909surface


2O4NPs



2O4NPs was mea-


80

84

88

92

96Tr

ansm

ittan

ce (

)

2350 20791984

1625 1316

20162331

3500 3000 2500 2000 1500 1000




temperature







0

10

20

30

40

50

05

1015202530

0 50 100 150 200 250 300

0 50 100 150 200 250 300

1000Oe

500Oe

250Oe100Oe

T (K)

T (K)

Powder

TmaxTp

H = 1000Oe

M(e

mu

g(C

oFe 2

O4))

M(e

mu

g(C

oFe 2

O4))









119901asymp 200K is


2O4


0

2

4

6

Temperature (K)

160 180 200 220 240

minus4

minus2

204K

164K

183K

Hea

t flow

(au

)



240K




119888









119888= 118 kOe 119872

119903= 391 emug(CoFe

2O4)

and 119872119904= 808 emug(CoFe




2O4








to 120583119904= 34120583



(Co1minus120575

Fe120575)[Co120575Fe2minus120575




119861and 3120583

119861 respectively


2O4NPs is




0

1

2

3

0

0

0

4

8

12

20 6040

70

80

H (kOe)

(a)

(b)

T (K)

60

minus3


minus2

minus1

5050 100150200250300

5K300K

M(120583

Bfo

rmul

a uni

t)

Ms

(em

ug(

CoF

e 2O4))

Hc(

kOe)










2O4nanoparticles


















0exp(119864119896

119861119879) that is





and frequency 119891 is119891 = 119891

0exp(minus119864119896

1198611198791198982) where 119891

0= 12120587120591

0 We find that

1198791198982


0=

121205871198910= 29 times 10

minus18 s and 119864119896119861= 5360K



Fulcher law

120591 = 1205910exp[ 119864

119896119861(119879 minus 119879

0)

] (4)



1198982and frequency 119891 is 119891 = 119891

0exp[minus119864119896

119861(1198791198982

minus





11Hz 1205910= 12120587119891

0=


119861= (2300 plusmn 300)K and 119879

0= (465 plusmn













000

001

002

003

004

005

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400

(em

uO

e g(C

oFe 2

O4))

(a)

0000

0001

0002

0003

0004

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400998400

(em

uO

e g(C

oFe 2

O4))

(b)



0

2

4

6

8

140 160 180 200 220

lnf

(Hz)

T (K)

Tm1

Tm2










= 1205911015840

0exp[1198641015840119896

119861(119879 minus 119879

1015840

0)] the relation

between 1198791198981

and frequency 119891 is 119891 = 1198911015840

0exp[minus1198641015840119896

119861(1198791198981

minus

1198791015840



parameters 12059110158400= 12120587119891

1015840


minus6 s 1198641015840119896119861

=


1015840



1015840












tens

ity (times

106

au)

334

336

338

340

342


Sextet 1

Doublet 1Doublet 2

SingletIcalcIobs





2O4powder sample








2O4particles with


4 Conclusion




Acknowledgment


References






3O4SiO
























2O3



Fe200

O362


Fe020

Co008

Fe2O4) rdquo




2O4powdersrdquo














2O4



2O4nanoparticles









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0

10

20

30

40

50

05

1015202530

0 50 100 150 200 250 300

0 50 100 150 200 250 300

1000Oe

500Oe

250Oe100Oe

T (K)

T (K)

Powder

TmaxTp

H = 1000Oe

M(e

mu

g(C

oFe 2

O4))

M(e

mu

g(C

oFe 2

O4))









119901asymp 200K is


2O4


0

2

4

6

Temperature (K)

160 180 200 220 240

minus4

minus2

204K

164K

183K

Hea

t flow

(au

)



240K




119888









119888= 118 kOe 119872

119903= 391 emug(CoFe

2O4)

and 119872119904= 808 emug(CoFe




2O4








to 120583119904= 34120583



(Co1minus120575

Fe120575)[Co120575Fe2minus120575




119861and 3120583

119861 respectively


2O4NPs is




0

1

2

3

0

0

0

4

8

12

20 6040

70

80

H (kOe)

(a)

(b)

T (K)

60

minus3


minus2

minus1

5050 100150200250300

5K300K

M(120583

Bfo

rmul

a uni

t)

Ms

(em

ug(

CoF

e 2O4))

Hc(

kOe)










2O4nanoparticles


















0exp(119864119896

119861119879) that is





and frequency 119891 is119891 = 119891

0exp(minus119864119896

1198611198791198982) where 119891

0= 12120587120591

0 We find that

1198791198982


0=

121205871198910= 29 times 10

minus18 s and 119864119896119861= 5360K



Fulcher law

120591 = 1205910exp[ 119864

119896119861(119879 minus 119879

0)

] (4)



1198982and frequency 119891 is 119891 = 119891

0exp[minus119864119896

119861(1198791198982

minus





11Hz 1205910= 12120587119891

0=


119861= (2300 plusmn 300)K and 119879

0= (465 plusmn













000

001

002

003

004

005

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400

(em

uO

e g(C

oFe 2

O4))

(a)

0000

0001

0002

0003

0004

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400998400

(em

uO

e g(C

oFe 2

O4))

(b)



0

2

4

6

8

140 160 180 200 220

lnf

(Hz)

T (K)

Tm1

Tm2










= 1205911015840

0exp[1198641015840119896

119861(119879 minus 119879

1015840

0)] the relation

between 1198791198981

and frequency 119891 is 119891 = 1198911015840

0exp[minus1198641015840119896

119861(1198791198981

minus

1198791015840



parameters 12059110158400= 12120587119891

1015840


minus6 s 1198641015840119896119861

=


1015840



1015840












tens

ity (times

106

au)

334

336

338

340

342


Sextet 1

Doublet 1Doublet 2

SingletIcalcIobs





2O4powder sample








2O4particles with


4 Conclusion




Acknowledgment


References






3O4SiO
























2O3



Fe200

O362


Fe020

Co008

Fe2O4) rdquo




2O4powdersrdquo














2O4



2O4nanoparticles









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




0

1

2

3

0

0

0

4

8

12

20 6040

70

80

H (kOe)

(a)

(b)

T (K)

60

minus3


minus2

minus1

5050 100150200250300

5K300K

M(120583

Bfo

rmul

a uni

t)

Ms

(em

ug(

CoF

e 2O4))

Hc(

kOe)










2O4nanoparticles


















0exp(119864119896

119861119879) that is





and frequency 119891 is119891 = 119891

0exp(minus119864119896

1198611198791198982) where 119891

0= 12120587120591

0 We find that

1198791198982


0=

121205871198910= 29 times 10

minus18 s and 119864119896119861= 5360K



Fulcher law

120591 = 1205910exp[ 119864

119896119861(119879 minus 119879

0)

] (4)



1198982and frequency 119891 is 119891 = 119891

0exp[minus119864119896

119861(1198791198982

minus





11Hz 1205910= 12120587119891

0=


119861= (2300 plusmn 300)K and 119879

0= (465 plusmn













000

001

002

003

004

005

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400

(em

uO

e g(C

oFe 2

O4))

(a)

0000

0001

0002

0003

0004

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400998400

(em

uO

e g(C

oFe 2

O4))

(b)



0

2

4

6

8

140 160 180 200 220

lnf

(Hz)

T (K)

Tm1

Tm2










= 1205911015840

0exp[1198641015840119896

119861(119879 minus 119879

1015840

0)] the relation

between 1198791198981

and frequency 119891 is 119891 = 1198911015840

0exp[minus1198641015840119896

119861(1198791198981

minus

1198791015840



parameters 12059110158400= 12120587119891

1015840


minus6 s 1198641015840119896119861

=


1015840



1015840












tens

ity (times

106

au)

334

336

338

340

342


Sextet 1

Doublet 1Doublet 2

SingletIcalcIobs





2O4powder sample








2O4particles with


4 Conclusion




Acknowledgment


References






3O4SiO
























2O3



Fe200

O362


Fe020

Co008

Fe2O4) rdquo




2O4powdersrdquo














2O4



2O4nanoparticles









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




000

001

002

003

004

005

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400

(em

uO

e g(C

oFe 2

O4))

(a)

0000

0001

0002

0003

0004

0 50 100 150 200 250 300T (K)

1Hz5Hz10Hz50Hz

100Hz500Hz1000Hz1500Hz

120594998400998400

(em

uO

e g(C

oFe 2

O4))

(b)



0

2

4

6

8

140 160 180 200 220

lnf

(Hz)

T (K)

Tm1

Tm2










= 1205911015840

0exp[1198641015840119896

119861(119879 minus 119879

1015840

0)] the relation

between 1198791198981

and frequency 119891 is 119891 = 1198911015840

0exp[minus1198641015840119896

119861(1198791198981

minus

1198791015840



parameters 12059110158400= 12120587119891

1015840


minus6 s 1198641015840119896119861

=


1015840



1015840












tens

ity (times

106

au)

334

336

338

340

342


Sextet 1

Doublet 1Doublet 2

SingletIcalcIobs





2O4powder sample








2O4particles with


4 Conclusion




Acknowledgment


References






3O4SiO
























2O3



Fe200

O362


Fe020

Co008

Fe2O4) rdquo




2O4powdersrdquo














2O4



2O4nanoparticles









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




tens

ity (times

106

au)

334

336

338

340

342


Sextet 1

Doublet 1Doublet 2

SingletIcalcIobs





2O4powder sample








2O4particles with


4 Conclusion




Acknowledgment


References






3O4SiO
























2O3



Fe200

O362


Fe020

Co008

Fe2O4) rdquo




2O4powdersrdquo














2O4



2O4nanoparticles









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



















2O3



Fe200

O362


Fe020

Co008

Fe2O4) rdquo




2O4powdersrdquo














2O4



2O4nanoparticles









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



research article magnetic and structural studies of cofe...

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