nb,

Received 22 September 2008

Received in revised form

26 November 2008

nick

stable ferric and nickel salts with sodium hydroxide as the precipitating agent and oleic acid as the

surfactant. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses conrmed

the formation of single-phase nickel ferrite nanoparticles in the range 828nm depending upon the

temperature) of these nanoparticles. These particles should be in

view, the superparamagnetic blocking temperature (TB) of

amicrtieshigh

and

hydrothermal process and template-assisted electrochemical

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Contents lists availab

.els

an

Journal of Magnetism and Magnetic Materials 321 (2009) 18381842nanometer-sized reactors for the formation of homogeneous Corresponding author at: Department of Physics, Quaid-i-Azam University,the nanoparticles used for recording devices should be well abovethe room temperature in order to have a stable data recording in

synthesis [1420]. Generally in most of these methods the precisecontrol over the size and its distribution is quite difcult [1]. Inorder to overcome these difculties, co-precipitation method hasbeen used for synthesis of these nanoparticles. In this method the

nanoparticles of nickel ferrite have been used. To protect theoxidation of these nanoparticles in the presence atmospheric

Islamabad, Pakistan. Tel.: +923215029820; fax: +92519290275.

E-mail addresses: maaz@impcas.ac.cn, maaz@phys.qau.edu.pk (K. Maaz).0304-88

doi:10.1single-domain state, of pure phase, having high coercivity (HC)and moderate magnetization. From the application point of

nanowires include solgel processing, evaporation condensation,microemulsion technique, combustion method, spray pyrolysis,technological interests [59]. The magnetic character of thenanoparticles used in medical, electronic and recording industriesdepends crucially on the size, shape, purity and magneticstability (e.g. blocked, unblocked state at particular operating

which form a major constituent of the magnetic cermaterials, nano-sized nickel ferrite possesses attractive propefor the application as soft magnets and low loss materials atfrequencies [13].

Conventional techniques for preparation of nanoparticlesIn the recent years much attention has been paid to theunderstanding of nanostructured materials (such as multi-layers,nanowires, nanoparticles, as well as composite materials), thatexhibit interesting optical, electrical, chemical and magneticproperties [14]. These properties along with their great chemicaland physical stability, that appear them different from their bulkcounterparts, make these materials of great scientic and

used for this purpose should be magnetically in the super-paramagnetic unblocked state with relatively low blockingtemperature. The most signicant properties of magnetic nano-particles (ferrites), namely magnetic saturation, coercivity,magnetization and loss, etc. change drastically as the particle sizereduces to the nanometric range [1012]. Among various ferrites,Available online 7 December 2008

PACS:

73.63b

75.50Gg

75.50Tt

75.70Rf

Keywords:

Magnetic properties

Ferrite nanoparticles

Surface anisotropy

Magnetic materials

1. Introduction53/$ - see front matter & 2008 Elsevier B.V. A

016/j.jmmm.2008.11.098goes through a maximum, peaking at 11nm and then decreases for larger particles. Typical blockingeffects were observed below 225K for all the prepared samples. The superparamagnetic blockingtemperature (TB) was found to be increasing with increasing particle size that has been attributed to the

increased effective anisotropy energy of the nanoparticles. The saturation moment of all the samples

was found much below the bulk value of nickel ferrite that has been attributed to the disordered surface

spins or dead/inert layer in these nanoparticles.

& 2008 Elsevier B.V. All rights reserved.

these devices. In biomedical applications the magnetic nanopar-ticles are used as drug carriers inside the body where conven-tional drug delivery systems may not work. The nanoparticlesannealing temperature of the samples during the synthesis. The size of the particles (d) was observed to

be increasing linearly with annealing temperature of the sample while the coercivity with particle sizeSynthesis and magnetic characterizatioprepared by co-precipitation route

K. Maaz a,b,, S. Karim a, A. Mumtaz b, S.K. Hasanaina PINSTECH, Post Ofce Nilore, Islamabad, Pakistanb Department of Physics, Quaid-i-Azam University, Islamabad, Pakistanc Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, PR China

a r t i c l e i n f o

Article history:

a b s t r a c t

Magnetic nanoparticles of

journal homepage: www

Journal of Magnetismll rights reserved.of nickel ferrite nanoparticles

J. Liu c, J.L. Duan c

el ferrite (NiFe2O4) have been synthesized by co-precipitation route using

le at ScienceDirect

evier.com/locate/jmmm

d Magnetic Materials

dellHighlight

73nm. The physical characterization was performed by X-ray

3. Results and discussion

The X-diffraction pattern (Fig. 1) of the sample annealed at1000 1C prepared by co-precipitation technique shows that thenal product is cubic NiFe2O4 with average crystallite size of28nm (the XRD peaks were compared to the standard PDF cardnumber 742081 for inverse cubic nickel ferrite). The average sizesof the particles annealed at 600, 700, 800, 900 and 1000 1C for 10hwere found to be 8, 11, 18, 24 and 2873nm. Fig. 2 shows the high-resolution TEM images of the same sample annealed at 1000 1C for10h. In the TEM image most of the particles appear spherical inshape, however, some elongated particles are also present in theimages. Some moderately agglomerated particles as well asseparated particles are also present in the sample. The inset of

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30

4500

(

2 (degree)40 50 60 70

Fig. 1. X-ray diffraction pattern of NiFe2O4 nanoparticles prepared by co-precipitation method annealed at 1000 1C for 10h with average grain size of28m.

Fig. 2. TEM micrograph of NiFe2O4 nanoparticles prepared by co-precipitationroute annealed at 1000 1C for 10h.

d Madiffractometer (Model: XPert Philips, Holland, with Cu-Kal 0.154056nm) and high-resolution transmission electronmicroscope (HRTEM, 300keV) (Model: JEM-3010, JEOL) whilethe magnetic characterization was done by vibrating samplemagnetometer (VSM, Model 7300 Lake Shore, USA) with anapplied eld of 710kOe.

In this article we have reported three new results that havenot been reported previously, to the best of our knowledge.Firstly, we have studied the superparamagnetic blocking effects indetail for the given size (828nm) of NiFe2O4 nanoparticlesthat has not been reported so far, at least for this size range.Secondly, we have used a theoretical model for calculating theratio of critical single-domain radii for bulk Ni- and Co-ferrite.This ratio has been used to calculate the single-domain limit forNi-ferrite nanoparticles using the previously reported values forCo-ferrite nanoparticles, that comes out to be 10.7 nm (the detailis given in Section 3). Comparison of the results shows that ourreported value for the single-domain limit (11nm) is in goodagreement with calculated (theoretical) value for Ni-ferritenanoparticles. The third and important point that makes thiswork different from the previous reports is that we havepresented a single-domain limit of around 11nm for NiFe2O4nanoparticles that has not been reported previously (the detail isgiven in Section 3).

2. Synthesis procedure

3M solution of sodium hydroxide (as the precipitating agent)was slowly mixed with salt solutions of 0.4M ferric chloride(FeCl3 6H2O) and 0.2M nickel chloride (NiCl2 6H2O). ThepH of the solution was constantly monitored as the NaOH solutionwas added dropwise. The reactants were constantly stirredusing a magnetic stirrer until a pH level of 412 was achieved. Aspecied amount of oleic acid (23 drops for total reactingsolution of 75ml) was added to the solution as the surfactant[23,24]. The liquid precipitate was then brought to a reactiontemperature of 80 1C and stirred for 40min. The product wascooled to room temperature and then washed twice with distilledwater and ethanol to remove unwanted impurities and the excesssurfactant from the prepared sample. The sample was centrifugedfor 15min at 2000 rpm and then dried overnight at above 80 1C.The acquired substance was then grinded into a ne powder andthen annealed for 10h at 600 1C. The nal product obtained asoxygen and also to stop their agglomeration, the particles areusually coated with some surfactant like sodium dodecylsulfate (NaDS) or the oleic acid [21,22] and then dispersed insome carrier liquid like ethanol, methanol, ammonia, etc.depending upon the nature of the materials (nanoparticles) tobe dispersed. The advantage of this method over the others is thatthe production rate of ferrite nanoparticles, its size and sizedistribution is relatively easy and there is no need of extramechanical or microwave heat treatments. In this work, nickelferrite nanoparticles (NiFe2O4) have been prepared by co-precipitation technique and then they were heat treated(annealed) at different temperatures (from 600 to 1000 1C) tosynthesize magnetic nanoparticles with different grain sizes.Various magnetic properties of nickel ferrite nanoparticleshave been explored as a function of particle size and temperature.Debey Scherrer formula was used for size determination usingthe strongest peak in the XRD pattern. The size variation in thisstudy was carried out from 8 to 28nm with a distribution of

K. Maaz et al. / Journal of Magnetism anconrmed by the X-ray diffraction and SAED was found to bemagnetic nanoparticles of nickel ferrite (NiFe2O4) with inversespinel structure.5000

5500

6000

6500

7000

7500

8000

8500

(440

)

(511

)

422)

(400

)

(311

)

(220

)Inte

nsity

(a.u

)

~28 nm NiFe2O4

gnetic Materials 321 (2009) 18381842 1839Fig. 2 shows the selected area electron diffraction (SAED) analysisof the sample indicating that the nanoparticles prepared arecrystalline. The same has also been conrmed from the XRD peaks

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-10000-50

-40

H (Oe)-7500 -5000 -2500 0 2500 5000 7500 10000

Fig. 4. Hysteresis loops for 28nm NiFe2O4 nanoparticles at room temperature(300K) and 77K at maximum applied eld of 10kOe. The two insets of the gure

show the expanded eld region around the origin for clear visibility of the readers.

100

120

140

160

180

Coe

rciv

ity (O

e)

Maindicating the ploycrystalline nature of the prepared sample. Fig. 3shows the dependence of size of the particles on annealingtemperature (Tann). The size of the particles was observed to beincreasing linearly with annealing temperature. It has beenreported earlier that annealing process generally decreases thelattice defects and strains; however, it can also cause coalescenceof smaller grains that results in increasing the average grain sizeof the nanoparticles [25]. The observed increase in particle sizewith annealing temperature is most likely due to the fact thathigher annealing temperature and time enhances the coalescenceprocess resulting in an increase in the grain size. Thus it appearsthat particle size may be controlled either by varying theannealing temperature of the sample or the annealing time

5000

5

10

15

20

25

30

35Linear fit to data points

Par

ticle

siz

e (n

m)

Tann (C)600 700 800 900 1000 1100

Fig. 3. Particle size as a function of annealing temperature for NiFe2O4nanoparticles. Samples were annealed at 600, 700, 800, 900 and 1000 1C withaverage sizes of 8, 11, 18, 24 and 2873nm, respectively.

K. Maaz et al. / Journal of Magnetism and1840during the synthesis process.Magnetic characterization of the particles was performed by

vibrating sample magnetometer (VSM) between the roomtemperature (300K) and 77K with maximum applied eld upto10kOe. Fig. 4 shows theM(H) loops of 28nm sample both at roomtemperature (300K) and 77K. The insets of the gure show theexpanded regions around the origin with different eld ranges(7400 and 71000Oe) in order to make the coercivities morevisible at these temperatures. For the 28nm size particles thecoercivity at room temperature as derived from the M(H) loopswas 89Oe, while at 77K it has increased to 175Oe. Thesaturation magnetization (MS) obtained at room temperature wasfound to be 40.5 emu/g smaller than the bulk value of 56 emu/gfor nickel ferrite, while at 77K this value has increased to45 emu/g. The relatively large coercivity and saturation magne-tization at 77K are consistent with the pronounced growth ofmagnetic anisotropy inhibiting the alignment of the momentalong the applied eld direction [24,26].

The coercivity of nanoparticles was also studied as a functionof particle size at room temperature (300K) as shown in Fig. 5.The gure shows that for smaller sizes (811nm) the coercivityincreases with size rapidly, attaining a maximum value of 175Oeat 11nm and then decreases with size of the particles, for largerparticles (1228nm). A ratio of the critical single-domain radii forNi-ferrite (dSD)Ni and Co-ferrite (dSD)Co has been calculated usingthe relation for critical single-domain radius dSD 36klex;where k is the magnetic hardness parameter dened byk (K1/moMS2)1/2 [27]. For very hard magnetic materials kb1,-30

-20

-10

0

10

20

30

40

50

77 K 300K

M (e

mu/

g)

gnetic Materials 321 (2009) 18381842while for very soft magnetic materials k51. The exchange length(lex) is dened by lex (A/moMS2)1/2 representing the length belowwhich the atomic exchange interactions dominate the typicalmagnetostatic elds. For a typical permanent magnet the value ofexchange length (lex) is of the order of 3 nm and A is the called theexchange constant or the exchange stiffness parameter. The valueof k was calculated by taking the values of K1 and MS for bulkmaterials from Skomsky [27], while the value of lex was calculatedby estimating the exchange constant (A) directly proportional tothe respective Tcs of Co- and Ni-ferrite (790 and 865K for Co- andNi-ferrite). The ratio [(dSD)Ni]/[(dSD)Co] was found to be 0.38. Fora single-domain limit of 28nm for CoFe2O4 as reported in one ofour previous papers [24], this suggests the value of single-domainlimit (dSD) for Ni-ferrite as 10.7 nm. We observed a maximum inHC-d curve (Fig. 5) for NiFe2O4 at 11nm. This value is smallerthan the previously reported value of 14nm for single-domainlimit of NiFe2O4 nanoparticles [12]. We noticed in Fig. 5 that the

580

Particle Size (nm)10 15 20 25 30

Fig. 5. The coercivity (HC) as a function of average particle diameter at roomtemperature. The peak of HC at 11nm is evident in the gure.

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coercivity of nickel ferrite shows a non-monotonic behaviorwith particle size, i.e. for small sizes the coercivity rst increases,goes through peaks (at around 11nm) and afterwards it decreasesfor larger particles (1228nm), above the peak. The initialincrease of coercivity for smaller particles, below the peak, withincreasing size may be assigned to the departure from thesuperparamagnetic state i.e., from unblocked to blocked state.This occurs for small particles when the thermal energydominates the volume-dependent anisotropy energy (EA KeffV).Hence in the lower d region (811nm) the coercivity mayincrease with increasing sizes as the larger-sized particleswould tend to show a blocked moment. The decline in HC withincreasing d, above the peak, can occur due to the two differentmechanisms. Firstly, it may occur as the particle sizes becomelarge enough to sustain a domain wall. In this situation themagnetization reversal would occur via domain wall motion andconsequently a lower coercivity would be observed. In NiFe2O4,however, this crossover is expected at much higher than 11nmwhere we observed the peak in our samples. Secondly, it may bedue to the varying role of the surface and the observed bulk

Fig. 6 shows the blocking temperature (Tb) as a function ofparticle size, derived from the M(T) curves of the samples. Theinset of the gure shows a typical zero-eld cooled (ZFC) M(T)curve for one of the representative samples (with average size of28nm). For a single particle, at nite temperature, the ferromag-netically aligned magnetic moments uctuate between their twoenergetically degenerate ground states on a time scale given bythe relation [31]

t to expKeff VpkBT

where t is the relaxation time and Keff VP the effective anisotropyenergy (EA) of the particles. The blocking temperature Tb of aparticle is the temperature at which t tm, the measurementtime of the instrument. For a nite temperature T4Tb the particlebehaves like a superparamagnet (unblocked state). On the otherhand, for ToTb the particle is said to be in the blocked state, i.e.behaves like a ferromagnet. Referring back to our data of (Tbd)curve for nickel ferrite (Fig. 6), we see that there is a clear increase

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K. Maaz et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 18381842 1841anisotropies as the size is diminished. Starting from larger sizes,with decreasing size of particles (from 28nm), the role of thesurface and its associated anisotropy energy is increased. Ithas been shown by Bodker et al. [28] that the effective aniso-tropy constant increases with decreasing particle sizes accordingto phenomenological expression (Keff KV+(6/d)KS) for the effec-tive anisotropy of spherical particles. Several other worksincluding simulation and experimental have also supported thisexpression [29]. If we assume similar behavior in NiFe2O4nanoparticles i.e. an increase in the effective anisotropy constantwith reducing particle sizes (from 28nm), this would tend toincrease the coercivity of the nanoparticles within the Stone-rWohlfarth picture (HC 2K/MS) consistent with the behavior ofHC above the peak (2811nm) as shown in Fig. 5. This increasewill, however, not continue indenitely and as the particlesize decreases to a small enough value (dSD), the thermal effectswill take over. For particles below the critical size (say dSD11nm)the thermal energy becomes sufcient to overcome theoverall anisotropy energy enabling the easier reversal of themoments leading to the lower critical elds for these small sizes[30]. This leads to the lowering of coercivity in the small sizeregime (811nm).

5

160

170

180

190

200

210

220

230 Linear fit to expt. data points

T B (K

)

Particle Size (nm)10 15 20 25 30Fig. 6. The dependence of superparamagnetic blocking temperature (TB) onparticle size for NiFe2O4 nanoparticles. The inset of the gure shows the

temperature dependence of zero-eld cooled (ZFC) magnetization.in the blocking temperature with size of the particles. The largerparticles seem to be blocked at high temperatures as compared tothe smaller particles. For larger particles, the larger volume causesincreased anisotropy energy, which decreases the probability of ajump across the anisotropy barrier and hence the blocking isshifted to higher temperatures. Fig. 7 shows the dependence ofsaturation magnetization on particle size. The MS values obtainedfor our samples varied between 9 and 40.5 emu/g for the sizesfrom 8 to 28nm. The saturation magnetization increases con-sistently with size of the nanoparticles. A similar trend has alsobeen reported for NiFe2O4 nanoparticles earlier [32,33]. Thedecrease in MS at smaller sizes is attributed to the pronouncedsurface effects in these nanoparticles. The surface of thenanoparticles is considered to be composed of some canted ordisordered spins that prevent the core spins to align along theeld direction resultantly decreasing the saturation magnetiza-tion of the nanoparticles for smaller sizes [3436]. The surfacemay also behaves like a dead or inert layer that has negligiblemagnetization [24,37,38]. This effect becomes more pronouncedfor smaller particles; due to the increased number of surface tovolume atoms that resultantly decreasing the saturation magne-tization for the smaller-sized NiFe2O4 nanoparticles.

50

5

10

15

20

25

30

35

40

45 Linear fit to the data points

MS (e

mu/

gm)

Particle Size (nm)10 15 20 25 30Fig. 7. Saturation magnetization (MS) as function of particle size for NiFe2O4nanoparticles. The observed values of MS lie much below the bulk value of

56 emu/g for NiFe2O4.

4. Conclusion

In this article we have presented synthesis of NiFe2O4nanoparticles by co-precipitation route in the range 828nm.The size of the particles was measured both by XRD and TEM andwas found in good agreement with each other. The size of theparticles appeared to increase linearly with annealing tempera-ture most probably due to the coalescence that increases withincreasing temperature of annealing. The relatively large coerciv-ity and saturation magnetization at 77K in comparison with roomtemperature appeared to be due to the pronounced growth ofmagnetic anisotropy at low temperatures. The coercivity showed apeak with particle size at a value smaller than the previouslyreported value of single-domain limit for NiFe2O4 that has beenattributed to the enhanced role of the surface anisotropy ascompared to the bulk for small sizes. The superparamagneticblocking temperature was found to increase linearly withincreasing particle sizes. This was assigned to the increasing

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K. Maaz et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 183818421842[1] M. Dariel, L.H. Bennett, D.S. Lashmore, P. Lubitz, M. Rubinstein, W.L. Lechter,M.Z. Harford, J. Appl. Phys. 61 (1987) 4067.attributed to the greater fraction of surface spins in thesenanoparticles that tend to be in a canted, spin glass like or deadlayer like state with a smaller net moment.

Acknowledgments

K. Maaz acknowledges the PCSIR of Pakistan for providing 6months fellowship titled Establishment of Nano-Tech Lab at PCSIRand some nancial support from the Material Science Group-II,Institute of Modern Physics, Chinese Academy of Sciences P.R.China. J. Liu and J.L. Duan acknowledge the National NaturalScience Foundation of China (No.10775161, 10775162, 10805062)and the West Light Foundation of Chinese Academy of Sciences fortheir nancial support to enable this work.[24] K. Maaz, A. Mumtaz, S.K. Hasanain, A. Ceylan, J. Magn. Magn. Mater. 308(2007) 289.

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(2005) 65.S[23] V. Pallai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996) 243.volume of the larger particles that resultantly increasing theeffective anisotropy energy and hence the blocking temperature ofnanoparticles. The smaller value of M for smaller particles was

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Synthesis and magnetic characterization of nickel ferrite nanoparticles prepared by co-precipitation routeIntroductionSynthesis procedureResults and discussionConclusionAcknowledgmentsReferences