Nanocrystalline nickel and zinc ferrites synthesised using a microemulsion technique were characterised by high resolution transmission electron microscopy and vibrating sample magnetometry. A narrow and uniform distribution of crystals of size range 5 – 8 nm, distinguished by a clear lack of saturation magnetisation at 9 kOe, were obtained. Also, no coercivity or remanence was observed.

Hosson, J.Th.M. De,

Kale, A.,

Kooi, B.J.,

Misra, R.D.K.,

English

University of Groningen Digital Archive

Some aspects of nanocrystalline nickel and
zinc ferrites processed using microemulsion
technique
R. D. K. Misra, A. Kale, B. J. Kooi and J. Th. M. De Hosson
Nanocrystalline nickel and zinc ferrites synthesised using a microemulsion technique were characterised by high
resolution transmission electron microscopy and vibrating sample magnetometry. A narrow and uniform distribution
of crystals of size range 5–8 nm, distinguishedby a clear lack of saturation magnetisationat 9 kOe, were obtained.
Also, no coercivity or remanence was observed. MST/5813
Professor Misra (dmisra@louisiana.edu)is in the Department of Chemical Engineering, University of Louisiana at Lafayette,
Lafayette, LA 70504–4130, USA, Mr Kale is at the University of Illinois at Urbana-Champaign, 1302 West Pennsylvania
Avenue, Urbana, IL 6801, USA, and Dr Kooi and Dr De Hosson are in the Department of Applied Physics, Materials Science
Centre and Netherlands Institute for Materials Research, University of Groningen, Nijenborgh 4, 9747AG, Groningen, The
Netherlands. At the time the work was carried out Mr Kale was at the University of Louisiana at Lafayette, USA. Manuscript
received 12 December 2002; accepted 21 March 2003.
# 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.
Introduction
The development of nanocrystalline materials is a subject
of considerable interest, both for the scienti￿c value of
understanding the unique properties of materials that have
relevance to condensed matter studies, and for the
technological signi￿cance of enhancing the performance
of existing materials. The ability to produce nanocrystalline
ferrites is expected to assist in controlling the ultimate
properties of ferrites, an advantage over those fabricated
by powder metallurgy process. The magnetic properties,
in particular, are strongly dependent on crystallite size,
crystallite size distribution, and morphology of crystallites.
High electrical resistance and lower saturation magneti-
sation are the characteristics of soft ferrites, which are not
found in traditional silicon steels and permalloys.1 This
makes them suitable for varied applications, including
magnetic and magneto-optical.Modern data storage appli-
cations require control over the morphology of particles
and a considerable reduction in crystallite size, to the size
of a domain. Nanosize magnetic materials are expected to
prove very useful in such applications. In recent years, a
number of chemical and physical methods have been
attempted to produce nanosize ferrites. Some of the main
physical methods include mechanical milling,
1 – 4 severe
plastic deformationconsolidation,5 and inert gas condensa-
tion.
6 The widely used chemical methods are electrodeposi-
tion,7 hydrothermal reaction,8 oxidative precipitation,9 the
organic precursor method,
10 sonochemical decomposi-
tion,11 and the sol–gel synthesis technique.12 The majority
of these methods suffer from a lack of control of particle
size distribution. The water in oil microemulsion chemical
synthesis method permits a narrow size distribution of
nanocrystalline particles to be obtained. It also has the
advantage of simple equipment and operating conditions.
In the water in oil emulsion system, the small droplets
of water that are encapsulated into reverse micelles act
as microreactors, providing a microstructured system to
produce well de￿ned nanosized crystallites
13 (micelle: a
colloidal aggregate of amphiphilic surfactant molecules
such that the hydrophobic ends are in contact; reverse
micelle: a colloidal aggregate of amphiphilic surfactant
molecules whereby hydrophilic ends are in contact). The
size of nanocrystallites depends on the size of water pools
and the water content. Thus, the success of this method
depends on the surfactant and solvent used, amount of
water in the system, and concentration of surfactant. In
the present paper, the potential of the water in oil micro-
emulsion method for producing nanocrystalline ferrites is
described, and the initial observations of high resolution
transmission electron microscopy and magnetic measure-
ments are outlined.
Experimental procedure
Nanocrystallinenickel and zinc ferrites based on the reverse
micelle concept were synthesised using two microemulsion
systems. The ￿ rst microemulsion system was an oil phase
microemulsion comprising iso-octane and surfactant
sodium diiso-octylsulphoccinate(AOT); the second micro-
emulsion was an aqueous phase emulsion consisting of
iso-octane and surfactant diiso-octylsulphoccinate with
reactant salts (hydrated iron sulphate and hydrated nickel
sulphate or zinc sulphate, depending on the nature of the
ferrite being processed).The [Fe
2z]/[Ni
2z] or [Fe
2z]/[Zn
2z ]
ratio in the metal containing solution was 2:1. In a typical
experimentfor the synthesisof nickelferrite, microemulsion
I consisted of 2 mL of 30% ammoniumhydroxidez2.4 mL
of water z66 mL of 0.5M AOT–iso-octane, and micro-
emulsion II contained 0.384 g of FeSO4.7H2Oz0.192 g
of NiSO4.6H2Oz66 mL of 0.5M AOT– iso-octane. The
constitutionof microemulsion systems for the processing of
zinc ferrites was identical to that for nickel ferrites except
for the replacement of nickel sulphate in microemulsion
II by zinc sulphate. Ammonium hydroxide acted as a
precipitating agent to form metal hydroxides. Ammonium
hydroxide also providedstability to the water in oil system.
The two microemulsionswere mixed and subjected to rapid
mechanical stirring. In initial experiments for the synthesis
of nickel ferrites, the mechanical stirring time was 10 min.
However, this led to the synthesis of amorphous nickel
ferrites (see below). The time was then increased to 15, 25,
50 min. The optimum mechanical stirring time required
to produce nanocrystalline ferrites was determined to be
25 min. This durationwas used for subsequentexperiments.
Hydroxides of nickel and iron were precipitated within the
water pools of reverse micelles and subsequently oxidised
to nickel ferrite. This was followed by separation of the
solid product by a centrifuging process. The resulting solid
DOI 10.1179/026708303225008040 Materials Science and Technology November 2003 Vol. 19 1617product was washed a number of times with methanol
followed by dryingin a low vacuum. The chemicals used for
synthesis of nanocrystallineferrite were of analyticalgrade,
obtained from Aldrich, and the water used was doubly
distilled.
The ferrites were characterised by transmission electron
microscopy (TEM) to determine crystallinity, extent of
segregation of the nanoparticles, and their particle size
(distribution).The TEM samples were prepared as follows.
Ferrite powder was pulverised and dispersed in isopropyl
alcohol, and a droplet of the suspension was put on a
carbon or formvar coated copper grid located on a glass
slide. After drying, the grids were inserted in the micro-
scope. A Jeol 4000 EX/II operating at 400 kV having a
point–point resolution of 0.165 nm, and a Jeol 2010FEG
(￿ eld emission gun) operating at 200 kV having an
information limit of 0.11 nm, equipped with an EDAX
EDS detector (with super ultrathin window), were used.
In situ heating was carried out in the Jeol 2010FEG using a
Gaton double tilt heating holder (model 652 with a model
901 SmartSethot stage controller).Magneticmeasurements
were conducted at room temperature using a vibrating
sample magnetometer with an applied ￿eld of 9 kOe.
Results and discussion
The anticipated chemical reaction expected to produce
nickel and zinc ferrite is
MeSO4:6H2Oz2FeSO4:7H2Oz6NH4OHz1=2O2
?MeFe2O4;z16H2Oz3(NH4)2SO4 : : : : : (1)
where Me~Ni or Zn.
The ￿rst observation was that the mechanical stirring
time of the microemulsion was critical for the synthesis of
nanocrystalline ferrites. As stated above, mechanical
stirring for less than the critical time of 25 min produced
amorphous ferrites. Micrographs (TEM) of nickel ferrites
produced by mechanical stirring of the two microemulsion
systems for 10 min are presented in Fig. 1, con￿rming the
amorphous nature of the nickel ferrites. Figure 1 gives no
evidenceofcrystalliteformation,and particlesareseen tobe
aggregated.The amorphousnickel ferriteswere heat treated
in situ in the microscope at 500, 600, and 700°C for 5 min,
and the progressin crystallisationof the amorphousferrites
was examined.Figures 2–4 clearlyindicatethat amorphous
nickel ferrites transform to nanocrystalline nickel ferrites.
Following heat treatment at 500°C for 5 min, nickel ferrite
crystals of 5–10 nm could be seen (Fig. 2b). However, the
heat treatmenttime of 5 min was not adequatefor complete
transformationof amorphousnickel ferrites to nanocrystal-
line ferrites, and the selected area diffraction pattern
predominantly indicated diffuse rings characteristic of an
amorphous nature (Fig. 2c). After heat treatment at 600°C
for 5 min, crystallisation was promoted, and a number of
crystals could be seen (Fig. 3a) and the diffraction pattern
(Fig. 3b) was representative of a crystalline material. Heat
treatment at 700°C for 5 min led to complete crystallisa-
tion of the amorphous material, but at the same time the
temperaturewas suf￿ciently high to promote crystalgrowth
(Fig. 4). Several lines, namely [220], [311], [400], [511], and
[440] could be identi￿ed in the diffraction pattern (Figs. 3b
and 4b). Their positions and relative intensities con￿rm
unambiguously the spinel crystallographic structure. The
observed behaviour at different temperatures suggests that
500°C is adequatefor transformationof an amorphousto a
crystallinenature;however,a heat treatmenttime of greater
a lower magni®cation; b,c higher magni®cation
1 Images (TEM) at different magni￿ cations of amorphous nickel ferrite powder produced by inadequate mechanical stir-
ring (10 min) of microemulsion systems
a,b TEM images; c SADP
2 Images (TEM) and selected area diffraction pattern (SADP) of amorphous nickel ferrite powder heat treated in situ in
TEM at 500°C for 5 min
1618 Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique
Materials Science and Technology November 2003 Vol. 19than 5 min (10 min) is required for transformation to be
complete.
Micrographs (TEM) of nanocrystalline nickel ferrites
processed by mechanicalstirring of the two microemulsions
for 25 min are presented in Fig. 5. According to Fig. 5a,
the particles were approximately spherical with a crystal-
ite size of 5–8 nm (Fig. 5b), and the rings on the diffrac-
tion pattern depicted a spinel crystal structure of nickel
ferrite. Several lines ([220], [311], [400], [511], and [440])
could be identi￿ed in the diffraction pattern in a manner
similar to Figs. 3b and 4b. Their positions and intensities
again con￿rm unambiguously the spinel structure. The
narrow distribution of crystallite size may be noted from
Fig. 5a.
Considering that the nucleation and growth of billions of
ferrite particles in a microreactor system requires a ￿nite
time for the chemical reaction to occur by a content
exchange mechanism, involving the transfer of reactants
following coalescenceof a pair of reverse micelles, or owing
to diffusion of the contents,
13 it is not surprising that an
optimal mechanical stirring time is required to obtain
nanocrystalline ferrites.
In line with the need for a critical time, zinc ferrites were
processed in a manner similar to that for nanocrystalline
a TEM image; b SADP
3 Image (TEM) and selected area diffraction pattern (SADP) of amorphous nickel ferrite powder heat treated in situ in
TEM at 600°C for 5 min
a TEM image; b,c SADPs
4 Image (TEM) and diffraction patterns (SADPs) of amorphous nickel ferrite powder heat treated in situ in TEM at
700°C for 5 min
a,b TEM images; c SADP
5 Low and high magni￿ cation TEM images and diffraction pattern (SADP) of nanocrystalline nickel ferrite processed by
mechanical stirring microemulsion for 25 min
Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique 1619
Materials Science and Technology November2003 Vol. 19nickel ferrites using a mechanicalstirring time of 25 min or
greater. A TEM image of zinc ferrite and corresponding
selected area electron diffraction pattern are presented in
Fig. 6. The TEM image (Fig. 6a) indicates that there was
some tendency for the particles to aggregate. The tendency
for agglomeration depends on the water/surfactant molar
ratio,13 and was (w~[H2O]/[surfactant]) 10 in the present
experiment. This agglomeration can be minimised by
minimising the water content/surfactant ratio. The diffrac-
tion pattern (Fig. 6b) was again characteristic of the spinel
structure of zinc ferrite. Several lines, [220], [311], [400],
[511], and [440], could be identi￿ed, similar to the case of
nickel ferrite. Their position and relative intensities again
con￿rmunambiguouslythespinelcrystallographicstructure.
Quantitative analysis of the EDS spectra taken for some
particles yielded an approximate stoichiometry of nickel
and zinc ferrite of NixFe32xO4/ZnxFe32xO4 (x:1.3, 1.2¡
0.2),implyingthattheconcentrationofnickeland zinc in the
resultantferrite was closeto thestoichiometriccomposition.
Initial magnetic measurements conducted at room tem-
perature on powder samples seemed to suggest that the
particles had a single domain structure and were super-
paramagnetic. Representative plots of magnetisation as
a function of applied ￿ eld are presented in Fig. 7 for
nanocrystallinenickel and zinc ferrites. The magnetisation–
￿ eld plot shows no hysteresis, and the higher ￿ eld part
doesnot indicateanyrealsaturation.Even at9 kOe, there is
visible lack of saturation. The maximum observed value of
magnetisation (6 emu g
21 for nickel ferrite, 0.75 emu g
21
at an applied ￿ eld of 9 kOe for zinc nanocrystallineferrite)
is signi￿cantly lower than that of multidomain bulk
particles. Almost no coercivity or remanence is observed.
The low value of saturation magnetisation is attributed to
the superparamagnetic nature of nickel and zinc ferrite,
and to the non-collinearity of the magnetic moments at or
near the surface of the particle. Nickel and cobalt ferrites,
which also show a ferromagneticinverse spinel, are known
to have a non-collinearspin structurewhen they are present
as ￿ ne crystallites.
14 The non-collinear structure attributed
to a surface effect is more pronounced for a smaller
crystallitesize of nickel and zinc ferrite (<10 nm). Detailed
magnetic studies are in progress.
Conclusions
Nanocrystalline nickel and zinc ferrites synthesised by a
microemulsion method exhibited a crystallite size of about
5–10 nm and an approximate stoichiometry of Mex-
Fe32xO4 (Me~Ni or Zn) (x:1.3, 1.2¡0.2). They lacked
saturation magnetisationat 9 kOe. A clear lack of coercive
force and lack of remanence was also observed.
References
1. m. gotic, i. czako-nagy, s. popovic and s. music: Philos. Mag.
Lett., 1998, 78, 193.
2. j. eckert, j. c. holzen, c. e. krill iii and w. l. johnson:
J. Mater. Res., 1992, 7, 1751.
3. c. n. chinnaswamy, a. narayanswamy, n. ponpandian and
k. chattopadhyay: Phys. Rev., B, 2001, B63, 184108.
4. j. guerveri-paz and d. paramillo-viguueras: Nanostruct.
Mater., 1999, 11, 195.
5. r. z. valiev, r. k. islamgaliev and i. v. alexandrov: Prog.
Mater. Sci., 2000, 45, 103.
6. h. e. schaefer, h. kisker, h. kronmuller and r. wurschum:
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7. u. erb: Nanostruct. Mater., 1995, 6, 533.
8. s. komarneni, m. c. d’arrigo, c. leionelli, g. c. pellacani
and h. katsuki: J. Am. Ceram. Soc., 1998, 81, 3041.
a TEM image; b SADP
6 Image (TEM) of nanocrystalline zinc ferrite and corresponding selected area diffraction pattern (SADP)
a nickel ferrite; b zinc ferrite
7 Magnetisation versus applied ￿ eld for nanocrystalline
nickel and zinc ferrite at 293 K
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Materials Science and Technology November 2003 Vol. 199. s. d. sartale and c. d. lokhande: Indian J. Eng. Mater. Sci.,
2000, 7, 404.
10. m. rajendran, a. k. bhattacharya, d. das, s. n. chintalpudi
and c. k. majumdar: Int. J. Mod. Phys. B, 2001, B15, 305.
11. k. v. p. m. shafi, y. koltypin, a. gedanken, r. prozorov,
j. balogh, j. lendvai and i. felner: J. Phys. Chem. B, 1997,
B101, 6409.
12. a. chatterjee, d. das, s. k. pradhan and d. chakravorty:
J. Magn. Magn. Mater., 1993, 127, 214.
13. m. p. pileni: J. Phys. Chem., 1993, 97, 6961.
Materials Science and Technology November2003 Vol. 19
Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique 1621

Some aspects of nanocrystalline nickel and zinc ferrites processed using microemulsion technique

Misra, RDK

Kale, A

Kooi, BJ

De Hosson, JTM

University of Groningen

   University of GroningenSome aspects of nanocrystalline nickel and zinc ferrites processed using microemulsiontechniqueMisra, RDK; Kale, A; Kooi, BJ; De Hosson, JTMPublished in:Materials Science and TechnologyDOI:10.1179/026708303225008040IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2003Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Misra, RDK., Kale, A., Kooi, BJ., & De Hosson, JTM. (2003). Some aspects of nanocrystalline nickel andzinc ferrites processed using microemulsion technique. Materials Science and Technology, 19(11), 1617-1621. https://doi.org/10.1179/026708303225008040CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Some aspects of nanocrystalline nickel andzinc ferrites processed using microemulsiontechniqueR. D. K. Misra, A. Kale, B. J. Kooi and J. Th. M. De HossonNanocrystalline nickel and zinc ferrites synthesised using a microemulsion technique were characterised by highresolution transmission electron microscopy and vibrating sample magnetometry. A narrow and uniform distributionof crystals of size range 5 – 8 nm, distinguished by a clear lack of saturation magnetisation at 9 kOe, were obtained.Also, no coercivity or remanence was observed. MST/5813Professor Misra (dmisra@louisiana.edu) is in the Department of Chemical Engineering, University of Louisiana at Lafayette,Lafayette, LA 70504–4130, USA, Mr Kale is at the University of Illinois at Urbana-Champaign, 1302 West PennsylvaniaAvenue, Urbana, IL 6801, USA, and Dr Kooi and Dr De Hosson are in the Department of Applied Physics, Materials ScienceCentre and Netherlands Institute for Materials Research, University of Groningen, Nijenborgh 4, 9747AG, Groningen, TheNetherlands. At the time the work was carried out Mr Kale was at the University of Louisiana at Lafayette, USA. Manuscriptreceived 12 December 2002; accepted 21 March 2003.# 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.IntroductionThe development of nanocrystalline materials is a subjectof considerable interest, both for the scienti c value ofunderstanding the unique properties of materials that haverelevance to condensed matter studies, and for thetechnological signi cance of enhancing the performanceof existing materials. The ability to produce nanocrystallineferrites is expected to assist in controlling the ultimateproperties of ferrites, an advantage over those fabricatedby powder metallurgy process. The magnetic properties,in particular, are strongly dependent on crystallite size,crystallite size distribution, and morphology of crystallites.High electrical resistance and lower saturation magneti-sation are the characteristics of soft ferrites, which are notfound in traditional silicon steels and permalloys.1 Thismakes them suitable for varied applications, includingmagnetic and magneto-optical.Modern data storage appli-cations require control over the morphology of particlesand a considerable reduction in crystallite size, to the sizeof a domain. Nanosize magnetic materials are expected toprove very useful in such applications. In recent years, anumber of chemical and physical methods have beenattempted to produce nanosize ferrites. Some of the mainphysical methods include mechanical milling,1 – 4 severeplastic deformation consolidation,5 and inert gas condensa-tion.6 The widely used chemical methods are electrodeposi-tion,7 hydrothermal reaction,8 oxidative precipitation,9 theorganic precursor method,10 sonochemical decomposi-tion,11 and the sol – gel synthesis technique.12 The majorityof these methods suffer from a lack of control of particlesize distribution. The water in oil microemulsion chemicalsynthesis method permits a narrow size distribution ofnanocrystalline particles to be obtained. It also has theadvantage of simple equipment and operating conditions.In the water in oil emulsion system, the small dropletsof water that are encapsulated into reverse micelles actas microreactors, providing a microstructured system toproduce well de ned nanosized crystallites13 (micelle: acolloidal aggregate of amphiphilic surfactant moleculessuch that the hydrophobic ends are in contact; reversemicelle: a colloidal aggregate of amphiphilic surfactantmolecules whereby hydrophilic ends are in contact). Thesize of nanocrystallites depends on the size of water poolsand the water content. Thus, the success of this methoddepends on the surfactant and solvent used, amount ofwater in the system, and concentration of surfactant. Inthe present paper, the potential of the water in oil micro-emulsion method for producing nanocrystalline ferrites isdescribed, and the initial observations of high resolutiontransmission electron microscopy and magnetic measure-ments are outlined.Experimental procedureNanocrystallinenickel and zinc ferrites based on the reversemicelle concept were synthesised using two microemulsionsystems. The  rst microemulsion system was an oil phasemicroemulsion comprising iso-octane and surfactantsodium diiso-octylsulphoccinate (AOT); the second micro-emulsion was an aqueous phase emulsion consisting ofiso-octane and surfactant diiso-octylsulphoccinate withreactant salts (hydrated iron sulphate and hydrated nickelsulphate or zinc sulphate, depending on the nature of theferrite being processed). The [Fe2z ]/[Ni2z ] or [Fe2z ]/[Zn2z ]ratio in the metal containing solution was 2 : 1. In a typicalexperiment for the synthesis of nickel ferrite, microemulsionI consisted of 2 mL of 30% ammoniumhydroxidez2.4 mLof water z66 mL of 0.5M AOT – iso-octane, and micro-emulsion II contained 0.384 g of FeSO4 .7H2Oz0.192 gof NiSO4 .6H2Oz66 mL of 0.5M AOT – iso-octane. Theconstitution of microemulsion systems for the processing ofzinc ferrites was identical to that for nickel ferrites exceptfor the replacement of nickel sulphate in microemulsionII by zinc sulphate. Ammonium hydroxide acted as aprecipitating agent to form metal hydroxides. Ammoniumhydroxide also provided stability to the water in oil system.The two microemulsions were mixed and subjected to rapidmechanical stirring. In initial experiments for the synthesisof nickel ferrites, the mechanical stirring time was 10 min.However, this led to the synthesis of amorphous nickelferrites (see below). The time was then increased to 15, 25,50 min. The optimum mechanical stirring time requiredto produce nanocrystalline ferrites was determined to be25 min. This durationwas used for subsequentexperiments.Hydroxides of nickel and iron were precipitated within thewater pools of reverse micelles and subsequently oxidisedto nickel ferrite. This was followed by separation of thesolid product by a centrifuging process. The resulting solidDOI 10.1179/026708303225008040 Materials Science and Technology November 2003 Vol. 19 1617product was washed a number of times with methanolfollowed by drying in a low vacuum. The chemicals used forsynthesis of nanocrystalline ferrite were of analytical grade,obtained from Aldrich, and the water used was doublydistilled.The ferrites were characterised by transmission electronmicroscopy (TEM) to determine crystallinity, extent ofsegregation of the nanoparticles, and their particle size(distribution). The TEM samples were prepared as follows.Ferrite powder was pulverised and dispersed in isopropylalcohol, and a droplet of the suspension was put on acarbon or formvar coated copper grid located on a glassslide. After drying, the grids were inserted in the micro-scope. A Jeol 4000 EX/II operating at 400 kV having apoint– point resolution of 0.165 nm, and a Jeol 2010FEG( eld emission gun) operating at 200 kV having aninformation limit of 0.11 nm, equipped with an EDAXEDS detector (with super ultrathin window), were used.In situ heating was carried out in the Jeol 2010FEG using aGaton double tilt heating holder (model 652 with a model901 SmartSet hot stage controller).Magnetic measurementswere conducted at room temperature using a vibratingsample magnetometer with an applied  eld of 9 kOe.Results and discussionThe anticipated chemical reaction expected to producenickel and zinc ferrite isMeSO4:6H2Oz2FeSO4:7H2Oz6NH4OHz1=2O2?MeFe2O4;z16H2Oz3(NH4)2SO4 : : : : : (1)where Me~Ni or Zn.The  rst observation was that the mechanical stirringtime of the microemulsion was critical for the synthesis ofnanocrystalline ferrites. As stated above, mechanicalstirring for less than the critical time of 25 min producedamorphous ferrites. Micrographs (TEM) of nickel ferritesproduced by mechanical stirring of the two microemulsionsystems for 10 min are presented in Fig. 1, con rming theamorphous nature of the nickel ferrites. Figure 1 gives noevidence of crystallite formation, and particles are seen to beaggregated.The amorphousnickel ferrites were heat treatedin situ in the microscope at 500, 600, and 700°C for 5 min,and the progress in crystallisation of the amorphous ferriteswas examined.Figures 2 – 4 clearly indicate that amorphousnickel ferrites transform to nanocrystalline nickel ferrites.Following heat treatment at 500°C for 5 min, nickel ferritecrystals of 5 – 10 nm could be seen (Fig. 2b). However, theheat treatment time of 5 min was not adequate for completetransformationof amorphous nickel ferrites to nanocrystal-line ferrites, and the selected area diffraction patternpredominantly indicated diffuse rings characteristic of anamorphous nature (Fig. 2c). After heat treatment at 600°Cfor 5 min, crystallisation was promoted, and a number ofcrystals could be seen (Fig. 3a) and the diffraction pattern(Fig. 3b) was representative of a crystalline material. Heattreatment at 700°C for 5 min led to complete crystallisa-tion of the amorphous material, but at the same time thetemperaturewas suf ciently high to promote crystal growth(Fig. 4). Several lines, namely [220], [311], [400], [511], and[440] could be identi ed in the diffraction pattern (Figs. 3band 4b). Their positions and relative intensities con rmunambiguously the spinel crystallographic structure. Theobserved behaviour at different temperatures suggests that500°C is adequate for transformation of an amorphous to acrystalline nature; however, a heat treatment time of greatera lower magni®cation; b,c higher magni®cation1 Images (TEM) at different magni cations of amorphous nickel ferrite powder produced by inadequate mechanical stir-ring (10 min) of microemulsion systemsa,b TEM images; c SADP2 Images (TEM) and selected area diffraction pattern (SADP) of amorphous nickel ferrite powder heat treated in situ inTEM at 500°C for 5 min1618 Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion techniqueMaterials Science and Technology November 2003 Vol. 19than 5 min (10 min) is required for transformation to becomplete.Micrographs (TEM) of nanocrystalline nickel ferritesprocessed by mechanical stirring of the two microemulsionsfor 25 min are presented in Fig. 5. According to Fig. 5a,the particles were approximately spherical with a crystal-ite size of 5 – 8 nm (Fig. 5b), and the rings on the diffrac-tion pattern depicted a spinel crystal structure of nickelferrite. Several lines ([220], [311], [400], [511], and [440])could be identi ed in the diffraction pattern in a mannersimilar to Figs. 3b and 4b. Their positions and intensitiesagain con rm unambiguously the spinel structure. Thenarrow distribution of crystallite size may be noted fromFig. 5a.Considering that the nucleation and growth of billions offerrite particles in a microreactor system requires a  nitetime for the chemical reaction to occur by a contentexchange mechanism, involving the transfer of reactantsfollowing coalescence of a pair of reverse micelles, or owingto diffusion of the contents,13 it is not surprising that anoptimal mechanical stirring time is required to obtainnanocrystalline ferrites.In line with the need for a critical time, zinc ferrites wereprocessed in a manner similar to that for nanocrystallinea TEM image; b SADP3 Image (TEM) and selected area diffraction pattern (SADP) of amorphous nickel ferrite powder heat treated in situ inTEM at 600°C for 5 mina TEM image; b,c SADPs4 Image (TEM) and diffraction patterns (SADPs) of amorphous nickel ferrite powder heat treated in situ in TEM at700°C for 5 mina,b TEM images; c SADP5 Low and high magni cation TEM images and diffraction pattern (SADP) of nanocrystalline nickel ferrite processed bymechanical stirring microemulsion for 25 minMisra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique 1619Materials Science and Technology November 2003 Vol. 19nickel ferrites using a mechanical stirring time of 25 min orgreater. A TEM image of zinc ferrite and correspondingselected area electron diffraction pattern are presented inFig. 6. The TEM image (Fig. 6a) indicates that there wassome tendency for the particles to aggregate. The tendencyfor agglomeration depends on the water/surfactant molarratio,13 and was (w~[H2O]/[surfactant]) 10 in the presentexperiment. This agglomeration can be minimised byminimising the water content/surfactant ratio. The diffrac-tion pattern (Fig. 6b) was again characteristic of the spinelstructure of zinc ferrite. Several lines, [220], [311], [400],[511], and [440], could be identi ed, similar to the case ofnickel ferrite. Their position and relative intensities againcon rm unambiguouslythe spinel crystallographicstructure.Quantitative analysis of the EDS spectra taken for someparticles yielded an approximate stoichiometry of nickeland zinc ferrite of NixFe32 xO4 /ZnxFe32xO4 (x:1.3, 1.2¡0.2), implying that the concentrationof nickel and zinc in theresultant ferrite was close to the stoichiometric composition.Initial magnetic measurements conducted at room tem-perature on powder samples seemed to suggest that theparticles had a single domain structure and were super-paramagnetic. Representative plots of magnetisation asa function of applied  eld are presented in Fig. 7 fornanocrystalline nickel and zinc ferrites. The magnetisation– eld plot shows no hysteresis, and the higher  eld partdoes not indicate any real saturation.Even at 9 kOe, there isvisible lack of saturation. The maximum observed value ofmagnetisation (6 emu g21 for nickel ferrite, 0.75 emu g21at an applied  eld of 9 kOe for zinc nanocrystalline ferrite)is signi cantly lower than that of multidomain bulkparticles. Almost no coercivity or remanence is observed.The low value of saturation magnetisation is attributed tothe superparamagnetic nature of nickel and zinc ferrite,and to the non-collinearity of the magnetic moments at ornear the surface of the particle. Nickel and cobalt ferrites,which also show a ferromagnetic inverse spinel, are knownto have a non-collinear spin structurewhen they are presentas  ne crystallites.14 The non-collinear structure attributedto a surface effect is more pronounced for a smallercrystallite size of nickel and zinc ferrite (<10 nm). Detailedmagnetic studies are in progress.ConclusionsNanocrystalline nickel and zinc ferrites synthesised by amicroemulsion method exhibited a crystallite size of about5 – 10 nm and an approximate stoichiometry of Mex -Fe32 xO4 (Me~Ni or Zn) (x:1.3, 1.2¡0.2). They lackedsaturation magnetisation at 9 kOe. A clear lack of coerciveforce and lack of remanence was also observed.References1. m. gotic, i. czako-nagy, s. popovic and s. music: Philos. Mag.Lett., 1998, 78, 193.2. j. eckert, j. c. holzen, c. e. krill iii and w. l. johnson:J. Mater. Res., 1992, 7, 1751.3. c. n. chinnaswamy, a. narayanswamy, n. ponpandian andk. chattopadhyay: Phys. Rev., B, 2001, B63, 184108.4. j. guerveri-paz and d. paramillo-viguueras: Nanostruct.Mater., 1999, 11, 195.5. r. z. valiev, r. k. islamgaliev and i. v. alexandrov: Prog.Mater. Sci., 2000, 45, 103.6. h. e. schaefer, h. kisker, h. kronmuller and r. wurschum:Nanostruct. Mater., 1992, 1, 523.7. u. erb: Nanostruct. Mater., 1995, 6, 533.8. s. komarneni, m. c. d’arrigo, c. leionelli, g. c. pellacaniand h. katsuki: J. Am. Ceram. Soc., 1998, 81, 3041.a TEM image; b SADP6 Image (TEM) of nanocrystalline zinc ferrite and corresponding selected area diffraction pattern (SADP)a nickel ferrite; b zinc ferrite7 Magnetisation versus applied  eld for nanocrystallinenickel and zinc ferrite at 293 K1620 Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion techniqueMaterials Science and Technology November 2003 Vol. 199. s. d. sartale and c. d. lokhande: Indian J. Eng. Mater. Sci.,2000, 7, 404.10. m. rajendran, a. k. bhattacharya, d. das, s. n. chintalpudiand c. k. majumdar: Int. J. Mod. Phys. B, 2001, B15, 305.11. k. v. p. m. shafi, y. koltypin, a. gedanken, r. prozorov,j. balogh, j. lendvai and i. felner: J. Phys. Chem. B, 1997,B101, 6409.12. a. chatterjee, d. das, s. k. pradhan and d. chakravorty:J. Magn. Magn. Mater., 1993, 127, 214.13. m. p. pileni: J. Phys. Chem., 1993, 97, 6961.Materials Science and Technology November 2003 Vol. 19Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique 1621

   University of GroningenSome aspects of nanocrystalline nickel and zinc ferrites processed using microemulsiontechniqueMisra, RDK; Kale, A; Kooi, BJ; De Hosson, JTMPublished in:Materials Science and TechnologyDOI:10.1179/026708303225008040IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2003Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Misra, RDK., Kale, A., Kooi, BJ., & De Hosson, JTM. (2003). Some aspects of nanocrystalline nickel andzinc ferrites processed using microemulsion technique. Materials Science and Technology, 19(11), 1617-1621. https://doi.org/10.1179/026708303225008040CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Some aspects of nanocrystalline nickel andzinc ferrites processed using microemulsiontechniqueR. D. K. Misra, A. Kale, B. J. Kooi and J. Th. M. De HossonNanocrystalline nickel and zinc ferrites synthesised using a microemulsion technique were characterised by highresolution transmission electron microscopy and vibrating sample magnetometry. A narrow and uniform distributionof crystals of size range 5 – 8 nm, distinguished by a clear lack of saturation magnetisation at 9 kOe, were obtained.Also, no coercivity or remanence was observed. MST/5813Professor Misra (dmisra@louisiana.edu) is in the Department of Chemical Engineering, University of Louisiana at Lafayette,Lafayette, LA 70504– 4130, USA, Mr Kale is at the University of Illinois at Urbana-Champaign, 1302 West PennsylvaniaAvenue, Urbana, IL 6801, USA, and Dr Kooi and Dr De Hosson are in the Department of Applied Physics, Materials ScienceCentre and Netherlands Institute for Materials Research, University of Groningen, Nijenborgh 4, 9747AG, Groningen, TheNetherlands. At the time the work was carried out Mr Kale was at the University of Louisiana at Lafayette, USA. Manuscriptreceived 12 December 2002; accepted 21 March 2003.# 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.IntroductionThe development of nanocrystalline materials is a subjectof considerable interest, both for the scienti c value ofunderstanding the unique properties of materials that haverelevance to condensed matter studies, and for thetechnological signi cance of enhancing the performanceof existing materials. The ability to produce nanocrystallineferrites is expected to assist in controlling the ultimateproperties of ferrites, an advantage over those fabricatedby powder metallurgy process. The magnetic properties,in particular, are strongly dependent on crystallite size,crystallite size distribution, and morphology of crystallites.High electrical resistance and lower saturation magneti-sation are the characteristics of soft ferrites, which are notfound in traditional silicon steels and permalloys.1 Thismakes them suitable for varied applications, includingmagnetic and magneto-optical. Modern data storage appli-cations require control over the morphology of particlesand a considerable reduction in crystallite size, to the sizeof a domain. Nanosize magnetic materials are expected toprove very useful in such applications. In recent years, anumber of chemical and physical methods have beenattempted to produce nanosize ferrites. Some of the mainphysical methods include mechanical milling,1 – 4 severeplastic deformation consolidation,5 and inert gas condensa-tion.6 The widely used chemical methods are electrodeposi-tion,7 hydrothermal reaction,8 oxidative precipitation,9 theorganic precursor method,1 0 sonochemical decomposi-tion,1 1 and the sol – gel synthesis technique.1 2 The majorityof these methods suffer from a lack of control of particlesize distribution. The water in oil microemulsion chemicalsynthesis method permits a narrow size distribution ofnanocrystalline particles to be obtained. It also has theadvantage of simple equipment and operating conditions.In the water in oil emulsion system, the small dropletsof water that are encapsulated into reverse micelles actas microreactors, providing a microstructured system toproduce well de ned nanosized crystallites1 3 (micelle: acolloidal aggregate of amphiphilic surfactant moleculessuch that the hydrophobic ends are in contact; reversemicelle: a colloidal aggregate of amphiphilic surfactantmolecules whereby hydrophilic ends are in contact). Thesize of nanocrystallites depends on the size of water poolsand the water content. Thus, the success of this methoddepends on the surfactant and solvent used, amount ofwater in the system, and concentration of surfactant. Inthe present paper, the potential of the water in oil micro-emulsion method for producing nanocrystalline ferrites isdescribed, and the initial observations of high resolutiontransmission electron microscopy and magnetic measure-ments are outlined.Experimental procedureNanocrystallinenickel and zinc ferrites based on the reversemicelle concept were synthesised using two microemulsionsystems. The  rst microemulsion system was an oil phasemicroemulsion comprising iso-octane and surfactantsodium diiso-octylsulphoccinate (AOT); the second micro-emulsion was an aqueous phase emulsion consisting ofiso-octane and surfactant diiso-octylsulphoccinate withreactant salts (hydrated iron sulphate and hydrated nickelsulphate or zinc sulphate, depending on the nature of theferrite being processed). The [Fe2 z ]/[Ni2 z ] or [Fe2 z ]/[Zn2 z ]ratio in the metal containing solution was 2 : 1. In a typicalexperiment for the synthesis of nickel ferrite, microemulsionI consisted of 2 mL of 30% ammonium hydroxide z2.4 mLof water z66 mL of 0.5M AOT – iso-octane, and micro-emulsion II contained 0.384 g of FeSO4 .7H2Oz0.192 gof NiSO4 .6H2Oz66 mL of 0.5M AOT – iso-octane. Theconstitution of microemulsion systems for the processing ofzinc ferrites was identical to that for nickel ferrites exceptfor the replacement of nickel sulphate in microemulsionII by zinc sulphate. Ammonium hydroxide acted as aprecipitating agent to form metal hydroxides. Ammoniumhydroxide also provided stability to the water in oil system.The two microemulsions were mixed and subjected to rapidmechanical stirring. In initial experiments for the synthesisof nickel ferrites, the mechanical stirring time was 10 min.However, this led to the synthesis of amorphous nickelferrites (see below). The time was then increased to 15, 25,50 min. The optimum mechanical stirring time requiredto produce nanocrystalline ferrites was determined to be25 min. This duration was used for subsequentexperiments.Hydroxides of nickel and iron were precipitated within thewater pools of reverse micelles and subsequently oxidisedto nickel ferrite. This was followed by separation of thesolid product by a centrifuging process. The resulting solidDOI 10.1179/026708303225008040 Materials Science and Technology November 2003 Vol. 19 1617product was washed a number of times with methanolfollowed by drying in a low vacuum. The chemicals used forsynthesis of nanocrystalline ferrite were of analytical grade,obtained from Aldrich, and the water used was doublydistilled.The ferrites were characterised by transmission electronmicroscopy (TEM) to determine crystallinity, extent ofsegregation of the nanoparticles, and their particle size(distribution). The TEM samples were prepared as follows.Ferrite powder was pulverised and dispersed in isopropylalcohol, and a droplet of the suspension was put on acarbon or formvar coated copper grid located on a glassslide. After drying, the grids were inserted in the micro-scope. A Jeol 4000 EX/II operating at 400 kV having apoint– point resolution of 0.165 nm, and a Jeol 2010FEG( eld emission gun) operating at 200 kV having aninformation limit of 0.11 nm, equipped with an EDAXEDS detector (with super ultrathin window), were used.In situ heating was carried out in the Jeol 2010FEG using aGaton double tilt heating holder (model 652 with a model901 SmartSet hot stage controller). Magnetic measurementswere conducted at room temperature using a vibratingsample magnetometer with an applied  eld of 9 kOe.Results and discussionThe anticipated chemical reaction expected to producenickel and zinc ferrite isMeSO4:6H2Oz2FeSO4:7H2Oz6NH4OHz1=2O2?MeFe2O4;z16H2Oz3(NH4)2SO4 : : : : : (1)where Me~Ni or Zn.The  rst observation was that the mechanical stirringtime of the microemulsion was critical for the synthesis ofnanocrystalline ferrites. As stated above, mechanicalstirring for less than the critical time of 25 min producedamorphous ferrites. Micrographs (TEM) of nickel ferritesproduced by mechanical stirring of the two microemulsionsystems for 10 min are presented in Fig. 1, con rming theamorphous nature of the nickel ferrites. Figure 1 gives noevidence of crystallite formation, and particles are seen to beaggregated.The amorphous nickel ferrites were heat treatedin situ in the microscope at 500, 600, and 700°C for 5 min,and the progress in crystallisation of the amorphous ferriteswas examined. Figures 2 – 4 clearly indicate that amorphousnickel ferrites transform to nanocrystalline nickel ferrites.Following heat treatment at 500°C for 5 min, nickel ferritecrystals of 5 – 10 nm could be seen (Fig. 2b). However, theheat treatment time of 5 min was not adequate for completetransformationof amorphous nickel ferrites to nanocrystal-line ferrites, and the selected area diffraction patternpredominantly indicated diffuse rings characteristic of anamorphous nature (Fig. 2c). After heat treatment at 600°Cfor 5 min, crystallisation was promoted, and a number ofcrystals could be seen (Fig. 3a) and the diffraction pattern(Fig. 3b) was representative of a crystalline material. Heattreatment at 700°C for 5 min led to complete crystallisa-tion of the amorphous material, but at the same time thetemperature was suf ciently high to promote crystal growth(Fig. 4). Several lines, namely [220], [311], [400], [511], and[440] could be identi ed in the diffraction pattern (Figs. 3band 4b). Their positions and relative intensities con rmunambiguously the spinel crystallographic structure. Theobserved behaviour at different temperatures suggests that500°C is adequate for transformation of an amorphous to acrystalline nature; however, a heat treatment time of greatera lower magni®cation; b,c higher magni®cation1 Images (TEM) at different magni cations of amorphous nickel ferrite powder produced by inadequate mechanical stir-ring (10 min) of microemulsion systemsa,b TEM images; c SADP2 Images (TEM) and selected area diffraction pattern (SADP) of amorphous nickel ferrite powder heat treated in situ inTEM at 500°C for 5 min1618 Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion techniqueMaterials Science and Technology November 2003 Vol. 19than 5 min (10 min) is required for transformation to becomplete.Micrographs (TEM) of nanocrystalline nickel ferritesprocessed by mechanical stirring of the two microemulsionsfor 25 min are presented in Fig. 5. According to Fig. 5a,the particles were approximately spherical with a crystal-ite size of 5 – 8 nm (Fig. 5b), and the rings on the diffrac-tion pattern depicted a spinel crystal structure of nickelferrite. Several lines ([220], [311], [400], [511], and [440])could be identi ed in the diffraction pattern in a mannersimilar to Figs. 3b and 4b. Their positions and intensitiesagain con rm unambiguously the spinel structure. Thenarrow distribution of crystallite size may be noted fromFig. 5a.Considering that the nucleation and growth of billions offerrite particles in a microreactor system requires a  nitetime for the chemical reaction to occur by a contentexchange mechanism, involving the transfer of reactantsfollowing coalescence of a pair of reverse micelles, or owingto diffusion of the contents,1 3 it is not surprising that anoptimal mechanical stirring time is required to obtainnanocrystalline ferrites.In line with the need for a critical time, zinc ferrites wereprocessed in a manner similar to that for nanocrystallinea TEM image; b SADP3 Image (TEM) and selected area diffraction pattern (SADP) of amorphous nickel ferrite powder heat treated in situ inTEM at 600°C for 5 mina TEM image; b,c SADPs4 Image (TEM) and diffraction patterns (SADPs) of amorphous nickel ferrite powder heat treated in situ in TEM at700°C for 5 mina,b TEM images; c SADP5 Low and high magni cation TEM images and diffraction pattern (SADP) of nanocrystalline nickel ferrite processed bymechanical stirring microemulsion for 25 minMisra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique 1619Materials Science and Technology November 2003 Vol. 19nickel ferrites using a mechanical stirring time of 25 min orgreater. A TEM image of zinc ferrite and correspondingselected area electron diffraction pattern are presented inFig. 6. The TEM image (Fig. 6a) indicates that there wassome tendency for the particles to aggregate. The tendencyfor agglomeration depends on the water/surfactant molarratio,1 3 and was (w~[H2O]/[surfactant]) 10 in the presentexperiment. This agglomeration can be minimised byminimising the water content/surfactant ratio. The diffrac-tion pattern (Fig. 6b) was again characteristic of the spinelstructure of zinc ferrite. Several lines, [220], [311], [400],[511], and [440], could be identi ed, similar to the case ofnickel ferrite. Their position and relative intensities againcon rm unambiguouslythe spinel crystallographicstructure.Quantitative analysis of the EDS spectra taken for someparticles yielded an approximate stoichiometry of nickeland zinc ferrite of NixFe3 2 xO4 /ZnxFe3 2xO4 (x:1.3, 1.2¡0.2), implying that the concentrationof nickel and zinc in theresultant ferrite was close to the stoichiometric composition.Initial magnetic measurements conducted at room tem-perature on powder samples seemed to suggest that theparticles had a single domain structure and were super-paramagnetic. Representative plots of magnetisation asa function of applied  eld are presented in Fig. 7 fornanocrystalline nickel and zinc ferrites. The magnetisation– eld plot shows no hysteresis, and the higher  eld partdoes not indicate any real saturation.Even at 9 kOe, there isvisible lack of saturation. The maximum observed value ofmagnetisation (6 emu g21 for nickel ferrite, 0.75 emu g21at an applied  eld of 9 kOe for zinc nanocrystalline ferrite)is signi cantly lower than that of multidomain bulkparticles. Almost no coercivity or remanence is observed.The low value of saturation magnetisation is attributed tothe superparamagnetic nature of nickel and zinc ferrite,and to the non-collinearity of the magnetic moments at ornear the surface of the particle. Nickel and cobalt ferrites,which also show a ferromagnetic inverse spinel, are knownto have a non-collinear spin structure when they are presentas  ne crystallites.1 4 The non-collinear structure attributedto a surface effect is more pronounced for a smallercrystallite size of nickel and zinc ferrite (<10 nm). Detailedmagnetic studies are in progress.ConclusionsNanocrystalline nickel and zinc ferrites synthesised by amicroemulsion method exhibited a crystallite size of about5 – 10 nm and an approximate stoichiometry of Mex -Fe3 2 xO4 (Me~Ni or Zn) (x:1.3, 1.2¡0.2). They lackedsaturation magnetisation at 9 kOe. A clear lack of coerciveforce and lack of remanence was also observed.References1. m. gotic, i. czako-nagy, s. popovic and s. music: Philos. Mag.Lett., 1998, 78, 193.2. j. eckert, j. c. holzen, c. e. krill iii and w. l. johnson:J. Mater. Res., 1992, 7, 1751.3. c. n. chinnaswamy, a. narayanswamy, n. ponpandian andk. chattopadhyay: Phys. Rev., B, 2001, B63, 184108.4. j. guerveri-paz and d. paramillo-viguueras: Nanostruct.Mater., 1999, 11, 195.5. r. z. valiev, r. k. islamgaliev and i. v. alexandrov: Prog.Mater. Sci., 2000, 45, 103.6. h. e. schaefer, h. kisker, h. kronmuller and r. wurschum:Nanostruct. Mater., 1992, 1, 523.7. u. erb: Nanostruct. Mater., 1995, 6, 533.8. s. komarneni, m. c. d’arrigo, c. leionelli, g. c. pellacaniand h. katsuki: J. Am. Ceram. Soc., 1998, 81, 3041.a TEM image; b SADP6 Image (TEM) of nanocrystalline zinc ferrite and corresponding selected area diffraction pattern (SADP)a nickel ferrite; b zinc ferrite7 Magnetisation versus applied  eld for nanocrystallinenickel and zinc ferrite at 293 K1620 Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion techniqueMaterials Science and Technology November 2003 Vol. 199. s. d. sartale and c. d. lokhande: Indian J. Eng. Mater. Sci.,2000, 7, 404.10. m. rajendran, a. k. bhattacharya, d. das, s. n. chintalpudiand c. k. majumdar: Int. J. Mod. Phys. B, 2001, B15, 305.11. k. v. p. m. shafi, y. koltypin, a. gedanken, r. prozorov,j. balogh, j. lendvai and i. felner: J. Phys. Chem. B, 1997,B101, 6409.12. a. chatterjee, d. das, s. k. pradhan and d. chakravorty:J. Magn. Magn. Mater., 1993, 127, 214.13. m. p. pileni: J. Phys. Chem., 1993, 97, 6961.Materials Science and Technology November 2003 Vol. 19Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique 1621

Nanocrystalline nickel and zinc ferrites synthesised using a microemulsion technique were characterised by high resolution transmission electron microscopy and vibrating sample magnetometry. A narrow and uniform distribution of crystals of size range 5-8 nm, distinguished by a clear lack of saturation magnetisation at 9 kOe, were obtained. Also, no coercivity or remanence was observed

NARCIS 

Nanocrystalline nickel and zinc ferrites synthesised using a microemulsion technique were characterised by high resolution transmission electron microscopy and vibrating sample magnetometry. A narrow and uniform distribution of crystals of size range 5-8 nm, distinguished by a clear lack of saturation magnetisation at 9 kOe, were obtained. Also, no coercivity or remanence was observed.</p

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 University of Groningen
Some aspects of nanocrystalline nickel and zinc ferrites processed using microemulsion
technique
Misra, RDK; Kale, A; Kooi, Bart; De Hosson, J.T.M.
Published in:
Materials Science and Technology
DOI:
10.1179/026708303225008040
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2003
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Misra, R. D. K., Kale, A., Kooi, B. J., & De Hosson, J. T. M. (2003). Some aspects of nanocrystalline nickel
and zinc ferrites processed using microemulsion technique. Materials Science and Technology, 19(11),
1617-1621. DOI: 10.1179/026708303225008040
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 10-02-2018
Some aspects of nanocrystalline nickel and
zinc ferrites processed using microemulsion
technique
R. D. K. Misra, A. Kale, B. J. Kooi and J. Th. M. De Hosson
Nanocrystalline nickel and zinc ferrites synthesised using a microemulsion technique were characterised by high
resolution transmission electron microscopy and vibrating sample magnetometry. A narrow and uniform distribution
of crystals of size range 5 – 8 nm, distinguished by a clear lack of saturation magnetisation at 9 kOe, were obtained.
Also, no coercivity or remanence was observed. MST/5813
Professor Misra (dmisra@louisiana.edu) is in the Department of Chemical Engineering, University of Louisiana at Lafayette,
Lafayette, LA 70504–4130, USA, Mr Kale is at the University of Illinois at Urbana-Champaign, 1302 West Pennsylvania
Avenue, Urbana, IL 6801, USA, and Dr Kooi and Dr De Hosson are in the Department of Applied Physics, Materials Science
Centre and Netherlands Institute for Materials Research, University of Groningen, Nijenborgh 4, 9747AG, Groningen, The
Netherlands. At the time the work was carried out Mr Kale was at the University of Louisiana at Lafayette, USA. Manuscript
received 12 December 2002; accepted 21 March 2003.
# 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining.
Introduction
The development of nanocrystalline materials is a subject
of considerable interest, both for the scienti c value of
understanding the unique properties of materials that have
relevance to condensed matter studies, and for the
technological signi cance of enhancing the performance
of existing materials. The ability to produce nanocrystalline
ferrites is expected to assist in controlling the ultimate
properties of ferrites, an advantage over those fabricated
by powder metallurgy process. The magnetic properties,
in particular, are strongly dependent on crystallite size,
crystallite size distribution, and morphology of crystallites.
High electrical resistance and lower saturation magneti-
sation are the characteristics of soft ferrites, which are not
found in traditional silicon steels and permalloys.1 This
makes them suitable for varied applications, including
magnetic and magneto-optical.Modern data storage appli-
cations require control over the morphology of particles
and a considerable reduction in crystallite size, to the size
of a domain. Nanosize magnetic materials are expected to
prove very useful in such applications. In recent years, a
number of chemical and physical methods have been
attempted to produce nanosize ferrites. Some of the main
physical methods include mechanical milling,1 – 4 severe
plastic deformation consolidation,5 and inert gas condensa-
tion.6 The widely used chemical methods are electrodeposi-
tion,7 hydrothermal reaction,8 oxidative precipitation,9 the
organic precursor method,10 sonochemical decomposi-
tion,11 and the sol – gel synthesis technique.12 The majority
of these methods suffer from a lack of control of particle
size distribution. The water in oil microemulsion chemical
synthesis method permits a narrow size distribution of
nanocrystalline particles to be obtained. It also has the
advantage of simple equipment and operating conditions.
In the water in oil emulsion system, the small droplets
of water that are encapsulated into reverse micelles act
as microreactors, providing a microstructured system to
produce well de ned nanosized crystallites13 (micelle: a
colloidal aggregate of amphiphilic surfactant molecules
such that the hydrophobic ends are in contact; reverse
micelle: a colloidal aggregate of amphiphilic surfactant
molecules whereby hydrophilic ends are in contact). The
size of nanocrystallites depends on the size of water pools
and the water content. Thus, the success of this method
depends on the surfactant and solvent used, amount of
water in the system, and concentration of surfactant. In
the present paper, the potential of the water in oil micro-
emulsion method for producing nanocrystalline ferrites is
described, and the initial observations of high resolution
transmission electron microscopy and magnetic measure-
ments are outlined.
Experimental procedure
Nanocrystallinenickel and zinc ferrites based on the reverse
micelle concept were synthesised using two microemulsion
systems. The  rst microemulsion system was an oil phase
microemulsion comprising iso-octane and surfactant
sodium diiso-octylsulphoccinate (AOT); the second micro-
emulsion was an aqueous phase emulsion consisting of
iso-octane and surfactant diiso-octylsulphoccinate with
reactant salts (hydrated iron sulphate and hydrated nickel
sulphate or zinc sulphate, depending on the nature of the
ferrite being processed). The [Fe2z ]/[Ni2z ] or [Fe2z ]/[Zn2z ]
ratio in the metal containing solution was 2 : 1. In a typical
experiment for the synthesis of nickel ferrite, microemulsion
I consisted of 2 mL of 30% ammoniumhydroxidez2.4 mL
of water z66 mL of 0.5M AOT – iso-octane, and micro-
emulsion II contained 0.384 g of FeSO4 .7H2Oz0.192 g
of NiSO4 .6H2Oz66 mL of 0.5M AOT – iso-octane. The
constitution of microemulsion systems for the processing of
zinc ferrites was identical to that for nickel ferrites except
for the replacement of nickel sulphate in microemulsion
II by zinc sulphate. Ammonium hydroxide acted as a
precipitating agent to form metal hydroxides. Ammonium
hydroxide also provided stability to the water in oil system.
The two microemulsions were mixed and subjected to rapid
mechanical stirring. In initial experiments for the synthesis
of nickel ferrites, the mechanical stirring time was 10 min.
However, this led to the synthesis of amorphous nickel
ferrites (see below). The time was then increased to 15, 25,
50 min. The optimum mechanical stirring time required
to produce nanocrystalline ferrites was determined to be
25 min. This durationwas used for subsequentexperiments.
Hydroxides of nickel and iron were precipitated within the
water pools of reverse micelles and subsequently oxidised
to nickel ferrite. This was followed by separation of the
solid product by a centrifuging process. The resulting solid
DOI 10.1179/026708303225008040 Materials Science and Technology November 2003 Vol. 19 1617
product was washed a number of times with methanol
followed by drying in a low vacuum. The chemicals used for
synthesis of nanocrystalline ferrite were of analytical grade,
obtained from Aldrich, and the water used was doubly
distilled.
The ferrites were characterised by transmission electron
microscopy (TEM) to determine crystallinity, extent of
segregation of the nanoparticles, and their particle size
(distribution). The TEM samples were prepared as follows.
Ferrite powder was pulverised and dispersed in isopropyl
alcohol, and a droplet of the suspension was put on a
carbon or formvar coated copper grid located on a glass
slide. After drying, the grids were inserted in the micro-
scope. A Jeol 4000 EX/II operating at 400 kV having a
point– point resolution of 0.165 nm, and a Jeol 2010FEG
( eld emission gun) operating at 200 kV having an
information limit of 0.11 nm, equipped with an EDAX
EDS detector (with super ultrathin window), were used.
In situ heating was carried out in the Jeol 2010FEG using a
Gaton double tilt heating holder (model 652 with a model
901 SmartSet hot stage controller).Magnetic measurements
were conducted at room temperature using a vibrating
sample magnetometer with an applied  eld of 9 kOe.
Results and discussion
The anticipated chemical reaction expected to produce
nickel and zinc ferrite is
MeSO4:6H2Oz2FeSO4:7H2Oz6NH4OHz1=2O2
?MeFe2O4;z16H2Oz3(NH4)2SO4 : : : : : (1)
where Me~Ni or Zn.
The  rst observation was that the mechanical stirring
time of the microemulsion was critical for the synthesis of
nanocrystalline ferrites. As stated above, mechanical
stirring for less than the critical time of 25 min produced
amorphous ferrites. Micrographs (TEM) of nickel ferrites
produced by mechanical stirring of the two microemulsion
systems for 10 min are presented in Fig. 1, con rming the
amorphous nature of the nickel ferrites. Figure 1 gives no
evidence of crystallite formation, and particles are seen to be
aggregated.The amorphousnickel ferrites were heat treated
in situ in the microscope at 500, 600, and 700°C for 5 min,
and the progress in crystallisation of the amorphous ferrites
was examined.Figures 2 – 4 clearly indicate that amorphous
nickel ferrites transform to nanocrystalline nickel ferrites.
Following heat treatment at 500°C for 5 min, nickel ferrite
crystals of 5 – 10 nm could be seen (Fig. 2b). However, the
heat treatment time of 5 min was not adequate for complete
transformationof amorphous nickel ferrites to nanocrystal-
line ferrites, and the selected area diffraction pattern
predominantly indicated diffuse rings characteristic of an
amorphous nature (Fig. 2c). After heat treatment at 600°C
for 5 min, crystallisation was promoted, and a number of
crystals could be seen (Fig. 3a) and the diffraction pattern
(Fig. 3b) was representative of a crystalline material. Heat
treatment at 700°C for 5 min led to complete crystallisa-
tion of the amorphous material, but at the same time the
temperaturewas suf ciently high to promote crystal growth
(Fig. 4). Several lines, namely [220], [311], [400], [511], and
[440] could be identi ed in the diffraction pattern (Figs. 3b
and 4b). Their positions and relative intensities con rm
unambiguously the spinel crystallographic structure. The
observed behaviour at different temperatures suggests that
500°C is adequate for transformation of an amorphous to a
crystalline nature; however, a heat treatment time of greater
a lower magni®cation; b,c higher magni®cation
1 Images (TEM) at different magni cations of amorphous nickel ferrite powder produced by inadequate mechanical stir-
ring (10 min) of microemulsion systems
a,b TEM images; c SADP
2 Images (TEM) and selected area diffraction pattern (SADP) of amorphous nickel ferrite powder heat treated in situ in
TEM at 500°C for 5 min
1618 Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique
Materials Science and Technology November 2003 Vol. 19
than 5 min (10 min) is required for transformation to be
complete.
Micrographs (TEM) of nanocrystalline nickel ferrites
processed by mechanical stirring of the two microemulsions
for 25 min are presented in Fig. 5. According to Fig. 5a,
the particles were approximately spherical with a crystal-
ite size of 5 – 8 nm (Fig. 5b), and the rings on the diffrac-
tion pattern depicted a spinel crystal structure of nickel
ferrite. Several lines ([220], [311], [400], [511], and [440])
could be identi ed in the diffraction pattern in a manner
similar to Figs. 3b and 4b. Their positions and intensities
again con rm unambiguously the spinel structure. The
narrow distribution of crystallite size may be noted from
Fig. 5a.
Considering that the nucleation and growth of billions of
ferrite particles in a microreactor system requires a  nite
time for the chemical reaction to occur by a content
exchange mechanism, involving the transfer of reactants
following coalescence of a pair of reverse micelles, or owing
to diffusion of the contents,13 it is not surprising that an
optimal mechanical stirring time is required to obtain
nanocrystalline ferrites.
In line with the need for a critical time, zinc ferrites were
processed in a manner similar to that for nanocrystalline
a TEM image; b SADP
3 Image (TEM) and selected area diffraction pattern (SADP) of amorphous nickel ferrite powder heat treated in situ in
TEM at 600°C for 5 min
a TEM image; b,c SADPs
4 Image (TEM) and diffraction patterns (SADPs) of amorphous nickel ferrite powder heat treated in situ in TEM at
700°C for 5 min
a,b TEM images; c SADP
5 Low and high magni cation TEM images and diffraction pattern (SADP) of nanocrystalline nickel ferrite processed by
mechanical stirring microemulsion for 25 min
Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique 1619
Materials Science and Technology November 2003 Vol. 19
nickel ferrites using a mechanical stirring time of 25 min or
greater. A TEM image of zinc ferrite and corresponding
selected area electron diffraction pattern are presented in
Fig. 6. The TEM image (Fig. 6a) indicates that there was
some tendency for the particles to aggregate. The tendency
for agglomeration depends on the water/surfactant molar
ratio,13 and was (w~[H2O]/[surfactant]) 10 in the present
experiment. This agglomeration can be minimised by
minimising the water content/surfactant ratio. The diffrac-
tion pattern (Fig. 6b) was again characteristic of the spinel
structure of zinc ferrite. Several lines, [220], [311], [400],
[511], and [440], could be identi ed, similar to the case of
nickel ferrite. Their position and relative intensities again
con rm unambiguouslythe spinel crystallographicstructure.
Quantitative analysis of the EDS spectra taken for some
particles yielded an approximate stoichiometry of nickel
and zinc ferrite of NixFe32 xO4 /ZnxFe32xO4 (x:1.3, 1.2¡
0.2), implying that the concentrationof nickel and zinc in the
resultant ferrite was close to the stoichiometric composition.
Initial magnetic measurements conducted at room tem-
perature on powder samples seemed to suggest that the
particles had a single domain structure and were super-
paramagnetic. Representative plots of magnetisation as
a function of applied  eld are presented in Fig. 7 for
nanocrystalline nickel and zinc ferrites. The magnetisation–
 eld plot shows no hysteresis, and the higher  eld part
does not indicate any real saturation.Even at 9 kOe, there is
visible lack of saturation. The maximum observed value of
magnetisation (6 emu g21 for nickel ferrite, 0.75 emu g21
at an applied  eld of 9 kOe for zinc nanocrystalline ferrite)
is signi cantly lower than that of multidomain bulk
particles. Almost no coercivity or remanence is observed.
The low value of saturation magnetisation is attributed to
the superparamagnetic nature of nickel and zinc ferrite,
and to the non-collinearity of the magnetic moments at or
near the surface of the particle. Nickel and cobalt ferrites,
which also show a ferromagnetic inverse spinel, are known
to have a non-collinear spin structurewhen they are present
as  ne crystallites.14 The non-collinear structure attributed
to a surface effect is more pronounced for a smaller
crystallite size of nickel and zinc ferrite (<10 nm). Detailed
magnetic studies are in progress.
Conclusions
Nanocrystalline nickel and zinc ferrites synthesised by a
microemulsion method exhibited a crystallite size of about
5 – 10 nm and an approximate stoichiometry of Mex -
Fe32 xO4 (Me~Ni or Zn) (x:1.3, 1.2¡0.2). They lacked
saturation magnetisation at 9 kOe. A clear lack of coercive
force and lack of remanence was also observed.
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Materials Science and Technology November 2003 Vol. 19
Misra et al. Nanocrystalline Ni and Zn ferrites processed using microoemulsion technique 1621



Some aspects of nanocrystalline nickel and zinc ferrites processed using microemulsion technique

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