Abstract
A Fe3O4 water-based ferrofluid was prepared by chemical co-precipitation technique, the magnetic particles were characterized using x-ray diffraction(XRD),transmission electron microscopy(TEM) and vibrating sample magnetometer)VSM)techniques.the results show that the shape of the nanoparticles is approximately spherical and they are superparamagnetic at room temperature.
1. Introduction
Ferrofluids are stable colloidal suspensions that consist of magnetic nanoparticles dispersed in base liquid such as water or oil [1].
A ferrofluid is a colloidal dispersion of monodomain magnetic particles (size of about 10 nm) of superparamagnetic nature. Superparamagnetic iron oxide nanoparticles are small synthetic Fe 3 O 4 or γ-Fe 2 O 3 particles with a core size of o10 nm and an organic or inorganic coating. The particles are well dispersed in a liquid, for medical application normally in water. Superparamagnetic magnetization is, compared to normal paramagnetic materials, much higher and can reach nearly the magnetization saturation (M s ) of ferromagnetic iron oxide. This behavior allows the tracking of such particles in a magnetic field gradient without loosing the advantage of a stable colloidal suspension.[S2]

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Ferrofluids are stable suspensions of magnetic nanoparticles and include Fe 3 O 4 , CoFe 2 O 4 , Mn–Zn , Co–Zn and lithium ferrite in a base liquid. The liquid can be polar or nonpolar . To avoid nanoparticles agglomeration, the magnetic particles have to be coated with an appropriate surfactant .This coating makes the ferrofluid stable even under intense magneticfields . ferrofluids are widely used in industry and clinical applications .[w2]
Magnetic fluids (ferrofluids) with a combined fluidic and magnetic properties have wide applications inindustry and biomedicine. Magnetic Nanoparticles ,such as magnetite(Fe 3 O 4 ),iron(Fe),nickel(Ni),andcobalt(Co),can be applied in magnetic fluid preparing.Among these magnetic NPs Fe 3 O 4 has been extensively adopted in magnetic fluids for its tunable magnetic properties.so far many synthetic strategies are proposed to prepare magnetic fluids of Fe 3 O 4 nanoparticles.Hereinto ,co-precipitation method in oil or water with the presence surfactants is traditional route to prepare magnetic fluids.[A1]
Magnetic nanofluids, also called ferrofluids, are stable colloidal solutions consisting of magnetic nanoparticles dispersed in a based fluid. The magnetic nanofluid behaves as a smart or functional fluid due to some of its unique features. They have some applications in a variety of fields such as electronic packing, mechanical engineering, aerospace, and bioengineering. [i1]
Water-based magnetic nanofluids are a special category of polar magnetic nanofluids with particular features of particle interactions and agglomerate formation processes. The interest in water-based magnetic nanofluids in the selected bioengineering and biomedical systems has been growing exponentially in the last decades .Surface coating of nanoparticles and colloidal stability of biocompatible water-based magnetic nanofluids are particularly important for biomedical applications such as magnetic cell separation, drug delivery, hyperthermia, and contrast enhancement in magnetic resonance imaging. They have been extensively applied to audio voice coil-damping, intertia-damping apparatuses,bearings, stepping motors, and vacuum seals.[i1]
Ferrofluids, which are magnetic colloids, can be synthesized by dispersing nanosized subdomain magnetic particles of ∼10 nm in diameter in a carrier liquid. Such fluids have physical properties that can be modified by an external magnetic field and are widely used as functional materials in engineering and technology applications.[j1]
In general, magnetic nanoparticles in ferrofluids are coated with a surfactant to prevent aggregation.In the 1980s, Massart proposed a method for chemical synthesis of aqueous ferrofluids with no surfactant .In this method, an appropriate surface treatment leads to adsorption of H+ or OH−ions on the surface of the particles, so stable aqueous ferrofluids can be obtained. These ferrofluids are known as ionic ferrofluids or electrical double-layered magnetic fluids .Ionic ferrofluids have attracted considerable attention because of their special behavior and this has led to new cross-disciplinary activities inchemistry, biomedicine and physics.[j1]
Ferrofluids of Fe3 O4 and γ-Fe2 O3 have attracted considerable attention for various biomedical applications including therapeutic magnetic hyperthermia,targeted drug delivery,and magnetic resonance imaging due to their bio-compatibility and desirable superparamagnetic properties with high saturation magnetization.[M1]
Transition metal oxide nanoparticles (NPs) such as magnetite (Fe 3 O 4) and maghemite (γ -Fe 2 O 3 ) are of general interest, due to their interesting magnetic, electrical, mechanical, catalytical and optical properties.[Q1]
Iron oxide nanoparticles can also be dispersed in specific carrier liquids, giving rise to the so called ferrofluids (FFs0. In particular, FFs are colloidal suspensions of magnetic NPs (usually magnetite or maghemite) with average size in the range from 8 to 20 nm and dispersed in organic/inorganic solvents or different oils(hydrocarbons, syntetics esters, etc). They combine the fluid related properties of the dispersing liquid with the magnetic properties of the solid NPs.[Q1]
FFs have proved to be useful for a wide variety of engineering applications such as: (i) ironless loudspeakers (ii) specific components for magnetic recording ,(iii) biomagnetics (e.g. hyperthermia ,tissue repairing ,target drug delivery ,cell separation ,magnetic resonance imaging, sensing )(iv) rotating seals and so on. Most of the applications of FFs are related to both their overall thermo/hydrodynamic properties and the magnetic properties of the constituent NPs.[Q1]
Fe3O4 nanoparticles are most frequently chosen because of following reasons:(i) Fe3O4 is biocompatible,(ii) Fe3O4 nanoparticles can be synthesized at large scale,(iii)the magnetization of Fe3O4 nanoparticles is significantly high,thus allowing these particles to be easily controlled by an external magnetic field.[R1]
Colloidal suspensions of magnetic nanoparticles dispersed in an organic or inorganic carrier liquid,so-called ferrofluids ,are being increasingly studied because of their peculiar physical properties and applications .Because of their small size (5–20 nm) the particles exhibit single magnetic domains and the magnetic fluid displays superparamagnetic behavior .Without external magnetic field the orientation of the magnetic moments of the particles is at random resulting in a vanishing macroscopic magnetization. An external field, however, easily orients the particle magnetic moment leading to large saturation values of the magnetization. Ferrofluids are classified into two groups ,ionic (IFF) and surfacted (SFF) ferrofluids, depending on the method used to avoid the aggregation of magnetic particles.[T1]
Ferrofluids are actually superparamagnetic, meaning that a ferrofluid reacts to a magnetic field in the same way as a ferromagnetic or ferrimagnetic solid, but magnetizes and demagnetizes more rapidly because in a ferrofluid the magnetic domains are the same size as the actual particles.[v1]
Ferrofluids are colloidal dispersions of small single domain magnetic particles suspended in a carrier fluid.Ferrofluids characteristically have both magnetic and fluid properties and have found a diverse range of applications,such as in audio devices, inertia dampers, stepper motors,sensors, vacuum seals, electromagnetic shielding, and high density digital storage.[W1]
Namely, besides the customary superparamagnetism a strong reduction of the magnetization of nanoparticles with respect to bulk value is observed. By both implicit and explicit tests, this reduction was proven to be due to a noncollinear spin structure.[s1]
Ferrofluids containing magnetic nanoparticles with single domains and superparamagnetic at room Temperature and well-dispersed in a carrier fluid are both of fundamental and applied interest in biomedicine,mechanical and sensor technologies,and room temperature magnetic refrigerators.[y1]
(In this research),(in the present work),(in this study) the magnetic Fe3O4 nanoparticles were synthesized by co-precipation method.the XRD,TEM and TEM techniques were used to characterize the structure and the size of the nanoparticles.the magnetic properties were evaluated by vibrating sample magnetometer.the Fe3O4 nanoparticles were dispersed into water to obtain the desired nanofluids.tetramethyl ammonium hydroxide was used as dispersant.
2. Experimental Details:
or
2. Synthesis and characterization
2.1 Materials
The starting materials used in this work were ferric chloride hexahydrate(Fe Cl3.6 H2O),ferrous chloride tetrahydrate(Fe Cl2.4H2O) aqueous ammonia and tetramethyl ammonium hydroxide(N(CH3)4OH).
2.2 Synthesis procedure
An acqeous ferrofluid material was prepared by a co-precipitation method . The synthesis is based on the reaction of iron ions in an aqueous ammonia solution to form magnetite Fe3O4 in the following form:
2Fe3++Fe2++8NH3.H2O=Fe3O4+8NH4+ +4H2O
To synthesis Fe3O4 nanoparticles, Fe Cl3.6 H2O (1M) and Fe Cl2.4H2O (2M) were prepared by dissolving iron salts in HCl (2M) solution.typically , 4ml of Fe Cl3 and 1ml of Fe Cl2 were mixed in a molar ratio of 2:1.then 50ml of ammonia aqueous solution was added into the solution with vigorous stirring at room temperature.a black precipitate was obtained.the precipitate was separated by applying a magnet and washing with distilled water several times until the PH decrease to7. tetramethyl ammonium hydroxide was used as dispersant.the nanoparticles are coated with hydroxyl ions of the tetramethyl ammonium hydroxide ,which themselves attract a sheath of strong positive ions.this surface structure creates electrostatic interparticle repulsion that can overcome coagulation forces of magnetic and van der waals attractions.
2.3 Characterization
X-ray powder diffraction(XRD)patterns were obtained by an X-ray diffractometer using Kα radiation(λ=1.54060Å) ( ´®µª ´ª± „² ³ªŸ)
The shape,size distribution and morphology of the particles were examined by using transmission electron microscope(TEM).( )(The nanoparticles are characterized in detail by TEM). TEM experiment was performed on a Philips CM30 electron microscope with an acceleration voltage of 150 kV.
Magnetization versus applied field were carried out withVSM (Lake shore 7404).
3. Result and Discussion
Fig. 1 shows the X-rays powder diffraction pattern of iron oxide nanoparicles in the ferrofluid sample.the spectral lines are broad because the grain are small.()relatively wide peaks are related to small size of nanoparticles.
all the observed peaks can be indexed to the Fe3O4 crystal structure .the peaks are characteristic of cubic system with Fd-3m space group.()the considered profile can be suitably fitted by considering the crystalline structure of either magnetite(cubic structure,Fd-3m group,ICSD reference code 98-011-1284)or maghemite(cubic structure P4132 group,ICSD reference code 00-39-1346).beacuse of overlapping reflections in the XRD pattern,it was not possible to obtain a good fit by taking simultaneously into account both phases.
using the Debye-scherer equation,the crystallite size was determined to be around 10 nm.

Fig .1. XRD spectra of the particle Fig .2. TEM image of Fe3O4 nanoparticles
TEM image(Fig. 2)show that the particles are roughly spherical and polydispersed))(approximately spherical in shape).The particle sizes, by considering about 115 particle, range from 8to30nm.
the average size of the particles observed in the TEM image is in the range of 13nm, which is good agreement with that estimated by Debye-Scherrer formula from the XRD pattern.( )This is larger than the crystallite size determined by XRD,perhaps due to the presence of an amorphous surface layer. (the difference between XRD and TEM may come from the surface coating layer).

fig .3. Magentic hysteresis of Fe3O4
Fig. 3 displays the magnetic hysteresis curve of the ferrofluid evaluated by VSM at room temperature,which is obtained from the magnetization cycle.we can see that the saturation magnetization(Ms)of the Fe3O4 nanoparticles was about 35 emu/g which is lower than that of the bulk Fe3O4 (92 emu/g).the Ms value decrease with decrease in crystallite size for mono-domain particles due to the surface spin canting (disorder) and thermal fluctuations.[i1]. ( Komada et al. have attributed the reduction of magnetization in magnetic oxide nanoparticles to the existence of canted spins and /or a spin-glass-like behavior of the surface spins.[M1]).
The saturation magnetion might decrease on Fe3O4 →γ Fe2O3 transformation because the magnetization of γ Fe2O3 is slightly less than that of Fe3O4 in bulk materials.[j1]
It is well known that the Fe3O4 nanoparticles show a superparamagnetic property. When a magnetic field was applied, the dipolar particles aligned themselves with the applied magnetic field, and resulted in a measurable magnetization. The saturation magnetization of theferrofluid was about 35 emu/g in the ferrofluid, which was lower than the saturation magnetization of pure Fe3O4 nanoparticles (50 emu/g), due to the fact that the saturation magnetization reduced significantly when the particle size was smaller than 10 nm. Also, the saturation magnetization of Fe2O3 nanoparticles was lower than that of Fe3O4 nanoparticles .So the saturation magnetization of nanoparticles in the ferrofluid, which consisted of a small amount of Fe2O3 nanoparticles, was lower than that of pure Fe3O4 nanoparticles.[w1]
Furthermore, zero remenance and nearly zero coercivity can be observed for the nanoparticles on the hysteresis curve, which indicates that the particles are superparamagnetism.this is related to the fine crystallite size of Fe3O4 particles,which are in the nanometer range.
4. Conclusion
In this study, superparamagnetic magnetite ferrofluid were successfully synthesized. The XRD,TEM and VSM techniques were used to characterized the structure size and magnetic properties of nanoparticles.
It was concluded that the spherical nanoparticles synthesized in this work were superparamagnetic