Synthesis and room temperature magnetic and magneto-optical characterization

IntroductionFerrite particles have been used extensively with grain sizes of the order of a few nanometers.In fact,at this scale,the size reduction induces the normal macro-scopic multidomain structure to convert into a single domain at a critical size,which typically lies below 100nm.The magnetism of ferrite nanoparticles exhibits properties such as superparamagnetism,enhanced anisotropy and surface effects,which are of great interest for industrial applications,particularly in the field of information storage[1].Nanoparticles also present some attractive possibilities in biomedicine since they have dimensions that are smaller than or comparable to those of the biological entities.Indeed, they can be coated with biological molecules and dispersed in a liquid medium,forming colloidal solu-tions called magneticfluids or ferrofluids.Owing to their liquid properties and sensitivity to an applied magneticfield,such materials can be,for example, made to deliver a package,such as an anticancer drug, or radionuclide atoms,to a targeted region of the body, such as a tumor[2].In this work,we propose the synthesis and investigation of the magnetic and mag-neto-optical properties of electrical double-layered magneticfluids(EDL-MF)based on size-controlled NiFe2O4nanoparticles,good candidates and precursors for future biomedical applications[3].Progr Colloid Polym Sci(2004)128:109–112 DOI10.1007/b97100ÓSpringer-Verlag2004Marcelo Henrique Sousa Francisco Augusto Tourinho Je ro me Depeyrot Emmanuelle DuboisRe gine Perzynski Synthesis and room temperature magnetic and magneto-optical characterizationof size-controlled nickel nanoferrite based ferrofluidsM.H.Sousa(&)ÆF.A.Tourinho Complex Fluids Group,Instituto deQuımica,Universidade de Brasılia,Caixa Postal04478,CEP70919-970,Brasılia,Brazile-mail:fralda@unb.brTel.:+55-61-3072163Fax:+55-61-2734149J.DepeyrotInstituto de Fısica,Universidade de Brasılia,Caixa Postal 04478,CEP70919-970,Brasılia,Brazil E.DuboisLaboratoire Liquides Ioniques et Interfaces Charge es,Universite Pierre et Marie Curie (Paris6),Case63,4Place Jussieu,75252Paris Cedex05,FranceR.PerzynskiLaboratoire des Milieux De sordonne s et He te roge nes,Universite Pierre et Marie Curie,Tour13,Case78,4place Jussieu, 75252Paris Cedex05,France Abstract Nickel ferrite nanoparti-cles are synthesized bottom up andthen peptized in aqueous mediaresulting in stable magneticfluids.X-ray powder diffraction and trans-mission electronic microscopy per-mit the investigation of thecrystalline structure and the mor-phological aspects of the nanoparti-cles.Magnetic and magneto-opticalproperties of the resulting materialsare investigated by static measure-ments performed on individual par-ticle solutions.Keywords Nickel ferriteÆFerrofluidÆMagnetizationÆBirefringenceExperimentalSample preparation and characterizationEDL-MF samples were synthesized by following the procedure described in Ref.[4].Ferrite nanoparticles were prepared by alkalinizing 1:2mixtures of Ni 2þand Fe 3þat 100°C,under vigorous stirring.Changing the velocity of mixing the reagents led to nanoparticles with different mean sizes.Then particles were peptized electrostatically in aqueous media,using an appropriate particle surface treatment.Then,in order to reduce the polydi-spersion,the samples synthesized were centrifuged at 4,000rpm for 15min.After centrifugation,the resulting precipitate and super-natant were redispersed,forming stable sols of high quality.Morphological and structural characterizationMicrographs and electronic diffraction patterns were obtained by transmission electronic microscopy (TEM).In this case,the samples were prepared by evaporating very dilute ferrofluid solutions onto carbon-coated grids.The particle size distribution was estimated by measuring the size of about 500particles,using a lognormal law of size distribution which corresponds to the probability density that a particle has a diameter d ,with a standard deviation s .The crystalline structure of the solids,their crystalline size,and the lattice constants were determined by X-ray diffraction (XRD).The average lattice constants were calculated from the five most intense diffraction lines,while the average crystal size (d XR )was deduced by means of the Scherrer formula from the width at half maximum of the diffraction line (311).Magnetic and magneto-optical characterizationTo characterize the ferrofluid samples the magnetization and magneto-optical birefringence were investigated as a function of the applied magnetic field at room temperature.Ferrofluid magnetiza-tions were measured with a Squid device at 300K.The setup and the method to measure the ferrofluid birefringence are described elsewhere [5].Results and discussionSample preparation and synthesis characterization The particles sizes as a function of the addition rate (d V /d t )of the metallic ions into the alkaline solution duringthe precipitation are shown in Table ually,in colloid synthesis the nucleation starts when the concen-tration of the species reaches a critical saturation and each nucleus may further grow by a diffusive mechanism [6].In this way,slow rates of addition yield large particles,while increasing d V /d t generates smaller par-ticles.A series of EDL-MF samples obtained after centrifugation and redispersion are listed in Table 1and are labeled A1–C2.The XRD powder patterns presented in Fig.1show only one system of lines,typical of nickel ferrite,with a characteristic d spacing of 0.833±0.05nm,in good agreement with the value of 0.834nm found in the literature [7].The same structure was also identified from TEM diffraction measurements.Moreover,the values of the size distribution obtained from TEM analysis (Table 1)compare well with the crystalline mean sizes since the diameter measured by XRD averages the size distribution as d XR ¼d 0exp ð2:5s 2Þ[8].Here,d 0is the characteristic diameter,with ln d 0being the mean value of ln d .Table 1Size sample characteristics:d XR is the crystalline size from X-ray diffraction (XRD )experiments;d 0and s are the parameters of the size distribution from transmission electron microscopy (TEM )measurements;d V /d t is the rate of addition of the reagents during the particle precipitation;snt and ppt are,respectively,the supernatant and precipitate fractions after centrifugation d V /d t (mL/s)XRD TEM d XR (nm)d 0(nm)s 125A 4.2CentrifugationA1 3.8snt ––A2 4.3ppt ––10B 7.1B1 5.9snt 5.40.18B27.3ppt ––3C 8.2C17.7snt 6.70.23C28.9ppt8.30.17110Magnetic and magneto-optical characterizationLet us consider an assembly of independent single-domain grains with a magnetic moment l¼p m s d3=6, where m s is the magnetization of the nanoparticle.In the presence of an externalfield,H,the colloidal solution behaves as an ideal superparamagnet with a magnetiza-tion given by the Langevin law:M¼m s/LðnÞ, L(n)=coth n–nÀ1,n¼l0l H=k B T,with/being the ferrite volume fraction,k B the Boltzmann constant and T the temperature.This equation shows that at zerofield, M=0and as thefield is turned on,the magnetic moments tend to align in thefield direction so that at highfields,M saturates at m s/.Thefield-induced birefringence behavior is based on the existence of an optical anisotropy axis along the magnetic anisotropy one[5].Then,in the presence of a field,one has a birefringence D n¼/d n0½1À3LðnÞ=n , with the optical anisotropy d n0¼c/fðrÞ,where C depends on the particle refractive index and shape eccentricity.The function f(r)increases from0to1as its argument r¼E a=k B T goes from zero to infinity,E a being the anisotropy energy.Nevertheless,since magneticfluids are always poly-disperse,the lognormal law of the size distribution must be taken into account in both measurements by consid-ering a volume-weighted superposition of the contribu-tions of all different particle volumes.Then,if m S (magnetization measurements)or d n0(birefringence measurements)can be determined by the high-field extrapolation,the normalized magnetization or birefrin-gence is reduced to a function of the size distribution parameters,d0and s.Typical normalized magnetization curves are shown in Fig.2a.The results obtained byfitting the experimen-tal data to the Langevin model are summarized inTable2.They are in qualitative agreement with the model,except in the case of the samples based on the smallest particles,where the magnetization does not saturate at the maximum value of thefield,H max.For these samples,the argument(n max)of the Langevin function at the maximum value of thefield is not sufficiently large to allow the saturation at H max.Then,in order to compensate the nonsaturation,a correction is made by replacing in the magnetization equation m S by m s=Lðn maxÞ,where n max is calculated by using the mean value of the particle magnetic moment at H max.Thus,the high-field extrapolation gives the m s values,which are all much lower than270kA/m,the bulk magnetization,and vary from161kA/m for smaller particles to183kA/m for bigger particles.In fact,under size reduction, NiFe2O4superparamagnetic particles show a strong reduction of their magnetization when compared with that of the bulk ferrite,probably owing toaTable2Birefringence and magnetization parameters:m s is the saturation magnetization and d n0the individual optical anisotropy of the particle,both determined from the high-field behaviorSample Magnetization Birefringencem s(kA/m)d0mag(nm)s mag d n0d0bir(nm)s birA2161 2.90.40.0137.70.4B1180 3.60.40.0189.90.4C2183 6.20.40.03814.00.4111noncollinearity of the surface spins[9].Moreover,thecharacteristic diameters d mag0and the polydispersion s magobtained from thefittings compare well with the crystalline mean sizes found by XRD experiments using the relation d XR¼d0expð2:5s2Þ.A typical normalized birefringence curve is presented in Fig.2b.As for the magnetization curves,the observed field-induced birefringence follows well the Langevin model:in particular,the curves readily saturate at high fields,a result which provides the value of the individual particle optical anisotropy d n0,also listed in Table2.The d n0measurements are close to previous determinations for NiFe2O4[3]:d n0=0.1for nanoparticles with d XR=4.4nm.This optical anisotropy shows a depen-dence on the grain size,increasing as the diameter of the particle increases,a result in good agreement with previous birefringence measurements made on c-Fe2O3 [5].This dependence of d n0on d shows that the nanoparticles are soft dipoles with an energy of anisot-ropy close to kB T.By adjusting the birefringence curves of our samples,as described in the Experimental section, we can easily determine with standard methods[5]the characteristic diameters d birand the polydispersity s bir ofthe particles(Table2).The values of d birare larger thanthe values of d magbecause the small particles are superparamagnetic and do not contribute to the mea-sured signal;therefore,only the big particles are detected.ConclusionSize-controlled nickel ferrite nanoparticles have been synthesized bottom up by chemical precipitation and dispersed in stable colloidal solutions.The structure of the particles,their crystalline size,and the lattice constants were determined by XRD experiments.The magnetization measurements show that m s determined for the nanoparticles is smaller than the bulk value and the birefringence measurements show that the nanopar-ticles behave as soft dipoles.These results will be compared in the future with those of measurements of the energy of anisotropy from magnetization measure-ments as a function of temperature.References1.Dormann JL,Fiorani D(1992)Mag-netic properties offine particles.North Holland,Amsterdam2.Pankhurst QA,Connolly J,Jones SK,Dobson J(2003)J Phys D Appl Phys36:R1673.Hasmonay E,Depeyrot J,Sousa MH,Tourinho FA,Bacri JC,Perzynski R(1999)J Magn Magn Mater201:1954.Sousa MH,Tourinho FA,Depeyrot J,da Silva GJ,Lara MCFL(2001)J PhysChem B105:11685.Hasmonay E,Dubois E,Bacri JC,Perzynski 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