Study of magnetic and structural properties of ferrofluids based on cobalt–zinc ferrite nanoparticles

Study of magnetic and structural properties of ferrofluids based on cobalt–zinc ferrite nanoparticles

Journal of Magnetism and Magnetic Materials 324 (2012) 394–402 Contents lists available at SciVerse ScienceDirect Journal of Magnetism and Magnetic ...

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Journal of Magnetism and Magnetic Materials 324 (2012) 394–402

Contents lists available at SciVerse ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Study of magnetic and structural properties of ferrofluids based on cobalt–zinc ferrite nanoparticles J. Lo´pez a,n, L.F. Gonza´lez-Bahamo´n b, J. Prado a, J.C. Caicedo a, G. Zambrano a, M.E. Go´mez a, J. Esteve c, P. Prieto d a

Thin Film Group, Universidad del Valle, A.A. 25360, Cali, Colombia Analytical Chemistry Laboratory, Universidad del Valle, A.A. 25360, Cali, Colombia c ´ ptica, Universitat de Barcelona, Catalunya, Spain Department de Fı´sica Aplicada i O d Center of Excellence for Novel Materials, Universidad del Valle, Cali, Colombia b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 30 May 2011 Received in revised form 15 July 2011 Available online 19 August 2011

Ferrofluids are colloidal systems composed of a single domain of magnetic nanoparticles with a mean diameter around 30 nm, dispersed in a liquid carrier. Magnetic Co(1  x)ZnxFe2O4 (x ¼0.25, 0.50, 0.75) ferrite nanoparticles were prepared via co-precipitation method from aqueous salt solutions in an alkaline medium. The composition and structure of the samples were characterized through Energy Dispersive X-ray Spectroscopy and X-ray diffraction, respectively. Transmission Electron Microscopy (TEM) studies permitted determining nanoparticle size; grain size of nanoparticle conglomerates was established via Atomic Force Microscopy. The magnetic behavior of ferrofluids was characterized by Vibrating Sample Magnetometer (VSM); and finally, a magnetic force microscope was used to visualize the magnetic domains of Co(1  x)ZnxFe2O4 nanoparticles. X-ray diffraction patterns of Co(1  x)ZnxFe2O4 show the presence of the most intense peak corresponding to the (311) crystallographic orientation of the spinel phase of CoFe2O4. Fourier Transform Infrared Spectroscopy confirmed the presence of the bonds associated to the spinel structures; particularly for ferrites. The mean size of the crystallite of nanoparticles determined from the full-width at half maximum of the strongest reflection of the (311) peak by using the Scherrer approximation diminished from (9.5 7 0.3) nm to (5.4 7 0.2) nm when the Zn concentration increases from 0.21 to 0.75. The size of the Co–Zn ferrite nanoparticles obtained by TEM is in good agreement with the crystallite size calculated from X-ray diffraction patterns, using Scherer’s formula. The magnetic properties investigated with the aid of a VSM at room temperature presented super-paramagnetic behavior, determined by the shape of the hysteresis loop. In this study, we established that the coercive field of Co(1  x)ZnxFe2O4 magnetic nanoparticles, the crystal and nanoparticle sizes determined by X-ray Diffraction and TEM, respectively, decrease with the increase of the Zn at%. Finally, our magnetic nanoparticles are not very hard magnetic materials given that the hysteresis loop is small and for this reason Co(1  x)ZnxFe2O4 nanoparticles are considered as soft magnetic material. Crown Copyright & 2011 Published by Elsevier B.V. All rights reserved.

Keywords: Chemical co-precipitation Ferrofluid Nanoparticle Single domain Spinel structure Super-paramagnetism

1. Introduction Magnetic nanoparticles offer attractive possibilities in biomedicine. Recently, ferrofluids (FFs) or magnetic fluids have been the subject of interest because of their unusual optical, electronic, and magnetic properties [1–3], which can be changed by applying an external magnetic field. But the use of nanometer-size magnetic particles has expanded their applications in fields such as biomedicine, which has been proposed, for example, as an alternative therapy and localized for treatment of malignant tumors, where

n

Corresponding author. E-mail address: [email protected] (J. Lo´pez).

the nanoparticles are injected directly into them. When this fluid is placed in an alternating magnetic field, the nanoparticles generate heat and destroy the tumor. It should be taken into consideration that, for medical applications, these materials must be biocompatible. Nowadays, FFs are promising materials for cancer diagnosis and therapy. Cobalt–Zinc ferrite nanoparticles have attracted considerable attention because of their broad applications in several technological fields, including electronic devices and ferrofluids, prospective material for biomedical applications in diagnostics and cancer therapy, magnetic drug delivery, microwave devices, crystal photonics, and high-density information storage systems [4–8]. Ferrofluids are colloidal systems composed of single domain of magnetic nanoparticles with a mean diameter of around 10 nm, dispersed in a liquid carrier.

0304-8853/$ - see front matter Crown Copyright & 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.07.040

´pez et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 394–402 J. Lo

The special feature of FF is the combination of normal liquid behavior with super-paramagnetic properties, due to the small particle size. There are various methods to prepare magnetic nanoparticles at nanometer-size range. Synthesis of Co(1  x)ZnxFe2O4 nanoparticles has been of great interest, especially in the form of ferrofluids. In the present work, magnetic Co(1  x)ZnxFe2O4 ferrite nanoparticles were prepared by co-precipitation method from aqueous salt solutions in an alkaline medium. We structurally characterized the samples by X-ray diffraction (XRD) and the magnetic nanoparticles size was determinate by Transmission Electron Microscopy (TEM). On the other hand, Atomic Force Microscopy (AFM) investigation provides a qualitative information of the surface morphology and a quantitative information of the scanned magnetic nanoparticles conglomerates, via statistical calculation of their diameter and height, in order to establish the grain size of conglomerates of nanoparticles. The compositions of the samples were calculated by Energy Dispersive X-ray Spectroscopy (EDS). Fourier Transform Infrared Spectroscopy (FTIR) was used to confirm the presence of bonds associated to the spinel structures for this ferrite in particular. The magnetic behavior of Co(1  x)ZnxFe2O4 nanoparticles was studied by Vibrating Sample Magnetometer (VSM) method, which allows us to make the magnetic moment curves as a function of applied field H, as well as perform magnetization as temperature function curves in the FC and ZFC process to find the blocking temperature of the material. Finally, Magnetic Force Microscopy (MFM) was used to visualize the magnetic domains of Co(1  x)ZnxFe2O4 nanoparticles.

2. Experimental details To prepare Co(1  x)ZnxFe2O4 magnetic nanoparticles, chemical co-precipitation is probably the most-common and the most-used method for synthesis of ferrofluids or magnetic fluids based on magnetic nanoparticles. The preparation of surfacted and ionic aqueous ferrofluids based on spinel ferrite nanoparticles has been reported by Massart [9,10]. Through this method, it is usually necessary to start from a mixture of FeCl3  6H2O, CoCl2  6H2O, and ZnSO4  7H2O salts in an aqueous alkaline medium. Then, the solution is subjected to different procedures such as decantation, magnetic separation, centrifugation, and dilution. To avoid the agglomeration of magnetic nanoparticles, the solution is usually covered with a shell of an appropriate material (steric stabilization).

2.1. Synthesis of Co(1  x)ZnxFe2O4 nanoparticles The synthesis of Co(1  x)ZnxFe2O4 magnetic nanoparticles in ferrofluids has two steps: the preparation of nano-sized magnetic particles and the respective dispersion—stabilization of the nanoparticles in a carrier liquid (ethanol). The magnetic ferrofluids based on Co(1  x)ZnxFe2O4 nanoparticles are prepared by co-precipitation technique from aqueous salt solutions FeCl3,  6H2O, CoCl2,  6H2O, and ZnSO4  7H2O in the molar ratio 1:2 [Me2 þ /Fe3 þ ], in an alkaline medium. The magnetic nanoparticles obtained are stabilized by a surfactant. Oleic acid is usually used as a surfactant, which forms the waterproofing shell around the magnetic nanoparticles. The treatment of the nanoparticles by oleic acid is a very important stage of magnetic ferrofluid preparation, because it is added to prevent the agglomeration of nanoparticles. The size and physical properties of the nanoparticles depend on preparation parameters such as, reaction temperature, pH of the suspension, initial molar concentration, and others.

395

2.2. Materials To prepare the ferrofluid, FeCl3  6H2O (97%), CoCl2  6H2O (97%), ZnSO4  7H2O (99%), and sodium hydroxide (NaOH) from MerckTM were used. HPLC-grade Oleic acid (C18H34O2) was used as surfactant. All the materials were reagent grade and used without further purification. Distilled and de-ionized water was used as a solvent. 2.3. Procedure Magnetic nanoparticles of Co(1  x)ZnxFe2O4, with nominal value x¼0.25–0.75, were prepared by co-precipitation technique from mixed solutions of CoCl2, ZnSO4, and FeCl3 in their respective stoichiometry at 80 1C. After dissolving 75 ml of 0.4 M FeCl3  6H2O, 75 ml of 0.1 M CoCl2,  6H2O, and ZnSO4  7H2O solution for Co0.5Zn0.5Fe2O4 sample, 75 ml of 3 M NaOH was added as co-precipitating agent. The mixed solution was added to the solution of NaOH drop-wise under constant stirring and then the precipitation occurred immediately to change the solution to a dark color (brown), characteristic of this ferrite. The precipitated liquid was brought to a reaction temperature of 80 1C for 1 h and cooled to room temperature. The pH of the solution was constantly monitored when NaOH solution was added to produce the precipitation of Co(1  x)ZnxFe2O4 ferrofluids with magnetic nanoparticles. The reactants were magnetically stirred during the precipitation, and the pH level was reduced from approximately 12 to 10.5 by washing with de-ionized water. As a result, the mixture of NaOH added slowly to the solution of salts promotes the formation of the respective metal hydroxide that subsequently becomes ferrites under temperature conditions. The reaction for the precipitation of Fe3 þ , Zn2 þ , and Co2 þ nanoparticles to obtain the Co0.5Zn0.5Fe2O4 sample, is described as follows: 4Fe3 þ þZn2 þ þCo2 þ þ16HO -2Co0:5 Zn0:5 Fe2 O4 þ8H2 O

ð1Þ

However, Co(1  x)ZnxFe2O4 nanoparticles synthesized by the co-precipitation method can easily form particle conglomerates. To prevent this phenomenon, we added 5 ml of oleic acid to the solution as a surfactant and coating material and then the solution was stirred for 2 h at 80 1C. Oleic acid is added when the pH of the precipitate has a value between 10 and 11 because the solution is completely ionized, permitting to bind the carboxylate group to the ferrite surface, creating an electrostatic repulsion that prevents the aggregation of particles, and therefore stabilizing the ferrofluid. To get particles free from sodium and chlorine compounds, the precipitate was washed twice with deionized and distilled water as a solvent and with ethanol and acetone to remove the surfactant excess from the solution. To isolate the supernatant liquid (ethanol, acetone, and water) the beaker content was then centrifuged for 15 min at 4000 rpm. Thus, the supernatant liquid is decanted and the precipitate remains. A part of the volume of this precipitate was dried at 100 1C for 10 h and ground into a fine powder to perform the XRD an FTIR analyses. The other part of the colloid was used to carry out the magnetic and AFM measurements [11–16]. Table 1 shows the Co, Zn, and Fe mol concentration for the three different values of x (Zn substitution). In all instances, we kept constant stirring speed and adjusted the volume of the final solution in the reaction vessel to keep the M/OH  -ratio around 0.20. 2.4. Characterization of Co(1  x)ZnxFe2O4 nanoparticles Chemical composition of magnetic nanoparticles obtained via co-precipitation method was performed in a Philips XL30 ESEM

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Table 1 Nominal values of zinc substitution in the Co(1 x)ZnxFe2O4 magnetic nanoparticles. Sample

Co0.75Zn0.25Fe2O4 Co0.5Zn0.5Fe2O4 Co0.25Zn0.75Fe2O4

Table 2 Atomic percent and ratios of x ¼Zn/(Znþ Co) and Fe/(Zn þCo) in the Co(1  x)ZnxFe2O4 magnetic nanoparticles for the three different values of Zn concentration.

Concentration mol/l Co

Zn

Fe

0.15 0.10 0.05

0.05 0.10 0.15

0.40 0.40 0.40

with an EDAX microprobe for chemical analysis (12 KV), equipped with a window for the detection of light elements. The crystal structure of Co(1 x)ZnxFe2O4 magnetic nanoparticles was recorded in the 2y scale with a RIGAKU (DMAX-2100) ˚ 30 kV, and diffractometer using CoKa radiation (l ¼1.78899 A, 16 mA) radiation at room temperature in the range of 15 to 951, with a scanning speed of 0.021 s  1 and step time of 0.2 s, and at 51 incidence angle to avoid the fluorescence from the Fe and Co present in the samples. The peaks of the diffraction pattern were indexed and analyzed by using the International Center for Diffraction Data (ICDD-PDF2) database and the WINJADE 8.5 software for identification and compared such with the CoFe2O4 standard. Fourier Transform Infrared Spectroscopy (FTIR) spectra were recorded for the dried sample of ferrite with an FTIR— Shimadsu 8400 spectrophotometer in wave range of 3500– 400 cm  1 with a resolution of 4 cm  1. The dried sample was placed on a silicon substrate transparent to the infrared radiation and spectra were recorded according to the transmittance method. Magnetic characterization of the nanoparticles was carried out through magnetic hysteresis loops by using a vibrating sample magnetometer (VSM) in the physical property measurements system (PPMS) from Quantum DesignTM. Magnetizations vs. applied field measurements were performed at room temperature with an applied magnetic field varying within the range of 15,000 Oe–15,000 Oe. Thermal demagnetization curves without applied magnetic field (Zero Field Cooling, ZFC) and cooled with applied magnetic field (Field Cooling, FC) of H ¼100 Oe were carried out to study the blocking temperature and also to obtain the profile of distribution of particle sizes of the Co(1  x)ZnxFe2O4 samples. The AFM used to visualize the morphology of nanoparticles was an Asylum Research MFP-3D Atomic Force Microscope in AC non-contact mode in air operating at room temperature. For magnetic imaging, we used an Asylum Research ASYMFM Si cantilever with a 50 nm Co–Cr coating, a spring constant between 1–2 N/m, resonant frequency of 55–90 KHz, and tip radius less than 20 nm. Transmission Electron Microscope (TEM Philips CM30) operating at 300 kV was used to determine nanoparticle size and their morphologies and crystal structure. A drop of the magnetic nanoparticle fluid at room temperature was placed on a copper grid with a graphite mask to support the magnetic nanoparticles and to reduce their magnetic effect on the TEM measurements.

3. Results and discussion 3.1. Chemical composition The EDS spectra for Co(1  x)ZnxFe2O4 magnetic nanoparticles confirms, in all instances, the presence of Co, Zn, Fe, and O2 in the samples. In Table 2, we present the atomic percent and relative ratios of x¼Zn/(Znþ Co) and Fe/(ZnþCo) in the Co(1 x)ZnxFe2O4 magnetic nanoparticles, as functions of the inclusion of zinc in the ferrite samples. Table 2 reveals that the metal ratio x¼Zn/(ZnþCo)

Sample

Co at%

Zn at%

Fe at%

Zn=ðZn þ CoÞ

FeðZnþ CoÞ

Co0.79Zn0.21Fe2O4 Co0.56Zn0.44Fe2O4 Co0.25Zn0.75Fe2O4

11 9 3

3 7 9

30 33 26

0.21 0.44 0.75

2.14 2.06 2.16

increased monotonically from 0.21 to 0.75; whereas, the Fe/(ZnþCo) ratio was preserved practically close to 1:2 with the increase of Zn concentration in the Co(1 x)ZnxFe2O4 samples. Note that the real x-values are slightly different from the nominal ones. From now on, we will use the real values extracted from the EDS chemical composition analysis. Therefore, EDS elemental concentrations were obtained by using the ZAF correction method; because certain factors related to the sample composition, called matrix effects associated with atomic number (Z), absorption (A), and fluorescence (F) can affect the X-ray spectrum produced during the analysis of the electron microprobe and, therefore, these effects should be corrected to ensure the development of a careful analysis. The correction factors for a standard specimen of known composition were initially determined by the ZAF routine. The relative intensity of the K peak was determined by dead time correction and a referent correction for the X-ray measured. Thus, before each quantitative analysis of an EDS spectrum, a manual background correction and an automated ZAF correction were carried out. 3.2. Structural and particle characterization Fig. 1 shows the X-ray diffraction spectra for (a) Co(1 x)ZnxFe2O4, (b) Co(1 x)ZnxFe2O4, and (c) Co(1 x)ZnxFe2O4 magnetic nanoparticles. In all cases, the peaks indexed (220), (311), (400), (422), (511), and (440) are presented. The strongest reflection coming from the (311) plane can also be observed, which denotes the formation of the characteristic cubic spinel structure [17–21]. According to Fontijn et al., [22,23] the substitution of Co by Zn in the Co(1 x)ZnxFe2O4 system, causes the Zn ions to begin to take up the sites of Fe ions found in tetrahedral sites and Fe ions migrate to octahedral sites, displacing Co ions thereby, the Fe ions in octahedral sites increase. Hence, the system changes from an inverse spinel to a normal spinel structure. This process produces a slight shift of peaks in the X-ray diffraction patterns. In our case, this effect has been detected. Thus, we confirm that the crystalline structure of Co(1 x)ZnxFe2O4 magnetic nanoparticles changes to a normal spinel structure when the Zn atomic concentration increases. The lattice parameters of the Co(1  x)ZnxFe2O4 were calculated from the position of the strongest reflection, the (311) peak, by fitting the whole pattern of the experimental data, according to (WPF) X-ray diffraction profiles and Rietveld refinement of crystal structures with Materials Data Jade 8.5 (MDJ) software. Also, Scherrer’s formula was used to assess the crystallite size of the particles from the broadening FWHM of the (311) peak in the XRD patterns, using the profile fitting and peak decomposition method of the Materials Data Jade 8.5 (MDJ) software. The lattice parameter and the crystal size are presented in Table 3. Fig. 2 displays the lattice parameter and the crystal size extracted from the fitted XRD data as functions of the Zn atom concentration. An increase of the Co(1  x)ZnxFe2O4 lattice parameter was observed with the inclusion of Zinc; the lattice parameter of CoFe2O4 is (0.838 nm), as expected, because the atomic radius for Zn is larger than the Co atomic radius. Also, the crystal size decreases with the increase of the Zn concentration, as seen in Fig. 2.

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Fig. 2. Variation of the lattice parameter and crystal size as functions of Zn concentration.

Fig. 3. FTIR spectra for Co(1  x)ZnxFe2O4/C18H34O2/ethanol based magnetic ferrofluids.

Fig. 1. X-ray diffraction patterns of (a) Co0.79Zn0.21Fe2O4, (b) Co0.56Zn0.44Fe2O4, and (c) Co0.25Zn0.75Fe2O magnetic ferrofluids at different Zn concentrations.

Table 3 Lattice parameter and crystal size of Co(1  x)ZnxFe2O4 magnetic nanoparticles as functions of Zn concentration. Sample

Lattice parameter [nm]

Crystal size [nm]

Co0.79Zn0.21Fe2O4 Co0.56Zn0.44Fe2O4 Co0.25Zn0.75Fe2O4

0.839 7 0.003 0.843 7 0.001 0.844 7 0.002

9.5 7 0.3 7.07 0.2 5.4 7 0.2

bands Fe–O bonds in the crystalline lattice of Co(1  x)ZnxFe2O4 [24]. They are characteristically pronounced for all spinel structures and for ferrites in particular. This occurs because the stretching vibration bands related to metal in the octahedral and tetrahedral sites are in this region. FTIR spectra also show an absorption band between 1247 cm  1 and 1650 cm  1, corresponding to the stretching vibration of the carboxyl group (C¼O), associated to the oleic acid molecule. In summary, FTIR absorption spectroscopy allows identifying the spinel structure and confirms the XRD structural characterization, as well as the presence of certain types of chemical substances adsorbed on the surface of nanoparticles [25,26]. 3.4. Magnetic characterization

3.3. FTIR spectral measurements FTIR spectra of the samples exhibit two intense bands between 590 cm  1 and 621 cm  1 (Fig. 3) belonging to the stretching vibration modes associated to the metal–oxygen absorption

Fig. 4 shows the M vs. H loops for the Co(1 x)ZnxFe2O4 ferrite samples based on ethanol-like carrier liquid, at different Zn concentrations taken at room temperature. From the hysteresis loops, one can see that when magnetic nanoparticles are suspended in

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Fig. 5. Coercive field (Hc) as a function of crystal size and Zn concentration. Fig. 4. M vs. H hysteresis loop of Co(1  x)ZnxFe2O4 magnetic ferrofluid as a function of Zn concentrations at room temperature.

ethanol and coated with oleic acid as surfactant agent, the difference in the magnetic properties of particles can be attributed to a weak interaction favored by the combined action of interfacial (Van der Waals of acid–base) magnetic attractions between the particles immersed in the carrier liquid and the steric repulsion produced between particles, which avoids the formation of conglomerates. Therefore, Co(1 x)ZnxFe2O4 nanoparticles can be considered single domain, showing the tendency to super-paramagnetic behavior at room temperature [27,28], characteristic of a soft ferromagnetic material like a Co(1 x)ZnxFe2O4 ferrite. The behavior of ferrofluids is mainly determined by their magnetic properties. Usually, they show super-paramagnetic behavior considering each particle as a thermally agitated permanent magnet in the carrier liquid. In the presence of a magnetic field, H, the magnetic moment (m) of the particles will try to align with the magnetic field direction leading to a macroscopic magnetization of the liquid. The magnetization, M, of the liquid behaves as the magnetization of a paramagnetic system. Furthermore, these types of materials present a behavior of the hysteresis M vs. H loops, which indicates the irreversibility in the magnetization process related to the pinning of the magnetic domain walls at impurities or grain boundaries within the material, as well as to the intrinsic effects such as the magnetic anisotropy of the crystalline lattice. On the other hand, the shapes of these loops are determined—in part—by particle size. At even smaller sizes (in order of tens of nanometers or less), one can see superparamagnetic behavior [29] where the magnetic moment of the particle as a whole is free to fluctuate in response to thermal energy, while the individual atomic moments maintain their ordered state relative to each other. This leads to the low an hysteretic behavior shown in Fig. 4. Finally, the coercive field (Hc) behavior obtained as a function of crystal size and Zn concentration at room temperature is presented in Fig. 5. The results related to the particle size and magnetic behavior of our Co(1  x)ZnxFe2O4 magnetic nanoparticles, agree well with the results previously reported for similar Fe3O4 and Co–Ni magnetic nanoparticles [30,31]. With respect to the coercive field Hc values reported for other systems, in the case of particles with similar or smaller particle sizes, it has been found Hc values greater than those obtained in this work. Taking into account the values of crystallite size of our Co (1  x) ZnxFe2O4 magnetic nanoparticles, probably a higher crystallinity in our

Fig. 6. Zero-field-cooled (ZFC) and field cooled (FC) magnetization curves, ZFC–FC curve and the derivative with respect to temperature of the difference between the FC and ZFC vs. T.

samples causes a decrease of the surface effects on their magnetic properties. The study of nanoparticle size from measurements of the blocking temperature for FC and ZFC processes is an interesting research topic [32]. Magnetization as a function of temperature in the applied field of 100 Oe was performed between 4 K and 300 K. From the FC and ZFC curves in Fig. 6, we can observe its irreversibility, typical of the blocking process assembly of superparamagnetic nanoparticles [33]. Additionally, above the blocking temperature, the magnetization decreases as the temperature increases. From the behavior of the ZFC and FC curves, it is possible to conclude that these curves are almost overlapped above the blocking temperature, TB, indicating the presence of the small-sized particles [34]. The measurements without field (ZFC) allow obtaining parameters such as blocking temperature, anisotropy constant, critical volume, and critical diameter of the nanoparticles by adjusting the dependence of magnetization with the temperature in the ZFC process. The expression used for adjustment by assuming a spherical shape of the particles is given by [35]: Z m2 H Vc ðT,tÞ MðH,TÞ ¼ 0 f ðvÞv2 dv ð2Þ 2T 0 where m0 is the magnetization of the nanoparticle, Vc critical volume or blocking and (V) is the volume distribution function.

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After the adjustment, the blocking temperature, TB, can be found. From the theory of magnetism, we know that for nanostructured materials the critical volume of super-paramagnetic materials is directly proportional to temperature. In other words, if the temperature increases, the critical size of the particles will be greater, and the particles having a size less than or equal to the critical can be considered in the super-paramagnetic regime. Finally, from the critical volume, for a nanoparticle, given by the expression: V¼

25KB T Ka

0.0

0.4

0.6

0.8µm

0.8µm

0.8µm

0.6

0.6

ð3Þ

where T is the blocking temperature, KB is Boltznmann’s constant, and Ka is anisotropy constant it is possible to calculate the critical diameter for the nanoparticles. Fig. 6 presents the ZFC and FC curves and the difference between ZFC and FC of Co0.79Zn0.21Fe2O4 magnetic nanoparticles dispersed in the carrier liquid. Additionally, the irreversibility temperature, which is the temperature where the ZFC magnetization curve is separated from the FC curve and the derivative with respect to the temperature of the difference between the FC and ZFC vs. T curves are also presented in Fig. 6. This temperature corresponds to the maximum temperature, which gives information on the profile of the distribution of sizes and obeys the lognormal distribution function. In our case, from the distribution of sizes, the TB and VC were found as 45 K and (151717) nm3, respectively. By using the critical volume, we obtained a value of 6.6 nm for the critical diameter for the nanoparticles, a value that is within the expected results and is consistent with the crystal size of the Co0.79Zn0.21Fe2O4 magnetic nanoparticles obtained from XRD and TEM measurements [36].

0.2

399

0.4

0.4

0.2

0.2

0.0

0.0 0.2

0.0 0.0

0.2

0.4 0.4

0.6 0.6

0.8µm 0.8µm

0.8µm

0.8µm

0.6

0.6

0.4

0.4

3.5. SPM measurements AFM measurements were performed on dried samples deposited on mica substrate. They were repeated on different sites of the deposited sample and prepared under the same conditions of room temperature and ambient atmosphere. The analysis of magnetic particles consisted of observing the surface of conglomerates of Co(1  x)ZnxFe2O4 nanoparticles, Fig. 7. The mean diameter from the height, measured by AFM, is larger than the crystal size of the nanoparticles measured by XRD. This can be explained because the analysis is statistical and some ‘particles’ are actually conglomerates, which may result in particle conglomerates with relatively larger height. The existence of such agglomerates requires improving the preparation of the samples given that the attractive electric and magnetic forces are not totally balanced by the steric repulsion conferred by the oleic acid coating molecules. The agglomeration of Co(1  x)ZnxFe2O4 nanoparticles is evident in the topography image (Fig. 7a). Fig. 7b shows a 2D MFM image, where it is possible to see the magnetic domains of the Co(1  x)ZnxFe2O4 nanoparticle conglomerates. The samples and the tip were magnetized by an external field oriented vertically to the mica substrate plane prior to the measurement. The interpretation of magnetic domains shows that a dark zone represents a repulsive force of domains, while a bright zone is an attractive force. Fig. 8 displays the particle-size histogram and the best-fit curves using the lognormal distribution function with a mostprobable particle diameter of conglomerates [37–40]. The lognormal distribution function for the different concentrations of zinc show that the conglomerate size determined by AFM statistical analysis decreases with the increasing of at% Zn, (62.970.3) nm for x¼0.21, (59.0 70.2) for x¼ 0.56, and (16.670.3) for x ¼0.75; similar behavior to that obtained for crystal size from X-ray diffraction [41].

0.2

0.2

0.0

0.0 0.0

0.2

0.4

0.6

0.8µm

Fig. 7. (a) 2D AFM topography image of conglomerates of Co56Zn0.44Fe2O4 nanoparticles, (b) 2D MFM image of magnetic domains of Co56Zn0.44Fe2O4 nanoparticle conglomerates.

3.6. Transmission electron microscopy (TEM) TEM images of Co0.79Zn0.21Fe2O4 nanoparticles at two different scales are shown in Fig. 9. In the HRTEM micrograph on a 10 nm scale (Fig. 9b) it may be observed that the nanoparticles prepared by this method are single crystals. On the other hand, the inset in the figure shows the selected area diffraction pattern (SAED pattern) obtained from a single nanoparticle. The lattice fringes of the image corresponds to a group of atomic planes within particles, demonstrating that the nanoparticles are structurally uniform. In other words, these fringe patterns indicate a highly crystalline structure in the sample, supported by well-pronounced diffraction rings [42]. Fig. 10 shows the inverse transform selected area (IFFT-SAED) of the image in Fig. 10b of a Co0.79Zn0.21Fe2O4 nanoparticle. The inverse Fourier transform indicates that the planes are preferentially aligned along the same direction and, thus, we can observe the periodicity of the crystal structure for this spinel ferrite. Fig. 11 shows the size-distribution histogram of the Co0.79Zn0.21Fe2O4 nanoparticles from TEM images, by using a

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Fig. 9. HRTEM images of Co0.79Zn0.21Fe2O4 nanoparticles at two different scales: (a) 50 nm and (b) 10 nm. The inset shows the selected area diffraction pattern (SAED) of the Co0.79Zn0.21Fe2O4 sample.

Fig. 8. Histogram constructed using the AFM data and the best fit of the lognormal function (a) Co0.79Zn0.21Fe2O4, (b) Co0.56Zn0.44Fe2O4, and (c) Co0.25Zn0.75Fe2O nanoparticles.

Gaussian fit of the particle diameter (D), from different parts of the grid for an average number of particles close to 40. Symmetry in the Gaussian fit of about 9.0 nm was obtained. This size of the Co–Zn ferrite nanoparticles obtained by TEM is in good agreement

Fig. 10. Inverse Fourier transform (IFFT) of the selected area in the image of Fig. 10b. The inset is the FFT of the selected TEM image of the sample.

´pez et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 394–402 J. Lo

Fig. 11. Size-distribution histogram of Co0.79Zn0.21Fe2O4 nanoparticles from TEM images.

with the crystallite size calculated from X-ray diffraction patterns by using Scherer’s formula (Table 3) [43,44].

4. Conclusions We synthesized Co1  xZnxFe2O4 magnetic nanoparticles by using a co-precipitation chemical method. XRD and FTIR analyses allowed determining the presence of characteristic spinel structure in the Co1  xZnxFe2O4 ferrofluid nanoparticles. Also, the crystal and conglomerate sizes determined by XRD and AFM decrease with increasing Zn at% concentration. On the other hand, the size distribution and the critical diameter of Co1  xZnxFe2O4 nanoparticles determined by TEM and from FC and ZFC measurements, respectively, are consistent with the crystal size obtained from XRD measurements. This determined size may be ideally suited for various technological applications. In addition, Co1  xZnxFe2O4 magnetic nanoparticles present a tendency to super-paramagnetic behavior at room temperature, because the coercive fields (Hc) are small, and Hc, as well as the crystal size of Co1  xZnxFe2O4 magnetic nanoparticles decreases with increasing Zn at%. In summary, due to the hysteresis loop of our magnetic nanoparticles measured at room temperature is very small, Co1  xZnxFe2O4 ferrite nanoparticles can be considered as a soft magnetic material given their very low coercive magnetic field.

Acknowledgments ´nomo Fondo This work was supported by ‘‘El Patrimonio Auto Nacional de Financiamiento para la Ciencia, la Tecnologı´a y la ´n Francisco Jose´ de Caldas’’ Contract RC-no. 275-2011 Innovacio and Universidad del Valle research project code 7703. Moreover, the authors acknowledge the Serveis Cientı´fico-Te´cnics of the Universitat de Barcelona for TEM analysis and J. Lopez thanks Colciencias for the doctoral fellowship ’’Francisco Jose´ de Caldas’’.

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