Nanoferrites of nickel doped with cobalt: influence of Co2+ on the structural and magnetic properties

Nanoferrites of nickel doped with cobalt: In

fluence of Co

2

þ

on the

structural and magnetic properties

A.P.G. Rodrigues

a

, D.K.S. Gomes

b,e,n

, J.H. Araújo

c

, D.M.A. Melo

a

, N.A.S. Oliveira

a

,

R.M. Braga

d

a

Federal University of Rio Grande do Norte, Chemical Institute, Natal-RN 59078-970, Brazil

b

Federal University of Rio Grande do Norte, Graduate Program in Materials Science and Engineering, Laboratory of Catalysis and Materials, Natal-RN 59078-970, Brazil

c

Federal University of Rio Grande do Norte, Department of Theoretical and Experimental Physics, Laboratory of Magnetism and Magnetic Materials, Natal-RN 59078-970, Brazil

d

Federal University of Paraíba, DEER-CEAR, João Pessoa–PB 58051-970, Brazil

e

Coordination of Improvement of Higher Education Personnel, CAPES/PNPD, Brazil

a r t i c l e i n f o

Article history: Received 23 July 2014 Received in revised form 3 September 2014

Available online 27 September 2014 Keywords: Ferrite Spinel Nickel Cobalt Combustion Magnetic propertie

a b s t r a c t

Nanoferrites of nickel substituted with cobalt of composition Ni1xCoxFe2O4 (0rxr0.75), were synthesized by combustion reaction assisted in microwaves. The influence of the substitution of Ni2þ_by Co2þcontent and the concentration of Co2þin the structural and magnetic properties was investigated. The powders were prepared by combustion according to the concept of chemical propellants and heated in a microwave oven with a power of 7000 kW. The synthesized powders were characterized by absorption spectroscopy in the infrared region (FTIR), X-ray diffraction (XRD) together with Rietveld refinement, surface area (BET) method, scanning electron microscopy (MEV) and magnetic measure-ments (MAV). The results indicated that it was possible to obtain nickel ferrite doped with cobalt in all compositions and that an increase of cobalt concentration caused an increase in particle size (9.78– 21.63 nm), a reduction in surface area, and reduction in magnetic concentrations greater than 50%.

& 2014 Published by Elsevier B.V.

1. Introduction

Results of numerous studies have aided in the establishment of the laws governing magnetism, allowing for the construction of more efficient and useful magnetic materials for applications in various electronic devices. In this context, nickel ferrite is con-sidered one of the most promising and versatile materials. Due to its high electrical resistivity, low coercivity, moderate saturation magnetization, and low hysteresis losses, it is located in the class of soft ferrites[1,2]. This material has been industrially utilized in radio-requency circuits, high-qualityfilters, antennas and cores of transformers, and other devices such as converters, inductors, and suppressor signals[3,4].

Particularly, the ability of certain materials such as ferrites doped with cobalt to acquire high magnetic momentum is of great importance. The applications of these magnetic materials are varied. In ferrites, the origin of magnetism is due to the presence of irregular 3d electrons distributed in uneven numbers in the tetrahedral and octahedral sites [5–8]. Therefore, the magnetic properties of ferrites depend on the electron spins of the consti-tuent ions and their interactions.

The performance of the ferrite is directly related to the quality and the technical processing of this material, since the morphol-ogy and microstructure of ferrites are critical to its performance. Factors such as grain size, porosity, density, impurities, constituent phases, and the structure of the cell unit generate the information needed to understand the relationship between structure, micro-structure, and the inherent magnetic properties of the end product [9–13].

The need for high performance ferrites led to the need to develop more appropriate methods for obtaining different com-positional possibilities of controlled structure with advanced magnetic characteristics. Among the existing methods, synthe-sized combustion reaction has been successfully employed to Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/jmmm

Journal of Magnetism and Magnetic Materials

http://dx.doi.org/10.1016/j.jmmm.2014.09.045

0304-8853/& 2014 Published by Elsevier B.V.

n_{Corresponding author at: Federal University of Rio Grande do Norte, Chemical}

Institute. Laboratório de Tecnologia Ambiental (LabTam). Senador Salgado Filho Av., 3000. Campus Universitário, Lagoa Nova, 59.072-970. Natal - RN, Brazil.

E-mail addresses:dkarinne@yahoo.com.br(D.K.S. Gomes),

humberto@dfte.ufrn.br(J.H. Araújo),daraujomelo@gmail.com(D.M.A. Melo),

obtain various types of materials. It enables us to obtain powders with nanometric particles, high specific area, and a high purity. The method of combustion is self-sustaining after initiating the reaction and reaches high temperatures which ensure the forma-tion and crystallizaforma-tion of powders in a short period of time with the release of a large quantity of gas. This tends to minimize the state of agglomeration of the particles formed. The combustion method does not involve many steps and produces powders of high purity, chemical homogeneity, and almost always of the nanometer scale[14_–16].

Material processing based on heating by means of microwave energy has gained increasing prominence and importance in many industrial applications due to a number of advantages compared to conventional heating methods. There are several benefits of using microwave energy, such as reduction in processing times and energy savings. This has caused multiple processes to be based on microwave heating in industrial applications[17].

Thus, this paper aims to synthesize nickel ferrite replaced in various proportions with cobalt, employing the methods of synth-esis by combustion in a microwave, and to evaluate the influence on the structural and magnetic properties of the synthesized compositions.

2. Material and methods 2.1. Preparation of ferrite

The synthesized combustion reaction for the preparation of Ni1xCoxFe2O4(0rxr0.75) system involved a mixture containing

metal ions such as oxidizing reagents (nitrates) and a fuel (urea) as reducing agent, prepared in accordance with stoichiometric com-positions according to the established concepts of propellant chemistry[18]. For this redox mixture, we used Ni(NO3)2 6H2O

(nickel nitrate hexahydrate, 97%), Co(NO3)2 6H2O (Cobalt nitrate

hexahydrate, 97%), Fe(NO3)3 9H2O (Iron nitrate nonahydrate, 98%)

and CO(NH2)2(urea, PA). The reagents were mixed and subjected

to a preheat temperature of 100°C. Then the solution was introduced to a Panasonic STYLEPANAGRILL microwave oven, preprogrammed for the synthesis conditions at a power of 7000 KW for 10 min, until the occurrence of auto-ignition (com-bustion). For didactic purposes the NiFe2O4, Ni0.75Co0.25Fe2O4,

Ni0.5Co0.5Fe2O4 and Ni0.25Co0.75Fe2O4, compositions are named

NF, NFC25, NFC50, and NFC75, respectively. 2.2. Characterization of ferrites

The powders resulting from the combustion reaction were characterized by absorption spectroscopy in the infrared spectral range of 4000–500 cm1_{in an ABB Bomem model MB104}

spectro-meter. The samples were dispersed in KBr and pressed into 10 mm in diameter pellets. The X-ray patterns were obtained using a Shimadzu XRD-7000 diffractometer with monochromatic CuK

α

radiation. The data was collected over a range of 2

θ

angular variation between 10° and 80° with a scan rate of 2° min1_and

0.02° step. The crystalline phases were identified using the International Centre for Diffraction Data (ICDD) database. The Rietveld method was used to refine the X-ray diffraction data using MAUD software (version 2.044). Specific area measurements were calculated using the BET method using Quantachrome NOVA 2000 model low temperature equipment. Micrographs were taken using the equipment Philips XL-30-EMEV, with a potential differ-ence of 20 kV. The samples were placed directly into the sample port, dispersed with acetone, and coated with a thinfilm of gold to ensure adequate electrical conductivity. The magnetic measure-ments were performed on a vibrating sample magnetometer

(MAV). The hysteresis curves were obtained at ambient tempera-ture using 30 mg of the powder sample and applying fields of 15000–15000 Oe.

3. Results and discussion

Fig. 1 shows the absorption spectra in the infrared (FTIR) spectrum of the Ni1xCoxFe2O4 system in the range of

4000–500 cm1_{. The absorption band at 2346 cm}1_corresponds

to the deformation of CO2molecules arising from the

decomposi-tion atmosphere or absorbed from the ambient atmosphere during the handling of the material[19–21]. Bands of low intensity that were observed at 1657 cm1 and 1547 cm1 are assigned to the angular deformation in the plane N–H, from combustible waste (urea) used in the synthesis method[19].

The region in the infrared where absorptions related to metal vibrations (oxygen in ceramic powders), are generally observed in the range of 1000–400 cm1_{, and are usually characterized by}

vibration of ions in the crystal lattice. The primary vibrations of this kind of material usually occur in the range of 600–400 cm1_,

corresponding to stretching of the tetrahedral and octahedral sites of the crystal structure. For spinel ferrite structures in particular, the most intense stretching is generally observed in the range between 600 and 550 cm1and less intensely between 450 and 385 cm1, which corresponds to the intrinsic vibration of the metal in the tetrahedral (Mtetrahedral2O) and octahedral

(Moctahedral2O) sites. The vibration in the tetrahedral site is more

intense than the octahedral site due to the values assigned to the shorter connection length in the tetrahedral towards octahedral [19,21].

In the infrared spectra of this work, it was not possible to view the vibration of the metal in the octahedral site, since this stretch is commonly observed in the range of 450–385 cm1 _{and the}

equipment has a limitation range of 4000–500 cm1_{. The}

pre-sence of stretches of the intrinsic vibrations of the metal (oxygen in tetrahedral sites at 573 cm1), indicates the formation of the Ni–Co ferrite phase spinel type.

Ferrites were identified as ttoheir crystalline structure by standard X-ray diffraction. Fig. 2 shows the X-ray patterns obtained for the Ni1xCoxFe2O4 system powders for NF, NFC25,

NFC50, and NFC75 compositions. In observing the diffraction patterns of the powders in all the compositions evaluated, the presence of well-defined peaks (2

θ

¼35.5°) is apparent, a

Fig. 1. Absorption Spectra in the Infrared compositions NF, NCF25, NCF50, and NCF75 Region.

characteristic of cubic ferrites belonging to the spatial group Fd3m:1 [22]. The structure has characteristics of inversed and partly-inversed spinels, due to the positioning of the ions in the crystal lattice. On the other hand, peaks relating to the phases

α

-Fe2O3, FeO, (FeCo)O, and Ni0 have been identified. Hematite

(

α

-Fe2O3), shown only in the NF composition, is a phase which

tends to appear when the material is subjected to a temperature above 500°C and an atmosphere rich in oxygen. A means of controlling the onset of this stage is to control the heating atmosphere and temperature[23,24]. The FeO and (FeCo) stages indicate the volatilization of metals resulting in compositional shifts favoring the precipitation of secondary phases[25–27].

The Nickel ferrite is an inverse spinel, whose cell unit is represented by the formula (Fe1x) [NiFe1þ x]O4, in which the ions

of Fe3þ are evenly distributed in tetrahedral and octahedral positions of the network. That is, they can be forming (tetrahedral sites) or modifying (octahedral sites). However, the precipitation of the metallic nickel phase in the ferrite suggests that nickel ions which typically occupy octahedral positions (network modifier) compete with iron ions which also occupy these modified network positions. This behavior leads to a segregation of nickel ions

forming in the metallic nickel phase. Therefore, the precipitation of the Ni0_{phase evidenced in all prepared systems is justified.}

In the X-ray patterns, it can also be seen that increasing the level of Co2þ_{in the structure caused a narrowing of the diffraction}

peaks which characterizes a more accentuated degree of crystalinity.

By treatment of the X-ray data by Rietveld refinement techni-que, it was possible to extract detailed information about the parameters of the crystal structure obtained.

The Rietveld analysis confirmed the formation of the spinel type phase for all compositions, the values in mass%, correspond-ing to 72%, 94%, 90%, and 87% for NF, NFC25, NFC50, and NFC75, respectively.Table 1shows the lattice parameters and crystallite size. Note that increasing the concentration of cobalt changed the parameter of the network of 8,357–8,375 Å with increasing Co. The spinel phase, however, kept cubic symmetry for all compositions. It was also observed that the average crystallite size increased from 9.78 nm to 21.3 nm with increasing concentration of cobalt. By observingTable 1, it was also realized that the addition of Co leads to an increased formation of the spinel phase, with NFC25 composition being the most significant. Furthermore, the addition of Co prevents the formation of

α

-Fe2O3, and decreases the

Fig. 2. Patterns of X-ray diffraction of Ni1xCoxFe2O4system for compositions (a) NF, (b) NFC25, (c) NFC50, and (d) NFC75.

Table 1

Data analysis of the Rietveld refinement of the system Ni1xCoxFe2O4for NF, NFC25, NFC50, and NFC75 compositions.

Composition Ni1xCoxFe2O4 RW Sig Phases (mass %) Lattice Parameter (Å) Dc(nm)

spinel α-Fe2O3 FeO (FeCo)O Ni°

NF 18.36 1.19 72.02% 5.4% 9.58% 0% 12% 8.357 9.78 NFC25 17.79 1.19 94% 0% 3.5% 0% 2.5% 8.357 20.23 NFC50 19.09 1.06 90% 0% 0% 7.5% 2.5% 8.366 18.96 NFC75 19.20 1.09 87.25% 0% 0% 8.42% 4.33% 8.375 21.63

formation of FeO and Ni0_{. Thus, it is possible that Co stabilizes the}

Fe forming the network, and the Ni as a modifier of the network. The Rw (indicator of numerical convergence) and Sig (optimi-zation refinement) parameters are related to the accuracy of the refinement and the agreement between the observed and calcu-lated profile. According to the literature, the Sig includes the number of variables under refinement whose Sig values r1.3 are favorable[28,29]. Observing the values obtained from the Sig inTable 1for the XRD patterns of the prepared compositions, it appears that the values are consistent with those published in the literature.

The results of specific area (SBET), particle size (DBET), crystallite

size (DDRX), and the relationship between the particle diameter

and the crystallite size DBET/DDRXare shown inTable 2. From the

data of BET, it is observed that the increase of Co2þ_{concentration}

in Ni1xCoxFe2O4system caused a decrease in specific surface area

and hence increased particle size.

According to DBET/DDRX, it was found that all samples have a

value greater than 1, indicating that the particles are polycrystal-line and easily agglomerate due to high surface tension. It was generally noticed that the increase of cobalt content led to a decrease of the ratio values DBET/DDRX, indicating that the powders

tend to form smaller clusters.

Fig. 3shows the morphology of the powders of Ni1xCoxFe2O4

system obtained by scanning (MEV) electron microscopy. A mor-phology with non-uniform particle size and irregular formation of agglomerates with a large number of pores can be seen in the micrographs, which arise due to the release of gases during combustion. The compositions exhibit different degrees of ag-glomeration, with irregular porous (not dense) block formats with soft or fragile characteristics (consisting of weak bonds) of easy disagglomeration.

Also based on the micrographs shown in Fig. 3, note the formation of a microstructure with grains of irregular size, where-in small particles are distributed between 1

μ

m and 2

μ

m, which favored the formation of soft agglomerates. It is known that the

Table 2

Specific area and particle size of the Ni1xCoxFe2O4system, prepared by

combus-tion reaccombus-tion.

Sample SBET(m2g1) DBET(nm)a DDRX(nm)b DBET/DDRX

NFC 3.395 33 9.78 3.37 NFC25 2.222 50 20.23 2.47 NFC50 2.206 51 19.79 2.58 NFC75 3.049 35 21.51 1.63 Theoretical density¼5373 g/cm3 a Calculated by BET. b Calculated by Rietyeld.

Fig. 3. Scanning electron microscopy (MEV) of Ni1xCoxFe2O4system for compositions (a) NF, (b) NFC25, (c) NFC50, and (d) NFC75.

smaller the particle size, the greater the surface tension, which generates driving force to promote the increase of the state of agglomeration and/or aggregation[30].

By MEV analysis, formation of precipitated phases observed in X-ray diffraction is clear, and within all compositions, there is the segregation of metallic components with subsequent volatilization.

Fig. 4 displays the dependence of the magnetization as a function of applied magnetic field through the hysteresis loop for the ferrite powders of NF, NFC25, NFC50, and NFC75 composi-tions. Hysteresis curves show a tendencys  H hysteresis with low energy losses. The low losses in the reverse magneticfield indicate that it is a soft magnetic material releasing little energy to reverse the magnetic momentum. Moreover, it is possible to observe that the increase in CO concentration caused a broadening of the hysteresis loop and by reducing the saturation magnetization, it affects the behavior (low coercivefield, low loss for hysteresis, and higher values of magnetization), with an increase in hysteresis loss and high coercivefield indicating a material with longer-lasting characteristics (intermediates).

The magnetic parameters (coercivefield, saturation magnetiza-tion, remanent magnetizamagnetiza-tion, and hysteresis loss) obtained from the curvess  H are shown inTable 3.

The increase in the cobalt concentration reduced the saturation magnetization and the coercivefield increased. The NFC25 com-position had the highest saturation magnetization and remanence in relation to NF, NFC50, and NFC75 compositions. The magnetic

field varied from 657 Oe to 96 Oe, being higher for the NFC75 composition, indicating a higher amount of energy to reverse the magneticfield of this ferrite. According to the Rietveld refinement, the crystallite size varied between 9.78; 20.23; 18.96; and 21.63 nm for NF, NFC25, NFC50, and NFC75 compositions, respec-tively. According to the literature, very small particles below the critical size of Dc, which is not well defined at around 100 nm, there was no formation of magnetic domains. In this case, the magnetization is explained by coherent rotation of magnetic momentum. Observing the hysteresis loop for the four composi-tions, it can be noted that the curves do not saturate even at high field strength, indicating that the phenomenon may be occurring in super paramagnetism, which is only considered the average size of the crystals (Tc)[31].In this case, the larger crystal size has the largest resulting magnetization. However, the order of magnetiza-tion obtained did not follow this criterion, since the NFC75 composition showed higher crystallite size compared to the other compositions. As already noted, the NFC25 composition had

Fig. 4. Hysteresis cycles of Ni1xCoxFe2O4system for compositions (a) NF, (b) NFC25, (c) NFC50 and (d) NFC75.

Table 3

Magnetic Parameters of the Ni1xCoxFe2O4system for compositions, NF, NFC25,

NFC50, and NFC75.

Sample (combustion) MS(emu/g) MR(emu/g) HC(Oe) MR/MS

NF 35.75 4.5 96 0.13

NFC25 41.18 15.8 363 0.38 NFC50 32.41 13.4 512 0.41 NFC75 29.30 12.6 657 0.43

higher magnetization compared to the other three compositions. This fact can be explained because it presents higher remnant magnetization compared to the other compositions, likely due to the crystalfield organization.

Fig. 5shows the saturation magnetization as a function of the concentration of the cobalt ferrite Ni1xCoxFe2O4. It can be seen

that the NFC25 composition had an increased saturation magne-tization, the maximum value is due to the smaller contribution of anti-magnetic iron phases FeO and Ni° present in the structure. In other compositions, the magnetization decreases linearly with the increase in the concentration of cobalt.

It is known that nickel ferrite is an inverse spinel, whose cell unit is represented by the formula (Fe1x) [NiFe1þ x]O4, where the

Fe3þ_{ions are evenly distributed in the tetrahedral and octahedral}

positions of the network. The variation of saturation magnetiza-tion, Ms, depends on the distribution of the cations in the spinel network. The ions contribute to the magnetization of the material according to their magnetic momentum, that is, the number of unpaired spins. As explained inTable 4, the ions Ni2þ _{and Co}2þ

have a preference for octahedral positions. The Ni2þ has 3d8

con_{figuration with two unpaired electrons, since the Co}2þ has 3d7_{configuration with three unpaired electrons and the ion Fe}3þ

has 3d5_{configuration with 5 unpaired electrons.}

Considering the strong preference of the ion Co2þ, the octahe-dral position of the spinel network[21], and assuming that cobalt partially replaces Ni2þ ions in this position, we will have the theoretical magnetization (intrinsic characteristic) calculated in the network of the spinels NF, NFC25, NFC50, and NFC75 as 2.6

μ

B, 3.34

μ

B, 2.79

μ

B, and 1.95

μ

B, respectively. Therefore, a reduction in the magnetic properties is expected when the simultaneous substitution of nickel for cobalt occurs in the spinel ferrite network of nickel. Since in concentrations greater than 25% cobalt this will also go to site A, increasing the contribution of the magnetic momentum of the iron in the B site, thus reducing net magnetic momentum. The estimated values for the total theoretical

magnetization of each ferrite was calculated based on the mag-netic contribution and the stoichiometric amount of each ion in the structure. On the other hand, it is known that the extrinsic features such as grain size and/or particle area in_{fluence the} magneticfield and can contribute to increasing the magnetization because the larger the particle size and/or grain, the smaller the number of barriers, thus achieving greater magnetization. In this work, we found that the concentration of cobalt led to an increase of particle size, however, with respect to the magnetization, we observed a linear decrease, and an increase in the coercivefield, which indicated that the intrinsic characteristic prevailed.

Thus, it became evident that there was a significant influence of cobalt substitution for nickel ions in the nickel ferrite network in synthesis, as morphological and magnetic characteristics of nano-powders were synthesized in a microwave.

4. Conclusions

Ferrites of Ni1xCoxFe2O4 system for NF, NFC25, NFC50, and

NFC75 compositions were obtained by combustion reaction as-sisted by microwave. The combustion reaction method can be recommended as a promising method, since it takes a large amount of spinel phase at all compositions and nano-sized particles with suitable structural properties for use as magnetic materials. The XRD patterns do not show monophasic systems however the desired spniel phase was dominant at all composi-tions. Structural parameters showed that increasing the amount of Co2þ caused an increase in crystallite size and hence a reduction in specific surface area, as shown by BET analysis. On the other hand, DBET/DDRXratio decreased, indicating that the particles are

polycrystalline and agglomerate easily due to high surface tension. The all compositions showed the non-uniform morphology with irregular-sized particles and the formation of clusters with lots of pores, characteristic of combustion reaction assisted in micro-waves. The dependence of the magnetization as a function of applied magnetic field through the hysteresis loop s  H to the ferrite powders of NF, NFC25, NFC50, and NFC75 composition showed that the concentration of Co2þcaused a linear decrease of the magnetization and increase in the coercivefield. The magnetic characteristics of ferrites obtained allow for the use of these materials as soft magnetic (permeable) and intermediate (medium magnetic recording), with cycles of relatively narrow hysteresis.

Acknowledgements

The authors would like to thank the Coordination of Improve-ment of Higher Education Personnel, CAPES/PNPD, the PPGCEM/ UFRN and Labtam for facilities. The laboratory of Magnetism and Magnetic Materials from Department of Theoretical and Experi-mental Physics (DFTE) of Federal University of Rio Grande do Norte (UFRN) for the magnetic measurements development.

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