Altering the properties of the magnetic Co ferrite nanoparticles fabricated by modified inverse coprecipitation for high-frequency applications

Magnetic Co ferrite nanoparticles doped with non-magnetic ions (Zn 2+ ) fabricated by modified inverse coprecipitation technique. X-ray calculations show that the average crystallite size (D) and the average lattice constant (a) of CoZn ferrite nanoparticles increase from 32.33 to 52.87 nm and from 8.39 to 8.41Ǻ respectively with increasing non-magnetic Zn 2+ ions from 0.00 to 0.55. Morphological forms and M-O at A and B sites studied by SEM and FT-IR spectroscopy. Measurements of the structural, optical, electrical and magnetic characterization of the CoZn ferrite nanoparticles strongly depend on non-magnetic Zn 2+ ions content (y). Non-magnetic ions transform Co ferrite from hard and dielectric nature to soft and semiconductor nature. Values of Coercivity and the remanence decrease as non-magnetic Zn 2+ ions increases to the minimum values 955 Oe and 6 emu /g for the sample with Zn = 0.55. Co 0.45 Zn 0.55 Fe 2 O 4 is might be suitable for high-frequency applications where it has the smallest value of optical gap, the largest value of resistivity and the lowest value of dielectric loss factor.


Introduction
CoFe2O4 receives great attention for distinctive transport and magnetic properties [1][2][3]. It is for instance considered in the varies applications of magneto-optical information storage media, medical diagnosis, magnetic sensors, medical resonance imaging magnetically controlled, drug delivery, catalysts, energy storage devices and optoelectronics [4][5][6][7][8][9][10][11]. In order to appropriate the properties of CoFe2O4 for these applications could be tuned by controlling the crystallite size (D) and by substituting subsequently the paramagnetic cobalt cations by diamagnetic cations [12]. Therefore, CoZn ferrites have attained very interest basis to the distinctive and varies properties of ZnFe2O4 and CoFe2O4 so they have excellent electromagnetic characterization and physical/chemical stability [13,14]. They can be used as active material in energy storage supercapacity, magnetic record medium, microwave absorption compounds and catalysis [15][16][17][18][19]. Waje et al. fabricated Co0.5Zn0.5Fe2O4 using mechanically alloyed and they studied influence of sintering temperature on its magnetic characterization [20]. M. Ajmal et al. conclude that variation of the sintering time on mixed Cu-Zn ferrites causes appreciable changes in its structural and magnetic parameters [21]. CoFe2O4 nano-particles fabricated using sol gel technique and influence of dopants and ball milling on magnetic characterization studied [22]. Koseoglu et al. [23] prepared ZnFe2O4 by microwave method and doping it by cobalt ions. They reported that ZnFe2O4 with less Co additives have superparamagnetic character at 300 K. The co-precipitation is an efficient technique to prepare ferrite nano-particles but, there are difficult to control size and morphology of the particles. So that, this paper aims to prepare non-magnetic ions Zn 2+ substituted CoFe2O4 nano-particles by modified inverse co-precipitation process in medium solvent of ethylene glycol and distilled water. In this method the solution of metal ions adds to the precipitant solution, which leads to precipitate 4

Characterization studies
The structural parameters of fabricated nano-powders carried out by using powder X-rays diffractometer (XRD) model a Shimadzu X-600 Japan with Cu Kα MHz at room temperature.    [26] and ZnFe2O4 (8.44 Å) [27] ferrites. This behavior of the investigated mixed spinel structure of cobalt ferrite can be explained based on the replacement of the smaller Co 2+ ions (r = 0.72 Å), with larger Zn 2+ ions (r = 0.82 A). Taking into account that the tetrahedral (A) sites are smaller than the octahedral (B), ones a higher occupancy of a sites by bigger metal Zn 2+ ions will leads to an expansion of the structure and consequently to an increase of the lattice parameter (a) this result is consistent with previous study [28].

Structural properties of CoZn ferrite nano-particles
Values of the lattice parameter (a) for the fabricated mixed cobalt ferrites increased by increasing zinc ions so the unit cell volume (V) of CoFe2O4 ferrite nanoparticles which came from V = a 3 relation [29] increases also as seen from Table 1. X-ray density (ρx) for all the prepared samples was determined by ρx = ZM/NAV [3034]; where Z is the number of molecules for spinel ferrite unit cell , M is the molecular weight of the sample, NA is Avogadro's number and V is the volume of unit cell and they are tabulated in Table1. Table 1 shows that ρx increases from 5.26 g/cm 3 to 5.31 g/cm 3 with increasing Zn 2+ ions content (y). Due to the increase in volume of the unit cell (V), ρx should be decreased but in the present case the molecular weight (M) increases where Zn 2+ has higher atomic weight (65.39 amu) than that of the Co 2+ (58.93 amu) which overtake the effect of V and as a result ρx increases with increasing Zn 2+ ions. Similar structural behavior noticed by Gul and Maqsood on the CoFe2-xAlxO4 ferrites prepared by sol-gel method [28]. The bulk density (ρb) was determined by ! = / " formula where m, r and l denote to mass, radius and thickness of a disc respectively. Determine the porosity for all the investigated compositions chivied according to = )1 − and density (ρ) by = $%%% #& relation [33]. The calculation of specific surface area (S) showed decreasing behavior with increasing Zn ions as illustrated in Table 1. This may be due to the strong correlation between S and D where D is inversely proportional to S. Table 1 The average lattice constant (Å), the unit cell volume(Å ' ), X-ray and bulk densities (g/cm 3 ), porosity and specific surface area (m 2 /g) of CoZn ferrite nanoparticles as a function of the Zn 2+ ions.

Morphological analysis of CoZn ferrite nano-particles
Scanning electron microscope (SEM) images and particles size distribution (PSD) histograms of all the synthesized CoZn ferrite samples are appeared in Fig. 3.
All CoZn ferrite have nearly homogeneous nano-size particles with particles size are increasing with increasing Zn 2+ ions. Nature of the surface shows aggregation coalescence character that may be refers to: the surface tension (ST) and magnetic dipoles interactions at the surface [24,34]. It can be also seen in Fig. 3 that the increase of Zn 2+ ions leads to decrease the agglomeration between nano-particles, this behavior agrees with the report of G. Raju et al [35]. This behavior can be explained as follow; increasing the nonmagnetic ions (Zn 2+ ) at the expense of magnetic ions (Co 2+ ) leads to decrease the magnetic dipoles at the surface and magneto-static actions and hence decrease the aggregation. In addition, increasing Zn 2+ ions leads to increase the crystallite size (D) and decrease the specific surface area (S) and hence decrease of surface tension (ST) and as a result decrease of the aggregation. In addition, statistical analysis of particles size (PS) and particles size distribution (PSD) of CoZn ferrite achieved using imageJ software which developed at the National Institutes of Health (NIH) [36]. From the inset histograms in Fig. 3 can be observed that the average particle size (APZ) of Co1-yZnyFe2O4 are 11    hydroxyl groups and adsorbed water on the surface of CoZn ferrite nano-particles [37]. Ferrites can be considered as continuously bonded crystals where the atoms are bonded to all nearest neighbors by equivalent ionic bonds [38]. Cations of ferrite are distributed at two sub-lattices designated by A and B sites according to the configuration geometry of the oxygen nearest neighbors [39]. bonds in A sites because the transition of the inverse spinel (Co ferrite) toward the normal spinel (Zn ferrite) [40]. In other words, the bands become sharper when moving from the mixed spinel ferrite CoFe2O4 and getting closer to the normal spinel ferrite ZnFe2O4, similar results reported for zinc ferrites doped with magnesium [42].

FTIR analyses of CoZn ferrite nano-particles
In the FTIR spectra of the fabricated CoFe2O4 nano-particles when Co 2+ ions are replaced by Zn 2+ ions that have larger ionic radius and higher molecular weight and they go to A sites, νA vibration shifts to lower wavenumber [43] from 595.93 cm -1 to 568.32 cm -1 (see Table 2). In the same time migrated Fe 3+ ions to B sites leads to shift of νB to higher wavenumber from 464.21 cm -1 to higher than 470.62 cm -1 (see Table 2). The phase transformation from mixed to normal spinel ferrites will be accompanied by decreasing the stretching frequencies [44]. It can be seen from Fig.4 and Table 2

Temperature of cobalt ferrites enhances the hopping of electrons between
Fe 2+ and Fe 3+ ions and jumping of holes Co 2+ and Co 3+ and therefore it increases σDC [48]. DC resistivity (ρDC) at 300 K is in order of 10 5 Ω. m, which make the fabricated CoZn ferrite nanoparticles samples suitable for applications of high frequency. Fig.   6 shows correlation of σDC of the prepared samples with the concentration of Zn 2+ ions. σDC is observed to decrease with Zn 2+ content (y). hopping process between Fe 2+ and Fe 3+ ions and Co 2+ and Co 3+ ions in B sites [48].
It is well known that Zn 2+ ions occupy tetrahedral (A) sites while Co 2+ and Fe 3+ ions occupy tetrahedral (A) and octahedral (B) sites [31]. Thus, increasing deficient of Fe 3+ ions from A sites to B sites with increasing Zn 2+ ions leads to decrease number of Co 2+ and Fe 3+ at B sites so this behavior gives a reason for decreasing σDC.  where NA is Avogadro's number, nFe is a number of iron atoms present in the chemical formula, ρb is the bulk density and M is the molecular weight of the compound.

AC electrical conductivity of CoZn ferrite nano-particles
It is well known that σAC in disordered solids is directly proportional to frequency . Alder and Feinleib [50] reported that σAC depends on . ln(σAC) versus ln( ) for CoZn ferrite is shown in Fig. 9. This because the hopping length between two metal ions on octahedral locations (B) smaller than that at tetrahedral locations (A). In addition, because Fe 2+ ions prefer occupancy B site so the hopping of charge carriers (CCs) among A sites is not possible [51]. Influence of grain boundaries (GBs) on σAC is clearer in low frequency range where hopping rate of the charge carriers (CCs) is less and hence σAC is less too. Where increases the conductive grains become more active according to Maxwell -Wigner model [52]. As a result, the hopping rate of charge carriers (CCs) increases and σAC increases. In addition, the higher frequencies lead to higher pumping force provided to charge carriers (CCs) so the value of σAC becomes higher.  ions present at the same site so the charge carriers at B sites decrease and hence it can be said the jumping rate of electrons between Fe 2+ and Fe 3+ ions and the mobility rate of holes between Co 2+ and Co 3+ ions decrease and as a result σAC decreases as can be observed [48].

3.6a Dielectric constant (ε/)
The room temperature dielectric constant (ε / ) for CoZn ferrite versus frequency from 100 kHz to 1 MHz as a function of Zn 2+ ions is shown in Fig. 11. where values of ε / decrease with the increase in values and at very high , ε / becomes constant this behaviour agrees with earlier study [53]. Koops has proposed that the influence of grains boundaries (GBs) is predominant at lower frequency ( ) region [54]. GBs work as trap states between valance and conduction bands. Thinner GB means higher ε / [55]. The large values of ε / at low are mainly due to the presence of various types of polarization including; space charge, directional, ionic and electronic polarizations. The decrease in ε / with is a natural because any species contributing to polarizability is bound to show lagging behind the external field at higher values [56]. The transition of charge carriers between cations may leads to the local displacement of electrons in the direction of oscillating field and up to reach a plateau because that above a specific value of the jumping charge carriers cannot follow the external alternating field. The correlation between ε / and Zn 2+ ions for CoZn ferrite is showed in Fig.12.

3.6b Dielectric loss factor (ε // )
The dielectric loss factor (ε // ) is considered to be the most important part of the total core loss in ferrites [57]. ε // measured how much amount of energy has been dissipated with the external ac electrical field [58]. ε // in ferrites mainly originates from electron hopping and defect dipoles [59]. Fig. 13 shows the variation of ε // of fabricated CoZn ferrite as a function of from 100 kHz to 1 MHz at 300 k. From Fig. 13 Dielectric loss factor of CoZn ferrite nanoparticles versus frequency as a function of Zn 2+ ions. Fig. 13 it can be seen that ε // has the same trend of ε / . The electron hopping contributes to ε // only in low frequency region. The hopping processes decrease with 23 the increase in and hence ε // decreases in a high frequency region for each sample as illustrated in Fig. 13. The decrease in ε // with the increase in is attributed to the fact that the hopping of charge carriers cannot follow the changes of the externally applied electric field beyond a certain limit [60].      [65]. From Fig. 16b observed that the squareness ratios R of all CoZn ferrite are lower than 0.5 which means that interactions between the particles are by magneto-statically and they have multi domain structure [66].

Magnetic properties of CoZn ferrite nano-particles
Magnetic anisotropy constant (K) of the samples calculated using = . / /0.98 [67]. The correlation between K of CoZn ferrite samples and Zn 2+ ions content is plotted in Fig 17a. Fig.17a

Optical properties of CoZn nanoferro-fluid
Optical data used to calculate the energy gap (Eg) of CoZn ferrite. The optical absorbance spectra of CoZn nanoferro-fluid studied from 200 to 800 nm using Uv-Vis spectrophotometer as illustrated in Fig. 18 which further used for Eg calculation.
The data analyzed by the relation: αhν = A(hν − E 4 ) ,/" [24] for near edge absorption. hν is the energy of incident photon, α is the absorbance coefficient, A is a constant and n is a number equal to one for direct and indirect optical gap respectively. The value of direct energy gap Eg calculated by extrapolating the straight-line portion of graph on hν axis as clarified in Fig. 19. The produced Eg for CoZn ferrite nano-particles is listed in Table 2. From Table 2 it can be observed that by increasing concentration of Zn 2+ ions, value of Eg decreases. It has been noticed that there is significant decrease in Eg with Zn 2+ ions which is may be due to increasing the crystallite size [68]. As the crystallite size increases the valance and conduction bands become closer and narrowing Eg.  Co0.45Zn0.55Fe2O4 has the largest value of resistivity and lowest value of dielectric loss factor so this sample is might be suitable for high-frequency and microwave applications.