Effect of Ni substitution on structural, optical and magnetic properties of ferrite nanoparticles synthesized by co-precipitation route

Materials Science & Engineering B 251 (2019) 114442

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Effect of Ni substitution on structural, optical and magnetic properties of ferrite nanoparticles synthesized by co-precipitation route

T

⁎

P. Iranmanesha, , Sh. Tabatabai Yazdib, M. Mehrana a b

Department of Physics, Vali-e-Asr University of Rafsanjan, Rafsanjan 77188-97111, Iran Department of Physics, Payame Noor University (PNU), Tehran 19395-3697, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: Ni ferrite nanoparticle Superparamagnetism Cation inversion Co-precipitation Band gap

In this research, fine nanoparticles of NixFe3−xO4 with Ni concentration up to x = 1.5 were synthesized via a simple one-step capping agent-free co-precipitation route. The effect of Ni substitution was investigated by details using X-ray diffractometer, transmission electron microscope, Fourier transform infrared and UV–Vis spectroscope, and vibrating sample magnetometer. The structural studies revealed that the particles have good crystallinity with diameters of 7.5–13 nm with cubic spinel structure. The FTIR spectra exhibiting a doublet band around 600 cm−1 confirms further the mixed spinel structure. It was found that the involved nanoparticles are indirect band gap materials that controlled by the size effect rather than the influence of the chemical composition. The magnetic characterization showed that the nanoparticles are superparamagnetic at room temperature with the superior saturation magnetizations of about 68–70.5 emu/g; the size effects dominate the influence of the sites occupancy in controlling the magnetization values, as well.

1. Introduction It is for about two decades that the nano-scale magnetic materials are of growing interest regarding their potential applications in various fields such as high-density magnetic recording, ferro-fluids, magnetic refrigeration, microwave devices, magnetic resonance imaging, magnetically guided drug delivery, etc [1,2]. Nano-structured magnetite (Fe3O4) and its cation substituted compounds (M,Fe)3O4, M being a divalent metal cation, which belong to the spinel ferrite group are of most extensively studied magnetic materials. Particularly, the transition metal doped Fe3O4 nanoparticles possessing enhanced saturation magnetization values meet the requirement of technical applications. Nickel ferrite (NiFe2O4) is one of such materials possessing attractive properties as a soft ferrimagnet [3–5] or superparamagnet [6,7] with low loss at high frequencies. The ferrites' structure is face centered cubic (fcc) made of oxygen ions with Fd3m space group, in which metal cations reside at two kinds of interstitial sites: tetrahedral or A sites surrounded by 4 oxygen anions and octahedral or B sites surrounded by 6 oxygen ions [8,9]. Based on the cations distribution among A and B sites, the spinel structure may adopt one of the three types. The general formula of a spinel ferrite can be written as (M12−+i Fe3i +)A (M2i +Fe32+−i)B O4 where “i” is the inversion parameter, which i = 0 and 1 correspond to the normal spinel and inverse spinel respectively, and for 0 < i < 1 the spinel is termed as the ⁎

mixed one. The inversion grade depends on the chemical composition, as well as the microstructural characteristics, particularly, the particle size. The bulk NiFe2O4 is a completely inverse spinel (i = 1) in which Ni2+ ions occupy preferentially B sites and Fe3+ ions are equally distributed among A and B sites [10,11]. However, the nanoparticles of Ni ferrite exhibit different structures of inverse [12] or mixed spinel [13], depending on the microstructural aspects. The magnetic properties of ferrites are determined by the superexchange interactions between the metal cations [8] and are so sensitive to the choice of M cation as well as the cations distribution depending in turn on the microstructural aspects, particularly the particle size and crystallinity. In our previous work, we studied the effect of preparative conditions, precisely pH value of the precipitating medium, on the microstructural properties and the cations distribution and so on the magnetic properties of NiFe2O4 nanoparticles [14]. The magnetic properties of Ni substituted magnetite can be tuned by changing the Ni and Fe atomic ratio, as well. A recent study on the magnetite nanoparticles doped with Ni (NixFe3−xO4 with x up to 1) [15] has revealed a decreasing trend for the saturation magnetization (Ms) values with the Ni content, independent of the particle size variations. On the other hand, in an earlier work it has been observed that on increasing the Ni content up to x = 0.4, the Ms values vary nonmonotonically but consistent with the particle size variations [16]. The literature reports are not in agreement and particularly miss the

Corresponding author. E-mail address: [email protected] (P. Iranmanesh).

https://doi.org/10.1016/j.mseb.2019.114442 Received 4 July 2018; Received in revised form 4 June 2019; Accepted 31 October 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. (a) XRD patterns of NixFe3−xO4 samples taken at room temperature by Cu-Kα radiation, and (b) enlarged view of 2θ = 34–37° region.

characterization was carried out by X-ray diffraction (XRD) method utilizing a Philips X'pert diffractometer system using a monochromatic Cu-Kα radiation (λ = 1.54056 Å). The XRD profiles were refined by CELREF software. The morphological characterization of the samples was performed by transmission electron microscopy (TEM) using a Zeiss EM10 TEM system. Their Fourier transform infrared (FTIR) spectra were recorded by a Thermo Scientific Nicolet iS10 spectrometer. The optical absorption spectra of the involved samples were recorded by a PG Instruments Ltd. T80 UV–Vis spectrometer. The room temperature magnetic measurements were carried out using a homemade vibrating sample magnetometer (VSM, Meghnatis Daghigh Kavir Kashan Co., Iran). For more details, see our previously published paper [14].

magnetic characterization over a wide range of Ni content in this system. Therefore, with the motivation of modifying the room temperature magnetic properties of Ni-substituted ferrites, in this research we prepare the NixFe3−xO4 nanoparticles with Ni content up to x = 1.5 (the Fe rich side of the series) via a simple one-step capping agent-free co-precipitation route and study their structural, morphological, optical and magnetic properties. Notably, the optical properties of this system have not been explored yet. 2. Experimental methods 2.1. Sample synthesis The NixFe3−xO4 nanoparticles with x = 0.25, 0.50, 0.75, 1.00 and 1.50 were synthesized by co-precipitation of chloride salts of Ni and Fe as the precursors and ammonia aqueous (NH4OH) as the precipitating agent. The reactants were stoichiometric amounts of iron III chloride hexahydrate (FeCl3·6H2O), iron II chloride tetrahydrate (FeCl2·4H2O) and nickel II chloride nonhydrate (NiCl2) (the molar ratio of trivalent to divalent cations was maintained at the ideal value of 2:1). So, to prepare the samples with x = 0.25, 0.50, 0.75, 1.00 and 1.50, the ratio of Fe(III), Fe(II) and Ni(II) salts was chosen as 8:3:1, 4:1:1, 8:1:3, 2:0:1 and 1:0:1, respectively. All the reactants used were of analytical grade with purity of better than 99% from Merck Co. Details of the synthesis method have been described in our earlier paper [14]. In brief, the salts dissolved in 25 ml of deionized water under continuous stirring were mixed together and kept at the reaction temperature of 60 °C for 5 min and then stirred for about half an hour to obtain a homogenous clear solution. A diluted ammonia solution was added drop by drop under continuous stirring to set the pH value at 11. Though the precipitate appeared at lower pH values, according to our previous study, pH = 11 was chosen which gives rise to the Ni ferrite particles with a superior magnetization [14]. To ensure the complete synthesis of the nanoparticles, the above conditions were kept for about half an hour. Then, after the solution was cooled to room temperature, the precipitated particles were collected with the help of a magnet and the by-products, mainly traces of the produced nitrogen and chlorides, were removed by the repeated rinsing of the resultant product with deionized water and ethanol. Afterwards, the particles were centrifuged and dried in an oven of 100 °C for 10 h. Finally, the acquired substance was ground into a fine powder in an agate mortar.

3. Results and discussion 3.1. Structural and morphological analysis The XRD patterns of NixFe3−xO4 samples with x = 0.25, 0.50, 0.75, 1.00 and 1.50 in the angular range of 2θ = 20–90° are shown in Fig. 1a. All the samples are well crystallized in the expected cubic spinel structure with S.G. Fd3m. The patterns reveal no detectable sign of the precursors or any secondary phases, i.e. the samples are single-phased. As seen from the enlarged view of the main (3 1 1) peak in Fig. 1b, by increasing the Ni content (x), the diffraction peaks shift slightly and continuously towards the higher angles indicating a gradual decrease in the interatomic distances. Similar gradual shifts occur for the other diffraction peaks, as well. Additionally, this supports the incorporation of Ni ions into the spinel structure. The lattice parameters obtained from the refinement of XRD patterns by CELREF software are summarized in Table 1 and depicted in Fig. 2a. As seen, the lattice parameter decreases on Ni substitution with two distinct slopes and do not obey Vegard's law. This suggests two different mechanisms for Ni substitution in these samples. The observed decreasing trend can be Table 1 Lattice constant (a), crystallite size (D), lattice microstrain (ε), dislocation density (δ), X-ray density (ρx), and particle size (D') of NixFe3−xO4 nanoparticles.

2.2. Characterization The synthesized (Ni,Fe)3O4 particles were characterized for structural, morphological, optical and magnetic properties. Their structural 2

Sample (x)

a (Å)

D (nm)

ε

δ (1011 line/ cm2)

ρx (g/ cm3)

D′ (nm)

0.25 0.50 0.75 1.00 1.50

8.374(2) 8.371(5) 8.367(2) 8.351(0) 8.333(7)

8.9 15.4 10.9 11.6 11.3

0.0033 0.0025 0.0042 0.0028 0.0039

12.62 4.22 8.42 7.43 7.83

5.255 5.277 5.301 5.348 5.414

7.5 ± 0.8 13 ± 1.6 8 ± 1.0 9 ± 1.2 8.5 ± 0.8

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Fig. 2. (a) Variation of the lattice parameter of NixFe3−xO4 nanoparticles with Ni content. (b) W-H plots for NixFe3−xO4 nanoparticles.

Fig. 3. TEM images of NixFe3−xO4 nanoparticles with Ni content of x = 0.25–1.50, along with their size distribution histograms.

3

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for similar compounds in literature [24–26], we considered the cations distribution for the samples with x ≤ 1 as:

explained based on the earlier Mössbauer studies on NixFe3−xO4 nanoparticles with x up to 0.5 suggesting that Ni2+ ions replace mainly Fe2+ ions at B sites [17], and the previous Mössbauer characterization of NiFe2O4 nanoparticles revealing that all iron atoms are in Fe3+ oxidation state [15]. Considering the values of the ionic radius at octahedral sites (Ni2+: 0.69, Fe2+: 0.78, Ni3+: 0.6 and Fe3+: 0.645 Å, [18]), the reduction observed in the lattice parameter of the samples with x < 1 is due to substitution of Ni2+ cations preferentially for Fe2+ ions at B sites. On further Ni substitution, Ni3+ ions should replace some Fe3+ ones and/or Ni2+ ions may replace Fe3+ cations resulting in a non-stoichiometric compound containing some oxygen vacancies. The presence of oxygen vacancies has been considered in the cation distribution of CoxFe3−xO4 nanoparticles with x ≥ 1 suggested according to their structural and magnetic data [19]. The crystallite (grain) size, D, of the synthesized particles was estimated from the peak width using Williamson-Hall equation as:

kλ β= + 4ε tan θ Dcos θ

(Ni2j +Fe3i +)A (Ni2x−+jFe12−+x Fe32+−i)B O24−

(2)

where i and j represent the inversion grades, i.e. A-site occupancy of Fe3+ and Ni2+ cations, respectively; The site balance requires that i + j = 1. On the other hand, for the samples of 1 ≤ x ≤ 1.5, we considered as: 2− (Ni2j +Fe3i +)A (Ni12−+jNi3x−+1Fe33 + − x−i) B O4

(3)

The inversion parameters were determined by matching the experimental lattice parameters obtained from XRD results (Table 1) with the theoretical ones calculated for these spinel structured compounds by Ref. [27]

ath = (1)

8 [(rA + rO) + 3 3

3 (rB + rO)]

(4)

Here, rO is the oxygen ionic radius (1.38 Å), rA and rB are the mean ionic radii at A and B sites, i.e.

where θ is the diffraction angle of a main reflection, β is the full width at half maximum of that peak, λ is the X-ray wavelength, 1.54056 Å, k is Scherrer's constant whose value for our particles of cubic symmetry being nearly spherical (see TEM micrographs) is assigned 0.94 [20] and ε is the microstrain parameter. The β cos θ values plotted against 4 sin θ (W-H plot) for all the samples are shown in Fig. 2b, whose linear fit's intercepts and slopes determine D and ε values, respectively. The low scattering of the data points reflects the uniformity of the lattice strain and the isotropic nature of the involved samples. In addition, the positive slope of the plots indicates tensile strains in the nanoparticles [21]. The tensile strain has been also reported for the Co, Mg and Mn nanoferrites synthesized by a combustion method [22]. The estimated crystallite size and microstrain values are listed in Table 1. No obvious correlation is observed between the crystallite size of the particles and their Ni concentration. This should be due to the complicated process of particle growth in the co-precipitation synthesis of the particles. However, the variations of ε and the dislocation density (δ = 1/D2) in the synthesized particles with different Ni contents are consistent with the variations of the crystallite size (see Table 1). The lower are the values of δ and ε in the particles (i.e. the better crystallinity), the larger is the crystallite size. It is worth mentioning that the microstrain parameters being of negligible amounts (of 10-3 order, the same as some reported values [22]) would have no significant influence on the physical properties of these particles. The theoretically calculated (X-ray) density, ρx, [14,23] of the synthesized NixFe3−xO4 samples is given in Table 1, as well. On increasing the Ni concentration, the molecular weight of the compound increases while the unit cell volume (or in other words, the lattice constant) decreases, so the observed increasing trend of the density with the Ni content was expected. The morphology of the synthesized particles was studied in more details by transmission electron microscopy (see Fig. 3). It was revealed that all the as-synthesized samples consist mostly of spherical-like nanoparticles of uniform size and slight agglomeration. On analyzing the particle size distribution (histograms) shown as insets in Fig. 3, a Gaussian function can be inferred showing well-dispersed particles. The mean particle size (D') of the involved samples determined by Gaussian fitting of the histograms is given in Table 1. The obtained values (with the estimated error) are around the crystallite sizes calculated from the XRD peaks widths. This suggests that the synthesized nanoparticles are of single crystalline nature.

rA = (1 − i) rA (Ni2 +) + i rA (Fe3 +) 1

x≤1 ⎧ 2 [( x + i − 1)rB (Ni2 +) + (1 − x) rB (Fe2 +) ⎪ + 3 ⎪ + (2 − i) rB (Fe )] rB = 1 ⎨ [i rB (Ni2 +) + ( x − 1)rB (Ni3 +) 1 ≤ x ≤ 1.5 ⎪2 ⎪ + (3 − x − i)r (Fe3 +)] B ⎩

(5)

The rA and rB values of the metal ions used were their radii in high spin state with the coordination number of 4 and 6, respectively, reported by Shannon [18]. The values of rA and rB (Eq. (5)) obtained by comparison of aexp (Table 1) and ath (Eq. (4)), as well as the sites occupancy in the synthesized (Ni,Fe)3O4 nanoparticles are given in Table 2. As seen, Ni ions occupy preferentially B sites and as a result, some Fe3+ ions migrate from B- to A-sites. Ni substitution decreases the inversion grade (i) or in other words, forces the structure to approach the normal spinel. Ni substitution also affects the ionic radii; Replacement of Fe ions by Ni ones causes an expansion in AO4 tetrahedra and simultaneously a contraction in BO6 octahedra, resulting in the observed increase in rA values and the decrease in rB ones. The spinel structure is characterized not only by the lattice constant “a” and the number of different cations in each site per formula unit, but by the oxygen positional parameter which often deviates from the ideal value of u = 3/8 to accommodate the cations of different sizes. The u values in the involved samples were determined from the mean cation to anion distance given by the following relation Eq. (6), for A site [27]:

rA + rO =

(6)

3 a ( u − 1/4)

The obtained u values listed in Table 2, reveal another effect of Ni substitution as a displacement of oxygen ions outward along the diagonal of the cube. This results in the mentioned tetrahedra expansion as well as the octahedra shrinkage. The variations of u, rA and rB against the Ni content in the samples are depicted in Fig. 4. Table 2 Mean ionic radii at A and B sites (rA and rB, respectively), oxygen positional parameter (u) and sites occupancy in NixFe3−xO4 nanoparticles.

3.2. Cations distribution The obtained lattice constants were used to suggest the possible sites occupancy of Ni and Fe ions in the synthesized NixFe3−xO4 nanoparticles. Recalling the discussion in Section 3.1 and the distributions 4

Sample (x)

rA (Å)

rB (Å)

u

A-site occupancy

B-site occupancy

0.25

0.4966

0.6988

0.3794

+ + 3+ Ni20.14 Fe20.75 Fe1.11

0.50

0.5020

0.6855

0.3798

0.75

0.5074

0.6722

0.3802

1.00

0.5121

0.6592

0.3808

1.50

0.5296

0.6526

0.3823

+ + Ni20.11 Fe30.89 2+ 3+ Ni0.20Fe0..80 + + Ni20.29 Fe30..71 + + Ni20.37 Fe30..63 2+ 3+ Ni0.66Fe0..34

+ + 3+ Ni20.30 Fe20.50 Fe1.20

3+ + + Ni20.46 Fe20.25 Fe1.29

+ 3+ Ni20.63 Fe1.37 + + 3+ Ni20.34 Ni30.50 Fe1.16

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Table 3 Bands observed in FTIR spectra of NixFe3−xO4 nanoparticles. Band position (cm-1)

Band assignment

x = 0.25

x = 0.50

x = 0.75

x = 1.00

x = 1.50

441 561,631 1052

442 578,631 1043

439 593,636 1054

439 578,631 1049

439 593,634 1053

1401 1624 3437

1401 1626 3435

1403 1630 3436

1401 1626 3433

1403 1629 3433

B-site groups stretching A-site groups stretching H-O bending (out-ofplane) H-O bending (in-plane) H-O bending H-O stretching

Additionally, the broad band around 3430 cm−1 and the peak around 1625 cm−1 correspond respectively to the stretching and bending modes of H-O bond due to the small amount of moisture in the particles. The doublet bands of low intensity around 1400 and 1040 cm−1 are attributed respectively to the in-plane and out-of-plane bending vibrations of H-O bond, as well [31]. All the IR active bands revealed in NixFe3−xO4 nanoparticles along with their assignment are tabulated in Table 3.

Fig. 4. Variations of the mean ionic radii at A and B sites and the oxygen positional parameter (u) with Ni content in NixFe3−xO4 nanoparticles.

3.4. Optical properties The UV–Vis absorption spectra of NixFe3−xO4 nanoparticles recorded in the wavelength range of 300–800 nm at room temperature are given in Fig. 6. They clearly show the prominent absorption in the visible light region. The variations of the absorbance (α) of the samples follow their particle size variations. The samples of x = 0.25 and 0.50 with the smallest and largest particles, respectively, exhibit the lowest and highest α values, respectively, and the absorbance values of the other samples lie between them. The energies of the optical band gap (Eg) of NixFe3−xO4 nanoparticles and their type were determined from analysis of the absorbance data using Tauc's model based on which the product of α and photon energy (hν) is proportional to (hν − Eg)n, where the exponent n accepts the values of 1/2 and 2 depending on the nature of the involving electronic transition being direct or indirect, respectively [32]. The plots of (αhν)n with n = 2 and 1/2 against the photon energy are presented in Fig. 6b-f. The (αhν)1/2 curves with two distinct slopes suggest the phonon participation in the involved transition processes [33]. Hence, the NixFe3−xO4 nanoparticles are the indirect band-gap materials. The optical band gap hierarchy has been reported for NiFe2O4 [34], possessing an indirect-gap assigning to the minority (spin-down) channel Χ → Γ, with two higher energy direct ones corresponding to the minority Χ → Χ and majority Γ → Γ channels. The optical band gap values estimated by extrapolation of the most linear part of the curves to α = 0 are listed in Table 4. The direct band gap values obtained for the synthesized Ni substituted magnetite nanoparticles are larger than that reported for the pure Fe3O4 particles of the same size (2.48 eV, [35]) and, except for the x = 0.5 particles being of larger size, Eg values show an uniform increasing trend with Ni substitution. The variations of both direct and indirect gap energies of the samples follow almost their particle size variations (Fig. 7a). Similar observation has been reported for Ni substitution in Mg-Zn nanoferrites, as well [36]. On the other hand, this behavior is in conflict with the more electron affinity of Ni (1.157 eV) compared to that of Fe (0.153 eV) lowering the conduction band minimum and with the reported decreasing trend of Eg values on Ni doping in NdFeO3 ceramics [37]. So, the observed variations in Eg values of our nanosized samples are predominantly influenced by their particle sizes. The band gap of the nanomaterials is controlled by two conflicting factors: the quantum size effect causing a blue shift and the surface and interface effects inducing a red shift in the band gap energy with decreasing the particle size [38]. It has been found that for ferrite particles

Fig. 5. FTIR spectra of NixFe3−xO4 nanoparticles, along with the enlarged view of 700–450 cm−1 region.

3.3. Ftir spectral analysis The FTIR transmittance spectra of NixFe3−xO4 nanoparticles recorded in the wavenumber range of 4000–400 cm−1 are presented in Fig. 5. The formation of spinel structure is further supported by these spectra exhibiting its characteristic absorption bands. Four IR active bands have been reported for a cubic spinel structure [28] among which two higher-frequency ones are accessible in the present spectral range: ν1 around 600 cm−1 and ν2 around 400 cm−1. These modes originate from the stretching vibrations of A- and B-site metal–oxygen bonds, respectively, and are triply degenerate for the normal spinel. In the involved Ni substituted nanoferrites, ν2 is observed in the range of 439–442 cm−1 and ν1 is revealed as two sub-bands in the regions of 561–593 and 631–636 cm−1 (see inset of Fig. 5). The existence of subbands confirms that the synthesized nanoparticles are not the fully inverse spinels, but the mixed ones, since the Jahn-Teller distortions due to the presence of Ni2+ cations in A sites lower the local symmetry and result in splitting of the band being triply degenerate for the normal spinels [29]. The variations of the peak positions can be explained regarding the dependence of the frequency of vibrational modes on the effective mass of the metal-oxygen bond (μ) and the force constant (k) as ν = 1/2πc k/μ , c being the velocity of light. In turn, k ∝ ZM ZO/d3M−O where ZM and ZO are the atomic numbers of the metallic cation and oxygen, respectively, and dM−O is the cation-oxygen bond length [30]. Hence, the peaks positions vary as a result of different cation distributions and cation-oxygen distances for each of the tetrahedral and octahedral groups.

5

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Fig. 6. (a) Optical absorbance, and (b–f) (αhν)2 and (αhν)1/2 versus incident photon energy hν for NixFe3−xO4 nanoparticles.

Fig. 8 reveals that the magnetization values of the Ni substituted nanoparticles at the maximum field of 12 kOe do not vary systematically with the Ni concentration, but they follow the particle size variations. The magnetization curves though tend to saturation, but do not completely saturate under the applied fields up to 12 kOe. The saturation magnetization (Ms) of the involved nanoparticles was obtained by the law of approach to magnetic saturation (Eq. (7)) being applicable within the high magnetic field region [42]:

of smaller than about 10 nm, like most of our synthesized particles with dimensions of about 7–13 nm, the surface effects dominate [39]. So, the wider band gap in the larger nanoparticles is due to their smaller surface to volume ratio. 3.5. Magnetic properties The field-dependence of magnetization (M) of NixFe3−xO4 nanoparticles measured under the applied magnetic field (H) up to ± 12 kOe at room temperature is presented in Fig. 8. From the enlarged view of the region around the origin (see upper left inset in Fig. 8), it is revealed that all the samples exhibit the typical M(H) loops of superparamagnetic (SPM) materials being anhysteristic. The absence of remanent magnetization may be due to the rapid Néel relaxation [40]. The SPM behavior is observed for the particles of a ferromagnetic material smaller than a certain size, as instead of the usual multi-domain structure, they consist of a single magnetic domain. For Fe3O4 and NiFe2O4 ferrites, this critical size has been reported to be about 14 nm [12,41]. So, the SPM behavior was expected for our ultrafine Ni ferrite nanoparticles. The involved superparamegntic nanoparticles can be good candidates for use in different applications e.g. in the field of biomedicine, especially in composite form for the drug delivery and targeting tissue. The enlarged view of the high field region in the upper right inset of

M(H, T) = Ms (T) (1 − −a/ H− −b/H2) + χhf H

(7)

Here, the coefficient “a” is related to the contribution of local inhomogeneities and “b” is correlated to the magnetocrystalline aniso2

tropy being

8 K eff 105 M2s

for polycrystalline cubic structured ferromagnets

[43] where Keff is the effective magnetic anisotropy constant. The last term is the forced magnetization, being negligible below TC. So, ignoring the last term at room temperature, the magnetization of the samples as a function of 1/H was well fitted by a second order polynomial function (see the lower inset in Fig. 8). The room temperature values of Ms and Keff obtained from the intercept and the fitting parameter b, respectively, are given in Table 4. The Ms values of our NixFe3−xO4 nanoparticles are relatively higher than those for the ferrimagnetic particles of these compounds with almost similar size [16], and even smaller particles [15]. The Keff values obtained are higher

Table 4 Direct and indirect band gap values (Eg), saturation magnetization (Ms), effective anisotropy constant (Keff), magnetic moment per formula unit (ηH(exp)), calculated magnetic moment of A and B sublattices (ηA and ηB) and maximum magnetic diameter (DM) of NixFe3−xO4 nanoparticles. Sample (x)

0.25 0.50 0.75 1.00 1.50

Eindirect (eV) g

1.06 1.17 1.07 1.09 1.10

Edirect (eV) g

2.50 2.58 2.52 2.53 2.56

Ms (emu/g)

70.14 70.55 68.06 70.12 68.97

Keff (105 erg/cm3)

1.85 1.89 1.78 1.66 1.73

ηH (exp) (μB)

2.92 2.94 2.85 2.94 2.91

6

ηH (cal)

DM (nm)

ηA (μB)

ηB (μB)

ηB -ηA

4.67 4.40 4.13 3.89 3.02

8.83 8.60 8.37 8.11 7.98

4.16 4.20 4.24 4.22 4.96

8.45 8.46 8.41 8.52 8.74

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Fig. 7. Variations of (a) direct and indirect band gap energies, (b) saturation magnetization and magnetic moment per formula unit of NixFe3−xO4 nanoparticles, as well as their particle size with Ni content.

than that of bulk NiFe2O4 material (0.68 × 105 erg/cm3, [44]), maybe due to the more pronounced surface contribution to the magnetic anisotropy of our nanoparticles. Another contribution may be from the presence of dipolar and exchange interactions among these single-domain ultrafine particles [45]. Substituting Ni2+ with the magnetic moment per atom of μ = 2 μB for Fe2+ with 4 μB (for x < 1), or for Fe3+ with 5 μB (for x ≥ 1) is theoretically supposed to decrease the magnetization. This prediction is quite satisfied by the magnetization variations observed for the Nisubstituted magnetite nanoparticles of almost the same size [15], but it fails for the involved samples. The nonmonotonic M variations of our nanoparticles on Ni substitution following their size variations highlights the important and predominant role played by the surface effects: As a result of the broken exchange bonds for the surface atoms, the surface layer of the nanoparticles is believed to be composed of some canted spins [46] having negligible magnetization (dead layer). The observed variations in M values can be explained regarding that this surface effect is more noticeable for the smaller particles having more surface to volume ratio. Besides the chemical composition and the size effects, Ms of the nanoparticles is also governed by the sites occupancy of metal ions

affecting the exchange interactions. According to Néel's two-sublattice model of a ferrimagnetic material with collinear structure, the magnetic moment per formula unit (ηH) is the difference between the resultant magnetic moments of the two sublattices (ηA and ηB). The resultant magnetic moment of each sublattice in the involved samples was calculated regarding the related cation occupancy of each one (see Table 2) and using the approximate ionic magnetic moment values of: Fe3+: 5 μB, Fe2+: 4 μB, Ni2+: 2 μB and Ni3+: 1 μB [47]. On the other hand, the magnetic moments per formula unit were also obtained based on the observed Ms values as ηH(exp) = Mw Ms , where Mw is the moleNa μB

cular weight of the compound. The calculated ηH values and the ones obtained from the experimental Ms data are given in Table 4. Their comparison confirms the applicability of Néel's model for the involved Ni substituted nanoferrites. (The deviation of the experimental values from the calculated ones may be due to the presence of the dead surface layer on the nanoparticles.) As seen from Fig. 7b, the dependence of Ms and the particle size of the samples on their Ni content match well, while Ms variations do not follow that of ηH. This indicats that the size effects dominate the role of the sites occupancy in controlling the Ms values of our nanoparticles. The existence of a canted layer on the surface of the involved

Fig. 8. Room temperature M-H loops of NixFe3−xO4 nanoparticles. The upper insets show their enlarged view around the origin and at high fields. The lower inset is the plot of M versus 1/H; The lines are the fitted second order polynomial functions. 7

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nanoparticles may be confirmed by determining their magnetic diameter [14,48]. The maximum magnetic diameter (DM) of the synthesized NixFe3−xO4 nanoparticles estimated based on the initial slope of M(H) curves near the origin is given in Table 4. This is the largest magnetic diameter, as the primary contribution to the initial slope for superparamagnetic particles originates from the largest ones [49]. So, the average magnetic size of the nanoparticles would be certainly smaller than the mean particle size (TEM results, with the estimated error). This can indicate the presence of a non-magnetic layer on the particles surface.

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4. Conclusion The nanoparticles of Ni-substituted ferrites (NixFe3−xO4, x ≤ 1.5) with diameters of 7.5–13 nm and good crystallinity were synthesized through a simple one-step capping agent-free coprecipitation route. The structural characterization demonstrates the formation of single-phased cubic spinel Ni nanoferrites whose lattice parameter and inversion grade decrease from a = 8.374 to 8.333 Å and from i = 0.89 to 0.34 with the Ni content. The FTIR spectroscopy of the nanoparticles exhibiting a doublet band around 600 cm−1 corresponding to the tetrahedral groups confirms further the mixed spinel structure of the involved nanoferrites. The optical characterization indicates that the Ni ferrite nanoparticles are indirect band gap materials with the indirect band gap energies of 1.06–1.17 eV and the higher energy direct-gaps of 2.50–2.58 eV whose variations on Ni substitution follow their size variations. The synthesized nanoparticles showed a superparamagnetic behavior at room temperature with superior saturation magnetization values of about 68–70.5 emu/g. The size effects dominate the role of the sites occupancy determining the resultant magnetic moment in controlling the magnetization values of our Ni ferrite nanoparticles. Resuming, this work has not only presented promising magnetic materials for different applications, but has also contributed to understanding the role of the composition factor (Ni and Fe atomic ratio) and the size effects in tuning different properties of these nanoparticles. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] J. Kudr, Y. Haddad, L. Richtera, Z. Heger, M. Cernak, V. Adam, O. Zitka, Magnetic nanoparticles: from design and synthesis to real world applications, Nanomaterials 243 (29) (2017). [2] A. Akbarzadeh, M. Samiei, S. Davaran, Magnetic nanoparticles: preparation, physical properties, and applications in biomedicine, Nanoscale Res. Lett. 144 (13) (2012). [3] D. Gherca, A. Pui, N. Cornei, A. Cojocariu, V. Nica, O. Caltun, Synthesis, characterization and magnetic properties of MFe2O4 (M = Co, Mg, Mn, Ni) nanoparticles using ricin oil as capping agent, J. Magn. Magn. Mater. 324 (2012) 3906–3911. [4] K. Maaz, S. Karim, A. Mumtaz, S.K. Hasanain, J. Liu, J.L. Duan, Synthesis and magnetic characterization of nickel ferrite nanoparticles prepared by co-precipitation route, J. Magn. Magn. Mater. 321 (2009) 1838–1842. [5] S. Joshi, M. Kumar, S. Chhoker, G. Srivastava, M. Jewariya, V.N. Singh, Structural, magnetic, dielectric and optical properties of nickel ferrite nanoparticles synthesized by co-precipitation method, J. Mol. Struct. 1076 (2014) 55–62. [6] S. Yáñez-Vilar, M. Sánchez-Andújar, C. Gómez-Aguirre, J. Mira, M.A. Señarísrodríguez, S. Castro-garcía, A simple solvothermal synthesis of MFe2O4 (M = Mn, Co and Ni) nanoparticles, J. Solid State Chem. 182 (2009) 2685–2690. [7] S.H. Lafta, Effect of pH on structural, magnetic and FMR properties of hydrothermally prepared nano Ni ferrite, Open Chem. 15 (2017) 53–60. [8] G. Blasse, Magnetic compounds with spinel structure, Philips Tech. Rev. 28 (1967) 23–30. [9] P. Lahiri, S.K. Sengupta, Physico-chemical properties and catalytic activities of the spinel series MnxFe3–xO4 towards peroxide decomposition, J. Chem. Soc., Faraday Trans. 91 (1995) 3489–3494. [10] A. Goldman, Modem Ferrite Technology, second ed., Springer, Pittsburgh (USA), 2006.

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