Journal of Nano Research Vol. 17 (2012) pp 99-114 Online available since 2012/Feb/03 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/JNanoR.17.99
Mg0.50Cu0.5-xNixFe2O4 Spinel Nanoferrites: Structural, Electrical, Magnetic and Y-K angle Studies Ahmad Faraz, Asghari Maqsood, Nasir M. Ahmad*, Fazal-Ur-Rehman, Shahid Ameer Department of Materials Engineering, School of Chemical and Materials Engineering (SCME) National University of Sciences and Technology (NUST) NUST H-12 Campus, Islamabad-44000, Pakistan *
Address correspondence to: Prof. Nasir Ahmad E-mail:
[email protected] Tel: +92+51+90855213
Keywords: Coercivity; magnetic moment; Mg0.50Cu0.5-xNixFe2O4, magnetization; nanoferrite; Y-K angle.
Abstract Spinel Nanoferrites of composition Mg0.50Cu0.5-xNixFe2O4 (0.00≤x≤0.50) were synthesized by chemical co-precipitation method. The structural, morphological and magnetically changes due to varying concentrations of metal ions of Cu and Ni in the prepared nanoferrites were studied. XRD confirmed the formation of single phase spinel ferrite with crystalline sizes in between 16-29 nm, and the lattice parameter (a) found to decreases with increase of Ni concentration. Electrical resistivity of the prepared nanoferrites with varying nickel and copper concentrations x observed to follow Arrhenius relation and also exhibited the semiconductor behavior. The magnetic hysteresis curves clearly indicate the soft nature of the prepared samples. Saturation magnetization (Ms) increases with Ni content. This effect is related to the magnetic moments of Ni+2 ions. The Y-K angles increase with increasing Ni content, and suggest a non-collinearity Néel type of ordering of the Y-K type. The increase in the Y-K angles also suggests the increase in triangular spin arrangements on B sites, which subsequently lead to increment in A-B interactions. 1 Introduction As the spinel ferrites size approaches nano scale its bulk properties changes remarkably. Considering this, in recent years, there are tremendous research efforts going on to synthesize and characterize novel ferrites of nanometric scale. In this direction, several phenomenons are attracting the attention of researcher such as magnetism, spin canting, cation distribution, Ferro fluids, Curie temperature, etc. Due to this, in many ways, spinel nanoferrites are unique as their chemical, physical, electrical and spontaneous magnetism characteristics can be tuned by various parameters such as particles size, porosity, processing conditions, method of preparation and composition [1-2]. Among these, choice of a specific composition for any particular nanoferrite is very critical to determine its ultimate properties and performance. Materials of the same compositions but with very different properties thus can be prepared. This is due the fact that in spinel ferrites MeFe2O4 (where Me can be a wide variety of metal cations, such as Fe, Co, Mg, Mn, Cu, Ni, etc.), metal ion Me can occupy few or all of the available tetrahedral site (A-site) and/or octahedral (B-site) arrangements in the lattice [3]. Depending on the cation distribution over the different crystallographic sites, the spinel compounds can be generally classified into two categories: normal and inverse spinels [4]. Normal spinels have the general formula At(B2)oO4 and contain all the trivalent metal ions (B3+) in the octahedral sites (o), whereas all the divalent metal ions (A2+) reside in the tetrahedral sites (t). In the inverse spinel, the divalent cations (A2+) and half of the trivalent cations (B3+) occupy the octahedral sites, whereas the other half of the B3+ metal ions lie in the All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 119.154.40.173-09/03/12,15:03:56)
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tetrahedral sites. It is also possible that metal ions can randomly distribute over both the tetrahedral and the octahedral interstices to yield mixed spinels. Thus using various metal ions, a wide range of compositions of spinel-ferrites can be developed and simple composition of MeFe2O4 ferrites exhibit characteristics, which can be tuned by the formation of mixed metal ions using various synthesis procedures, compositions, sintering temperatures and time, concentrations of Fe2+ and Fe3+ ions, choice of the other metal cations and their distribution between tetrahedral (A) and octahedral (B) sites in the spinel lattice. For example, MgFe2O4 has a spinel structure which is mostly of normal-type [5]. Contrary to this, NiFe2O4 has a fully inverse spinel structure to exhibit ferrimagnetism, which originated from magnetic moments of anti-parallel spins between Fe3+ ions at tetrahedral sites and Ni2+ ions at octahedral sites [6]. Similarly, CuFe2O4 is a unique spinel because of its cation distribution over the octahedral and tetrahedral sites is variable and dependent on temperature [7]. Considering above, it would be interesting to investigate the compositions of those ferrites, which contain the combination of Mg, Cu and Ni to synthesize Mg0.5Cu0.5-xNixFe2O4 (0.00≤x≤0.50) nanoferrites, and study their structural, electrical and magnetic characteristics. The substitution of Cu and Ni can lead to interesting effects in general properties such as electrical, magnetic, Curie temperature, and can provide opportunities for potential applications in area such as sensors, transducers, actuators, etc. In addition, the fraction of Cu ions occupying A-sites can be changed by heat treatment to produce a variation of A-B exchange interaction, and hence can affect the Curie temperature (Tc) [8].In the current work, we adopted an alternative approach to tune the structural and magnetic properties of ferrites by incorporating foreign ions, which have preference for either the A or the B sites. We employ substitution of Cu ions by Ni ions that has a strong B site preference and therefore interesting from this perspective. Finally, use of Mg is particularly interesting as well, because Mg ions is known to occupy the tetrahedral A-sites [9], and it is one of the most important soft magnetic semiconducting materials of important applications in areas such as sensors and magnetic technologies [9]. In view of above, current work focuses on the preparation of relatively complex ferrites and study of their structural and magnetic properties by the simultaneous substitution of Cu and Ni ions in the system Mg0.5Cu0.5-xNixFe2O4. To the best of our knowledge, mostly complex compositions based on Zn have been reported earlier [10]; however, this is the 1st report on the detailed syntheses, structural and magnetic properties of Mg0.5Cu0.5-xNixFe2O4 (0.00≤x≤0.50) nanoferrites. Current compositions can be considered more complex as compare to previously reported compositions based on Zn, which itself is a non-magnetic and known to exclusively occupy site A in the spinel structure. The substitution of Cu and Ni ions in ferrites of Mg may lead to the modification of the structural as well as possible cations distributions, electrical and magnetic properties. Detailed electrical and dielectric properties of these ferrites will be the subject of future work. In the current work, synthesis, compositional, structural, morphological, dc resistivity and magnetic properties of Mg0.5Cu0.5-xNixFe2O4 (0.00≤x≤0.50) have been discussed. Nanoferrites of Mg0.5Cu0.5-xNixFe2O4 with systematic variation of Ni concentration x were prepared by chemical co-precipitation method. The characteristics of the prepared nanoferrites were studied by X-rays diffraction (XRD), scanning electron microscope (SEM), FTIR, resistivity and vibrating sample magnetometer (VSM) to study the structural, morphological, electrical and magnetic changes taking place with varying Ni concentrations x in the composition of the nanoferrites. The present work has as also specific aim to discuss the influence of the distribution of the metallic ions on the tetrahedral and octahedral sites upon the various magnetic parameters, such as Ms, Mr, Ms/Mr and Y-K angle in mixed ferrites containing copper, magnesium, and nickel ions.
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2. Experimental 2.1 Synthesis A series of Mg0.5Cu0.5-xNixFe2O4 (0.00≤x≤0.50) nanoferrites were prepared using analytical grade reagents and chemical co-precipitation method. Experimental synthesis procedure involved the preparation of aqueous solutions of Ni (NO3)2.6H2O, Mg(NO3)2.4H2O , Cu(NO3)2.1/2H2O and Fe(NO3)3.9H2O in alkaline medium of NaOH. NaOH with 3M concentration was used as the coprecipitation agent. Separate solutions of Ni (NO3)2.6H2O, Mg(NO3)2.4H2O and Cu(NO3)2.1/2H2O (with total concentration of 0.1 M), and Fe(NO3)3.9H2O (0.2M) were prepared in 100 ml of deionized water. An example to prepare ferrites with the composition of Mg0.5Cu0.4Ni0.1Fe2O4 (with x = 0.10) is outlined below. For this specific composition, the stiochiometric reagent ratio of 0.05 M of Mg(NO3)2.4H2O, 0.04 M of Cu(NO3)2.1/2H2O and 0.01M of Ni(NO3)2.6H2O and 0.2 M of Fe(NO3)3.9H2O were used. These reagents were taken separately in 100 ml de-ionized water followed by mixing the prepared solutions in a beaker at 85 °C with constant magnetic stirring until a clear solution was obtained. Solution of co-precipitating agent of NaOH (3M) was made separately in 100 ml of de-ionized water and heated up to 85 °C. The solution of co-precipitating agent then mixed rapidly with the prepared salt solutions. It is important to mention here that rapid mixing of metal solution with co-precipitating agent gives relatively smaller size and homogenous nano-particles. For transforming metallic hydroxide into ferrites, temperature was increased to 310 °C. After mixing metallic solution with NaOH solution, both solutions were heated and stirred for further 40 minutes. This was followed by stoppage of heating while constant stirring remained continue for further three hours. The pH value was kept between 11.3-12.3. Precipitates were washed 5 times with de-ionized water. Products were dried with the help of electric oven over night at 110 °C. At the end, a black colored ferrite powder was obtained. Similarly, all remaining compositions were synthesized by varying the desired stoichiometric amounts of salts. Dried samples were sintered in a furnace for about six hours at 800 °C followed by cooling of the furnace at 10 °C/min. 2.2 Analysis and characterization The prepared Mg0.5Cu0.5-xNixFe2O4(0.00≤x≤0.50) ferrites were characterized by various techniques. Structural and related characteristics of the samples were determined by X-ray diffraction (XRD) using a Stoe diffractometer to determine the lattice parameter, phases, particle size and x-rays density etc. XRD patterns were obtained using CuKα (λ = 1.5406 Å) radiation at room temperature. The mean crystallite sizes were estimated using the standard Scherer formula. Further analyses were then performed on the XRD data to obtain the lattice constants, crystallite size, and XRD density ρx using the standard relations as described elsewhere [11]. Transmission IR spectra were obtained by Perkin Elmer FTIR spectrometer using KBr pellet. Morphology of the nanoferrites was studied using scanning electron microscopy (SEM), Jeol JSM 6490A by depositing gold coating on their circular discs. The dc electrical resistivity (ρdc) was measured by a custom-made two probe method set-up in the temperature range of 373-573 K for various compositions of Mg-Cu-Ni nanoferrites. Disc shaped samples of approximately 8 mm diameter and 3 mm thickness were used for measurements. Standard Arrhenius relation was used to obtain dc electrical resistivity (ρdc) at specific temperatures. From the slope of the Arrhenius plot, activation energy data was obtained [13].Magnetic measurements were made using a commercial vibrating sample magnetometer (VSM) model, BHV-50, Riken Denshi Co. Ltd. Japan.
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3. Results and discussion 3.1 Structural and morphological studies The indexed XRD patterns of the prepared samples of Mg0.5Cu0.5-xNixFe2O4 are presented in Fig1. The presence of planes (220), (311), (222), (400), (422), (440), and (511) in the diffractograms confirmed the cubic spinel structure. As it is evident from the figure that Mg0.5Cu0.5-xNixFe2O4 with varying compositions exhibit single phase spinel structure without showing any other detectable impurity. The peaks broadening indicate lower crystallite size of the synthesized samples. The average crystallite size for each composition was calculated using the standard Scherer formula by taking half peak width at maximum for all observed peaks and then average particle size of all peaks of the sample was obtained [12]. The crystallite size remained within the range 16–27 nm and the particle size gradually increases as the Ni concentration is increased. This can be explained on the basis of cation stoichiometry. In a complex spinel ferrites like the current Mg0.5Cu0.5-xNixFe2O4 ferrites, there are several cations are incorporated in spinel structure. Considering this, the nucleation and growth of the nano-particles can be affected by the probability, capability and affinity of cations to occupy available in equivalent sites [13]. XRD data was also used to calculate various characteristics of the prepared nanoferrites such as lattice constant (a), average crystallite size (t (ave)), volume of the cell (V), x ray density (ρx), and porosity (P) using standard relationships as discussed elsewhere [11,14]. Table 1 summarized the values of these parameters for different compositions. The variation of crystallize sizes with composition can also be discussed with respect to the ionic radii of metal ions of Cu+2 (0.73Å) and Ni+2 (0.69Å) [15]. It is observed that the lattice constant decreases with the increase of Ni concentration, and thus can be attributed to the smaller ionic radii of Ni+2relative to Cu+2. Therefore, the decrease of the lattice constant a is a consequence of the replacement of the larger Cu+2 by the smaller Ni+2 and in good agreement with related studies [16].
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Fig. 1. The XRD patterns of Mg0.5Cu0.5-xNixFe2O4 nanoferrites with varying concentrations (x).
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Table. 1: Summary of results of various properties of Mg0.5Cu0.5-xNixFe2O4 (0.00≤x≤0.50): lattice constant (a), volume (V), measured density (ρm), X-ray density (ρx), porosity (P), activation energy (∆E), correlation factor (R), electrical properties of dc resistivity (ρdc), drift mobility (µd), and magnetic parameters of saturation magnetization (Ms), remanent magnetization (Mr), squareness ratio (Mr/Ms), and coercivity (Hc), Bohr magneton (nB), and Y-K angle (deg). Ferrites Composition Mg0.5Cu0.5-xNixFe2O4
x=0.00
x=0.10
x=0.20
x=0.30
x=0.40
x=0.50
a(Å) V(Å)3 ρm (g cm-3)
8.47 607 3.21
8.41 599 3.38
8.37 596 3.56
8.32 592 3.71
8.28 586 3.84
8.24 580 4.13
ρx (gcm-3) P (fraction) ρdc (×106 Ω-cm)
5.23 0.39 3.21
5.26 0.36 4.32
5.29 0.34 5.42
5.36 0.31 6.17
5.42 0.30 7.41
5.51 0.26 8.17
0.33
0.30
0.29
0.24
0.22
0.21
0.98
0.98
0.98
0.98
0.98
0.98
16.2
13.4
10.6
8.26
7.51
6.42
Hc
90.5
86.4
82
77.6
73.2
68.8
Ms Mr Mr/Ms nB Y-K angle (deg)
11.79 2.23 0.19 0.09 32.85
12.86 2.34 0.18 0.097 41.4
13.98 2.437 0.17 0.10 48.71
15.15 2.52 0.16 0.11 55.24
16.21 2.61 0.16 0.12 61
17.27 2.71 0.15 0.13 66
Parameter
(373 K) ∆E (eV) R µd (x10
-11
2
cm /(V.s)
(373K)
Fig. 2(i) represents variation in lattice constant and x-rays density with composition x. Both the X-ray and measured densities are found to be dependent on the Ni concentration and increases with its concentration. Densities of ferrites are dependent on the ionic size and atomic mass of the metal ions. The Ni atom (58.69 amu) is lighter than the Cu atom (63.55 amu), while the size of the Ni2+ ion is also smaller than that of the Cu 2+ion. Considering this, although Ni is lighter than Cu to contribute to decrease in density, however, in the current work it seems that smaller ionic radii of Ni ions may have contributed more to produce the increase in the observed density. Due to similar reasons, the porosity of the prepared ferrites found to decrease with Ni concentration x, and thus again can be attributed to the difference in ionic radius of Cu and Ni as well as higher density and smaller particle size (see Table 1). Fig.2(ii) represents a relationship between measured density (ρm) and X-rays density (ρx) vs. concentration.
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Fig. 2. Variation of physical properties of Mg0.5Cu0.5-xNixFe2O4 ferrites with Ni concentration x: (i) lattice constant (a) and X-ray density (ρx), and (ii) measured and X-ray density. In addition to structural studies, morphological analyses were carried out with the aid of scanning electron microscopy (SEM). A representative SEM micrographs of Mg0.5Cu0.5-xNixFe2O4 (x= 0.0) nanoferrites is presented in Figure 3, and average size determined by the SEM is found in good agreement with that obtained from XRD technique. Typically, at high resolutions (~ 100,000) the dimension of the particle observed to increase with Ni concentration and found thus in agreement with the XRD study.
Fig. 3. Scanning electron microscopy (SEM) of the nanoferrites of Mg0.5Cu0.5-xNixFe2O4 with concentration of x =0.00 at low (i) and high (ii) resolution
3.2 Infrared Spectroscopy Studies Study of the vibrational spectra of ferrites provide a useful tool to understand their properties as later depends on the precise configuration of the ions in their crystal lattice. It should be emphasized that MgFe2O4 can be considered as mostly a normal mixed spinel, whose structure is more precisely represented by the formula MgIIO[FeIII2O3], while NiFe2O4 belongs to the class of
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fully inverse spinels, whose structure is more precisely represented by the formula FeIIIO[NiIIFeIIIO3], where brackets enclose the ions in the octahedral interstices, and roman numbers in superscripts indicate the metal atom oxidation state. Furthermore, structure of CuFe2O4 can be of mixed types, where Cu ions can occupy both the tetrahedral and octahedral sites. This means that, during the course of preparation of ferrites of Mg0.5Cu0.5-xNixFe2O4, x=0.00 to 0.50, there is an induction in the structural changes in both the octahedral and tetrahedral units of considered samples. In the current work, IR spectra of the prepared ferrites were examined in the frequency range of 350-1500 cm-1 as shown in Fig4. Two main broad metal–oxygen bands are important in the IR spectra of all spinels, and specifically in ferrites [18-20]. The IR band, ν1, usually observed in the higher frequency range of 600–550 cm−1. The ν1 band corresponds to intrinsic stretching vibrations of the metal atom at the tetrahedral site, Mtetra ↔ O. The second band is ν2, a lowest frequency band which is observed in the range of 450–385 cm−1. This is assigned to the octahedrally-metal atom stretching vibrations, Mocta↔ O. As shown in Figure 2, both ν1 and v2 stretching vibrations were observed with the normal mode of vibration of tetrahedral cluster is higher than that of octahedral cluster. This can be attributed to the shorter bond length of tetrahedral cluster and longer bond length of octahedral cluster. Ni2+ ions have the preference to occupy octahedral-site, while Mg+2 possesses much larger preference to tetrahedral geometry than Fe+3, and Cu+2and these later ions can occupy both the octahedral and tetrahedral sites. Both ν2 and ν1 vibrational bands observed are the characteristics of the prepared ferrites [18-20].
Fig. 4. IR spectra of the prepared nanoferrites of Mg0.5Cu0.5-xNixFe2O4 with varying Ni concentrations. ν1and ν2 designate high and low energy frequency bands. 3.3 DC Electrical Properties Standard Arrhenius relation in accordance to Eq. 1 was used to obtain dc electrical resistivity (ρdc) at specific temperatures. From the slope of the Arrhenius plot, activation energy data was obtained [13]: ∆
ρ = ρ exp
(1)
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Where ρT is electrical resistivity at a particular temperature T and ρo is the resistivity at room temperature, kb (8.616 eV/K) is the Boltzmann constant, T is the absolute temperature and ∆E is the activation energy. In general increase in temperature results in increment in the resistivity of metals, while the semiconductor exhibit opposite behavior with temperature. The prepared Mg0.5Cu0.5-xNixFe2O4 are insulators to exhibit semiconductor like characteristics with the increase of temperature. The resistivity behavior is investigated in the temperature range of 373–573 K from the consideration of influence of Ni concentrations x in the nanoferrites of Mg0.5Cu0.5-xNixFe2O4. Table 1 summarized the electrical properties of the nanoferrites Mg0.5Cu0.5-xNixFe2O4. Electrical resistivity behavior of the prepared nanoferrites of Mg0.5Cu0.5-xNixFe2O4 with varying nickel and copper concentrations x observed to follow Arrhenius relation in accordance to Equation (1) as presented in Figure 5. It is well known that the electrical resistivity (ρdc) of ferrite decreases with increase in temperature and thus confirmed the semiconductor nature of the prepared nanoferrites in the present work [17-18]. This observation is attributed to the increase in the thermally activated drift mobility (µd) of charge carriers in accordance to the conduction mechanism of the hopping of electron [19]. In the hopping conduction mechanism, the charge carrier mobility is temperature dependent, and band-gap between the conduction band minima and valence band maxima can be tuned by appropriate choice of a dopant. The later characteristics is clearly exhibited by incorporating Ni in the prepared nanoferrites and indicated by the decrease in resistivity as x content is increased. The observed extent of variation of ρdc in the current work also found to highly dependent on temperature. The variation in resistivity of the prepared ferrites is in agreement with related studies of ferrites which indicated that addition of certain metal ions resulted in change of their resistivity values [20-22]. The above mentioned observations regarding the ρdc in the prepared nanoferrites can be explained from the consideration of the Verwey mechanism due to electron hopping between Fe+2 tetrahedral (A-site) and Fe+3 at octahedral site (B-site) [23-24]. Cations which are capable to promote the increment in this transition, thus results in increases in the conduction. Therefore, the observed decrease in the dc resistivity (ρdc) with the increase of Ni may be attributed to the presence of relatively higher electron hopping between Fe+2 ions in A-sites and Fe+3 ions in B-site ions. The improvements in resistivity upon increment in Ni contents appeared to the replacement of the Fe+2-Fe+3 conduction mechanisms by a more complex conduction mechanism. Presumably, replacing Fe and/or Ni ions with the addition of Cu and Mg ions seem to eliminate the easy conduction paths otherwise provided by the former ions. In the current work, ∆E values were also determined using Equation (1) from the slopes of the Arrhenius relationship in the linear plots of ln ρdc against 1/kbT given in Figure 1 and values are summarized in Table 1. ∆E values found to decrease approximately linearly with concentrations x increase in Mg0.50Cu0.5-xNixFe2O4 (0.00≤x≤0.50). The values of ∆E decrease from 0.33 to 0.21 eV, which correspond to about 36% change as the Ni concentration increased from 0.0 to 1.0. In ferrites, the activation energy is not dependent on concentration of charges but associated with the variation of charge carrier mobility localized at the ions or vacant sites to give electrical energy to overcome the barrier experienced by the electrons during the process of conduction via the hopping mechanism [19].
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Fig. 5. Linear dependence of temperature on the electrical resistivity (ln ρdc) in accordance to Arrhenius plot for the nano-ferrites of Mg0.5Cu0.5-xNixFe2O4 (0.00≤x≤0.50): (i) Graph shows the resistivity as a function of temperature at various concentrations x. (ii) Straight lines are the best fit to Equation 1; 3.4 Magnetic properties Fig. 6 represents the variation of magnetization M (emu/g) versus the applied magnetic field H (Oe) for the prepared Mg0.5Cu0.5-xNixFe2O4 (0.00≤x≤0.50) spinel nanoferrites. Vibrating sample magnetometer (VSM) was used to obtain hysteresis loops at room temperature. The curves appear to behave normally, and the soft nature of the ferrite nano-materials is indicated from the narrow loops. A clear hysteretic behavior under applied magnetic field was observed. With increasing in the strength of applied magnetic field, the magnetization also observed to increase and reaches maxima at 1.25 Tesla. Dependence of the various magnetic parameters such as saturation magnetization (Ms), remanence (Mr) and coercivity (Hc) are calculated from the hysteresis loop. These parameters are summarized in Table 1, and schematically represented by Fig. 6 as a function of Ni concentration x.
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Fig. 6. Room temperature magnetic hysteresis curve of Mg0.5Cu0.5-xNixFe2O4 (0≤x≤0.5). From the magnetization curve at Hmax, the saturation magnetization Ms values were obtained. Figure 7(i) indicates that Ms increases with x. The remanent magnetization Mr was determined by the magnetization curve at H = 0.0 Oe, and its dependence on x is shown in Figure 7(i). Both Table 1 and Fig. 7(i) indicate that the Ms and Mr increase as Ni concentration increased. This can be explained on the basis of Neel’s theory [17]. The results obtained are also in reasonable agreement to previous studies of related ferrites including recent studies of MgFe2-xCrxO4 and Ni1-xCuxFe2-yMnyO4 [18-19]. Respective magnetic moment of nickel ferrite and copper ferrite are 2.3 and 1.3 [20]. Thus, the substitution of Ni of higher magnetic moment in the ferrite is expected to enhance the value of Ms in the prepared for Mg0.50Cu0.5-xNixFe2O4. The observed variation in magnetization can also be explained from the consideration of the cation distribution and the exchange interactions between A and B-sites in a spinel lattice [20]. In the Mg0.5Cu0.5-xNixFe2O4 ferrites, the stable Ni2+ ions occupy the B-sites only, while Cu ions can occupy both A and B sites. Therefore, substitution of Cu2+ with Ni2+ ions on the octahedral B-sites produce the lowest value of the saturation magnetization for all compositions. The observed decrease in MS with Cu ions can be discussed if one assumes the cation distribution that involved forcing out of certain Fe3+ ions from B-site to A-site by Cu2+ ion. It may be possible that Mg3+ ions can also migrate between A and B-sites due to heat treatment. So, the Ms for the investigated samples can be attributed to the decrease in the concentration of Fe3+cations at (B) sites to produce a reduction in the number of magnetic Fe3+(A)–O2–Fe3+ [B] linkages. This effect subsequently can lead to the weaking of A-O-B super exchange interaction. Therefore, the saturation magnetization appeared to depend in a complex way on the cation distribution in the prepared samples. The remanence ratio (Mr/Ms) as a function of concentrations x is illustrated in the Figure 7(ii). This ratio expresses the squareness (SQR) of the hysteresis loop, and indicates complex behavior in the prepared ferrites. The values of Mr/Ms were found to small, and may suggest that a significant amount of nanoparticles are still superparamagnetically fast relaxing at room temperature, when the external magnetic field is turned off. The SQR of the prepared ferrites is equal to 0.17±0.02. When the value of SQR is bigger than zero, it can be useful in applications such as in permanent magnets and recording media of high density.
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The coercive force (Hc) values were also obtained for the prepared ferrites and observed to be in between 90-68 Oe (see Fig. 7(iii). The coercivity studies are critical to determine the strength of the magnetic field that allows the magnetization of the nanoparticle by surpassing the anisotropy barrier. A lower applied field is required for spin reversal for lower coercivity Hc as it lower the anisotropy and the activation energy barrier in the nanoparticles. The role of temperature is also crucial since in superparamagnetic specimens, the corcivity Hc tends to disappear with temperature [21]. In the current work, however, it is present even at room temperature to suggest that anisotropy in the prepared Mg0.5CuxNi0.5-xFe2O4 nanoparticles. The Hc values obtained are also found to decrease with concentrations x. The magnetic properties observed for the prepared nanoferrites are due to combination of several mechanisms to influence surface anisotropy. Since the prepared nanoparticles possess variable sizes, porosities, and densities, therefore these parameters may have differently influenced the contributions of the inter particle interactions and surface anisotropy to the net anisotropy to able to induce the notable decrease in the Hc values with concentrations x [18]. Bohr magneton constant (nB), which express an electron magnetic dipole moment, was also calculated in accordance to Eq. (1) [21] and results obtained are shown in Fig. 7(iv) and given in Table 1.
=
. .
.
(1)
Where M.W, Ms and ρm are molecular weight, saturation magnetization and measured density, respectively. In order to discuss magnetic moment (nB) of spinel ferrites, there is a need to consider octahedral-tetrahedral (A-B) interactions and distribution of cations. It is beyond the scope of the current work to go into the details of these two effects. Nonetheless as mentioned earlier, it is well known that Ni2+ ions, have strong preferences for Octahedral site (B-site), while Cu+2 can occupy both A and B sites. On substitution of Ni for Cu in Mg0.50Cu0.5-xNixFe2O4, Ni ions can force an equal amount of Fe+3 ions, which have the preference for B-site.
Using nB values, the Yafet–Kittel (Y–K) angles have been also calculated for concentration x [21-23]:
nB = (6 + x) cos αY-K – 5 (1-x)
(2)
The vales of Y–K angles are given in Table 1 and effect of composition on Y-K angle at room temperature is represented in Fig. 7(iv). The occurrence of noncollinear arrangements in ferrites systems has been suggested by Yafet and Kittel [22]. The non-zero Y-K angles values suggest that the compositional variation of Ms cannot be explained in accordance to Neel two sub-lattice model [21]. In the prepared ferrite compositions, the Y-K angles increases with the increase in concentration x. Therefore, Cu and Ni substitution brings change in magnetization, which can be attributed to the presence of Y-K angles in the spin system on B sites. In other compositional series, such as Ni-Zn ferrites, the condition for Y-K angles to occur has been discussed from the consideration of the molecular field approximation using a noncolinear three sub-lattice model. In the current work, the decrease in the Y-K angles suggest a decrease of triangular spin arrangements on B sites to produce the increment in A-B interaction. The effect of B-B interaction can be masked by strong A-B interaction to cause the spin on B sites to be aligned parallel to each other. However, the substitution of Ni seems to lead to canted type of arrangements
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on B sites to enhance the A-B interactions. This indicates that in the present compositions of ferrites, randomness is affected as Ni is substituted and thus exhibits a departure from Neel collinear model. These results are also in conformity with earlier studies, with the observations of variation of A-B interactions in two sub-lattices because of substitution of various metal ions in mixed ferrites [21-23].
Fig. 7. Magnetic properties of Mg0.5Cu0.5-xNixFe2O4 (0 ≤ x ≤ 0.5): (i) Saturation magnetization (Ms) and remanence, Mr vs. x; (ii) Mr/Ms vs. x; (iii) coercivity (Hc) vs. x; (iv) Y-K angle and Bohr magneton (nB) vs. x.
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4 Conclusions Co-precipitation method was used for the synthesis of nanoferrites of Mg0.5Mg0.5-xNixFe2O4 with concentration 0.00≤x≤0.50. The prepared nanoferrites were characterized for their structural, morphological, and magnetic properties. XRD confirmed the formation of single phase spinel ferrite structure with size of the nanoferrites in the range of 16-29 nm. The lattice constant and porosity were found to decrease with decrease in Cu contents. These observations are attributed due to the relative larger ionic radii of Cu2+ as compared to Ni+2. It is noted that the prepared ferrites are insulators but exhibited semiconductor like characteristics with the increase of temperature. The electrical resistivity behavior of the prepared nanoferrites with varying nickel and copper concentrations x observed to follow Arrhenius relation. The improvements in resistivity upon increment in Ni contents appeared to the replacement of the Fe+2-Fe+3 conduction mechanisms by a more complex conduction mechanism. Hysteresis Loop indicates the soft nature of the prepared nanoferrites. Saturation magnetization (Ms), remanence (Mr), Hc and Bohr magneton (nB) are calculated from the hysteresis loop and found to increase with Ni concentration (X) in the prepared nanoferrites. Y-K angle strongly varies with composition and affects the curie temperature of synthesized material. The decrease in the Y-K angles with Ni concentration indicates the decrease in favor of triangular spin arrangements on B sites leading to increment in A-B interaction, which subsequently increase the Curie temperature as a result of replacement of Cu2+ ions in B sites by Ni2+ ions.
Acknowledgements Authors are thankful to Mr. Noor Ahmed, and Mr. Shmasudin from School of Chemical and Materials Engineering (SCME), NUST for their support to carry out characterization of the prepared samples.
References [1]. A. Goldman, Handbook of Modern Ferromagnetic Materials (The Springer International Series in Engineering and Computer Science) (Springer-Verlag, New York, 1999). [2]. M. A. Willard, L. K. Kurihara, E. E. Carpenter, S. Calvin, V. G. Harris, Chemically prepared magnetic nanoparticles, Int. Mater. Rev., 49 (2004)125-170. [3]. C. Venkataraju, G.Sathishkumar, K.Sivakumar, Effect of cation distributionon on the structural and magnetic properties of nickel substituted nanosized Mn–Zn ferrites prepared by co-precipitation method, J. Magnetism and Magnetic Materials, 322 (2010) 230–233. [4]. P. Muthukumarasamy, T. Nagarajan, A. Narayanasamy, Mossbauer study of copper-nickel and copper-manganese ferritesj. Phys. C: Solid State Phys., 15 (1982) 2519-2528. [5]. R. A. Candeia, M. A. F. Souza, M. I. B. Bernardi, S. C. Maestrelli, I. M. G. Santos, A. G. Souza, E. Longo, MgFe2O4 pigment obtained at low temperature, Materials Research Bulletin 41 (2006) 183-190. [6]. H. V. Kiran, A. I. Shashimohan, D. K. Chakrabarty, A. B. Biswas, Structural and magnetic properties of copper-nickel ferrites, Phys. Stat. Sol., A66 (1981) 743-747.
Journal of Nano Research Vol. 17
113
[7]. P. B. Pandya, H. H. Joshi, R. G. Kulkarni, Magnetic and structural properties of CuFe204 prepared by the co-precipitation method, J of Materials Science Letters 10 (1991) 474-476. [8]. P. Muthukumarasamy, T. Nagarajani, A. Narayanasamy, Mossbauer study of copper-nickel and copper-manganese ferrites. J. Phys. C: Solid State Phys., 15 (1982) 2519-2528. [9]. S. Maensiri, M. Sangmanee, A. Wiengmoon, Magnesium Ferrite (MgFe2O4) Nanostructures Fabricated by Electrospinning, Nanoscale Res. Lett. 4 (2009) 221–228. [10]. N. Chu, X. Wang, Y. Liu, H. Jin, Q. Wu, L. Li, Z. Wang, H. Ge, Magnetic properties of low Mn-doped NiCuZn nanocrystalline ferrites, J. of Alloys Comp. 470 (2009) 438-442. [11]. M. N. Ashiq, S. Saleem, M. A. Malana, Anis-Ur Rehman, Physical, electrical and magnetic properties of nanocrystalline Zr–Ni doped Mn-ferrite synthesized by the co-precipitation method, J. Alloys Comp. 486 (2009) 640–644. [12]. B. D. Cullity, Elements of X-ray Diffraction, Addison–Wesley Pub. Co. Inc., Reading, Mass., USA, 1956. [13]. A. Faraz, A. Maqsood, N. M. Ahmad, Synthesis, structural and electrical properties of Co1-xCuxFe2O4 spinel nanoferrites,Advances in Applied Ceramics, Accepted (2011). [14]. M. Ajmal, N. A. Shah, A. Maqsood, M. S. Awan, M. Arif, Influence of sintering time on the structural, electrical and magnetic properties of polycrystalline Cu0.6Zn0.4Fe2O4 ferrites, J. Alloys Comp. 508 (2010) 226–232. [15]. P.A. Jadhav, R.S. Devan, Y.D. Kolekar, B.K. Chougule,Structural, electrical and magnetic characterizations of Ni–Cu–Zn ferrite synthesized by citrate precursor method, J. Physics and Chemistry of Solids, 70 (2009) 396-400. [16]. C. S. Kim, W. C. Kim, S. Y. An, S. W. Lee, Structure and Mossbauer studies of Cu-doped Ni-Zn ferrite, Journal of Magnetism and Magnetic Materials 215-216 (2000) 213-216. [17]. A. T. Raghavender, K. Zadro, D. Pajic, Z. Skoko, N. Biliškov, Effect of grain size on the Neel temperature of nanocrystalline nickel ferrite, Materials Letters, 64 (2010) 1144-1146. [18]. A. Faraz, M. Saqib, N. M. Ahmad, Fazal-ur-Rehman, A. Maqsood, M. Usman, A. Mumtaz, M. A. Hassan, Synthesis, Structural, and Magnetic Characterization of Mn1−xNixFe2O4 Spinel Nanoferrites, J. Supercond. Nov. Magn., DOI 10.1007/s10948-011-1212-7. [19]. P. P. Hankare, V. T. Vader, N.M. Patil , S. D. Jadhav, U. B. Sankpal, M.R. Kadam,B.K. Chougule, N.S. Gajbhiye, Synthesis, characterization and studies on electrical and magnetic properties of Mg ferrite with Cr substitution, Materials Chemistry and Physics 113 (2009) 233–238. [20]. G. K. Joshi, A. Y. Khot, S. R. Sawant, Magnetisation, curie temperature and Y-K angle studies of Cu substitute and non-substituted Ni-Zn mixed ferrites, Solid State Communication, 65 (1988) 1593-1595.
114
Journal of Nano Research Vol. 17
[21]. J. G. S. Duquea, E. A. Souzaa, C. T. Menesesa, and L. Kubota, Magnetic properties of NiFe2O4 nanoparticles produced by a new chemical method, Physica B 398, 287-290 (2007). [22]. M. U. Rana, M. Ul-Islam, I. Ahmad, T. Abbas, Determination of magneticproperties and Y-K angles in Cu-Zn-Fe-O system, J. Magnetism and Magnetic Materials, 187 (1999) 242-246. [23]. N. S. S. Murthy, M. G. Natera, S. I Youssef, R. J. Begum, Yafet-Kittel angles in Zinc-Nickel ferrites, Physical Review, 181 (1969) 969-977. [24]. C. Venkataraju, G. Sathishkumar, K. Sivakumar, Effect of cation distribution on the structural and magnetic properties of nickel substituted nanosized Mn–Zn ferrites prepared by co-precipitation method, J. Magnetism and Magnetic Materials 322 (2010) 230–233.
Journal of Nano Research Vol. 17 10.4028/www.scientific.net/JNanoR.17
Mg0.50Cu0.5xNixFe2O4 Spinel Nanoferrites: Structural, Electrical, Magnetic and Y-K Angle Studies 10.4028/www.scientific.net/JNanoR.17.99
