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Substitution effect on structural, electrical and magnetic properties of NiFe2−2xAlxCrxO4 (x = 0–0.6) nano-crystalline ferrites via oxalate precursor route
Materials Research Bulletin 60 (2014) 433–440
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Substitution effect on structural, electrical and magnetic properties of NiFe22xAlxCrxO4 (x = 0–0.6) nano-crystalline ferrites via oxalate precursor route M.A. Gabal a, * ,1, A.Y. Obaid a , M. Abdel Salam a , W.A. Bayoumy b a b
Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 March 2014 Received in revised form 9 August 2014 Accepted 5 September 2014 Available online 6 September 2014
This manuscript reports on the structural, magnetic and electrical properties of Al–Cr co-substituted nickel ferrites; NiAlxCrxFe22xO4 (0.0 x 0.6), synthesized via oxalate decomposition route. The decomposition process was monitored using DTA-TG techniques, and the obtained ferrites were characterized using XRD, FT-IR, VSM, AC-magnetic susceptibility and electrical properties measurements. The single-phase structure was confirmed using XRD and FT-IR spectroscopic measurements. With increasing Al–Cr substitution, the obtained saturation magnetizations, via VSM measurements, showed a gradual decrease while coercivity exhibited a steady increase. The electrical properties as a function of temperature and frequency showed a semiconducting behavior with conductivity decreasing by increasing substitution, which enhances the use of these materials in microwave devices applications. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Magnetic materials A. Semiconductors C. X-ray diffraction D. Electrical properties
1. Introduction The interest of scientists and researchers toward ferrites materials with spinel structure arises from the interesting theoretical as well as technological applications in many fields, such as satellite communication, memory devices, computer components, antenna rod, transformers, etc. [1]. NiFe2O4 is an inverse spinel ferrite taken to be collinear ferrimagnet [2], where degree of inversion depends on the cooling rate and heat treatment. It has been the subject of many investigations under different treatment [3–5]. The effect of Cr-substitution in NiFe2O4 has received a lot of attention. Fayek and Ata Allah [6] reported that Cr3+ content occupy the octahedral sites for a maximum of x = 0.6, and the excess Cr3+ replaces the Fe3+ at the tetrahedral site. Mössbauer measurements [7] showed well-resolved magnetic spectra for the tetrahedral and octahedral sites, which suggest that as Cr substitution increases, the system slowly converted into a normal spinel structure. Singh et al. [8] synthesized spinel-type ternary
* Corresponding author. Tel.: +966 557071572. E-mail address: [email protected] (M.A. Gabal). 1 Permanent address: Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. http://dx.doi.org/10.1016/j.materresbull.2014.09.017 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
ferrites with composition NiFe2xCrxO4 (0 x 1) by a precipitation method. The samples were characterized using IR, XRD, BET surface area, and XPS techniques. Singhal et al. [9] have investigated the cation distribution in chromium substituted nickel ferrites, prepared by aerosol route, using XRD, magnetic and Mössbauer spectral studies. Chromium-substituted nickel ferrites synthesized through oxalate precursors [10] showed a decrease in the magnetic properties with increasing Cr content. In Al-containing ferrites, aluminum ions are preferred to occupy both tetrahedral (A) and octahedral [B] sites depending on the amount of Al, which will affect the amount of iron ions in the two sites [1]. NiFe2xAlxO4 ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements [11]. Nano-crystalline Al-doped nickel ferrites have been synthesized by sol–gel auto-ignition method, and the effect of magnetic dilution on the structural and magnetic properties has been studied [12]. The structural and electrical properties of NiFe2xAlxO4, synthesized through co-precipitation method, were discussed as a function of Al substitution [1]. In the literature, there are very few works dealing with Al and Cr co-substituted NiFe2O4. Chhaya et al. [13] studied crystal structure and the magnetic properties of the mixed spinel NiAlxCrxFe22xO4 (x = 0.0–0.9) using XRD, Mössbauer spectroscopy and magnetic susceptibility measurements.
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Polycrystalline Al and Cr co-substituted disordered spinel series NiAlxCrxFe22xO4 (0.1 x 0.9) were prepared through solid-state method and studied by XRD, magnetization and electrical resistivity measurements [14]. The aim of the present study is to prepare single-phase nano-crystalline NiAlxCrxFe22xO4 ferrites (0.0 x 0.6) through the thermal decomposition of their corresponding metal oxalates. The oxalate decomposition reaction was followed using thermal analysis techniques (DTA-TG). The structural characterization was investigated using XRD and FT-IR techniques. The magnetic properties were measured using vibrating sample magnetometer (VSM) and DC-magnetic susceptibility techniques. The electrical properties as a function of temperature and frequency were also characterized. The effect of the magnetic dilution on the electrical and magnetic properties was discussed. To the best of our knowledge, no systematic investigations of the structural, magnetic and electrical properties of Al and Cr co-substitution of NiFe2O4 are reported in the literature. In addition, the entire preparation method seems to be novel for the preparation of these types of ferrites. 2. Experimental 2.1. Synthesis of ferrites Nano-crystalline ferrites of the general formula NiAlxCrxFe22xO4 (0.0 x 0.6) were synthesized through thermal decomposition process. AR grade FeSO47H2O, Ni(NO3)26H2O and H2C2O42H2O were used as supplied for the preparation of individual metal oxalates; FeC2O42H2O and NiC2O42H2O through coprecipitation method [15]. The impregnation technique, previously described [16,17], was then used for the preparation of ferrites precursors. In this process, stoichiometric amounts of metal oxalates along with Al2O3 and Cr2O3 are weighed and thoroughly mixed in a porcelain mortar using drops of bi-distilled water. After drying the wet precursors in an electrical oven, they were annealed in a muffle furnace at 1000 C for 2 h under static air atmosphere. 2.2. Techniques Differential thermal analysis-thermogravimetry (DTA-TG) measurements were investigated in air atmosphere using a Shimadzu DT-60 thermal analyzer. The experiments were carried out at a heating rate of 5 C min1 up to 1000 C. The powder X-ray diffraction (XRD) analysis of the calcined precursors were conducted at room temperature using a D8 Advanced diffractometer, Bruker AXS, using Cu Ka1 radiation (l = 0.15406 nm). FT-IR spectra were measured in the range of 1000–200 cm1 using a Jasco FTIR 310 spectrometer. The hysteresis measurements at room temperature were performed using a vibrating sample magnetometer (VSM9600M) with a maximum applied field of 5 kOe. The DC-magnetic susceptibility measurements were carried out using Faraday’s method [10,16]. The measurements were performed up to 850 K as a function of different magnetic field intensities (1010, 1340 and 1660 Oe). The temperature dependence of electrical properties as a function of frequency (100 Hz–5 MHz) was measured by a Hioki 3531 LCR bridge using the two-probe method [10]. The samples were palletized, and the two surfaces of each pellet (1 cm in diameter and about 1 mm in thickness) are polished, coated with silver paste and checked for good conduction. A K-type thermocouple was used to measure the sample temperature with accuracy better than 1 C.
3. Results and discussion 3.1. Precursor decomposition and ferrite formation processes The thermal decomposition process of the entire precursors, up to ferrite formation, was monitored in air using DTA-TG measurements. Typical DTA-TG curves of the precursor with x = 0.2 at heating rate of 5 C min1 are shown in Fig. 1. From the figure, it is clear that the decomposition proceeds through three defined steps giving a total weight loss of 54.4% at 343 C. According to the accompanied DTA peaks behavior, the first step can be attributed to the dehydration of the precursor contents whereas the following steps can be assigned to the decomposition of the oxalate contents. The dehydration step showed an experimental weight loss of 19.0% agrees well with the theoretically calculated one of 18.9% attributed to the loss of 5.2 water molecules from nickel and iron oxalate contents. Two endothermic DTA peaks are found to characterize this dehydration process. According to the obtained endothermicity, these peaks at 162 and 189 C can be assigned to the loss of water content from nickel oxalate and iron oxalate, respectively. The second step follows immediately after the dehydration step, showing a weight loss of 20.9% at 255 C. This weight loss agrees well with calculated weight loss of 20.7% attributed to the decomposition of ferrous oxalate content into Fe2O3. This oxidative decomposition reaction is accompanied by a sharp exothermic DTA peak at 231 C due to the oxidation of ferrous oxalate in the oxidizing atmosphere which is followed, immediately by the oxidation of the decomposition products with the evolution of CO2 [15]. The decomposition products are thermally stable up to 310 C and then decomposed exothermically in the third step with a weight loss of 14.6%. This weight loss can be assigned to the decomposition of nickel oxalate content into NiO with the oxidation of the decomposition products into CO2 [18] (calc. weight loss = 14.5%). The DTA curve exhibits a sharp exothermic peak at 334 C, due to oxidative decomposition of nickel oxalate, followed by another weak broad exothermic one, which can be attributed to the oxidation of the residual CO. At this stage, only oxides due to iron(III), Ni(II), Al(III) and Cr(III) are only present and no further weight loss change can be observed up to 1100 C. The broad endothermic DTA peak starting at 1000 C can be assigned to the beginning of the ferrite formation [10]. Accordingly, this temperature can be taken as the minimum calcination temperature for ferrites formation. 3.2. Structural properties 3.2.1. X-ray diffraction X-ray diffraction patterns of calcined precursors at 1000 C are shown in Fig. 2. Analysis of these patterns confirms the complete formation of spinel cubic structure with plains reflections of (111),
Fig. 1. DTA-TG curves in air of precursor with Al–Cr content of 0.2. Heating rate = 5 C min1.
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Fig. 3. Variation of the experimental and theoretical lattice parameters with Al–Cr content (x).
cell volume, thus the obvious decrease in the obtained values with gradual substitution can be attributed to the decrease in the volume on the substitution of higher atomic weight iron (55.845 amu) with lower atomic weight aluminum (26.982 amu) and chromium (51.996 amu).
Fig. 2. XRD patterns of the precursors with different Al–Cr content calcined at 1000 C.
(2 2 0), (3 11), (2 2 2), (4 0 0), (4 2 2), (5 11) and (4 4 0). The lattice parameters (a) of the investigated samples have been calculated from different diffraction lines according to the equation [16]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 (1) a ¼ dhkl h þ k þ l
3.2.2. FT-IR spectral study FT-IR spectral measurements of NiAlxCrxFe22xO4 system in the range 200–1000 cm1 (Fig. 4) revealed the appearance of two strong absorption bands y1 (602–607 cm1) and y2 (400–453 cm1) which are considered as a common features of all spinel ferrites [20]. The band positions as a function of Al–Cr content are listed in Table 1. These two bands can be assigned to the stretching vibration mode of Fe3+O2 in the tetrahedral and octahedral sites, respectively. The obtained data showed an obvious increase in the band positions attributed to the octahedral sites with increasing Al–Cr substitution. On the other hand, no obvious change in the band position assigned to the tetrahedral sites can be observed. This variation in the band positions can be due to the decrease in the cation oxygen bond length due to substitution. The metal oxygen bonds will strengthen due to the replacement of larger ionic radii Fe3+ ions in the octahedral sites with smaller Al3+ and Cr3+ ions. On the other hand, the constant band positions obtained for the tetrahedral sites indicate constant composition with progress of substitution. The presence of Fe2+ ions in the spinel lattices can cause a local deformation attributed to the Jahn–Teller distortion effect. This effect appears as splitting or shoulders on the absorption bands as can be seen for the Al–Cr rich samples in the range 450–500 cm1. The intensity of this shoulder increases with increasing substitution causing an obvious broadening of the octahedral site band (y2). Similar behaviors are reported in the literature for other related ferrites systems [10,21,22]. In addition, the obvious change in the intensity of the y2 absorption band with increasing Al–Cr concentration can be attributed to the perturbation occurring in FeO bonds due to
The results are tabulated in Table 1. From the table, it is obvious that the lattice constant gradually decreases with increasing Al and Cr-substitution. This behavior can be explained based on the difference between ionic radii of Fe3+, Cr3+ and Al3+. The replacement of the large Fe3+ ions (0.645 Å) by smaller Al3+ (0.535 Å) and Cr3+ (0.615 Å) [19] results in the decrease of the unit cell volume and consequently decreases the lattice parameter. The average particle sizes were calculated from the broadening of the diffraction peaks using Scherer’s equation [16]: 0:94l (2) bcosu where l is the wavelength of the X-ray radiation used, and b is the D¼
full width at half intensity maximum (FWHM) in radian. The obtained values revealed the nano-sized characteristics of the prepared ferrites. The results summarized in Table 1 exhibited average particle sizes between 54 and 90 nm. X-ray density was also calculated using the experimentally obtained lattice parameters (Fig. 3) according to the equation [16]:
rx ¼
ZM Na3
(3)
where Z is the number of molecules per unit cell (Z = 8), M is the molecular weight of the ferrite sample, and N is Avogadro’s number. The results are listed in Table 1. Since, the density is dependent on the atomic weight of the entire ferrites and their unit Table 1 Structural and electromagnetic parameters of NiAlxCrxFe22xO4 system. x
0.0 0.1 0.2 0.3 0.4 0.5 0.6
a
L
DX
y2
y1
Hysteresis loop
Magnetic susceptibility
T1
T2
ln s
E
(Å)
(nm)
(g cm3)
(cm1)
(cm1)
MS
Mr
HC
hB
xM
TC
(K)
(K)
(ohm1 cm1)
(eV)
8.3347 8.3274 8.3230 8.3182 8.3170 8.3140 8.3062
90 87 75 69 62 58 54
5.38 5.32 5.24 5.19 5.10 5.04 4.98
400 406 410 415 429 445 453
602 604 602 606 605 604 607
47.7 42.4 33.8 25.3 21.4 13.4 9.8
13.2 12.4 9.3 7.9 5.9 4.2 3.4
10 35 45 69 118 189 186
2.00 1.75 1.38 1.02 0.85 0.52 0.38
12.7 12.6 13.3 9.1 7.0 4.0 –
863 866 851 815 – – –
483 503 523 543 – – –
603 643 650 – – – –
-12.41 -12.72 -13.75 -13.83 -16.39 -14.35 -15.20
– – – 0.62 0.70 0.82 0.88
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hB ¼ MW
Ms 5585
(4)
where MW is the molecular weight of the sample. The figure shows the S-type shape loops indicating ordered magnetic structure of the entire samples. The dependence of the saturation magnetization as well as the coercivity on the amount of Al–Cr substitution is illustrated in Fig. 6. From the figure, it is clear that, the saturation magnetization gradually decreases with increasing Al–Cr substitution while coercivity shows an inverse behavior. The remanent magnetization shows a similar trend to that of saturation magnetization. The experimental magnetic moment (hB) calculated from the saturation magnetization data [16] (Table 1), exhibits a decreasing trend with increasing cationic substitution. The magnetization values are apparently higher than those obtained by Chhaya et al. [13] for Al–Cr co-substituted nickel ferrites synthesized via ceramic route. The decrease in the values of Ms and consequently the accompanying magnetic moments due to the present successive magnetic dilution can be explained in the light of Neel’s two sub-lattice model of ferrimagnetism [23]. In this model, the net magnetization of the ferrite lattice is given by the difference between magnetic moments of B and A sub-lattices. Since the dopant ions (i.e., Al3+ and Cr3+ ions), according to the estimated cation distribution, are replacing Fe3+ ions situated in the B sub-lattices [24–26] (ionic magnetic moments of Fe3+, Ni2+, Al3+ and Cr3+ are 5.0, 2.3, 0.0, 3.0 BM, respectively) thus, the calculated Neel’s magnetic moments per formula unit (Table 1) are found to show a decreasing trend similar to that of experimental magnetic moment (hB). The obvious increase in the coercivity values with increasing Al–Cr content can be interpreted based on the following relation [27]: Fig. 4. FT-IR spectra of NiAlxCrxFe22xO4 system.
changing of the dipole moment as well as the internuclear distance by the substitution [22].
Hc ¼
2K 1
m0 Ms
(5)
3.3.1. VSM studies The different magnetic parameters such as saturation magnetization (Ms), magnetic remanent (Mr), magnetic moment (hB) in Bohr magneton and coercivity (Hc) were elucidated from the hysteresis measurements (Fig. 5) and were tabulated in Table 1. The experimental magnetic moment (hB) is determined from the saturation magnetization data using the following formula [16]:
in which coercivity is inversely proportional to saturation magnetization. Another reason for this coercivity behavior can be interpreted through its striking dependence on the particle size estimated by Cullity and Graham [28] and Igarashi and Ogasaki [29] in which an inverse relation was observed. This observation agrees well with the obtained particle sizes and coercivities of the entire system (Table 1). A similar behavior was reported for Alsubstituted nickel ferrite by Al-Haj [11]. On the other hand, the effect of magneto-crystalline anisotropy of the entire ions on the coercivity, which must lead to an obvious decrease in the coercivity values with Al–Cr substitution [26] was overcome in the present study by the particle size effect [16].
Fig. 5. Hysteresis loops for NiAlxCrxFe22xO4 system.
Fig. 6. Variation of the saturation magnetization and coercivity with x in NiAlxCrxFe22xO4 system.
3.3. Magnetic properties
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3.3.2. AC-magnetic susceptibility measurements The magnetic field dependence of the AC-magnetic susceptibility (xM) vs. absolute temperature for the investigated ferrite samples are shown in Fig. 7. The magnetic parameters such as molar magnetic susceptibility (xM) and Curie temperature (TC) are reported in Table 1. The obvious decrease in the susceptibility with increasing magnetic field intensity can be attributed to the saturation of the ferrimagnetic domains, which is a normal magnetic behavior [16]. From the figure it is clear that, only the ferrite samples with Al–Cr content up to x = 0.3 exhibits normal ferrimagnetic behavior in which xM decreases gradually with increasing temperature until it reaches the Curie point (TC) after which paramagnetic characters are obtained. The samples with higher Al–Cr contents showed very week ferrimagnetic behavior or nearly behaved as paramagnetic materials. The sample with x = 0.6 shows a very slight response to the magnetic field, and in this case the Curie temperature measurements could not be carried out as the sample became paramagnetic at room temperature. This indicates that the ferrimagnetic grains are widely separated and enclosed by the low magnetic moment aluminum and chromium ions. The gradual decrease in the susceptibility values (xM) with increasing Al–Cr content could be attributed to the decrease in the magnetic moment interactions due to the spin canting. The obtained Curie temperatures are slightly higher than those obtained for similar system prepared through usual ceramic method [13]. The decrease in the Curie temperatures (TC) with increasing substitution can be attributed to the increase in the paramagnetic region (disordered state) on the expense of the
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ferrimagnetic region (ordered state). The Curie temperature is generally affected by the number of Fe3+ ions, the separation between them and Fe3+ O Fe3+ bond angle and the exchange interaction [30]. The preferential occupancy of zero magnetic moment, Al3+ and low magnetic moment, Cr3+ ions by octahedral sites [24–26] replaces Fe3+ ions and weakens the A–B exchange interaction and consequently decreases the Curie temperature. Similar results are obtained in the literature for similarly investigated ferrites systems [10,13,22,31]. 3.4. Electrical properties 3.4.1. AC-conductivity Fig. 8 illustrates temperature dependence of the AC-electrical conductivity, for the entire investigated samples, as a function of frequency. From the figure, it is clear that a semiconducting behavior, in which conductivity increases with increasing temperature, characterizes all the samples. The conductivity appeared to be frequency dependent at low temperatures and frequency independent at high temperature regions. The pumping force of the applied frequency helps in transferring charge carriers between different localized states as well as liberating the trapped charges and thus increasing conductivity. The simultaneous increase in temperature will cause a large lattice vibration, which scatters the charge carriers and vanishes the frequency effect. The metallic-like behavior, in which conductivity is slightly changed with temperature, appeared in the low temperature region can be attributed to the low thermal energy which is not enough to liberate charge carriers. The changes in the gradient of
Fig. 7. Relation between molar magnetic susceptibility and absolute temperature as a function of different magnetic field intensities for NiAlxCrxFe22xO4 system.
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Fig. 8. Relation between ln s and reciprocal of absolute temperature at different Al Cr content as a function of applied frequency for NiAlxCrxFe22xO4 system.
the lines, by increasing temperature, with the appearance of more than one transition are observed to be the main characteristics of the samples with x 0.3. Depending on the Al–Cr content, two transitions (T1 and T2) are appeared around 500 K and above 600 K, respectively. The temperature positions of these transitions are listed in Table 1. The first transition may be due to the presence of Ni2+ ions, which are expected to play a significant role in the conduction process at this temperature region [32]. The second transition can be attributed to the cation–anion–cation interactions over the octahedral sites, occurring between 3d and 2p orbitals of the transition element and oxygen, respectively [33]. These interactions can be expressed, for example in the case of NiFe2O4, as: [Ni2+ O2 Ni2+] and [Fe3+ O2Fe3+]. The conduction in the spinels is suggested to be due to the charge transfer of electrons between cations of different valencies
on the B sites. For example, in the case of NiFe2O4, the conduction will be due to the electron exchange between Fe2+ (formed during processing) and Fe3+ and the hole exchange between Ni3+ and Ni2+. The A–A hopping does not participate in the conduction, as there are only Fe3+ ions on this sub-lattice since, all Fe2+ ions preferentially occupy the B sites. Thus the B–B hopping is considered to be more dominant than A–B and A–A hoppings [34]. This explains the observed decrease in the conductivity (s ) with increasing Al–Cr-contents (Table 1). The gradual replacement of the Fe3+ ions situated in the octahedral sites by Al3+ and Cr3+ ions (have strong octahedral sites preference) limit the degree of Fe2+ Fe3+ conduction by blocking up this transformation since, they do not participate in the conduction mechanism [26]. This decrease in conductivity with increasing Al–Cr content reflects that the synthesized materials can be used in microwave devices applications, which required highly resistive materials.
M.A. Gabal et al. / Materials Research Bulletin 60 (2014) 433–440
At higher Al–Cr substitution (x 0.4), both transitions completely disappeared due the successive replacement of Al3+ and Cr3+ ions for Fe3+ ions in the octahedral sites, and only the steeply rising behavior of conductivity is observed with rising temperature. This behavior can be attributed to the increase in drift mobility of the thermally activated charge carriers (electron and hole) according to hopping conduction mechanism [35]. The hopping probability will depend on the activation energy associated with the electrical energy barrier experienced by the electrons during hopping. These activation energies (E) were calculated using Arrhenius relation (in the temperature range 563–723 K) and reported in Table 1. From the table it is clear that
439
the activation energy increases with Al–Cr concentration. This behavior corresponds well to the decreasing trend of conductivity. 3.4.2. Dielectric constant The variation of dielectric constant (e0 ) as a function of temperature and frequency is shown in Fig. 9. From the figure, it is noticed that the dielectric constant increases with increasing temperature especially in the high temperature region. In this region, the increase in temperature liberates more localized dipoles from their atomic bonds so the number of charge carriers increases and under the applied field, their contribution in the polarization increases. In addition, the decrease in the dielectric
Fig. 9. Relation between dielectric constant and the absolute temperature as a function of applied frequency for NiAlxCrxFe22xO4 system.
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decrease in conductivity with increasing Al–Cr content enhances the use of these materials in microwave devices applications. Acknowledgment This paper was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant no. 78-1301433. The authors therefore, thank DSR technical for financial support. References
Fig. 10. Relation between dielectric loss and the absolute temperature as a function of applied frequency for the sample with x = 0.2.
constant with increasing frequency can be considered as a normal dielectric behavior [32]. Fig. 9 and its insets also exhibit the presence of two transitions having positions agreeing well with those obtained via conductivity behavior. These transitions are observed to be nearly vanished with increasing frequency due to the disturbance occurs in the system by increasing frequency. These transitions cannot be assigned to the Curie temperature, as they are lower than that of TC already measured (Table 1). This low temperature transitions may be attributed to defects present in the sample. Similar results are reported by Joshi et al. [36] for NiFe2O4 prepared via coprecipitation method. The imaginary part of dielectric constant as a function of temperature and frequency also shows similar behavior as that represented by dielectric constant. Fig. 10 represents typical dielectric loss behavior as a function of temperature and frequency for the sample with x = 0.2. 4. Conclusions XRD of NiAlxCrxFe22xO4 (0.0 x 0.6) system, prepared via oxalate decomposition route, revealed the formation of singlephase cubic ferrites. The introduction of Al3+ and Cr3+ ions in NiFe2O4 structure resulted in important changes in the structural, magnetic and electrical properties. An appropriate cation distribution for the system was suggested based on the obtained structural data, which was fortified through the magnetic and electrical measurements. The magnetization values obtained are apparently higher than those reported in the literature for similar compositions prepared via other alternative methods. The
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Substitution effect on structural, electrical and magnetic properties of NiFe22xAlxCrxO4 (x = 0–0.6) nano-crystalline ferrites via oxalate precursor route M.A. Gabal a, * ,1, A.Y. Obaid a , M. Abdel Salam a , W.A. Bayoumy b a b
Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia Chemistry Department, Faculty of Science, Benha University, Benha, Egypt
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 March 2014 Received in revised form 9 August 2014 Accepted 5 September 2014 Available online 6 September 2014
This manuscript reports on the structural, magnetic and electrical properties of Al–Cr co-substituted nickel ferrites; NiAlxCrxFe22xO4 (0.0 x 0.6), synthesized via oxalate decomposition route. The decomposition process was monitored using DTA-TG techniques, and the obtained ferrites were characterized using XRD, FT-IR, VSM, AC-magnetic susceptibility and electrical properties measurements. The single-phase structure was confirmed using XRD and FT-IR spectroscopic measurements. With increasing Al–Cr substitution, the obtained saturation magnetizations, via VSM measurements, showed a gradual decrease while coercivity exhibited a steady increase. The electrical properties as a function of temperature and frequency showed a semiconducting behavior with conductivity decreasing by increasing substitution, which enhances the use of these materials in microwave devices applications. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Magnetic materials A. Semiconductors C. X-ray diffraction D. Electrical properties
1. Introduction The interest of scientists and researchers toward ferrites materials with spinel structure arises from the interesting theoretical as well as technological applications in many fields, such as satellite communication, memory devices, computer components, antenna rod, transformers, etc. [1]. NiFe2O4 is an inverse spinel ferrite taken to be collinear ferrimagnet [2], where degree of inversion depends on the cooling rate and heat treatment. It has been the subject of many investigations under different treatment [3–5]. The effect of Cr-substitution in NiFe2O4 has received a lot of attention. Fayek and Ata Allah [6] reported that Cr3+ content occupy the octahedral sites for a maximum of x = 0.6, and the excess Cr3+ replaces the Fe3+ at the tetrahedral site. Mössbauer measurements [7] showed well-resolved magnetic spectra for the tetrahedral and octahedral sites, which suggest that as Cr substitution increases, the system slowly converted into a normal spinel structure. Singh et al. [8] synthesized spinel-type ternary
* Corresponding author. Tel.: +966 557071572. E-mail address: [email protected] (M.A. Gabal). 1 Permanent address: Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. http://dx.doi.org/10.1016/j.materresbull.2014.09.017 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
ferrites with composition NiFe2xCrxO4 (0 x 1) by a precipitation method. The samples were characterized using IR, XRD, BET surface area, and XPS techniques. Singhal et al. [9] have investigated the cation distribution in chromium substituted nickel ferrites, prepared by aerosol route, using XRD, magnetic and Mössbauer spectral studies. Chromium-substituted nickel ferrites synthesized through oxalate precursors [10] showed a decrease in the magnetic properties with increasing Cr content. In Al-containing ferrites, aluminum ions are preferred to occupy both tetrahedral (A) and octahedral [B] sites depending on the amount of Al, which will affect the amount of iron ions in the two sites [1]. NiFe2xAlxO4 ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements [11]. Nano-crystalline Al-doped nickel ferrites have been synthesized by sol–gel auto-ignition method, and the effect of magnetic dilution on the structural and magnetic properties has been studied [12]. The structural and electrical properties of NiFe2xAlxO4, synthesized through co-precipitation method, were discussed as a function of Al substitution [1]. In the literature, there are very few works dealing with Al and Cr co-substituted NiFe2O4. Chhaya et al. [13] studied crystal structure and the magnetic properties of the mixed spinel NiAlxCrxFe22xO4 (x = 0.0–0.9) using XRD, Mössbauer spectroscopy and magnetic susceptibility measurements.
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Polycrystalline Al and Cr co-substituted disordered spinel series NiAlxCrxFe22xO4 (0.1 x 0.9) were prepared through solid-state method and studied by XRD, magnetization and electrical resistivity measurements [14]. The aim of the present study is to prepare single-phase nano-crystalline NiAlxCrxFe22xO4 ferrites (0.0 x 0.6) through the thermal decomposition of their corresponding metal oxalates. The oxalate decomposition reaction was followed using thermal analysis techniques (DTA-TG). The structural characterization was investigated using XRD and FT-IR techniques. The magnetic properties were measured using vibrating sample magnetometer (VSM) and DC-magnetic susceptibility techniques. The electrical properties as a function of temperature and frequency were also characterized. The effect of the magnetic dilution on the electrical and magnetic properties was discussed. To the best of our knowledge, no systematic investigations of the structural, magnetic and electrical properties of Al and Cr co-substitution of NiFe2O4 are reported in the literature. In addition, the entire preparation method seems to be novel for the preparation of these types of ferrites. 2. Experimental 2.1. Synthesis of ferrites Nano-crystalline ferrites of the general formula NiAlxCrxFe22xO4 (0.0 x 0.6) were synthesized through thermal decomposition process. AR grade FeSO47H2O, Ni(NO3)26H2O and H2C2O42H2O were used as supplied for the preparation of individual metal oxalates; FeC2O42H2O and NiC2O42H2O through coprecipitation method [15]. The impregnation technique, previously described [16,17], was then used for the preparation of ferrites precursors. In this process, stoichiometric amounts of metal oxalates along with Al2O3 and Cr2O3 are weighed and thoroughly mixed in a porcelain mortar using drops of bi-distilled water. After drying the wet precursors in an electrical oven, they were annealed in a muffle furnace at 1000 C for 2 h under static air atmosphere. 2.2. Techniques Differential thermal analysis-thermogravimetry (DTA-TG) measurements were investigated in air atmosphere using a Shimadzu DT-60 thermal analyzer. The experiments were carried out at a heating rate of 5 C min1 up to 1000 C. The powder X-ray diffraction (XRD) analysis of the calcined precursors were conducted at room temperature using a D8 Advanced diffractometer, Bruker AXS, using Cu Ka1 radiation (l = 0.15406 nm). FT-IR spectra were measured in the range of 1000–200 cm1 using a Jasco FTIR 310 spectrometer. The hysteresis measurements at room temperature were performed using a vibrating sample magnetometer (VSM9600M) with a maximum applied field of 5 kOe. The DC-magnetic susceptibility measurements were carried out using Faraday’s method [10,16]. The measurements were performed up to 850 K as a function of different magnetic field intensities (1010, 1340 and 1660 Oe). The temperature dependence of electrical properties as a function of frequency (100 Hz–5 MHz) was measured by a Hioki 3531 LCR bridge using the two-probe method [10]. The samples were palletized, and the two surfaces of each pellet (1 cm in diameter and about 1 mm in thickness) are polished, coated with silver paste and checked for good conduction. A K-type thermocouple was used to measure the sample temperature with accuracy better than 1 C.
3. Results and discussion 3.1. Precursor decomposition and ferrite formation processes The thermal decomposition process of the entire precursors, up to ferrite formation, was monitored in air using DTA-TG measurements. Typical DTA-TG curves of the precursor with x = 0.2 at heating rate of 5 C min1 are shown in Fig. 1. From the figure, it is clear that the decomposition proceeds through three defined steps giving a total weight loss of 54.4% at 343 C. According to the accompanied DTA peaks behavior, the first step can be attributed to the dehydration of the precursor contents whereas the following steps can be assigned to the decomposition of the oxalate contents. The dehydration step showed an experimental weight loss of 19.0% agrees well with the theoretically calculated one of 18.9% attributed to the loss of 5.2 water molecules from nickel and iron oxalate contents. Two endothermic DTA peaks are found to characterize this dehydration process. According to the obtained endothermicity, these peaks at 162 and 189 C can be assigned to the loss of water content from nickel oxalate and iron oxalate, respectively. The second step follows immediately after the dehydration step, showing a weight loss of 20.9% at 255 C. This weight loss agrees well with calculated weight loss of 20.7% attributed to the decomposition of ferrous oxalate content into Fe2O3. This oxidative decomposition reaction is accompanied by a sharp exothermic DTA peak at 231 C due to the oxidation of ferrous oxalate in the oxidizing atmosphere which is followed, immediately by the oxidation of the decomposition products with the evolution of CO2 [15]. The decomposition products are thermally stable up to 310 C and then decomposed exothermically in the third step with a weight loss of 14.6%. This weight loss can be assigned to the decomposition of nickel oxalate content into NiO with the oxidation of the decomposition products into CO2 [18] (calc. weight loss = 14.5%). The DTA curve exhibits a sharp exothermic peak at 334 C, due to oxidative decomposition of nickel oxalate, followed by another weak broad exothermic one, which can be attributed to the oxidation of the residual CO. At this stage, only oxides due to iron(III), Ni(II), Al(III) and Cr(III) are only present and no further weight loss change can be observed up to 1100 C. The broad endothermic DTA peak starting at 1000 C can be assigned to the beginning of the ferrite formation [10]. Accordingly, this temperature can be taken as the minimum calcination temperature for ferrites formation. 3.2. Structural properties 3.2.1. X-ray diffraction X-ray diffraction patterns of calcined precursors at 1000 C are shown in Fig. 2. Analysis of these patterns confirms the complete formation of spinel cubic structure with plains reflections of (111),
Fig. 1. DTA-TG curves in air of precursor with Al–Cr content of 0.2. Heating rate = 5 C min1.
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Fig. 3. Variation of the experimental and theoretical lattice parameters with Al–Cr content (x).
cell volume, thus the obvious decrease in the obtained values with gradual substitution can be attributed to the decrease in the volume on the substitution of higher atomic weight iron (55.845 amu) with lower atomic weight aluminum (26.982 amu) and chromium (51.996 amu).
Fig. 2. XRD patterns of the precursors with different Al–Cr content calcined at 1000 C.
(2 2 0), (3 11), (2 2 2), (4 0 0), (4 2 2), (5 11) and (4 4 0). The lattice parameters (a) of the investigated samples have been calculated from different diffraction lines according to the equation [16]: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 (1) a ¼ dhkl h þ k þ l
3.2.2. FT-IR spectral study FT-IR spectral measurements of NiAlxCrxFe22xO4 system in the range 200–1000 cm1 (Fig. 4) revealed the appearance of two strong absorption bands y1 (602–607 cm1) and y2 (400–453 cm1) which are considered as a common features of all spinel ferrites [20]. The band positions as a function of Al–Cr content are listed in Table 1. These two bands can be assigned to the stretching vibration mode of Fe3+O2 in the tetrahedral and octahedral sites, respectively. The obtained data showed an obvious increase in the band positions attributed to the octahedral sites with increasing Al–Cr substitution. On the other hand, no obvious change in the band position assigned to the tetrahedral sites can be observed. This variation in the band positions can be due to the decrease in the cation oxygen bond length due to substitution. The metal oxygen bonds will strengthen due to the replacement of larger ionic radii Fe3+ ions in the octahedral sites with smaller Al3+ and Cr3+ ions. On the other hand, the constant band positions obtained for the tetrahedral sites indicate constant composition with progress of substitution. The presence of Fe2+ ions in the spinel lattices can cause a local deformation attributed to the Jahn–Teller distortion effect. This effect appears as splitting or shoulders on the absorption bands as can be seen for the Al–Cr rich samples in the range 450–500 cm1. The intensity of this shoulder increases with increasing substitution causing an obvious broadening of the octahedral site band (y2). Similar behaviors are reported in the literature for other related ferrites systems [10,21,22]. In addition, the obvious change in the intensity of the y2 absorption band with increasing Al–Cr concentration can be attributed to the perturbation occurring in FeO bonds due to
The results are tabulated in Table 1. From the table, it is obvious that the lattice constant gradually decreases with increasing Al and Cr-substitution. This behavior can be explained based on the difference between ionic radii of Fe3+, Cr3+ and Al3+. The replacement of the large Fe3+ ions (0.645 Å) by smaller Al3+ (0.535 Å) and Cr3+ (0.615 Å) [19] results in the decrease of the unit cell volume and consequently decreases the lattice parameter. The average particle sizes were calculated from the broadening of the diffraction peaks using Scherer’s equation [16]: 0:94l (2) bcosu where l is the wavelength of the X-ray radiation used, and b is the D¼
full width at half intensity maximum (FWHM) in radian. The obtained values revealed the nano-sized characteristics of the prepared ferrites. The results summarized in Table 1 exhibited average particle sizes between 54 and 90 nm. X-ray density was also calculated using the experimentally obtained lattice parameters (Fig. 3) according to the equation [16]:
rx ¼
ZM Na3
(3)
where Z is the number of molecules per unit cell (Z = 8), M is the molecular weight of the ferrite sample, and N is Avogadro’s number. The results are listed in Table 1. Since, the density is dependent on the atomic weight of the entire ferrites and their unit Table 1 Structural and electromagnetic parameters of NiAlxCrxFe22xO4 system. x
0.0 0.1 0.2 0.3 0.4 0.5 0.6
a
L
DX
y2
y1
Hysteresis loop
Magnetic susceptibility
T1
T2
ln s
E
(Å)
(nm)
(g cm3)
(cm1)
(cm1)
MS
Mr
HC
hB
xM
TC
(K)
(K)
(ohm1 cm1)
(eV)
8.3347 8.3274 8.3230 8.3182 8.3170 8.3140 8.3062
90 87 75 69 62 58 54
5.38 5.32 5.24 5.19 5.10 5.04 4.98
400 406 410 415 429 445 453
602 604 602 606 605 604 607
47.7 42.4 33.8 25.3 21.4 13.4 9.8
13.2 12.4 9.3 7.9 5.9 4.2 3.4
10 35 45 69 118 189 186
2.00 1.75 1.38 1.02 0.85 0.52 0.38
12.7 12.6 13.3 9.1 7.0 4.0 –
863 866 851 815 – – –
483 503 523 543 – – –
603 643 650 – – – –
-12.41 -12.72 -13.75 -13.83 -16.39 -14.35 -15.20
– – – 0.62 0.70 0.82 0.88
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hB ¼ MW
Ms 5585
(4)
where MW is the molecular weight of the sample. The figure shows the S-type shape loops indicating ordered magnetic structure of the entire samples. The dependence of the saturation magnetization as well as the coercivity on the amount of Al–Cr substitution is illustrated in Fig. 6. From the figure, it is clear that, the saturation magnetization gradually decreases with increasing Al–Cr substitution while coercivity shows an inverse behavior. The remanent magnetization shows a similar trend to that of saturation magnetization. The experimental magnetic moment (hB) calculated from the saturation magnetization data [16] (Table 1), exhibits a decreasing trend with increasing cationic substitution. The magnetization values are apparently higher than those obtained by Chhaya et al. [13] for Al–Cr co-substituted nickel ferrites synthesized via ceramic route. The decrease in the values of Ms and consequently the accompanying magnetic moments due to the present successive magnetic dilution can be explained in the light of Neel’s two sub-lattice model of ferrimagnetism [23]. In this model, the net magnetization of the ferrite lattice is given by the difference between magnetic moments of B and A sub-lattices. Since the dopant ions (i.e., Al3+ and Cr3+ ions), according to the estimated cation distribution, are replacing Fe3+ ions situated in the B sub-lattices [24–26] (ionic magnetic moments of Fe3+, Ni2+, Al3+ and Cr3+ are 5.0, 2.3, 0.0, 3.0 BM, respectively) thus, the calculated Neel’s magnetic moments per formula unit (Table 1) are found to show a decreasing trend similar to that of experimental magnetic moment (hB). The obvious increase in the coercivity values with increasing Al–Cr content can be interpreted based on the following relation [27]: Fig. 4. FT-IR spectra of NiAlxCrxFe22xO4 system.
changing of the dipole moment as well as the internuclear distance by the substitution [22].
Hc ¼
2K 1
m0 Ms
(5)
3.3.1. VSM studies The different magnetic parameters such as saturation magnetization (Ms), magnetic remanent (Mr), magnetic moment (hB) in Bohr magneton and coercivity (Hc) were elucidated from the hysteresis measurements (Fig. 5) and were tabulated in Table 1. The experimental magnetic moment (hB) is determined from the saturation magnetization data using the following formula [16]:
in which coercivity is inversely proportional to saturation magnetization. Another reason for this coercivity behavior can be interpreted through its striking dependence on the particle size estimated by Cullity and Graham [28] and Igarashi and Ogasaki [29] in which an inverse relation was observed. This observation agrees well with the obtained particle sizes and coercivities of the entire system (Table 1). A similar behavior was reported for Alsubstituted nickel ferrite by Al-Haj [11]. On the other hand, the effect of magneto-crystalline anisotropy of the entire ions on the coercivity, which must lead to an obvious decrease in the coercivity values with Al–Cr substitution [26] was overcome in the present study by the particle size effect [16].
Fig. 5. Hysteresis loops for NiAlxCrxFe22xO4 system.
Fig. 6. Variation of the saturation magnetization and coercivity with x in NiAlxCrxFe22xO4 system.
3.3. Magnetic properties
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3.3.2. AC-magnetic susceptibility measurements The magnetic field dependence of the AC-magnetic susceptibility (xM) vs. absolute temperature for the investigated ferrite samples are shown in Fig. 7. The magnetic parameters such as molar magnetic susceptibility (xM) and Curie temperature (TC) are reported in Table 1. The obvious decrease in the susceptibility with increasing magnetic field intensity can be attributed to the saturation of the ferrimagnetic domains, which is a normal magnetic behavior [16]. From the figure it is clear that, only the ferrite samples with Al–Cr content up to x = 0.3 exhibits normal ferrimagnetic behavior in which xM decreases gradually with increasing temperature until it reaches the Curie point (TC) after which paramagnetic characters are obtained. The samples with higher Al–Cr contents showed very week ferrimagnetic behavior or nearly behaved as paramagnetic materials. The sample with x = 0.6 shows a very slight response to the magnetic field, and in this case the Curie temperature measurements could not be carried out as the sample became paramagnetic at room temperature. This indicates that the ferrimagnetic grains are widely separated and enclosed by the low magnetic moment aluminum and chromium ions. The gradual decrease in the susceptibility values (xM) with increasing Al–Cr content could be attributed to the decrease in the magnetic moment interactions due to the spin canting. The obtained Curie temperatures are slightly higher than those obtained for similar system prepared through usual ceramic method [13]. The decrease in the Curie temperatures (TC) with increasing substitution can be attributed to the increase in the paramagnetic region (disordered state) on the expense of the
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ferrimagnetic region (ordered state). The Curie temperature is generally affected by the number of Fe3+ ions, the separation between them and Fe3+ O Fe3+ bond angle and the exchange interaction [30]. The preferential occupancy of zero magnetic moment, Al3+ and low magnetic moment, Cr3+ ions by octahedral sites [24–26] replaces Fe3+ ions and weakens the A–B exchange interaction and consequently decreases the Curie temperature. Similar results are obtained in the literature for similarly investigated ferrites systems [10,13,22,31]. 3.4. Electrical properties 3.4.1. AC-conductivity Fig. 8 illustrates temperature dependence of the AC-electrical conductivity, for the entire investigated samples, as a function of frequency. From the figure, it is clear that a semiconducting behavior, in which conductivity increases with increasing temperature, characterizes all the samples. The conductivity appeared to be frequency dependent at low temperatures and frequency independent at high temperature regions. The pumping force of the applied frequency helps in transferring charge carriers between different localized states as well as liberating the trapped charges and thus increasing conductivity. The simultaneous increase in temperature will cause a large lattice vibration, which scatters the charge carriers and vanishes the frequency effect. The metallic-like behavior, in which conductivity is slightly changed with temperature, appeared in the low temperature region can be attributed to the low thermal energy which is not enough to liberate charge carriers. The changes in the gradient of
Fig. 7. Relation between molar magnetic susceptibility and absolute temperature as a function of different magnetic field intensities for NiAlxCrxFe22xO4 system.
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Fig. 8. Relation between ln s and reciprocal of absolute temperature at different Al Cr content as a function of applied frequency for NiAlxCrxFe22xO4 system.
the lines, by increasing temperature, with the appearance of more than one transition are observed to be the main characteristics of the samples with x 0.3. Depending on the Al–Cr content, two transitions (T1 and T2) are appeared around 500 K and above 600 K, respectively. The temperature positions of these transitions are listed in Table 1. The first transition may be due to the presence of Ni2+ ions, which are expected to play a significant role in the conduction process at this temperature region [32]. The second transition can be attributed to the cation–anion–cation interactions over the octahedral sites, occurring between 3d and 2p orbitals of the transition element and oxygen, respectively [33]. These interactions can be expressed, for example in the case of NiFe2O4, as: [Ni2+ O2 Ni2+] and [Fe3+ O2Fe3+]. The conduction in the spinels is suggested to be due to the charge transfer of electrons between cations of different valencies
on the B sites. For example, in the case of NiFe2O4, the conduction will be due to the electron exchange between Fe2+ (formed during processing) and Fe3+ and the hole exchange between Ni3+ and Ni2+. The A–A hopping does not participate in the conduction, as there are only Fe3+ ions on this sub-lattice since, all Fe2+ ions preferentially occupy the B sites. Thus the B–B hopping is considered to be more dominant than A–B and A–A hoppings [34]. This explains the observed decrease in the conductivity (s ) with increasing Al–Cr-contents (Table 1). The gradual replacement of the Fe3+ ions situated in the octahedral sites by Al3+ and Cr3+ ions (have strong octahedral sites preference) limit the degree of Fe2+ Fe3+ conduction by blocking up this transformation since, they do not participate in the conduction mechanism [26]. This decrease in conductivity with increasing Al–Cr content reflects that the synthesized materials can be used in microwave devices applications, which required highly resistive materials.
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At higher Al–Cr substitution (x 0.4), both transitions completely disappeared due the successive replacement of Al3+ and Cr3+ ions for Fe3+ ions in the octahedral sites, and only the steeply rising behavior of conductivity is observed with rising temperature. This behavior can be attributed to the increase in drift mobility of the thermally activated charge carriers (electron and hole) according to hopping conduction mechanism [35]. The hopping probability will depend on the activation energy associated with the electrical energy barrier experienced by the electrons during hopping. These activation energies (E) were calculated using Arrhenius relation (in the temperature range 563–723 K) and reported in Table 1. From the table it is clear that
439
the activation energy increases with Al–Cr concentration. This behavior corresponds well to the decreasing trend of conductivity. 3.4.2. Dielectric constant The variation of dielectric constant (e0 ) as a function of temperature and frequency is shown in Fig. 9. From the figure, it is noticed that the dielectric constant increases with increasing temperature especially in the high temperature region. In this region, the increase in temperature liberates more localized dipoles from their atomic bonds so the number of charge carriers increases and under the applied field, their contribution in the polarization increases. In addition, the decrease in the dielectric
Fig. 9. Relation between dielectric constant and the absolute temperature as a function of applied frequency for NiAlxCrxFe22xO4 system.
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decrease in conductivity with increasing Al–Cr content enhances the use of these materials in microwave devices applications. Acknowledgment This paper was funded by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant no. 78-1301433. The authors therefore, thank DSR technical for financial support. References
Fig. 10. Relation between dielectric loss and the absolute temperature as a function of applied frequency for the sample with x = 0.2.
constant with increasing frequency can be considered as a normal dielectric behavior [32]. Fig. 9 and its insets also exhibit the presence of two transitions having positions agreeing well with those obtained via conductivity behavior. These transitions are observed to be nearly vanished with increasing frequency due to the disturbance occurs in the system by increasing frequency. These transitions cannot be assigned to the Curie temperature, as they are lower than that of TC already measured (Table 1). This low temperature transitions may be attributed to defects present in the sample. Similar results are reported by Joshi et al. [36] for NiFe2O4 prepared via coprecipitation method. The imaginary part of dielectric constant as a function of temperature and frequency also shows similar behavior as that represented by dielectric constant. Fig. 10 represents typical dielectric loss behavior as a function of temperature and frequency for the sample with x = 0.2. 4. Conclusions XRD of NiAlxCrxFe22xO4 (0.0 x 0.6) system, prepared via oxalate decomposition route, revealed the formation of singlephase cubic ferrites. The introduction of Al3+ and Cr3+ ions in NiFe2O4 structure resulted in important changes in the structural, magnetic and electrical properties. An appropriate cation distribution for the system was suggested based on the obtained structural data, which was fortified through the magnetic and electrical measurements. The magnetization values obtained are apparently higher than those reported in the literature for similar compositions prepared via other alternative methods. The
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