Improved electrical, dielectric and magnetic properties of Al-Sm co-doped NiFe2O4 spinel ferrites nanoparticles

Materials Science & Engineering B 243 (2019) 47–53

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Improved electrical, dielectric and magnetic properties of Al-Sm co-doped NiFe2O4 spinel ferrites nanoparticles

T

Hafiz Sartaj Aziza, Rafaqat Ali Khana, , Faheem Shaha, Bushra Ismaila, Jan Nisarb, ⁎ Syed Mujtaba Shahc, Abdur Rahimd, Abdur Rahman Khana, ⁎

a

Applied and Analytical Chemistry Laboratory, Department of Chemistry, COMSATS University Islamabad, Abbottabad Campus, Abbottabad 22060, Khyber Pakhtunkhwa, Pakistan b National Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar 25120, Pakistan c Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan d Interdisciplinary Research Center in Biomedical Materials, COMSATS University Islamabad, Lahore Campus, Lahore, Pakistan

ARTICLE INFO

ABSTRACT

Keywords: Magnetic materials Chemical synthesis X-ray diffraction Dielectric properties Magnetic properties

This study reports the effects of aluminum and samarium on the structural, electrical and magnetic properties of NiFe2O4 spinel ferrites. Nanoparticles were prepared by hydrothermal method in the presence of ascorbic acid and urea. Thermogravimetric analysis revealed no weight losses above 923 K and therefore all samples were annealed at 1023 K to avoid extra phases. X-ray diffraction analysis confirmed well crystallized single-phase spinel structure for all the compositions. The morphology of the particles did not change significantly with dopant contents as evident from scanning electron microscopy. The room temperature electrical resistivity was observed to increase with content of Al3+ and Sm3+ ions following the Verwey’s hopping mechanism. The dispersion in the dielectric constant and dielectric loss with applied frequency in the range of 0.0–3.0 GHz, exhibited normal behavior of ferrites. The magnetic measurements indicated increase in saturation magnetization till x = 0.15. Coercivity and remanence were observed to decrease with Al-Sm contents.

1. Introduction

high spin transition metal ions and the rare earth ions in place of Fe3+ ions in ferrites results in the strong spin–orbital (3d–4f) coupling. A regular difference in magnetic moments arising from the sequential filling of electrons in 4f shells is also observed that results in structural variation and improvement in properties. As far as the magnetic moment of rare earth ions is concerned, it varies from 0 (La3+) to 10.5μB (Dy3+) that make them able to exhibit a range of magnetic variations. The rare earth ions may be designated as isotropic or anisotropic based on their relation with the variation in the f electron orbitals contribution to magnetic interactions [14]. Earlier investigations have many complications in synthesizing single phase rare-earth doped ferrites. Rare earth ions having larger radii than Fe3+, have the tendency to diffuse on grains boundaries of spinel network even with a very little content, which cause the precipitation of a crystalline or extra amorphous phases e.g. (RFeO3) [15]. The properties of nickel spinel ferrites can be enhanced by doping with rare-earth ions, as their 4f orbitals are completely screened by 5s and 5p orbitals and have a central role in determining the electrical and magnetic properties of the ferrites due to their interaction with 3d electrons of transition metals [16]. The current study is focused on the dielectric, magnetic and

Nickel spinel ferrites being soft magnetic materials have recently attracted great number of researchers due to their good chemical stability and magnetic characteristics i.e. high saturation magnetization [1], low power losses and high resistivity [2]. Therefore, they have been extensively utilized for a variety of purposes such as multilayer chip inductor (MLCI) [3], magnetically activated drug delivery [4], microwave frequency devices [5], storage media [6], transducers [7], and transformers [8]. Due to their aforementioned possible uses and functions, various research groups have investigated and reported the influence of rare earth cations on the properties of nickel ferrites and cobalt ferrites in bulk form [9], thin films [10] or nanoparticles [11]. Previous studies indicated that doped spinel ferrites with rare earth and transition ions have appreciable magnetic and electrical properties [12]. A spinel ferrite has a general formula AB2O4, where A is a divalent metal ion and B a trivalent metal ion respectively. The oxygen atoms are cubic close-packed and the A and B ions occupy both tetrahedral and octahedral positions [13]. The preference of a cation for occupying (A) sites or (B) sites depends generally on a size of cation. The doping of

⁎

Corresponding authors. E-mail addresses: [email protected] (R. Ali Khan), [email protected] (A. Rahman Khan).

https://doi.org/10.1016/j.mseb.2019.03.021 Received 11 April 2018; Received in revised form 22 January 2019; Accepted 24 March 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.

Materials Science & Engineering B 243 (2019) 47–53

H. Sartaj Aziz, et al.

electrical properties of Al3+ and Sm3+ ions doped NiFe2O4 spinel ferrite system prepared via hydrothermal route. Aluminum ions have considerable effects on properties of nickel ferrites; particularly on the enhancement of electrical resistivity and hence decreasing eddy current losses [17]. Moreover, substitution of Al3+ions in Ni-Zn–ferrite not only influenced grain refinement but also changed the magnetic properties up to a prominent extent [18]. In this work, by doping the non-magnetic Al3+ and Sm3+ ions at octahedral B sites of nickel ferrite, we are able to improve the magnetization for the doped nickel spinel ferrite samples. Six parameters namely thermogravometric analysis, X-ray diffraction, microstructure analysis, room temperature resistivity, dielectric and magnetic properties are correlated in great detail. The results obtained in this work would provide some useful insight in to the R&D for the development of ferrite based high performance devices.

100

0.01

Residual water loss 99

Hydrated water loss

0.00 97

DTG

Weight (%)

98

96

Transition metal hydroxide decomposition

95

-0.01

Rare earth metal hydroxide decomposition

94 93

2. Experimental procedure

400

600

800

1000

1200

-0.02

Temperature (K)

NiFe2−2xAlxSmxO4 (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25) spinel ferrite powders were prepared via hydrothermal method. Ni (NO3)2·6H2O, Fe(NO3)3·9H2O, Sm(NO3)3·6H2O, Al(NO3)3·6H2O, ascorbic acid (AA) C6H8O6 and urea CO(NH2)2 were the chemicals (purity 98–99%) used for the synthesis. Initially, Ni(NO3)3·6H2O and Fe (NO3)3·9H2O in a molar ratio of 1:2 and the stoichiometric quantities of Sm(NO3)3·6H2O, Al(NO3)3·6H2O, C6H8O6 and CO(NH2)2 were dissolved into 130 mL distilled water and the solution was kept in stirring condition for 15 min. The homogeneous mixture was shifted into a Teflon autoclave and then to an electric oven for heating at 453 K for 6 h. The precipitates formed were collected through filtration and washed. Subsequently, the washed precipitates were dried at 363 K in a drying oven for overnight. At last, the obtained powders samples were collected and thermally characterized by thermogravimetric analysis (TG/ DTA, model: SDT Q600) in order to form an opinion regarding the best possible annealing temperature. All samples were then annealed for 4 h at 1023 K in an electric furnace to get the NiFe2−2xAlxSmxO4 nanoparticles. The phase purity was ascertained using XRD (Phillips X’Pert PRO 3040/60). Using tungsten filament based scanning electron microscope (VEGA3 LM, TESCAN, Czech Republic); the scanning images were developed in secondary electron mode. The electrical resistivity measurements at room temperature were performed by means of twopoint probe technique (Keithley source meter-2400). For measuring dielectric parameters, RF Impedance material analyzer (Agilent 4991A) was used. The magnetic parameters (remanent magnetization, saturation magnetization, coercivity) were measured using a vibrating sample magnetometer.

Fig. 1. Thermogram of the un-annealed NiFe1.9Al0.05Sm0.05O4 sample.

Intensity (a.u)

(311)

(220)

(400)

(440) (511) (422) x=0.00 x=0.05 x=0.10 x=0.15 x=0.20 x=0.25

20

30

40

50

60

70

80

2θ Fig. 2. Powder XRD patterns for un-doped NiFe2−2xAlxSmxO4 spinel ferrite samples.

NiFe2O4

and

doped

3.2. Structural analysis XRD patterns for NiFe2−2xAlxSmxO4 (x = 0.00, 0.05, 0.1, 0.15, 0.20, 0.25) system annealed at 1023 K are presented in Fig. 2. The results confirmed the formation of well crystallized single-phase spinel ferrite (Fd3m). No extra peak except for that of spinal ferrites was observed thus confirming successful substitution of Al-Sm in the NiFe2O4. The patterns of all the peaks in the figure perfectly matched with the standard pattern (ICDD-00-003-0875), therefore, confirming spinal phase purity in the samples. The structural parameters e.g. lattice constant (a), cell volume (Vcell), X-ray density (ρx-ray), and crystallite size (D) were determined from the XRD data using the following expressions [21].

3. Results and discussion 3.1. Thermal analysis A thermogram of a synthesized un-annealed doped compound NiFe1.9Al0.05Sm0.05O4 is presented in Fig. 1. Four different weight losses are observed in the temperature range of 293–923 K. The first weight loss in the temperature range 293–423 K is ascribed to residual moisture loss. The second weight loss in the temperature range of 423–485 K can be assigned to the hydrated water present in the compound [19]. The third weight loss in the temperature range of 485–678 K indicates the formation of oxides from the decomposition of their corresponding transition metal hydroxides. The fourth and maximum weight loss from 773 K to 923 K corresponds to the decomposition of samarium hydroxides to samarium oxide [20]. As no weight loss was observed beyond 923 K, hence all the samples were annealed at 1023 K for 4 h to avoid potential chances of appearing any of the extra phases.

a = dhkl h2 + k 2 + l2

(1)

where d is the inter planer spacing and hkl is the indices of planes. The unit cell volume (Vcell) and X-ray density (ρX-ray) were determined as follows: (2)

Vcell = a3 X ray

48

=

2M NA Vcell

(3)

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quite different properties of Al-Sm substituted nickel ferrite as compared to un-doped nickel ferrite. Grains of samples NiFe1.9Al0.05Sm0.05O4 and NiFe1.5Al0.25Sm0.25O4 are smaller than the pure NiFe2O4. The same may be justified in terms of large ionic radii of rare earth ions (Sm3+ = 0.96 Å). Sm3+ ions when diffuse into nickel ferrite grains create more residual stress due to their larger size, and this leads into the formation of smaller sized spherical grains. The average crystallite size as determined by XRD is considerably small as can be seen in SEM. Such a variation is due to the fact that SEM value is based on the size of secondary particles, which consist of many crystallites formed by soft reunion, whereas XRD take into account only the size of a single crystallite.

Table 1 Crystallite size (D), lattice constant (a), cell volume (V) and X-ray density of NiFe2−2xAlxSmxO4 spinel ferrite samples. Parameters

Crystallite sizes (D/nm) Lattice constant (a/Å) Cell volume (V/Å3) X-ray density (ρx/gcm−3)

AlxSmx content, x 0.00

0.05

0.10

0.15

0.20

0.25

12 8.344 580.9 5.34

13 8.346 581.3 5.40

11 8.347 581.5 5.60

10 8.347 581.5 5.55

9 8.348 581.7 5.61

9 8.351 582.3 5.58

where ρx-ray is the X-ray density, Z is the number of molecules per unit cell (for spinel system Z = 8), M is the molecular weight of the sample, and NA is an Avogadro’s number. The crystallite size (D) was calculated using the following expression [22]:

0.9 D= cos

3.4. Electrical resistivity measurements Ferrites exhibit high room temperature dc electrical resistivity and low eddy current losses that are very important for microwave frequencies, electrical devices and high density recording media. The electrical resistivity can be calculated using the following expression [25]:

(4)

where λ is the wavelength of X-ray, β is the full width at half maxima of the corresponding reflection and θ is the Bragg’s diffraction angle. Table 1 indicates that the values of lattice constant and cell volume slightly increase with raise in Al–Sm contents. The increment in these parameters has a direct relationship with ionic radii of respective substituent. As reported previously, the ionic radii of Fe3+, Al3+and Sm3+ are 0.64 Å, 0.53 Å [18] and 0.964 Å [23], respectively. The increase in lattice constant and cell volume is due to the partial substitution of Sm3+ion with greater ionic radius at Fe3+ site. Table 1 also shows that the value of X-ray density (ρX-ray) is increased with addition of Al-Sm contents from 5.34 (x = 0.00) to 5.61 (x = 0.20) g cm−3. As evident from Eq. (3), ρX-ray has direct relationship to the molar mass (M) of the sample, therefore, with the occupation of Sm3+ ions, an increase in the X-ray density is observed as the atomic weight of substituent Sm (150.40 a.m.u) is higher than that of Fe (55.85 a.m.u). In case of Al-Sm, at first the total molar mass of the sample increases with increase in Al–Sm content and consequently the X-ray density increases. However, at Al-Sm content x = 0.15 and x = 0.25, a slight decrease in X-ray density is observed and this can be attributed to the smaller atomic weight of the Al (26.98 a.m.u). The porous nature of the sample as observed in SEM images also contributed to larger value of X-ray density (D) at lower content level of dopants [24]. It is clear from the Table 1 that crystallite size (D) of the synthesized samples is decreased and was found between 13 and 9 nm. The crystallite size achieved in this study is relatively small due to the larger ionic radius of Sm3+ion (0.964 Å) as compared to Fe3+ion (0.64 Å), which is not easily dissolved in the spinel lattice and perhaps the grain development is restricted [24].

RT

=

RA L

(5) 2

where R = resistance, A = πr and L = width of the pellet. Room temperature electrical resistivity (ρRT) as a function of Al–Sm content is shown in Fig. 4. The un-doped sample has a room temperature resistivity of 2.6 × 107 Ω cm (minimum) whereas the doped sample with Al-Sm content of x = 0.10 has a resistivity of 4.9 × 107 Ω cm (maximum). The figure clearly indicates an enhancement in electrical resistivity (ρRT) with rise in the Al–Sm content as dopant. The enhancement in electrical resistivity with increase in the Al3+and Sm3+ions content in a system can be attributed to a decrease in Fe3+ ions that limit the hopping probability between Fe3+ and Fe2+ ions and increase the intragranular porosity that hinders the motion of charge carriers. Moreover, smaller grain structure contains greater number of grains boundaries that act as energy barriers to the electron flow [26]. Further, small grain size helps in reducing Fe2+ ions in a crystal structure as oxygen moves faster in smaller grains for keeping iron in the Fe3+ state. According to the Verwey’s hopping mechanism, electrical conduction in ferrites is due to the hopping of the electrons between the ions of the same element, having different valence state at B-sites. Similarly, the electric bonds formed between Fe2+ and Al3+ions limit Fe2+ charge carriers that ultimately hinder Verwey mechanism, due to which an increase in resistivity is observed. Hopping between the Sm3+ and Sm4+ ions is initiated due to Sm ions at the B-site and this behavior results in making a small contribution to the conductivity. Moreover, there may be equal possibility of exchange of an electron among Ni2+ and Ni3+ ions although it involves higher energy as compared to Fe3+↔Fe2+ exchange. The existence of a very lower amount of Fe2+ and Ni3+ions in these ferrites, may result either in the sintering process or the electron exchange between Fe2+ ions and Ni3+ ions [27] as illustrated below

3.3. Microstructure analysis Microstructure analysis is a useful tool to study the surface morphology of materials with desired dielectric and electrical properties for specific uses. The microstructure and surface morphology of the doped and un-doped samples with nominal compositions as NiFe2O4, NiFe1.9Al0.05Sm0.05O4, and NiFe1.5Al0.25Sm0.25O4 are shown in Fig. 3. The images indicate that dopants have diminutive effect on morphology and hence, the particles morphology has not been affected to a noticeable level and remained the same (almost spherical). However, indistinguishable grains boundaries are observed in the samples doped with Al-Sm. It seems that the grains of the doped samples have mingled into one other in order to lower the free energy of system. Moreover, with enhancement in the Al-Sm content, the coagulation phenomena seem to decrease. The substituted samples are smaller whereas the agglomerates produced are bulky in the pure nickel ferrite. The texture of grains of the sample (c) NiFe1.5Al0.25Sm0.25O4 is significantly different from that of samples (a) NiFe2O4, which may be the reason for

Ni2 + + Fe3 +

Ni3 + + Fe2 +

Furthermore, Al3+ ions have strong preference for B-sites and transfer Fe3+ ions to A-site, and this mechanism leads to an increase in the resistivity. Al3+ ions have no role in the conduction mechanism and restrict conduction by blocking transformation of Fe2+ ↔ Fe3+. This trend obstructs the Verwey mechanism amid Fe2+ and Fe3+ ions and as a result, resistivity is increased. The decrease in resistivity at concentration x = 0.15 to 9.7 × 106 Ω cm is shown in Fig. 4. The possible reason for this decrease in resistivity is the presence of divalent Fe2+ ions in excess and due to the collective effect of both Al3+ and Sm3+ at the octahedral site, the divalent ions cause a sharp drop in the resistivity of ferrites e.g. (Fe2+, O2− etc.). In addition, the larger atomic radius of Sm3+ ion is also 49

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H. Sartaj Aziz, et al.

(a)

(b)

(c)

Fig. 3. SEM images for the samples (a) NiFe2O4, (b) NiFe1.9Al0.05Sm0.05O4, and (c) NiFe1.5Al0.25Sm0.25O4.

accountable for bringing down the resistivity as the possibility of hopping electrons and hole transfer is raised when the neighboring oxygen ions come nearer to each other in the crystal lattice [28]. Further, increase in Al-Sm content x = 0.20 and x = 0.25 causes increase in resistivity to 1.5 × 107 Ω cm and 3.5 × 107 Ω cm respectively due to the excess of both the non-magnetic Al3+ ions and Sm3+ ions as well as reduction of Fe3+ ions at octahedral B sites. 3.5. Dielectric measurements Dielectric constant is the ability of a dielectric material to store charge. The calculation of dielectric parameters gives valuable information about the performance of electric charge carriers and is useful to understand the frequency dependent conduction mechanism in ferrites. The variation in dielectric constant versus frequency is presented in Fig. 5. The figure indicates that the values of dielectric constant (ε′) for all the compositions decrease abruptly at low frequency and in the intermediate region become independent of frequency up to a frequency of 1.5 because of the general behavior of studied ferrites. Furthermore, at frequency 1.79 GHz, a resonance phenomenon is observed for all the samples of NiFe2−2xAlxSmxO4

Fig. 4. Variation among the resistivity, dielectric constant and dielectric loss of NiFe2−2xAlxSmxO4 spinel ferrite samples.

50

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Fig. 5. Effect of frequency on the dielectric constant of NiFe2−2xAlxSmxO4 spinel ferrite samples.

Fig. 6. Effect of frequency on the dielectric loss of NiFe2−2xAlxSmxO4 spinel ferrite samples.

system because of the intrinsic property associated to crystal structure of our synthesized material. The energy coupling generates resonance phenomenon when rotational frequency of electron around their axis becomes equal to applied frequency [29]. The generated resonance behavior is due to the vacancy or pores leading to the space charge polarization in low-frequency regions while high frequency resonance is due to the atomic and electronic polarization. However, the shifting of resonance peak towards higher frequencies with Al-Sm content signals the presence of electronic polarization in the material. It is clear from Fig. 5 that all the samples of NiFe2−2xAlxSmxO4 series show high resonance at around 2.5 GHz due to the presence of atomic and ionic polarization. These ionic relaxation peaks can be attributed to the existence of variable valencies of iron and samarium ions, and are accountable for the creation of oxygen vacancies for overcoming the local electroneutrality. The resonance peaks can only be observed when the jumping frequency of electrons between two ionic pairs of samarium and iron ions becomes equal to the frequency of applied ac field. In addition, this type of resonance behavior can also be due to the lattice distortion produced by the greater radii of Sm3+ ions. The same distortion yields distance modification among nearby oxygen sites and at higher frequencies cause the relaxation peaks to displace. In the present work, the un-doped sample NiFe2O4 exhibits ionic relaxation peak caused by variable valencies of (Fe2+/Fe3+) iron ions so as to create the oxygen vacancies for maintaining the overall local electroneutrality. These created oxygen vacancies could also assist in thermally-activated hopping and ultimately lead to larger broadening of electron levels in fine particle [28]. The relaxation time (τ) for all the samples is calculated using Debye equation, ° = / Â2° , where γ is the width of resonance peaks and ω is the angular resonance frequency [30]. The calculated relaxation times

shown in Table 2 are found in the range from 1.0 × 10−12 s to 1.86 × 10−12 s. The low relaxation time observed in these samples confirms the simultaneous loss of excitation in the ac field. The Fig. 5 shows that the value of dielectric constant is high at low frequency, however, increase in frequency results a decrease in the value of the dielectric constant due to the reason that the dielectric poles follow the current at lower frequency instead of higher frequency. Table 2 further shows the values of dielectric constant at selected frequencies. The NiFe2O4 nanoparticles without Al3+ and Sm3+ doping has dielectric constant value of 4.42 (x = 0.00) at 0.5 GHz, whereas this value is dropped to 2.98 at (x = 0.25) for NiAl0.25Sm0.25Fe1.5O4 nanoparticles at the same frequency. This twofold drop off in dielectric constant indicates prospective utilization of Al-Sm doped NiFe2O4 nanoparticles in electromagnetic instruments in future. The dielectric loss parameter has also been observed to show the same trend. The variation in dielectric loss (ε'') with frequency in the frequency range of 1 MHz–3 GHz is shown in Fig. 6 indicating similar tendency as observed in case of dielectric constant. At lower frequencies, the dielectric loss is constant and then decreases suddenly with two relaxation peaks at 1.79 and 2.75 GHz respectively. This is due to interfacial polarization, whereas, the one at higher frequency is ascribed to ionic relaxation [31]. In the present study, a decline in the intensity of the peak and a small shift towards higher frequencies are observed with dopants content. This happens as a result of reduction in oxygen vacancies when Al3+and Sm3+ ions replace iron ions. This sort of behavior might be due to lattice distortion as the Sm3+ ions increase the lattice parameters and shift the relaxation peaks to a bit higher frequencies. A relationship between dc resistivity, dielectric loss and dielectric constant with Al–Sm content for NiFe2−2xAlxSmxO4 series is shown in Fig. 4. Initially, the pure sample (x = 0.00) has maximum value of dielectric constant (ε′ = 4.42) as the resistivity of the system is minimum (ρRT = 2.6 × 107 Ω cm). However, with enhancement in Al-Sm content to x = 0.05 and x = 0.10, dielectric constant is decreased to 3.00 and 2.76, respectively, at a frequency of 0.5 GHz. This is due to the fact that, when Fe3+ ions are replaced at octahedral sites by Al3+ and Sm3+ ions a reduction in electron hopping occurs between iron ions (Fe2+/Fe3+) and consequently a fall in oxygen vacancies take place. Moreover, at x = 0.10, the dielectric loss value is raised to the level of 1.77 at 0.5 GHz as presented in Fig. 4, which may be attributed to enhancement in resistivity (ρRT = 4.9 × 107 Ω cm) of the samples. Yet, at content level of x = 0.15, an enhancement in dielectric constant is noted from 2.76 to 2.87 and this can be attributed to the larger atomic radii of

Table 2 Relaxation time and dielectric parameters at selected frequencies for NiFe2−2xAlxSmxO4 nanoparticles. Parameters

Dielectric constant Dielectric loss Relaxation time (τ/10−12 s)

Frequency (GHz)

AlxSmx content, x 0.00

0.05

0.10

0.15

0.20

0.25

0.5 1.5 3.0 0.5 1.5 3.0

4.42 4.58 4.30 1.42 4.33 8.11 1.86

3.00 3.08 3.03 1.36 1.98 6.22 1.27

2.76 2.76 2.71 1.77 1.66 3.16 1.00

2.87 2.89 2.75 1.79 1.86 4.83 1.12

3.14 3.18 3.09 2.34 3.10 5.54 1.08

2.98 2.99 2.84 1.84 1.86 5.67 1.13

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preservation of local electroneutrality, the transformation of Fe3+ to Fe2+ ions takes place which can produce the local structural distortion and results in an increased saturation magnetization [28]. ii) Replacement of Fe3+ by Sm3+ at B site causes the formation of a local ferromagnetic spin configuration because of the variation in magnetic moment among B site occupants [33]. The higher remanence magnetization and saturation magnetization shown in Table 3 may be linked to the average size of crystallites. Therefore, high temperature sintering of the sample leads to an ample distribution of the crystallites and lower temperature sintering of sample usually results in singledomain crystallites. This causes the crystallites diameter outside the critical diameter and relates it to the sintering temperature of the single-domain state. The single-domain crystallites generally show disordered surface spins as compared to that of particle core. Therefore, the saturation magnetization reduces with reduction in diameter of crystallites owing to surface effects. Furthermore, the non-collinearity of magnetic moments on the surface is also caused by the finite-size scaling of crystallites and may be attributed to the surface effects. These effects are more powerful in ferrimagnetic systems due to oxygen ion (O2−), which promote exchange interaction (superexchange). Consequently, a very small saturation magnetization is shown by the crystallite scale system as compared to the ferrite in bulk. This abrupt reduction in remanence magnetization and saturation magnetization at x = 0.20 and x = 0.25 is shown in Table 3. With decrease in grain size, an increase in grain boundaries occurs that results in breaking up of magnetic ordering [33]. As the magnetization of doped ferrite is smaller vis-à-vis un-doped one, therefore, the domain wall displacement contributes less towards magnetization process due to the presence of nonmagnetic pores at the grain boundaries. Therefore, the domain walls have to move and share in the magnetization process when the applied magnetic field H is increased [34]. The saturation magnetization increases from (x = 0.00) 44.62 emu/g to (x = 0.15) 53.79 emu/g, and then decreases to 36.39 emu/g at content x = 0.25. The reason is, the magnetization is originated from the super exchange interaction among the magnetic moment of cations at tetrahedral (A) and octahedral (B) sites [35]. This is in accord with Neel model, which states that A–B super exchange interactions are more powerful than intra-sublattice A–A and B–B interactions [36]. As Al3+ and Sm3+cations have no contribution to the overall magnetization due to their non-magnetic nature, hence, the magnetization is controlled by the distribution of magnetic Fe3+cations among the (A) and (B) sublattices and this occurs due to the misbalance of Fe3+cations at tetrahedral (A). As the octahedral (B) sites are preferentially occupied by Al3+ and Sm3+cations (Table 3), hence, this will decrease the number of Fe3+cations and thus the corresponding interactions between (B–B) and (A–B) sites. Furthermore, presence of large amount of nonmagnetic Al3+ and Sm3+ ions at concentration (x = 0.20 and x = 0.25 shown in Table 3) is also responsible for the decrease in remanence and saturation magnetization as they replace the Fe3+ ions. The increase in saturation and remanence magnetization, whereas, the simultaneous decrease in coercivity are clear indications of the fact that the obtained electromagnetic properties will be very helpful in the design of efficient passive magnetic components at very high frequencies.

Fig. 7. Hysteresis loops for NiFe2−2xAlxSmxO4 spinel ferrite samples.

Sm3+ ions, which are very instrumental in bringing the oxygen ions near to each other in the crystal lattice and as such the resistivity (ρRT = 9.7 × 106 Ω cm) is decreased due to the hopping of electrons. At content level of x = 0.20, increase in dielectric loss (2.34) and dielectric constant (3.14) is observed due to conduction behavior. Further, increase in Al-Sm content (x = 0.25), both the dielectric loss and dielectric constant is decreased to a value of 1.84 and 2.98 respectively, due to the collective effect of both the nonmagnetic Al3+ and Sm3+ ions and this results in increase in resistivity (ρRT = 3.5 × 107 Ω cm). The variations in the dielectric parameters with increase in resistivity indicate that the synthesized materials have better chances of utilization in microwave appliances. 3.6. Magnetic properties The hysteresis loops at ambient temperature for all samples of NiFe2−2xAlxSmxO4 series are presented in Fig. 7. The corresponding variations in magnetic parameters are shown in Table 3. It can be clearly seen from Table 3 that the values of coercivity decreases with the Al-Sm content (x = 0.00 and x = 0.25) from 451.9 to 179.9 kOe. This decrease in coercivity may be attributed to the presence of nonmagnetic Al3+ and Sm3+ ions in the samples, indicating their magnetically softer nature. Additionally, in pure nickel ferrite (NiFe2O4) the magnetocrystalline anisotropy stems initially from the strong spin-orbit coupling from minute concentration of Ni3+cations residing in the Bsites [32]. Accordingly, in spinel ferrites with high Ni content, it can be expected that the partial removal of Ni3+cations from the octahedral sites of the Al3+ and Sm3+ doped nickel ferrites will diminish anisotropy, which is evident in the form of coercivity fall. The doped samples primarily exhibit higher saturation magnetization and remanence magnetization than pure NiFe2O4 and this behavior increases with increase in Al–Sm content due to the following reasons. i) For the

4. Conclusions

Table 3 Magnetic parameters calculated from the hysteresis loops of the prepared NiFe2−2xAlxSmxO4 spinel ferrite samples. NiFe2−2xAlxSmxO4

Coercivity (Hc/Oe) Saturation magnetization (Ms/emug−1) Remanence (Mr/emu g−1)

The hydrothermal method is a simple and convenient method for the preparation of Al-Sm doped nickel ferrites precursors, which are then subjected to annealing at 1023 K. The XRD analysis confirmed the formation of single phase of the cubic spinel ferrite and the average crystallite sizes produced were in the range of 13–9 nm, a size suitable for the miniaturization of materials and for getting better signal-tonoise ratio in high density recording media. A constant but minor increase was found out in the lattice parameters due to the presence of rare earth ions (larger ionic radii than Fe3+ ions). The room temperature dc electrical resistivity was observed to increase due to the

AlxSmx content, x 0.00

0.05

0.10

0.15

0.20

0.25

451.9 44.62

410.6 47.57

312.8 51.68

286.8 53.79

282.0 47.28

179.9 36.39

13.22

16.77

12.14

13.32

11.41

8.990

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collective contribution from Al3+and Sm3+ ions that are responsible for retarding the hopping of electrons among octahedral sites. The dielectric constant and loss factors were decreased with applied frequency in the low frequency region showing normal behavior of ferrites. Both these parameters also showed an enhancement with the Al-Sm content, when comparison is made at a frequency of 0.5 GHz, which indicates that these materials can successfully be utilized in microwave and electrical devices. The lower magnetocrystalline anisotropy and larger grains of the doped Al-Sm samples facilitated the reversible magnetization processes with ultimate reduction in measured coercivity, HC. The remanence magnetization (Mr) and saturation magnetization (Ms) were observed to decrease due to nonmagnetic behavior of Al3+ ions and rare earth ions (Sm). The enhancement in remanence and saturation magnetization for some synthesized samples with improved resistivity shows that these materials can successfully be applied in high density recording media and microwave devices.

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