Electrical and morphological properties of magnetocaloric nano ZnNi ferrite

Journal of Magnetism and Magnetic Materials 394 (2015) 96–104

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Electrical and morphological properties of magnetocaloric nano ZnNi ferrite O.M. Hemeda a,n, Nasser Y. Mostafa b,c, Omar H. Abd Elkader d,e, D.M. Hemeda a, A. Tawfik a, M. Mostafa a a

Physics Department, Faculty of Science, Tanta University, Tanta, Egypt Materials and Corrosion Group, Department of Chemistry, Faculty of Science, Taif University, Saudi Arabia c Chemistry Department, Faculty of Science, Suez Canal University, Ismailia 41522, Egypt d Electron Microscope & Thin Films Department, Physics Division, National Research Center, Dokki 12622, Cairo, Egypt e Electron Microscope Unit, Zoology Department, College of Science, King Saud University, Riyadh, Saudi Arabia b

art ic l e i nf o

a b s t r a c t

Article history: Received 24 March 2015 Received in revised form 6 May 2015 Accepted 21 May 2015 Available online 17 June 2015

A series of Zn1–xNixFe2O4 nano ferrite (with x ¼0, 0.2, 0.4, 0.6, 0.8, and 1) compositions were synthesized using the combustion technique. The powder samples were characterized by XRD. The X-ray analysis showed that the samples were single phase spinel cubic structure. The AC resistivity decreases by increasing the frequency from 1 kHz to 10 kHz. As the frequency of the applied field increases the hopping of charge carrier also increase, thereby decreasing the resistivity. A shift in dielectric maximum is observed toward higher temperature with increasing the Ni content from 536 K to 560 K at 1 kHz. The HRTEM (high resolution TEM) images of four compositions have lattice spacing which confirms the crystalline nature of the samples. The surface morphology SEM of the sample consists of some grains with relatively homogenies distribution with an average size varying from 0.85 to 0.92 μm. The values for entropy change in this work are still small but are significally higher than the values that have been reported for iron oxide nanoparticle. The magnetic entropy change was calculated from measurements of M (H, T) where H is the magnetic field and T is the temperature. The maximum value of entropy change (ΔS) obtained near Curie temperature which makes these material candidates for magnetocaloric applications. & 2015 Elsevier B.V. All rights reserved.

Keywords: Nano ZnNi ferrite combustion technique Electrical and morphological properties Magnetic entropy HRTEM

1. Introduction The magnetocaloric effect (MCE) is definned as the heating or cooling (i.e., the temperature change) of a magnetic material due to the application of a magnetic field. This effect has been called adiabatic demagnetization for years, though this fenomenon is one practical application of the MCE in magnetic materials. For excellent reviews on the magnetocaloric effect, see references [1,2]. The magnetocaloric effect was discovered in 1881, when Warburg observed it in iron [3]. The origin of the MCE was explained independently by Debye [4] and Giauque [5]. They also suggested the first practical use of the MCE: the adiabatic demagnetization, used to reach temperatures lower than that of liquid helium, which had been the lowest achievable experimental temperature. Nowadays, there is a great deal of interest in using the MCE as an alternative technology for refrigeration, from room temperature to n

Corresponding author. E-mail address: [email protected] (O.M. Hemeda).

http://dx.doi.org/10.1016/j.jmmm.2015.05.059 0304-8853/& 2015 Elsevier B.V. All rights reserved.

the temperatures of hydrogen and helium liquefaction (300– 4.2 K). The magnetic refrigeration offers the prospect of an energyefficient and environtment friendly alternative to the common vapor-cycle refrigeration technology in use today [6,7]. Magnetic refrigeration is rapidly becoming competitive with conventional gas compression technology because it offers considerable operating cost savings, mechanical stability, light weight and better performance [8]. For many years magnetocaloric effect (MCE) has been successfully used to reach ultra-low temperatures in a research environment. Gd5Si1.5Ge2.5 is the best known example of a material that exhibits giant MCE [9]. Additionally, most of the current bulk MCE materials rely on the entropy change associated with the paramagnetic–ferromagnetic transition at the Curie temperature. The MCE shows a maximum around the Curie temperature and drops rapidly on either side [10]. It has been theoretically shown that by reducing the average particle size close to the single magnetic domain, magnetic entropy change increases by several orders of magnitude as compared to the entropy change in bulk materials [11]. In addition, the large surface

O.M. Hemeda et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 96–104

(111)

(400)

NiFe2O4(x=1.0)

Zn0.2Ni0.8Fe2O4(x=0.8)

Intenesity (a.u.)

Where, M¼ Zn and/or Ni and z¼ 4.555, is the multiplication factor in order to get fuel lean (ϕ ¼1). In case ϕ ¼ 1 represents the stoichiometric condition at which oxygen content of oxidizer can be reacted to consume fuel entirely [14]. In typical procedure, the stoichiometric amounts of cobalt nitrate, ferric nitrate and glycine (1:2:4.555 M ratios) were taken in a glass beaker. A minimum amount of distilled water was added with stirring to completely dissolve all constituents. The beaker was then kept on furnace preheated to 400 °C. The whole combustion reaction process was complete in less than 5 min whereas actual time of ignition was less than 5 s. the remaining fluffy product was collected from the beaker. This synthesis route needs no subsequent heat step. X-ray diffraction patterns (XRD) were obtained using Bruker diffractometer D8-advance with Cu-Kα radiation. Determination of the lattice constants were made by least squares refinement of the X-ray diffraction data. Indexing of the powder patterns and least squares fitting of the unit cell parameters was possible using the software X'Pert HighScore Plus [15]. The powders morphology was investigated using SEM (JOEL, Model: JSM-5600, Japan.) equipped with secondary electron detector and EDX. All samples were coated with gold. The high resolution transmission electron microscope was performed using Jeol -2100 HRTM in NRC. RLC Bridge of type BM 591 was used for the measurement the dielectric constant (έ), dielectric loss (tanδ), AC resistivity and magnetic permeability (μi) for all toroidal samples at 1 kHz and different temperature. The temperature of the sample was measured by Ni Cr–Ni thermocouple. The measurements were carried from room temperature up to 825 K. The magnetization as a

(333) (511)

M(NO3)2 þ2Fe (NO3)3 þ z NH5C2O2 (aq)-MFe2O4 (s) þz CO2(g) þ (5 þz)N2(g)þ (9 þ2 z) H2O(g)

(422)

Nanoparticles of cobalt ferrite were prepared by solution combustion method. Analytical grade nickel nitrate (Ni (NO3)2  6H2O), zinc nitrate (Zn(NO3)2  6H2O) and ferric nitrate (Fe (NO3)3  9H2O) was taken as oxidants while glycine (CH2NH2COOH) (98%) was employed as fuel to drive the combustion reaction. According to propellant chemistry, the total oxidizing valences the metal nitrates should be balanced by total reducing valences of fuel in order to release maximum energy. A stoichiometric mixture of fuel and oxidant is one in which the quantity of oxidant present is theoretically correct for complete oxidation. The combustion reactions can be represented as follows:

The x-ray diffraction patterns for the nano ferrite Zn1  xNixFe2O4 synthesized by solution combustion technique are shown in Fig. 1. The existence of (111) (220) (311) (222) (400) (422) (511) and (440) major lattice planes in the XRD patterns confirms the formation of a single phase spinel cubic structure of space group Fd-3 m for all Ni1–x ZnxFe2O4 samples with x values up to 1.0. The strongest reflection comes from (311) which characterize the spinel structure. A shift occurs of (311) XRD peak to higher angle (2θ) with the increase of Ni content because the sample with higher Ni content has a smaller lattice parameter. These shifts are larger for Ni nano crystal ferrite compared to the same compositions in bulk state. This means that the present combustion synthesis resulted in complete conversion of reactants to yield ferrite as a single phase without any calcination processes. The cell parameters were obtained from the fitting of x-ray diffraction patterns carried out applying the XPert HighScore Plus program, are given in Fig. 2. Results showed that increasing the Ni content (x) decreases cell parameter. The small changes in the lattice parameters are due to the incorporation of the slightly small Ni ion in place of the Zn ion.

(400)

2. Materials and methods

3.1. structral analysis

(311)

In this paper, we have presented our results on the entropy change in Ni and zinc ferrite nanoparticles. The present work is mainly focused on the morphological and electrical properties of ferrite. The effect of Ni on MCE and resistivity is analyzed. The aim of the present work is to study the electrical and magnetic properties of Zn1  xNix Fe2O4 nanoparticles prepared by solution combustion method, and to indicate its ability as magnetocaloric material.

3. Result and discussions

(222)

⎛ ∂S ⎞ ⎛ ∂M ⎞ ⎟ ⎜ ⎟ =⎜ ⎝ ∂H ⎠T ⎝ ∂T ⎠H

function of temperature and magnetic field has been measured in the previous work using Faraday method [16]. The entropy changes were calculated from magnetization crves using Maxwell's equation.

(220)

area in nanostructured materials has the potential to provide better heat exchange with the surrounding materials. The MCE can be measured directly or can be calculated via different methods using the field dependent specific heat values [12]. Also, the MCE can be calculated from magnetization (M) measurements, through a thermodynamic Maxwell equation, which can be derived from the first law of thermodynamics [13]

97

Zn0.4Ni0.6Fe2O4(x=0.6)

Zn0.6Ni0.4Fe2O4(x=0.4)

Zn0.8Ni0.2Fe2O4(x=0.2) ZnFe2O4(x=0.0)

10

20

30

40 2Θ

50

60

70

Fig. 1. XRD patterns of the system Zn1  xNixFe2O4, x¼ 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0.

98

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Table 1 Curie temperature of samples Zn1–x Nix Fe2O4 (x¼ 0, 0.2, 0.4, 0.6, 0.8, and 1)

8.40

a(A)

8.36

8.32

8.28 0.0

0.2

0.4

0.6

0.8

1.0

X Fig. 2. Variation of the lattice parameter a of the system Zn1  xNixFe2O4, x ¼ 0.0 to 1.0.

3.2. A.c resistivity Fig. 3. shows the Ac resistivity as a function of reciprocal temperature, the figure shows a break at a particular temperature indicating the sample changing from ferrimagnetism to paramagnetism this temperature is known as Curie temperature. The Curie temperature of a sample is given in the Table 1, it is clear from the table that Curie temperature changes from 558 to 537 K by increasing Ni content up to x¼ 1. The activation energy of the sample was determined by the slope of the linear part of ln(ρ) and 1000/T. The value of activation energy increases and reaches maximum at x¼ 0.2 and minimum at x¼ 0.4 Ni content and then decrease at higher Ni concentration. The variation of activation energy with Ni content is shown in Fig. 4. The decrease of activation energy may be due to the creation of smaller number of oxygen vacancies. It may be also due to the decrease in resistivity with increasing Ni concentration. It is also in the figure that the activation energy of paramagnetic region EP is higher than that of

X

TC (K)

0 0.2 0.4 0.6 0.8 1

558.97 440.72 485.42 460.44 435.81 537.63

ferrimagnetic region EF, indicating the influence of magnetic order on the conductivity. The behavior of activation energy Vs Ni content has the same trend as the variation of lnρ Vs Ni content [17]. The conductivity in ferrite is reported to occur as a result of electron hopping between ions of the same elements existing in different valence state. The occurrence of ions in more than one valence state is caused by the sintering temperature.Some vacancies and Fe2 þ ions in the lattice are created due to Zn loss during the sintering process. The number of Fe2 þ ions in the lattice depends on the amount of Zn lost from the sample. The lattice constant “a” calculated from XRD was found to decrease with increasing Ni content, the radius of Zn ion (0.82 Å) being larger than that of Ni ion (0.78 Å) so the addition of Ni at the expense of Zn is expected to decrease the lattice constant. This is indeed observed in the present work. The decrease in the value of “a” manifests itself as decrease in the inter-ionic distance, consequently in a decrease in barrier height of the hopping process. The activation energy is therefore expected to decrease with increasing the value of Ni concentration [5]. It is observed from Fig. 3 that the Ac resistivity decrease by increasing frequency from 1 kHz to 10 kHz. As the frequency of the applied field increase hopping of charge carrier also increase, thereby decreasing the resistivity. The real part of electrical resistivity consists of two terms [18]

ρ = ρ1(T ) + ρ2 (ω, T ) The first term is temperature dependence dc resistivity which is related to the thermal activation of mobility of electric charge and follows Arrhenius relation given by:

At 1KHZ

16

At 10KHZ

14

14 12

12 10

ln ρ

ln ρ

10

8

8

x=0 (1KHZ) 6

x=0.2 (1KHZ)

6

x=0 (10KHZ) x=0.2(10KHZ) x=0.4(10 KHZ) x=0.6(10KHZ) x=0.8(10KHZ) x=1 (10KHZ)

x=0.4 (1KHZ) x=0.6(1KHZ) x=0.8(1KHZ)

4

4

x=1 (1KHZ)

2 1.4

1.6

1.8

2.0

2.2

2.4

2.6

1000/T(K)

2.8

3.0

3.2

3.4

2 1.4

1.6

1.8

2.0

2.2

2.4

2.6

1000/T(K)

Fig. 3. shows the variation of lnρ vs 1000/T(K) at different frequencies 1 kHz and 10 kHz.

2.8

3.0

3.2

3.4

O.M. Hemeda et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 96–104

Activation energy at 1KHZ

Activation energy at 10KHZ 0.70

0.70 Ef 0.65

Ef

0.65

Ep

0.60

0.60

0.55

0.55

E (ev)

E (ev)

99

0.50

Ep

0.50

0.45

0.45

0.40

0.40 0.35

0.35 0.0

0.2

0.4

0.6

0.8

0.0

1.0

0.2

0.4

x

0.6

0.8

1.0

x

Fig. 4. Shows activation energy vs Ni content (x) at different frequencies.

with that reported in the previous work [19].

Ea

ρ1 = ρo e kpT 3.3. Dielectric constant

Where Ea is the activation energy for electric conduction, ρo the pre-exponential factor. The temperature and frequency dependent Ac resistivity which is related to the dielectric relaxation caused by localized charge carriers is given by

The dielectric constant as a function of temperature for all nano ferrite samples are shown in Fig. 6. The dielectric constant for all samples gradually increases with temperature up to 550–600 K. Behind this temperature the dielectric constant sharply increase. The sample Zn Fe2O4 has a peak at 536 K. At high temperature the increased thermal energy provided to the sample decrease the relaxation time and consequently the dielectric constant increases. In the sample ZnFe2O4 at higher temperature the value ωτ becomes very smaller than unity and at certain temperature equal 536 K a maximum value in dielectric constant is observed. The temperature dependence of the dielectric constant can further be explained as follows: Dielectric constant in ferrite is attributed to four types of polarization [20] interfacial, dipolar, atomic and electronic at lower frequencies and high temperature at which all four types of polarization contributes, the dielectric constant is mainly attributed to interfacial and dipolar polarization are strongly dependent on temperature. In case of interfacial polarization which is due to the accumulation of charges at the grain boundary, an increase of polarization results as more and more charge reach the grain boundary with increase in temperature. For the first sample Zn Fe2O4 beyond a certain temperature the charges acquire an adequate thermal energy to overcome the

ρ2 (ω, T ) = B(T ) ωn(T ) Where B is the parameter having the unit of resistivity and n is dimensionless Parameter. Since AC resistivity is frequency dependent in the low temperature region, so we can calculate the power n for different composition. The value of n is estimated from the relation

n=

ln ρ ln ω

And the dependence of n with temperature is shown in Fig. 5. It is known that variation of n with temperature can throw light on the type of conduction mechanism. If n increases with temperature, the conduction obeys small Polaron tunneling mechanism. It can be seen that the behavior of n is changed when Ni content increases. This observation indicated the presence of hopping conduction mechanism in the sample by different hopping rates. We can exclude the presence of small Polran tunneling model in Ac conduction. The value of n ranged from 1.6 to 0.4 through the temperature range 250 to 630 K. The values of n are in agreement

n at 1KHZ

1.8

x=0 x=0.2 x=0.4 x=0.6 x=0.8 x=1

1.6 1.4

x=0 x=0.2 x=0.4 x=0.6 x=0.8 x=1

1.2 1.0

1.2

0.8

1.0

0.6

n

n

n at 10KHZ

1.4

0.8

0.4

0.6

0.2

0.4

0.0

0.2

250

300

350

400

450

500

550

600

650

250

300

350

400

T (K) Fig. 5. Shows the variation of n, vs. temperature T (K) at different frequencies.

450

T (K)

500

550

600

650

100

O.M. Hemeda et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 96–104

at 1KHZ x=0 x=0.2 x=0.4 x=0.6 x=0.8 x=1

2000

x=0 x=0.2 x=0.4 x=0.6 x=0.8 x=1

350 300 250

ε

ε

1500

at 10KHZ

400

2500

1000

200 150

500

100 0

50 0 250

300

350

400

450

500

550

600

650

250

300

350

400

450

500

550

600

650

T (K)

T (K)

Fig. 6. Shows the dielectric constant of Zn1–x Nix Fe2O4 (x ¼0, 0.2, 0.4, 0.6, 0.8, and 1) at 1 kHz and 10 kHz.

resist barrier at the grain boundary and conduction take place resulting a decrease in the polarization and consequently dielectric constant. The interfacial polarization occurs up to a frequency of about 1 kHz with a possibility some contribution from the dipole polarization as a temperature increase small rise in the curve at 10 kHz shows that there is small contribution of dipolar polarization at this frequency. A shift in dielectric maximum is observed toward higher temperature with increasing the Ni content from 536 K to 560 K at 1 kHz. As the Ni content increase the polarization decrease, consequently the dielectric maximum shift to higher temperature. The rapid increase in dielectric constant with temperature was seen to begin at relatively higher temperature in the case of the sample (x ¼0.2, 0.8, and 1) than for the samples (x ¼0, 0.4, and 0.6). This observation can be correlated to the activation energy observed for all composition. The activation energy of composition with (x ¼ 0.2, 0.8, and 1) is higher than that of composition with (x ¼0, 0.4 and 0.6). 3.4. Transmission electron microscopy (TEM) The morphology and the extent of dispersion of Ni–Zn ferrite nanoparticles were determined using HRTEM. High resolution transmission electron microscopy was employed to confirm the results of XRD studies. TEM images of Zn1–xNixFe2O4 (x ¼0, 0.2, 0.8, and 1) are shown in Fig. 7. The particles exhibited cubic octahedral shapes nanoparticles. It is apparent that the cubic nanoparticles diameters were ranging from 25 to 36 nm. The increase of particle size with Ni content is evident in TEM images. The HRTEM images of four compositions have lattice spacing which confirms the crystalline nature of the samples. The inter planner distance obtained from HRTEM for Zn0.2Ni0.8Fe2O4 was found to be 0.13 nm which coincide with the planes (620) given from XRD. The HRTEM for other samples give D-spacing equal 0.08 nm which is referred to Bragg angle 2θ higher than 80 degree. The particle size determined from TEM was found to be in agreement with that obtained from XRD as given in Table 1. In addition, most of the nanoparticles are agglomerated and few are detached. This behavior suggests the presence of high magnetic dipole interaction among the Ni–Zn ferrite nanoparticles [21]. It is indicated from the TEM images that the Ni–Zn ferrite nanoparticles obtained are uniform in both morphology and crystalline size but having agglomeration to some extent.

3.5. Scanning electron microscopy analysis (SEM) SEM of samples Zn1  x Nix Fe2O4 (x¼ 0, 0.2, 0.6, 0.8, and 1) are shown in Fig. 8. The morphology of ferrite powders were investigated using SEM analysis. A fluffy like cotton morphology and sponge-like with porous structure for all samples. This morphology is similar to that observed in Mn–Zn powder prepared by microwave [22–24]. The porous structure has a large specific area that implies a much more active surface required for many applications. The chemical composition of the sample is checked using EDX spectrum obtained along with the SEM micrographs. Fig. (8) depicts the resulting EDX spectrum for each sample, where in sample ZnFe2O4, Fe, Zn and O peaks are clearly seen without any other peak for Ni. The Ni peak appeared in the other sample and its intensity increase as the value of x increase. The surface morphology SEM of the sample consists of some grains with relatively homogenies distribution with an average size varying from 0.85 to 0.92 μm as given in Table 2. Elemental analysis employing EDX determined the elemental composition of each sample. The estimated stoichiometry is very closed to the anticipated values. The grain size decreases by increasing Ni content, consequently the grain size boundaries increase. From SEM it is shown that the porosity decreases by increasing Ni content, leading to the decrease of the grain diameter [4]. 3.6. Magnetic permeability The initial permeability μi vs. temperature for the sample Zn0.6Ni0.4Fe2O4 nanoferrite is shown in Fig. 9.The initial permeability was measured at 1 KHZ which decreases sharply with increasing temperature and reaches a minimum value due to the transition from ferromagnetic to paramagnetic. The thermal variation of the initial permeability was recorded to find the Curie temperature Tc. The Curie temperature can be obtained from the μi–T curve by extraplolation the linear part to the temperature axis. The obtained value of the Curie temperature has the same value that obtained from the conductivity measurements. The behavior of magnetic permeability can be explained according to Globus model, according to this model μi is given by the relation [25,26]

O.M. Hemeda et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 96–104

Fig. 7. TEM images of Zn1–x Nix Fe2O4, (a) x ¼0, (b) x ¼0.2, (c) x¼ 0.8 and (d) x ¼ 1.

101

102

O.M. Hemeda et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 96–104

Fig. 8. SEM of Zn1  xNixFe2O4 prepared using hydrothermal technique. (a) x¼ 0, (b) x¼ 0.2, (c) x ¼ 0.6,(d) x ¼0.8 and (e) x ¼1.

620

Table 2 show the particle size determined from TEM and X-ray and grain size from SEM.

600

X

D (from TEM) nm

D (from X-ray) nm

Grain size from SEM mm.

0 0.2 0.8 1

25.22 28.06 36.66 31.08

32.57 31.98 31.23 30.46

0.92 0.90 0.85 0.88

580

μi =

Ms2 D k

Where Ms is saturation magnetization, D is the particle size and k is the magnetic anisotropy. The sudden drop of μi is due to the transformation from ferromagnetic to paramagnetic state. 3.7. Entropy change Following the thermodynamic Maxwell's equations the magnetic entropy change can be calculated from measurements M [H, T] where H is the magnetic field and T is the independent temperature magnetization curve. Magnetization (M) vs Temperature (T) at magnetic fields 16.8 kGs for sample Zn0.6Ni0.4Fe2O4 is shown

μ

560 540 520 500 480 250

300

350

400

450

500

550

T (K)

Fig. 9. shows the permeability vs. temperature of Zn0.6Ni0.4Fe2O4 at 1 kHz.

in Fig. 10. As shown from figure the Tc is around 460 K. As we well known, the adiabatic magnetic entropy change ΔSm, is determined by Maxwell’s fundamental relation [27]:

⎛ ΔSm(T , ΔH ) ⎞ ⎛ ∂M (T , H ) ⎞ ⎟ ⎟ =⎜ ⎜ ⎠H ⎠T ⎝ ⎝ ∂H ∂T

O.M. Hemeda et al. / Journal of Magnetism and Magnetic Materials 394 (2015) 96–104

Fig. 10. Magnetization as a function of temperature for ample Zn0.6Ni0.4Fe2O4 at 16.8 kGs.

To study the MCE of samples, a series of isothermal magnetization curves around their respective TC has been measured in a magnetic field up to 13.5 kOe. Fig. 11 shows these curves of ferrite Zn0.6Ni0.4Fe2O4.When magnetization is measured in a small discrete field and temperature interval, ΔSm could be determined from the following Eq:

ΔSm =

∑

Mi − Mi + 1 Ti − Ti + 1

ΔH

where Mi and Mi þ 1 are the experimental values of magnetization at Ti and Ti þ1, respectively, under magnetic field variation of ΔH. The |ΔSm|(T) curve of ferrite Zn0.6Ni0.4Fe2O4 is illustrated in Fig. 12. The |ΔSm| reached a maximum value of 0.0.025 J/kg K near Curie temperature. Similar behavior was observed for other samples under study. The values of |ΔSm|max in our samples are identify with that firstly examined by Chaudhary et al. [28] for cobaltite perovskites La1–xSrxCoO3. Thus Ni1–xZnxFe2O4 ferrites could be considered as active magnetic refrigerant materials working in quite wide temperature range.

4. Conclusion Nanoparticles of Ni–Zn ferrite were prepared by solution

Fig. 11. A series of isothermal magnetization curves of sample Zn0.6Ni0.4Fe2O4.

Fig. 12. magnetic Zn0.6Ni0.4Fe2O4.

entropy

change

103

|ΔSm|

versus

temperature

of

sample

combustion method. The small changes in the lattice parameters are due to the incorporation of the slightly small Ni ion in place of the Zn ion.The Ac resistivity decrease by increasing frequency from 1 kHz to 10 kHz.A shift in dielectric maximum is observed toward higher temperature with increasing the Ni content from 536 K to 560 K at 1 kHz. The inter planner distance obtained from HRTEM for Zn0.2Ni0.8Fe2O4 was found to be 0.13 nm which coincide with the planes (620) given from XRD. The grain size decreases by increasing Ni content, consequently the grain size boundaries increase. The thermal variation of the initial permeability and magnetization were recorded to find the Curie temperature Tc. The entropy change Vs temperature shows maximum near Curie teperaure. This behavior would be potentially useful for the operation of cooling devices over a broad temperature range.

Acknowledgements The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the research Group projects no. RGP-306.

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