^{1}

^{2}

^{*}

^{2}

^{3}

^{3}

The effect of Zn
^{2+} ions on the microstructure and electrical properties of Mn
_{1-x}Zn
_{x}Fe
_{2}O
_{4} (0.0 ≤ x ≤ 0.5 in steps of 0.1) through a solid state reaction has been investigated. The structural properties
have been investigated using X-ray diffraction (XRD) technique. The XRD analysis confirms that
all samples are in a single-phase cubic spinel structure. The experimental lattice parameter (a_{exp})
was decreased with increasing Zn
^{2+} ions substitution due to the smaller ionic radius of zinc content. The crystallite size (
t) of samples was estimated by Scherrer’s formula and found in the range (90 - 115 nm). Dc electrical resistivity and Seebeck voltage coefficients were measured as a function of temperature using the two probe methods. The temperature variation of resistivity exhibits two breaks, each break referring to a change in the activation energy. The Curie temperature estimated from dc resistivity measurement decreases with increasing Zn
^{2+} ions. Seebeck voltage coefficient measurements reveal n-type conduction for all samples.

Soft ferromagnetic oxides are of great importance as high-frequency magnetic materials. The general formula for these compounds is MOFe_{2}O_{3}, where M is a divalent metallic ion such as Fe^{2+}, Ni^{2+}, Mg^{2+}, Mn^{2+}, Zn^{2+} or a mixture of these ions [_{2}O_{4}, where (A) and [B] refer to tetrahedral and octahedral cation sites, respectively, in a FCC anion (oxygen) sub-lattice. The type of cations and their distribution between the two interstitial sites in these ferrites determine many of the intrinsic magnetic properties of the ferrites [

Manganese zinc ferrites represent an important class of soft ferromagnetic materials characterized by high magnetic permeabilities, wide range of temperature and frequency, and high stability which are widely used in anti-electromagnetic interference (EMI) noise filters, broad band electronic circuits transformers, integrated services digital network (ISDN), local area network (LAN), wide area network (WAN), pulse transformer, and background lighting, etc. [^{2+} ions by Zn^{2+} ions to reduce the electric losses. M.M. Hessien et al. [_{s}) was obtained at Mn_{0.8}Zn_{0.2}Fe_{2}O_{4} phase. S.J. Kim et al. [^{3+} and Fe^{2+} valence states and explained that the saturation magnetization was increased by Fe^{2+} ion.

Electrical transport properties of ferrites provide information suitable for the selection of these materials for specific application and they are used in the interpretation of the conduction mechanism in semiconductors. The measurement of thermoelectric power is simple and its sign gives vital information about the type of conduction in semiconductors, i.e., whether they are n- or p-type. Electrical conductivity, which gives valuable information about conduction mechanism, is one of the important properties of ferrites [

In this work, we study the influence of Zn^{2+} ions substitution on the microstructure and transport properties of Mn_{1−x}Zn_{x}Fe_{2}O_{4} (0.0 ≤ x ≤ 0.5 in steps of 0.1) using X-ray diffraction (XRD), infrared spectroscopic analysis (IR), dc electrical resistivity and Seebeck coefficient which are utilized in order to study the effect of variation of zinc substitution to get the desired concentrations of Mn^{2+} and Zn^{2+} ions to obtain the desired Curie temperature and electric properties to reduce losses to open new era of applications in telecommunications and electronics.

Polycrystalline spinel ferrite with formula Mn_{1−x}Zn_{x}Fe_{2}O_{4}, (0.0 ≤ x ≤ 0.5 in steps of 0.1) was prepared by a solid state reaction. In this method high purity oxides MnCO_{3}, ZnO and Fe_{2}O_{3} were used in stoichiometric ratio and well ground in agate mortar for 4 h, and pre-sintered in air at 900˚C for 5 h with heating rate 4˚C/min using Lenton UAF 16/5 furnace then slow cooled to room temperature. After that, the samples were grounded again for 3 h and the mixture was pressed into pellets using unaxial press of pressure 1.9 × 10^{8} N·m^{−2}, then finally sintered in air at 1300˚C for 15 h with heating rate 2˚C/min. Crystalline phases in different annealed samples were identified using XRD on a Brucker axis D8 diffractometer using the Cu-K_{α} radiation (λ = 1.5418 Å) radiation and a secondary monochromator in the range 2θ from 10˚ to 80˚. The experimental lattice parameter a_{exp}

is calculated by using the relations,

Miller indices of each plane and lattice parameter can be calculated from the slope of linear relation between

1/d and_{x} (g/cm^{3}) was calculated from the relation,

cular weight of the sample, a the lattice parameter of the ferrite and N_{A} is Avogadro’s number. The percentage porosity P% was calculated from the relation, _{app} is the apparent density. The two surfaces of each pellet were coated with silver paste and checked for good electrical contact. The dc resistivity of the samples was measured using the two probe method. The thermoelectric power (Seebeck coefficient) as a function of temperature was measured across the pellet to determine the type of charge carriers.

Sintering process is necessary step in the preparation of ferrite by standard ceramic method, and the sintering temperature plays a key role in crystalline type and particle size of ferrite [^{2+} ions, indicating that the lattice parameter for the cubic Mn_{1−x}Zn_{x}Fe_{2}O_{4} samples decreased with Zn-content (x). The crystallite size (t) of the prepared samples was calculated using Scherrer’s Equation [

where β is the peak width at half maximum, λ is wavelength and θ corresponds to the peak position. The most intense peak at (220), (311) and (400) are used for calculation. The calculated crystallite size (t) as a function of Zn content (x) shows that the substitution the Mn^{2+} ions by Zn^{2+} ions results in decrease in crystallite size from 115 nm to 90 nm as shown in

The experimental lattice parameter (a_{exp}) of the prepared samples was calculated and plotted as a function of Zn content (x) which decreases with increasing Zn content (x) in accordance to gradual replacement of bigger Mn^{2+} ions of higher ionic radius (0.75 Å) with smaller Zn^{2+} ions of ionic radius (0.68 Å) as shown in _{th}) is calculated using the equation [

Zn-content (x) | Apparent density (D_{app}) g/cm^{3} | X-ray density (D_{x}) g/cm^{3} | Crystallite size (t) nm |
---|---|---|---|

0 | 4.788 | 4.978 | 115 |

0.1 | 4.781 | 5.009 | 108 |

0.2 | 4.741 | 5.04 | 100.3 |

0.3 | 4.743 | 5.071 | 96.6 |

0.4 | 4.713 | 5.106 | 96.5 |

0.5 | 4.632 | 5.135 | 90.15 |

where R_{o} is the radius of the oxygen ion (1.38 Å), r_{A} and r_{B} are the ionic radii of tetrahedral A- and octahedral B- site, respectively. In order to calculate r_{A} and r_{B} it is necessary to know the cation distribution for a given system. In general, Zn^{2+} ions have higher preference for A-sites while Mn^{2+} and Fe^{3+} ions are distributed between the tetrahedral A- and octahedral B-sites, so the formula of the cation distribution can be written as:

where the brackets ( ) and [ ] denote to A- and B-sites respectively. The ionic radius of each site was calculated according to the following equations:

where^{2+}, Mn^{2+} and Fe^{3+} ions respectively. The theoretical and experimental values of lattice parameter (a_{th} and a_{exp}) are plotted against Zn content (x) in _{th} and a_{exp}) decreased slightly with increasing Zn content (x). This decrease may be due to substitutional occupancy since the ionic radii of Zn^{2+}, Mn^{2+} and Fe^{3+} ions are 0.68, 0.75 and 0.64 Å respectively. ^{2+} ions during the sintering process, which have an ionic radius greater than Fe^{3+}.

The X-ray density (D_{x}) was calculated using the relation [

where 8 is the number of molecules in a unit cell of spinel lattice, M is the molecular weight of the sample, a_{exp} is the experimental lattice parameter of the ferrite and N is the Avogadro’s number. The theoretical density D_{x} depends on the lattice constant and molecular weight of the sample. The theoretical density D_{x} as a function of Zn concentration is shown in _{x}. Pores trapped inside the grains must be explained in terms of rapid grain growth, with increasing the Zn^{2+} ions concentration. It is clear from _{intra}) and intergranular porosity (P_{inter}), so that the total porosity (P%) could be written as the sum of two types

The percentage porosity (P%) was calculated from the relation

where D_{app} is the apparent density, it was found that the apparent density D_{app} values are less than those of X-ray density D_{x} as shown in ^{2+} ions by Zn^{2+} ions caused a relative decrease in apparent density D_{app} of the sample and a corresponding increase in porosity P%. In addition the decrease of density and the

increase of porosity with increasing Zn content (x) are due to the increase of oxygen vacancies which plays a predominant role in accelerating densification; i.e. the decrease in oxygen ion (anion) diffusion would retard the densification. The presence of Zn^{2+} ions reduces the population of Fe^{3+} ions in B-sites resulting in the decrease of density as well as the increase in porosity [

_{1−x}Zn_{x}Fe_{2}O_{4} (0.0 ≤ x ≤ 0.5), from which it can be seen that there are five bands characterizing spinel ferrites in the range 200 - 1000 cm^{−1} for the observed samples [_{1}) is in range of 547 - 555 cm^{−1} and is related to intrinsic vibrations of tetrahedral group, the second band (υ_{2}) is in the range of 407 - 430 cm^{−1} and is related to intrinsic vibrations of octahedral group. Band (υ_{1}) (near 600 cm^{−1}) arises due to tetrahedral complexes (the stretching vibration of tetrahedral metal-oxygen band) and band (υ_{2})_{ }(near 400 cm^{−1}) is due to octahedral complexes (metal-oxygen vibration in octahedral sites). The difference in position of two strong bands (υ_{1}) and (υ_{2}) could be related to difference in Fe^{3+}-O^{2−} distance for A-sites (0.189 nm) and that of the B-sites (0.199 nm). The three low-frequency bands, (υ_{3}) is in the range 380 - 387 cm^{−1} and is related to the divalent octahedral metal-oxygen ion complexes. The fourth band (υ_{4}) and fifth band (υ_{5}) in range 215 - 284 cm^{−1} were observed and can be assigned to the divalent tetrahedral vibrations (lattice vibration). The presence of shoulder (υ’) could be ascribed to the linear combination of the bands (υ_{2}) at 407 cm^{−1} and (υ_{3}) at 380 cm^{−1} (407 + 380) = 787 cm^{−1} at x = 0.1, (υ_{2}) at 423 cm^{−1} and (υ_{3}) at 380 cm^{−1} (423 + 380) = 803 cm^{−1} at x = 0.4 [

The electric properties of the ferrite to spinel system Mn_{1−x}Zn_{x}Fe_{2}O_{4}, was studied for (0.0 ≤ x ≤ 0.5 in steps of 0.1), over temperature range from room temperature to 650 K and represented in

where E is the activation energy, K is the Boltzmann constant, ρ_{o} is the temperature independent constant. The temperature dependence of resistivity exhibits two breaks and distinct regions (I, II and III). Such a break was associated with a change in the slope which is attributed to the change of magnetic order and lowering the generation of charge carrier [

Zn-content (x) | υ’ cm^{−}^{1} | υ_{1} cm^{−}^{1} | υ_{2} cm^{−}^{1} | υ_{3} cm^{−}^{1} | υ_{4} cm^{−}^{1} | υ_{5} cm^{−}^{1} |
---|---|---|---|---|---|---|

0.0 | 783 | 550 | 409 | 383 | 261 | 223 |

0.1 | 793 | 550 | 407 | 380 | 284 | 223 |

0.2 | 795 | 547 | 411 | 381 | 268 | 224 |

0.3 | 795 | 549 | 410 | 386 | 273 | 226 |

0.4 | 799 | 551 | 423 | 380 | 269 | 219 |

0.5 | 795 | 555 | 430 | 387 | 255 | 215 |

linked with magnetic ordering or with a conduction mechanism. The first region is attributed to extrinsic conduction mechanism (impurities); it extends from room temperature up to the first transition temperature (T_{1}) ≈ 305:355 K for all studied samples. The second transition temperature (T_{c})_{elec.} is always attributed to the magnetic phase transition from ferrimagnetic to paramagnetic state. The activation energies for the conduction process were calculated from slopes of each line according to Equation (8). The activation energies (E_{ferri.}) for region II and (E_{para.}) for region III and the transition temperature (T_{c})_{elec.} between ferrimagnetic and paramagnetic region are given in _{0.8}Zn_{0.2}Fe_{2}O_{4}, as an example of studied samples to illustrate the transition temperatures. The change in

Zn-content (x) | E_{ferri.}(eV)II | E_{para.}(eV)III | T_{1}(K) | T_{c.elect.}(K) | ρ(Ω.m) × 10^{3} |
---|---|---|---|---|---|

0.0 | 0.45 | 0.67 | 355 | 515 | 6.3 |

0.1 | 0.43 | 0.81 | 343 | 541 | 2.3 |

0.2 | 0.40 | 0.55 | 333 | 464 | 7.6 |

0.3 | 0.39 | 0.50 | 333 | 409 | 31.9 |

0.4 | 0.37 | 0.50 | 305 | 396 | 13.9 |

0.5 | ---- | 0.41 | 314 | ---- | 36.9 |

activation energies between region II and region III is attributed to magnetic transition from order to disorder state. It is observed that (E_{para.}) is greater than (E_{ferri.}) for all investigated samples as shown _{c}. The lower activation energy in the ferrimagnetic region is attributed to the magnetic spin disordering due to decrease in the concentration of current carriers [

The temperature dependence of Seebeck voltage coefficient for the investigated samples for Mn_{1−x}Zn_{x}Fe_{2}O_{4} with 0.0 ≤ x ≤ 0.5 is shown in ^{2+} to Fe^{3+} ions (Fe^{2+} ↔ Fe^{3+} + e^{−}) [^{2+} ion substitution leads to the migration of more Fe^{3+} from tetrahedral site to octahedral one, hence increasing hopping electron between Fe^{2+} ↔ Fe^{3+}. This means that the variation in the conductivity is due to thermally activated mobility and not due to thermal activated creation of charge carriers.

1) XRD measurements confirm the formation of single-phase cubic spinel structure;

2) The experimental lattice parameter (a_{exp}) and crystallite size (t) decrease while the porosity (P) increases with increasing Zn^{2+} ions;

3) Dc electrical resistivity and Seebeck voltage coefficients were measured as a function of temperature using the two probe method; it is also found that the temperature variation of resistivity exhibits two breaks, each break referring to a change in the activation energy;

4) The Curie temperature decreases with increasing Zn^{2+} ions;

5) Seebeck voltage coefficients measurements reveal n-type conduction for all samples.

The investigated samples Mn_{1−x}Zn_{x}Fe_{2}O_{4} (0.0 ≤ x ≤ 0.5 in steps of 0.1) showed that:

· Dc electrical resistivity increased from (2.3 - 36.9) × 10^{3} Ω.m with increasing Zn^{2+} ions;

· The Curie temperature decreases with increasing Zn^{2+} ions;

· Seebeck voltage coefficients measurements reveal n-type conduction for all samples.

So we recommend sample x = 0.5 in many applications due to higher resistivity.