Magnetic transformation of Zn-substituted Mg-Co ferrite nanoparticles: Hard magnetism → soft magnetism

Journal Pre-proofs Research articles Magnetic transformation of Zn-substituted Mg-Co ferrite nanoparticles: hard magnetism→soft magnetism Wei Zhang, Aimin Sun, Xiaoguang Pan, Yingqiang Han, Xiqian Zhao, Lichao Yu, Zhuo Zuo, Nanzhaxi Suo PII: DOI: Reference:

S0304-8853(19)32936-1 https://doi.org/10.1016/j.jmmm.2020.166623 MAGMA 166623

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Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

21 August 2019 22 December 2019 14 February 2020

Please cite this article as: W. Zhang, A. Sun, X. Pan, Y. Han, X. Zhao, L. Yu, Z. Zuo, N. Suo, Magnetic transformation of Zn-substituted Mg-Co ferrite nanoparticles: hard magnetism→soft magnetism, Journal of Magnetism and Magnetic Materials (2020), doi: https://doi.org/10.1016/j.jmmm.2020.166623

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Magnetic transformation of Zn-substituted Mg-Co ferrite nanoparticles: hard magnetism→soft magnetism Wei Zhang1, Aimin Sun1,2 *, Xiaoguang Pan, Yingqiang Han, Xiqian Zhao1, Lichao Yu1, Zhuo Zuo1and Nanzhaxi Suo1 1 College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China 2 Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, Lanzhou 730070, China

Abstract: Zinc substituted magnesium-cobalt ferrite nanoparticles having the basic composition Mg0.2Co0.8-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6 and 0.8) were successfully synthesized using sol-gel auto combustion method. The results show that the doping amount of Zn2+ ions has a great influence on its structure and magnetic properties. X-ray diffraction (XRD) measurements show that Mg0.2Co0.8-xZnxFe2O4 has a good phase formation, and all samples have single-phase cubic spinel structure. The average crystallite size is calculated from Scheller's formula to be between 49-54 nm. And the lattice constant increases from 8.38 to 8.43 Å with the increase of Zn2+ ions content. Fourier transform infrared (FTIR) measurements also confirm the formation of the cubic spinel structure of ferrite. The ferrite samples were observed by scanning electron microscopy (SEM) to be spherical nanoparticles. The magnetic properties of the samples have been determined at room temperature by vibrating sample magnetometer (VSM). The results show that the magnetic properties of Mg-Co ferrite are significantly affected by the doping amount of Zn2+ ions. For Zn-substituted Mg-Co ferrites, the magnetic properties decrease obviously with the increase of Zn content. This rapid decrease of magnetic properties reveals that the magnetic properties of Zn-substituted magnesium-cobalt ferrite have realized the transition from hard magnetic to soft magnetic. Meanwhile, the coercivity is Hc = 21.26 Oe, the saturation magnetization is 4.56 emu/g and the residual magnetization is 0.03 emu/g for

*

Corresponding Author: Aimin Sun

[email protected]

Mg0.2Zn0.8Fe2O4 sample. This also indicates that the ferrite samples prepared have a transition from ferromagnetic behavior to superparamagnetic behavior. The smaller coercivity value confirms that soft ferrite has been obtained. Meanwhile, this transition from hard magnetism to soft magnetism can be used as a potential high frequency soft magnetic material. Keywords: Mg-Co ferrites; Zinc substituted; Sol-gel auto-combustion; Structural; Magnetic properties.

1. Introduction Nanomaterials are different from other materials because of their unique size effect, surface and boundary effect, quantum effect and macroscopic quantum tunnel effect. Nano materials because of its unique size effect, surface and boundary effect, quantum effect and macroscopic quantum tunneling effect and different from other materials.It shows excellent special properties and has broad application prospects. It has excellent special performance and broad application prospect.The physical, chemical and magnetic properties of nanomaterials change with the change of their size and morphology. Nanomaterials of physical, chemical and magnetic changes over its size and shape.Therefore, many researchers have been trying to use a more simple and effective experimental method to prepare nano-magnetic materials and adjust their particle size and micro-morphology.Therefore, many researchers have been trying to use a more simple and effective experimental method to the preparation of nanometer magnetic materials, and adjust its size and microstructure. Spinel nanoferrites have excellent magnetic properties. It has important applications in many fields, such as magnetic devices, switching devices, magnetic recording, read-write magnetic head, magnetic refrigeration, magnetic resonance (MRI), magnetic cell separation, detoxification of biological fluids and magnetic targeting drug delivery [1-6]. Spinel ferrite is a nano-magnetic material with chemical general formula M2+Fe2O4, in which M2+ indicates that tetrahedral (A-site) is occupied by divalent metal cations [7]. In ferrite, Mg2+ and Co2+ ions mainly occupy the octahedral position (B-sites), however, a small number of Mg2+ and Co2+ ions occupy the tetrahedron position (A-site). Therefore, magnesium-cobalt ferrite shows mixed spinel structure. Kulkarni et al. [8] synthesized the spinel solid solution series Mg-Co ferrite by

co-precipitation method. The results of magnetic susceptibility and Mossbauer spectra show that the prepared fine particles can cause unusual magnetic properties in the system, such as superparamagnetism. In the past, many considerations have been established for diamagnetic exchange in mixed ferrites [9]. The change of magnetic and electrical properties of nanoferrite is due to the existence of nonmagnetic ions in spinel lattice [10]. Meanwhile, it also has high coercivity, large magnetocrystalline anisotropy, relatively large saturation magnetization and large magnetostriction coefficient at room temperature [11-14]. Moreover, Mg-Co ferrite has a series of excellent properties such as relatively good flexibility, mechanical hardness and chemical stability. These excellent characteristics make it widely used in magneto-optical recording, electronic material devices, high-density magnetic recording devices, supercapacitors and photocatalysis [15-19]. The saturation magnetization of Mg0.2Co0.8Fe2O4 is lower than that of metal, but the resistivity of Mg0.2Co0.8Fe2O4 is much higher than that of metal soft magnetic materials. Therefore, if the magnetic transformation can be realized: hard magnetism – soft magnetism, it will have good high frequency characteristics. In addition, many researchers generally believe that doping Mg-Co ferrite with non-magnetic metal ion Zn2+ can counteract the coupling between magnetic ions, which will effectively refine the grain size, reduce the coercivity and improve the saturation magnetization. The magnetic properties of Mg0.2Co0.8Fe2O4 doped with Zn2+ ions are changed: hard magnetism – soft magnetism, which has good application prospects in high and ultra-high frequencies. It is precisely because of the strong occupy tetrahedral position (A-site) of Zn2+ ions in the spinel ferrite structure. Therefore, in extreme cases, ZnFe2O4 has a normal spinel structure. For soft magnetic materials such as ZnFe2O4, its main characteristics are easy magnetization and demagnetization, as well as high permeability, high saturation magnetization, high resistance, low magnetic loss and good stability. It is mainly used in making inductance winding coil, small transformer, pulse transformer, recording head and magnetic amplifier, etc. Meanwhile, it is possible that the spinel structure will be distorted accordingly for Zn2+ ion replaces Mg-Co ferrite [20]. It also depends on the concentration of cations. Therefore, Mg-Co-Zn ferrite has mixed spinel structure [21]. The use of zinc ions to replace

Mg-Co mixed ferrites is due to their high magnetization temperature sensitivity. This characteristic has a wide range of applications for such as self-controlled hyperthermia. There are many methods for the preparation of spinel nanocrystalline ferrite, such as coordination compounds method and organic metal thermal decomposition method, microwave assisted synthesis method, ultrasonic radiation method, the freeze drying method, hydrothermal synthesis method, co-precipitation method and sol-gel auto-combustion method, etc. In this experiment, sol gel method was used to prepare spinel ferrite with required properties. This is because sol-gel method has many unique advantages compared with other methods. The raw materials used in sol-gel method are first dispersed into solvents to form low viscosity solutions. Therefore, the uniformity of molecular level can be obtained in a short time. When forming gel, the reactants are likely to be evenly mixed on the molecular level. However, up to now, there are few reports on the structure and electromagnetic properties of Mg-Co ferrites doped with non-magnetic Zn2+ ion and few studies have been done on the causes of their magnetic transition. Therefore, the sol-gel self ignition method was used to prepare magnetic nano particles of Mg0.2Co0.8-xZnxFe2O4 (x=0.0，0.2，0.4，0.6，0.8) in this paper. The effects of different Zn2+ ions substitution contents on the structure and magnetic properties of Mg-Co ferrite nanoparticles were studied. The structure characteristics of the prepared nanoparticles were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The micro-morphology of the prepared nanoparticles was characterized by Scanning Electron Microscope (SEM). The chemical composition for the constituent elements of the synthesized nanoparticles by Energy dispersive x-ray (EDX). By Vibrating sample magnetometer (VSM) characterization of the preparation of magnetic nanoparticles. The adjustment of subferromagnetic parameters (magnetization, rectangularity, coercivity) of spinel ferrite is very important for its possible application in magnetic data storage [22-23].

2. Experimental Mg0.2Co0.8-xZnxFe2O4 (Where, x=0.0, 0.2, 0.4, 0.6 and 0.8) nano powder was prepared by sol-gel auto combustion method. Using the analytical balance, the

appropriate amount of high nitrate magnesium nitrate (Mg(NO3)2·6H2O), cobalt nitrate (Co(NO3)2·6H2O), Zinc nitrate (Zn(NO3)2·6H2O) and ferric nitrate (Fe(NO3)3·9H2O) were weighed and the molar ratio was 0.2: (0.8-x): x: 2. The nitrate was dissolved in 120 mL deionized water and added with the proper amount of citric acid (C6H8O7·H2O). Citric acid in a molar ratio of 1.2:1 to metal ions was used in the synthesis. Slowly add the appropriate amount of ammonium hydroxide (NH3·H2O) to adjust the pH value of the solution to 7 under constant agitation.The precursor solution was stirred by a magnetic force for 3-4 hours under 80 ℃ water baths to make nitrate and citrate fully dissolved. The obtained precursor solution evaporates water slowly under 80 ℃ water baths until a viscous sol is formed. The obtained sol was placed in a blast dryer and heated at a constant temperature for 2 h at 120 ℃, until fluffy xerogels were formed. Prepared xerogels were sintered using alcohol blowlamp, then leaving a lot of loose product. These flocs were ground into powder in an agate mortar, and the precursor powder was sintered at 900 ℃ for 3 h in a muffle furnace. After calcination, the samples were cooled to room temperature, and then take out the Mg0.2Co0.8-xZnxFe2O4 powder samples. These powders were used for characterization. The typical flow chart of preparation of Mg-Co-Zn ferrite system by sol-gel auto combustion method is shown in Fig. 1.

3. Result and discussion 3.1 Structural characterization The unit cell structure of ferrite is shown in Fig. 2. The lattice of spinel ferrite has a complex face-centered cubic structure. There are 56 ions in one cell of ferrite, including 32 oxygen ions and 24 metal cations, which are equivalent to 8 molecules of MFe2O4. In the spinel ferrite cell, oxygen ions are densely packed to form 32 octahedral and 64 tetrahedral gaps. As a result of the equilibrium of valence states, only 8 of the 24 metal ions occupy A-sites, while 16 occupy B-sites. the remaining 16 positions in the octahedral gap and 56 positions in the tetrahedral gap. There are two kinds of gap between oxygen ions: tetrahedral gap and octahedral gap, as showed in Figure 2. The tetrahedral gap is surrounded by four nearest oxygen atoms. It consists of four triangular planes connected by four nearest oxygen atom centers, so it is called tetrahedrons. Because of its small gap, it is called A-sites. The octahedral gap is

surrounded by six nearest oxygen atoms. It consists of six nearest neighbor oxygen atom centers connected to form eight triangular planes, so it is known as octahedrons. Because of its large gap, it is called B-sites. The maximum radius of metal ions that tetrahedron and octahedron can accommodate is related to lattice constant and oxygen parameter. In spinel ferrite lattices, metal ions at adjacent A and B sites are separated by nonmagnetic oxygen ions with larger ion radius. The relatively large distance between metal ions makes it impossible for electrons to exchange directly, so they can only be carried out through non-magnetic ions in the middle. This exchange interaction through oxygen ions is called super-exchange interaction. For spinel structure, there are three types of super-exchange interaction of metal ions: A-O-B, A-O-A and B-O-B. Meanwhile, among the three super-exchange functions mentioned above, they can be further divided into five types. Configuration of ion pairs in spinel ferrites with favourable distances is shown in Fig. 3. According to Anderson's calculation, when the angle between M-O-M is β =90°, the super-exchange interaction is the weakest, and when the angle between M-O-M is β =180°, the super-exchange interaction is the strongest. According to the principle of super-exchange and the five relative positions in Fig. 3, it can be seen that the A-B type super-exchange is the strongest, followed by the B-B type super-exchange, and the A-A type super-exchange is the weakest. The required properties of Mg0.2Co0.8-xZnxFe2O4 (where, x = 0.0, 0.2, 0.4, 0.6 and 0.8) spinel ferrite nanoparticles were synthesized by sol-gel auto combustion method. The powder X-ray diffraction (XRD) patterns of further grinded spinel ferrite nanoparticles are shown in Fig. 4(a). The diffraction peaks of the prepared samples were compared with the standard diffraction patterns, and the phase identification was carried out. The measured diffraction peaks of all the synthesized ferrite samples match well with the standard pattern (JCPDS reference card #22-1086). Moreover, no other foreign phase was observed in all samples, which indicated that the prepared spinel ferrite nanoparticles had a single-phase cubic spinel structure. Meanwhile, it can be observed that the substituted Mg-Co ferrites with different Zn content do not change the structure of cubic spinel. The sample shows all characteristic reflections of the ferritic material, with the strongest (311) reflections, confirming the formation of

the cubic spinel structure. Diffraction peaks were observed in all synthesized ferrite samples and successfully indexed as (111), (220), (222), (400), (422), (511), (440) and (311). These measured diffraction peaks are the cubic spinel phase plane of Mg0.2Co0.8-xZnxFe2O4 [24-26]. The lattice of spinel Mg-Co ferrite will eventually expand when the smaller ion radius of Co2+ ions are substituted by the larger ion radius of Zn2+ ions. At the same time, the substitution of Zn2+ ions with larger ion radius will cause uniform strain and elastic deformation in the lattice structure. This effect is caused by doping will change the lattice plane spacing. The diffraction peak (311) moves to a lower 2θ positions as the substitution of Zn2+ ions increases (Fig. 4b). The lattice parameter was calculated according to the formula: a  d h2  k 2  l 2

(1)

Where ‘a’ is lattice constant, ‘d’ is spacing between the planes, (h, k, l) are the miller indices. In order to calculate the average crystallite size from Scherrer equation, the half-width of the strongest diffraction peak (311) measured by XRD was used:

D

0.9  cos 

(2)

Where ‘λ’ is wavelength of the X-ray radiation, ‘β’ is the full width at half maximum of the strongest diffraction peak (311) [27], and ‘θ’ is the Bragg's angle. The X-ray density (ρx) was calculated using the following relation [28, 29]:

x 

8M N Aa 3

(3)

Where ‘M’ is the molecular weight, ‘NA’ is the Avogadro's parameter and ‘a’ is the lattice parameter. The packing density of the disk-shaped particulate samples was calculated by considering the size and weight of the sample [30]:

 b  M s R 2 t 

(4)

Where, Ms is the sample’s mass, ‘r’ is the sample’s radius and ‘t’ is the sample’s thickness.

The porosity (P) of samples was deduced from bulk (ρb) and X-ray densities (ρx) using the formula: Porosity P (%) =  x -  b   x 100

(5)

The dislocation line density of the prepared samples has been calculated using the following formula [31]: δ

1 D2

(6)

The XRD parameters calculated from the above formulas are recorded in Table 1 and Table 2. Relation diagram of the lattice constants and the average crystallite sizes with the substitution of Zn2+ ions are shown in Fig. 5 (a). Observation of Fig. 5 (a) and Table 1, the lattice constant increases gradually with the increase of substitution amount of Zn and follows Vegard's law [32]. The lattice constant increases gradually from 8.3858 Å to 8.4352 Å when Zn2+ content reaches 0.8 of Mg0.2Zn0.8Fe2O4. The change of lattice constant may be due to the difference of ion size between Zn2+ and Co2+ ions. Because the smaller ionic size of Co2+ (0.72 Å) is substituted by the larger ionic size of Zn2+ (0.82 Å) [33-36]. Therefore, it is predicted that the lattice constants will be increased by the substitution of Co2+ ions by Zn2+ ions in ferrite lattices. It can be clearly seen from Table 1 that the experimental values of average crystalline size decreases first and then increases with the increase of Zn content, which is calculated by the Debye-Scherrer equation. It has been observed that the average crystalline size decreases from 52.46 nm to 49.63 nm with the increase of Zn2+ ions content up to x = 0.0-0.4, and further increases of Zn2+ ions content (x = 0.6-0.8), thus increasing the average crystalline size from 51.46 to 54.13 nm. The experimental results show that the dopant of Zn2+ ions with lower concentration (x = 0.0, 0.2 and 0.4) controls and retards the growth of grain size, while the dopant of Zn2+ ions with higher concentration (x = 0.6 and 0.8) favors the growth of grain size at the nucleation centers, which resulted in higher crystallite size [37]. This change of cation distribution and magnetism is attributed to the dimension decreases from bulk to nanosize. As a striking example, zinc ferrite belongs to normal spinel ferrite in bulk, and its nanoparticles have ferrimagnetic at low temperature. The low concentration of zinc dopant will hinder the growth of crystal, and the entropy of formation will reflect

any configurational disorder present in the spinel, and at high temperature a considerable "entropy stability" may result from this disorder [38]. Meanwhile, the intensity of the diffraction peak increased significantly with the further increase of Zn2+ ions content, which indicated that the growth of the grain size was promoted by the higher concentration of Zn2+ dopant (x = 0.6 and 0.8). It can be seen from relevant literature that the relationship between the dislocation density and the square of the average crystallite size is inversely proportional . And the reduction of dislocation line density indicates the reduction of defect density, which means the improvement of crystallinity [39]. The linear dislocation density increased from 3.63 to 4.06 10-4 nm-2 at low concentration of Zn dopant (x = 0.0, 0.2 and 0.4), which indicated that the crystallinity of the sample decreased, and further revealed that the low concentration of Zn dopant would hinder the growth of the crystal. However, when the substitution amount of Zn is further increased to 0.8, the linear dislocation density decreases and the crystallinity of the samples increases. Moreover, the line dislocation density of Mg0.2Zn0.8Fe2O4 is the smallest, which also shows that it has the best crystallinity, which is consistent with the intensity of the diffraction peak observed in XRD. It is interesting to note that these features of surface defect material is very advanced in technology application [40]. In Fig. 5(b) and Table 2, the porosity of the prepared samples increased gradually with the increase of Zn content. The increase of porosity and X-ray density may be attributed to the increase of lattice constants of ferrite samples, which is also due to the irregular shape of grains [41]. At the same time, the discontinuous grain growth occurs in the sintering process of ferrite powder. It is the discontinuous grain growth that leads to the increase of intergranular porosity [42]. 3.2 Cation distribution In ferrite with anti-spinel structure, 8 bivalent metal ions (Me2+) occupy octahedral (B) position, while the remaining 16 Fe3+ ions distribute evenly between tetrahedral (A) and octahedral (B) sites. This is based on Neel's two sublattice models. As shown in the cation distribution formula [43]: (Fe3+ ) A [Me 2+ Fe3+ ]B O 4

(7)

Neutron diffraction and Mosdobauer spectroscopy of bulk CoFe2O4 reveal the

structure of antispinel. By consulting relevant references [44], the ionic fraction of MgFe2O4 is as follows: (Mg0.1Fe0.9)A[Mg0.9Fe1.1]BO4. This is because in the anti-spinel CoFe2O4 material, the strong tendency of Co2+ ions to occupy the octahedral lattice B site. Although Mg2+ ions exist in both tetrahedral and octahedral space, only a small number of ions occupy tetrahedral A-site, and more cations also strongly tend to occupy octahedral B-site. Among them, Fe3+ ions tend to occupy tetrahedral and octahedral sites [45-47]. Therefore, for Mg0.2Co0.8Fe2O4 with Zn content x=0, it can be interpreted the cation distribution by Neel's two sublattice models as follows: (Fe3+1.0 ) A [Mg 2+ 0.2 Co 2+ 0.8 Fe3+1.0 ]B O 4

(8)

It is precisely because of the strongly tend to occupy tetrahedral position (A-site) of Zn2+ ions in the spinel ferrite structure. Therefore, in extreme cases, ZnFe2O4 has a normal spinel structure. When the Zn2+ ion replaces the Co2+ ion, because the Zn2+ ion occupies the tetrahedral (A-site), some of the original tetrahedral Fe3+ ions enter the octahedron (B-site). Therefore, in the case of Mg-Co-Zn ferrite, the cation distribution can be expressed as [48]: (Zn 2+ x Fe3+1.0- x ) A [Mg 2+ 0.2 Co 2+ (0.8- x) Fe3+ (1.0+ x) ]B O 4

(9)

According to the cation distribution given by Zn instead of Mg-Co ferrite, the average ion radius (rB) and oxygen parameters (u) at B-site can be calculated by the following formula [49]: 1 rB = [0.2 rMg 2+ + (0.8 - x) rCo 2+ + (1 + x) rFe3+ ] 2 5 (r + r ) u= - B 0 8 a

(10) (11)

Where, ‘a’ is lattice constant, ‘r0’ is the radius of oxygen ion (0.140 nm), and the radius of Mg2+ ion is 0.065 nm, that of Co2+ ion is 0.072 nm, that of Zn2+ ion is 0.082 nm, and that of Fe3+ ion is 0.064 nm. And the calculated data are recorded in Table 3. The configurations of A-O-A, B-O-B and A-O-B ion pairs in spinel ferrites have favorable distances for effective magnetic interaction, as shown in Fig. 3. The distance between cations and anions (Me-O: p, qr, r and s) and the distance between cations (Me-Me: b, c, d, e and f) can be calculated by equation (12) and (13) [50]:

1 p = a( - u ) 2 3 1 s= a (u + ) 3 2 b=

2 a 4

1 q r = 3a (u - ） 8 1 r = 11a (u - ) 8 c=

3 3 a e= 8

11 a 8

d=

6 a f= 4

3 a 4

(12)

(13)

The experimental values of lattice constant 'a' and oxygen parameter 'u' are used to calculate the distance between ions(Me-O and Me-Me). Here, b and d represent octahedral position (B-site) and tetrahedral position (A-site) which can be used for the distance (jump length) between magnetic ions. The jump lengths d and b of tetrahedral and octahedral positions change with the increase of Zn content as shown in Table 3. And other this distance between anions (Me-O) and cations (Me-Me) and the relationship between cations and Zn content were recorded in Table 3. The results show that the jump length increases in the ferrite lattice with the increase of Zn2+ ions content. This is related to the change of lattice constant of Mg-Co ferrite. The lattice constant mainly depends on the cation radius and the cation distribution of A and B sites in the spinel structure. It is also attributed to the larger radius of Zn2+ ions, which result in lattice expansion when doped into the lattice, and further increases the jump length. At the same time, it is also attributed to the lattice expansion of ferrite caused by the substitution of larger Zn2+ ions for smaller Co2+ radius. Table 3 also shows that the variation of the distance between cations exhibits similar behavior to that of lattice constants. The distance between cation and oxygen ion at tetrahedral position (qr) increases with the increase of Zn2+ ion content, while the distance between cation and oxygen ion at octahedral position (p) decreases with the increase of Zn2+ ion content. With the increase of Zn2+ ions content, the distance between the cations at different positions (tetrahedral A-sites and octahedral B-sites) and oxygen ions has a different linear relationship. This may also be attributed to the substitution of Zn2+ ions with larger ion radius for Co2+ ions with smaller ion radius. Meanwhile, in terms of cation occupancy, Zn2+ ions have a strong tendency to occupy tetrahedron position (A-sites). It preferentially enters the tetrahedral position when Zn2+ ions are incorporated into

the lattice. This will lead to a part of the original Fe3+ ions in the tetrahedral position into the octahedral position. This will cause some Fe3+ ions in tetrahedron position (A-sites) to enter octahedron position (B-sites). In this way, B-site sublattice is compressed by A-site sublattice, so the value of qr increases while the value of p decreases. 3.3 Fourier transform infrared spectroscopy analysis (FT-IR) In the synthesis process of ferrite samples, the information of chemical residues, attached molecular bands and the presence of functional groups can be identified by Fourier transform infrared (FTIR) spectra. The Fourier transform infrared spectrum of the sintered Zn-substituted Mg-Co ferrite system (for x= 0.0, 0.2, 0.4, 0.6 and 0.8) were measured in the wave number range of 400-4000 cm-1. The FT-IR spectrum of Mg0.2Co0.8-xZnxFe2O4 (x= 0.0, 0.2, 0.4, 0.6, 0.8) nanoparticles after high temperature sintering is shown in Fig. 6. The structure of spinel ferrite was determined by measuring the samples and the chemical properties of the ferrite were studied. The FT-IR absorption band of the prepared ferrite samples is usually attributed to the vibration of the chemical bond between the ions in the lattice [51]. In all spinels, it can be observed that there are two main broad metal oxygen energy bands in the FT-IR spectrum, and ferrites in particular. The FT-IR spectrum shows two main absorption bands, ranging from 400 to 600 cm-1. The first absorption band is near 400 cm-1 and the second is near 580 cm-1 for the prepared Zn2+ ions substituted Mg-Co ferrite. In the spinel structure, the absorption bands (υ1 and υ2) correspond to the intrinsic lattice vibrations of tetrahedral and octahedral coordination, respectively. For the absorption band (υ2) of 380-400 cm-1, this is due to the stretching vibration of Moctahedral ↔ O in octahedral position and the stretching vibration of the chemical bonds between tetrahedral position Mtetrahedral↔O shows an absorption band (υ1) of 500-600 cm-1 [52, 53]. There is no characteristic peak υ2 in the infrared spectrum of Fig. 6 and because it is beyond the measuring range machine. In Fig. 6, the prominent peaks (υ1) were observed indicating the formation of spinel ferrite. Among all the samples sintered at 950 ℃ for 3 hours, there were no hydroxyl and carboxyl groups (at 3200-3700 cm-1) indicating that the chemical reaction had been completed by sol-gel auto combustion method. The bands observed at 3420 and 1630 cm-1 attributed

to H-O-H stretching and bending patterns were interpreted as evidence of free (or absorbed) water [54]. The peak at 1120 cm-1 may be due to the vibration of the remaining C-O bonds. The absorption band at 1460 cm-1 is caused by C-H stretching vibration. The peak was observed near 1380 cm-1 may be due to the stretching vibrations of the antisymmetric NO3-1 and ferrite tetrahedral complexes. The vibration frequency of the chemical bond is attributed to cation mass, cation oxygen distance and bonding force in ferrite lattice. The wave numbers of Zn2+ ions substituted Mg-Co ferrite samples are recorded in Table 4. It was also observed that the intensity of the absorption band decreased obviously, especially the substitution content of Zn2+ ions was x= 0.6 and 0.8. This result can be explained by the fact that Zn2+ ions are added to the spinel structure. This is due to the strong tendency of divalent Zn2+ ions to occupy the tetrahedral position of spinel ferrite. Therefore, with the increase of Zn2+ ion content in tetrahedron position, it will replace some Fe3+ ions in the original tetrahedron position and force them to migrate to octahedron position, thus changing and affecting the absorption efficiency [55]. In the case of Zn-substituted Mg-Co ferrite, the frequencies of samples υ1 of Mg0.2Co0.8-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8) are 590, 588, 586, 577 and 555 cm-1, respectively. It can be seen that the absorption band υ1 at high frequency moves to low frequency obviously with the increase of Zn content. The change of frequency caused by this doping is due to the change of bond length of oxygen ion O2- and metal ion Mg2+, Co2+, Zn2+ and Fe3+ at the tetrahedral position [56-58]. This is also due to the substitution of larger atomic weight Zn2+ ions for smaller atomic weight Co2+ ions in ferrite. The difference of cation mass results in the change of vibration frequency of chemical bond. Meanwhile, the change of υ1 frequency after doping of Zn2+ ions also strengthens cation redistribution. Therefore, the formation of the spinel structure of the sample was also confirmed from the FTIR measurement results, and the XRD measurement results were also well supported. 3.4 Surface Morphology analysis (SEM) Scanning electron microscopy (SEM) is widely used to observe the surface morphology and composition of various solid materials. Morphology and structure have a certain impact on the performance of nano magnetic materials, so it is very

important to study the morphology and structure changes of magnetic materials. The surface morphology of ferrite nanoparticles was studied by SEM. Fig. 7a–e shows a TEM image of typical sample Mg0.2Co0.8-xZnxFe2O4 (x= 0.0, 0.2, 0.4, 0.6 and 0.8) , this image was used to study the particle size. Figure 7 also shows nano-sized high-concentration particles of Mg-Co ferrite samples with different zinc substitutes. It can be observed that the particles are uniformly distributed and slightly agglomerated with spherical shape. The surface morphology of SEM shows that the ferrite nanoparticles are fully crystallized and have the agglomeration behavior. It is observed that the agglomeration of nanoparticles may be due to the magnetic interaction between particles. The magnetic nanoparticles are gradually densified and the larger clusters are less with the increase of Zn2+ content. When x= 0.4, i.e. Mg0.2Co0.4Zn0.4Fe2O4, the surface of the sample becomes dense. SEM results show that the doped Zn2+ ions affect the morphology of the samples to some extent. It can also be seen that all the samples are high-density, agglomerated and porous, and the target length of ferrite nanoparticles was selected at 100 nm. The particle size distribution for Mg0.2Co0.8-xZnxFe2O4 ferrite nano particles: (a) x=0.0, (b) x=0.2, (c) x=0.4, (d) x=0.6, (e) x=0.8, as shown in Figure 8. The average particle sizes of the prepared ferrites Mg0.2Co0.8-xZnxFe2O4 (x= 0.0, 0.2, 0.4, 0.6 and 0.8) were 95, 87, 86, 88 and 100 nm, respectively. The average particle size increases first and then decreases with the increase of Zn2+ substitution. It is observed that the change of the average particle size is similar to that calculated by XRD. 3.5 Elemental analysis (EDX) The chemical composition of the constituent elements of the synthesized nanoparticles was studied and analyzed by using Energy dispersive X-ray (EDX). EDX spectra of pure Mg-Co ferrite sample and magnesium substituted Mg-Co ferrites samples of Mg0.2Co0.8-xZnxFe2O4 system (x = 0.0, 0.2, 0.4, 0.6 and 0.8) are shown in Fig. 9a–e. Four main elements of Mg, Co, Fe and O were measured in pure Mg-Co ferrite sample by EDX spectroscopy, while Zn was detected in Zn-substituted Mg-Co ferrite sample. It has been proved that the synthesized ferrite samples had pure phase and structure, and successfully realized the substitution of Zn2+ ions. 3.6 Magnetic measurements

The prepared samples were magnetized by applying an external magnetic field. The relationship between magnetization (M) and magnetic field (H) can be described as:

M = χH

(14)

Where, the χ represents the susceptibility. This magnetized material is called magnetic material. Similarly, the magnetic properties of materials can be classified into five categories according to their susceptibility and symbols: diamagnetism, paramagnetism, ferromagnetism, antiferromagnetism and ferrimagnetism. The basic structure of five types of magnetism is shown in Fig. 10. For diamagnetic materials, they are a kind of weak magnetic material. Its relative susceptibility is negative and very small. So the susceptibility χ(d) < 0. And the magnitude of susceptibility of diamagnetic materials is generally 10-5. Paramagnetism refers to the weak magnetic response of materials to magnetic field. For example, the magnetic susceptibility χ = M/H (M and H are respectively magnetization and magnetic field strength). From this relationship, the susceptibility χ(p) > 0, that is, the direction of the magnetization intensity is the same as that of the magnetic field intensity, and the value is 10-6 ⁓ 10-3 order of magnitude. In magnetic materials with orderly arrangement of atomic spins (magnetic moments) by exchange, if the spins of adjacent atoms are negatively exchanged and the spins are antiparallel, the magnetic moments are in an ordered state (called ordered magnetism), but the total net magnetic moments are still zero when they are not affected by the external field. This magnetic ordered state is called antiferromagnetism. When a magnetic field is added to the material, its magnetic moment tends to align along the direction of the magnetic field, that is, the material shows a small positive susceptibility. However, the susceptibility is temperature dependent and has a maximum at the Nair point. The explanation for the existence of the Neil point is that at very low temperature, the magnetic moment of the adjacent atoms almost completely cancels out because the spin of the atoms is completely reversed, so the susceptibility is almost zero. For ferrimagnetic materials, their macromagnetism is similar to that of ferromagnetism, except that their susceptibility is slightly lower. This order of magnitude is about 100 ⁓ 103. For ferromagnetic materials, they can be magnetized to saturation only under a small magnetic field. Not

only is their susceptibility χ(f) > 0, but their magnitude is about 10 ⁓ 105. At the same time, magnetic susceptibility also indicates a production of magnetic materials that are difficult to magnetize in a certain magnetic field. Ferromagnetic materials have ferromagnetic only under the Curie temperature. Above the Curie temperature, due to the interference of thermal motion of crystals, the directional arrangement of magnetic moments of atoms is destroyed, resulting in the disappearance of ferromagnetism and the transformation of matter into paramagnetism. Different magnets have different magnetization curves according to the basic structure of magnetism. The variation curves of magnetization with magnetic field for different magnets are shown in Fig. 11. It is also observed from Fig. 11 that only diamagnetic materials have susceptibility less than zero. However, the susceptibility of the other four magnets is positive. Meanwhile, it is also observed that the susceptibility of diamagnetic materials, paramagnetic materials and antiferromagnetic materials are relatively small, especially the susceptibility of antiferromagnetic materials is almost zero. Vibration sample magnetometer (VSM) is a commonly used magnetic measuring device. It can directly measure the variation curve of magnetization with temperature, magnetization curve and hysteresis loop of magnetic materials. Meanwhile, it can also directly measure the relevant magnetic parameters of magnetic materials, such as coercivity Hc, saturation magnetization Ms and residual magnetization Mr. When the magnetic field changes periodically, the relationship between magnetic induction intensity and magnetic field intensity in a ferromagnet is a closed line, which is called hysteresis loop. The magnetic properties of the ferrite samples were measured at room temperature using a vibrating sample magnetometer (VSM). And the applied filed was measured in the range of -20 to +20 kOe. Figure 12 shows the hysteresis loops of Mg0.2Co0.8-xZnxFe2O4 (where, x = 0.0, 0.2, 0.4, 0.6 and 0.8) spinel ferrites. The data obtained through the hysteresis loop are recorded in Table 5. The hysteresis loops of samples measured by VSM can be considered as pure Mg0.2Co0.8Fe2O4 nanoparticles and Zn2+ ion doped ferrites. The hysteresis loops of the samples show that the ferrite samples prepared have ferromagnetic behavior. Moreover, when the Zn2+ ion completely substituted the Co2+ ion, the prepared ferrite sample of Mg0.2Zn0.8Fe2O4 shows superparamagnetic behavior. This is attributed to the fact that magnetic metal

cations occupy octahedral and tetrahedral positions in ferrite samples. In spinel ferrite structure, octahedral position is occupied by trivalent metal cations and tetrahedral position is occupied by bivalent metal cations. However, the reverse spinel ferrite is the opposite [59]. For mixed spinel ferrite structure, tetrahedral and octahedral positions are occupied by divalent and trivalent cations simultaneously. In ferrite, Mg2+ and Co2+ ions mainly occupy octahedral position (B-site). It is precisely because of the strongly tend to occupy tetrahedral position (A-site) of Zn2+ ions in the spinel ferrite structure. Therefore, in extreme cases, ZnFe2O4 has a normal spinel structure. When the Zn2+ ion substituted the Co2+ ion, because the Zn2+ ion occupies the tetrahedral (A-site), some of the original tetrahedral Fe3+ ions enter the octahedron (B-site). The crystal structure of Zn2+ ion substituted magnesium-cobalt ferrite, which reveals that magnesium-cobalt-zinc-iron oxide nano particles have mixed crystal structure. Ferromagnets are in the original demagnetization state below Curie temperature. Because the magnetization vectors of each domain in ferromagnet have different orientations, they do not show magnetism on the macro level. When there is an external magnetic field, the ferromagnet is magnetized and in a state of magnetization, showing magnetism in macroscopic. At the same time, the change of domain structure in ferromagnet is due to the effect of external magnetic field on domain. This will also lead to changes in ferrite magnetism. In ferromagnets, the magnetization mechanism can be divided into three types: (Ⅰ) displacement magnetization process of domain wall, (Ⅱ) domain rotation magnetization process, (Ⅲ) paramagnetization process. The process of magnetization is essentially a process of domain structure change in ferromagnet under the action of external magnetic field. To further illustrate the physical nature of these three magnetization mechanisms, Fig. 13 shows three magnetization mechanisms in the process of magnetization. Neel's two sublattices model proposed the cationic distribution between A and B sublattices [60, 61]. The ion occupancy ratio of their cations at position A and B are [ZnxFe1-x]A[Mg0.2Co0.8-xFe1+x]BO4 for Mg0.2Co0.8-xZnxFe2O4. The theory of the spinel ferrite magnetization by the position of A and B of the difference of the total magnetic moment of the decision. According to Neel's double sublattice model, the theoretical

magnetic moment of the ferrite sample is calculated through the following formula:

 B Cal.  M B  M A

(14)

Among them, MB is the magnetic moment of B-site sublattice and MA is the magnetic moment of A-site sublattice. Co2+ ion magnetic moment is 3 µB, Fe3+ ion magnetic moment is 5 µB, Zn2+ and Mg2+ ions are non-magnetic ions, magnetic moment is 0 µB. By using the following relationship to calculate magnetic moment of each formula in the Bohr magneton units of the experimental value [62]:

 B exp. 

MW  M S 5585

(15)

Where, Ms is the saturation magnetization (emu/g) of Mg0.2Co0.8-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8) samples. MW is the relative molecular mass (g/mol) of samples with different manganese ion content. It is generally believed that the main factors affecting coercivity are the resistance of domain wall displacement and domain rotation, including magnetocrystalline anisotropy, stress effect and impurity concentration. In this paper, magnetocrystalline anisotropy is the main factor. The magnitude of the magnetocrystalline anisotropy constant K depends not only on the properties of the magnetic particles themselves, but also on the symmetry of the crystal position where the particles are located and the intensity of the crystal field. The expressions of coercivity Hc and magnetocrystalline anisotropy constant K are as follows [63]:

HC 

2K 0  M S

(16)

Where, ‘µ0’ is the vacuum permeability, and the value in the Gauss unit system (CGS) is 1.‘K’ is anisotropic, ‘Ms’ is saturated magnetization,‘Hc’ is coercivity. They are given by the Stoner-Wohlfarth theory from the M-H loop. Data in Table 6 are calculated from the above formulate. The effects of different Zn content on the saturation magnetization, magnetic moment, remanent magnetization and squareness (Mr/Ms) of Mg0.2Co0.8-xZnxFe2O4 are given in Fig. 14. The magnetic properties of spinel ferrite come from the super-exchange between magnetic metal ions separated by oxygen ions, which make the magnetic moments of metal ions in different lattice positions arrange in reverse. The exchange of A and B

sites is transferred through the oxygen ion P orbital, that is to say, the A-O-B bond is formed, and the maximum bond angle is 180°. The magnetization and Curie temperature of ferrites are related to the strength of super-exchange, which depends on the occupancy ratio of magnetic particles at A and B sites. Figure 12 (a) visually shows the saturation magnetization and magnetic moment of sintered samples of Mg0.2Co0.8-xZnxFe2O4 (x = 0.0, 0.2, 0.4, 0.6 and 0.8) with different zinc content. As is known to all, Mg0.2Co0.8Fe2O4 is a hard magnetic material while ZnFe2O4 is a soft magnetic material. It can be seen from the Fig. 12(a) and Table 5 that the saturation magnetization (Ms) of the samples first increases and then decreases with the increase of Zn2+ ions content. The saturation magnetization increased from 66.40 to 77.09 emu/g at low concentration of Zn dopant (x = 0.0 and 0.2). The influence of magnetic moment plays a leading role in the samples with less Zn2+ ions content. According to Neel's molecular field theory, the magnetic moments of A and B sites in the spinel structure of Mg-Co ferrite are antiparallel, and the net magnetic moments of the lattice are expressed as µB(Cal.)=MB-MA [64, 65]. The nonmagnetic metal ion Zn2+ ions occupies the tetrahedral position (A-sites) of spinel structure at low concentration of Zn dopant. This affects the net magnetic moment of A-sites, which leads to the increase of saturation magnetization (Ms). The saturation magnetization decreased from 71.99 to 4.56 emu/g at high concentration of Zn dopant (x = 0.4, 0.6 and 0.8). This is mainly due to the following two aspects: On the one hand, with the further increase of Zn2+ ions content, some Zn2+ ions will enter the B-sites, and the net magnetic moment will be affected, resulting in the decrease of Ms. On the other hand, Zn2+ ions enter into the lattice of Mg-Co ferrite, weakening the A-O-B super exchange between metal ions, resulting in the decrease of Ms. According to equation (14) and the occupancy ratio of oxygen ions in ferrite (9), the theoretical magnetic moment of the samples can be calculated as the content of Zn2+ ions increases. After the corresponding theoretical calculation, when x = 0.0, 0.2, 0.4, 0.6 and 0.8, the theoretical magnetic moments of Mg0.2Co0.8-xZnxFe2O4 are estimated to be 2.4, 3.8, 5.2, 6.6 and 8.0 µB, respectively. However, the experimental magnetic moments of the prepared ferrite samples are 2.71, 3.16, 2.97, 1.51 and 0.19 µB, respectively. It can be found that the theoretical and experimental magnetic moments of Zn2+ ions at low

concentration have similar changes. And when the content of Zn is more than 0.4, the experimental magnetic moment decreases rapidly, i.e. x = 0.6 and 0.8 (as showed in Table 6). Especially when x = 0.8, the experimental magnetic moment of Mg0.2Zn0.8Fe2O4 prepared is very small, only 0.19 µB. When a small amount of non-magnetic Zn2+ ions are doped with Mg-Co ferrite, Zn2+ ions will squeeze part of Fe3+ ions into B-sites after occupying A-sites, which will reduce the magnetic moment of A-sites, increase the magnetic moment of B-sites, and increase the molecular net magnetic moment. At this time, the super exchange between A-B is still dominant, far greater than that between A-A and B-B. Meanwhile, with a large number of Zn2+ ions doping, the amount of A-sites nonmagnetic ions increases, which will lead to the weakening of A-B super-exchange. At the same time, Fe3+ ions are squeezed into the B-sites enhance the super-exchange between B-B and decrease the total magnetic moment of the molecule, which leads to spin order disturbance and magnetic order instability. Ni0.2Zn0.8Fe2O4 samples show superparamagnetism, which may be due to the redistribution of metal cations in ferrite at high concentration of Zn2+ ions, which lead to the transition from ferromagnetism to paramagnetism. Fig. 14(b) depicts the relationship between remanent magnetization (Mr) and squareness (Mr/Ms) with Zn content. It can be seen that the remanent magnetization and squareness (Mr/Ms) decrease with the increase of Zn content. In Table 5, the remanent magnetization (Mr) decreased from 28.45 to 0.03 emu/g. In a cubic system of ferromagnetic spinels, the magnetic order is mainly explained on the basis of cation distribution and super exchange interaction mechanism occurring between iron and cobalt ions between the metal ion at tetrahedral A and octahedral B sites. The substitution of non-magnetic ion such as zinc, which has a preferentially A site occupancy results in the reduction of the exchange interaction between A and B sites, resulting in strengthening of B-B interaction. When Zn2+ ions are introduced at the expense of cobalt ions, some of the Fe3+ ions migrate from A to the B sites in view of the site preferences for different ions. This increases the Fe3+ ions concentration at B-sites. As Zn2+ concentration increases, the iron ions left at A-site, being small in number, the A-B interaction experienced by B-site iron ions decreases. Also, the increased number of Fe3+ ions at the B-site increases the B-B interaction, resulting in

spin canting [66]. The low values of Mr/Ms ratio indicate an appreciable fraction of supermagnetic particles. Mr/Ms decreases to 0.006 indicating a fraction of the particles in the blocked state. In Tables 5 and 6, it is observed that coercivity (Hc) and anisotropy constant (K) decrease with the increase of substitution of Zn. With the increase of Zn content from 0.0 to 0.8, the coercivity of the samples were 948.49, 166.05, 50.29, 2.65 and 21.26 Oe, respectively. Meanwhile, with the increase of Zn content from 0.0 to 0.8, the anisotropy constant of the samples were 314.90×102, 64.00×102, 18.10×102, 0.481×102 and 0.485×102 erg/g, respectively. For Mg0.2Co0.8Fe2O4 spinel ferrite, the magnetocrystalline anisotropy is greater than zero. The magnetocrystalline anisotropy of Mg-Co ferrite is mainly attributed to the spin orbit coupling of Co2+ and Mg2+ ions occupying the center of octahedron. It is because of the strong tendency of Zn2+ ions to occupy the A-sites that the concentration of Zn2+ ions in the A-sites increases while the concentration of Co2+ ions in the B-sites decreases after doping Zn2+ ions in the Mg-Co ferrite lattice. This change of cation occupation ratio weakens the indirect super exchange between A-B, which leads to the decrease of magnetocrystalline anisotropy and the decrease of the coercive force. The results show an interesting phenomenon that the coercivity of Mg0.2Zn0.8Fe2O4 is 21.26 Oe, while that of Mg0.2Co0.2Zn0.6Fe2O4 is 2.65 Oe. Manikandan et al. [67] reported that the optical and magnetic properties of Mg-doped ZnFe2O4 nanoparticles prepared by rapid microwave combustion method. The coercive force of Mg0.2Zn0.8Fe2O4 reported by them is 20.76 Oe, which is approximately similar to our measurement of coercive force. This may be due to the transition of the magnetic behavior of the sample, which leads to the change of the domain in the ferrite (the displacement of the domain wall and the rotation of the domain): the transition of ferromagnetic to superparamagnetic behavior. This change in magnetic properties for magnetic materials such as Hc, Ms and Mr is related to the stoichiometry of cations and their special occupancy. In addition, the formation of surface energy non-inductive layer, the existence of arbitrary tilt of particle surface spin, the arbitrary distribution of particle size and the phenomenon of water absorption also cause the decrease of magnetic properties of Mg0.2Co0.8-xZnxFe2O4 doped with Zn2+ ions [68]. The interior of ferrite magnetic

materials can be divided into three regions: multi-domain (MD) state, single domain (SD)/pseudo-single domain (PSD) state and superparamagnetism. It forms single domain particles when the particle size is smaller than the critical dimension. However, the prepared ferrite has different size. This fact shows that there are many single domain structures in the prepared multi domain particles. For hard magnetic materials, part or most of the magnetic domains remains in their original direction after withdrawing the external magnetic field. The irreversible displacement of the magnetic domain wall or the irreversible rotation of the magnetic domain results in the low initial susceptibility of the hard magnetic material. Meanwhile, the magnetic domain walls are easy to move when the magnitude and direction of the external magnetic field change for soft magnetic materials. Therefore, it is easy to be magnetized and demagnetized, and has large permeability and initial susceptibility. The decrease of the magnetic parameters (remanent magnetization, saturation magnetization, magnetic moment, etc.) of the prepared ferrites indicates that the magnetic particles change from multi-domain (MD) state to single-domain (SD)/pseudo-single-domain (PSD) state in ferromagnetic materials, while the compositions for x=0.8 and Mg0.2Zn0.8Fe2O4 are paramagnetic at the room temperature [69]. The dM/dH curve is obtained by first order differential treatment of the hysteresis loop for Mg0.2Co0.8-xZnxFe2O4 (x=0.0, 0.2, 0.4 and 0.6), as showed in Fig. 15. The value of dM/dH represents the magnetic susceptibility (χ) of ferrite and all magnetic data are recorded in Table 6. For an ideal single domain particle with square shaped M-H loop, the χ (dM/dH) is technically infinite (very large) at the coercive field (Hc) and zero at H → 0 [69]. At H→0, finite values of χ (dM/dH) are 18.19, 84.99, 134.18, 132.78 and 0.30 in 10-3 emu/g Oe unit for all samples, respectively. The multi-domain grains and single domain/pseudo-single domain of regimes are further confirmed by the finite values of the studied samples. Meanwhile, at the coercive field (Hm), the χ (dM/dH) values of pure samples and substituted samples with different zinc contents are 50.50, 133.43, 164.17, 133.16 and 0.30 in 10-3 emu/g Oe unit for all samples, respectively. It is found that the gap between the two peaks of Mg0.2Co0.8-xZnxFe2O4 (x=0.4 and 0.6) samples at Hm is very narrow, which indicates

that there are some unstable superparamagnetic domains in ferrite. At the same time, this also shows that the ferrite samples are transformed from hard magnetic materials to soft magnetic materials. The magnetization mechanism in soft ferrite is caused by domain wall motion, which may be affected by grain size, sintering density and spin domain rotation [70]. The values of Hm calculated from M(H) data are 1048.7, 198.3, 59.6 and 19.5 Oe for pure and zinc-substituted samples (x=0.0, 0.2, 0.4 and 0.6). And it is found that the coercive field (Hc) values are less than Hm values. This is attributed to the switching field distribution caused by the disordered shell contribution in multi domain or pseudo single domain grains. The values of χ (dM/dH) are very small, i.e. both are 0.03×10-3 emu/g Oe at x=0.8. This superparamagnetic behavior of larger particle size was observed in samples with high concentration of Zn2+ ions, which is attributed to the weakening of A-B exchange interaction in ferrite lattice due to replacement of Fe3+ in tetrahedral site by Zn2+ ions [71]. The structure and magnetic properties of Mg-Co-Zn ferrite nanoparticles strongly depend on the percentage of cations doped with Zn2+ ions. Meanwhile, this also indicates that the Zn substituted Mg-Co nano-ferrite has a low magnetic of multi ferric material at high concentrations.

4. Conclusions Zn substituted magnesium-cobalt ferrite nanoparticles having formula of Mg0.2Co0.8-xZnxFe2O4 (x = 0.0-0.8) were prepared by sol-gel auto combustion method. The X-ray diffraction and Fourier transform infrared (FTIR) analysis confirmed a single-phase cubic spinel structure. The lattice constant of magnesium-cobalt ferrite increases with the increase of Zn content. And the average crystallite size is about 49-54 nm calculated by the Scherrer equation. It can be seen that the absorption band υ1 at high frequency moves to low frequency obviously with the increase of Zn content. These prepared ferrite nanoparticles have spherical cubic shaped particles by Scanning electron microscopy (SEM). And the prepared particles are uniform and the grain size is small. The particle size of the prepared samples ranged from 80 to 100 nm. The influence of Zn2+ ions substitution on the magnetic characteristics of the Mg-Co ferrite has been investigated by vibration sample magnetometer (VSM). Mg0.2Co0.8Fe2O4 is a typical hard magnetic curve with a coercive force of Hc = 948.49

Oe. The hysteretic loop tends to soft magnetic with the increase of Zn2+ ions content. And the saturation magnetization (Ms) and magnetic moment of the samples first increased and then decreased with the increase of Zn2+ ion substitution content. When the content of Zn2+ ions is x = 0.6, the hysteretic loop of the sample shows a soft magnetic curve. This indicates that Zn2+ ions doped Mg-Co-Zn ferrite had better soft magnetic properties. Meanwhile, the hysteretic loop of the sample shows a paramagnetism curve when the Mg2+ ions are completely replaced by Zn2+ ions (x = 0.8). The saturation magnetization and the remanent magnetization are 4.56 and 0.56 emu/g for Zn0.8Co0.2Fe2O4, respectively. This also indicates that the ferrite samples prepared have a transition from ferromagnetic behavior to superparamagnetic behavior. This transition from hard magnetism to soft magnetism can be used as a potential high frequency and ultra-high frequency soft magnetic material.

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Physical Journal B-Condensed Matter and Complex Systems. 39 (2004) 417–425.

Statement: The authors claim that none of the material in the paper has been published or is under consideration for publication elsewhere.

Dear editor, The author declares that this article does not involve any conflict of interest and claim that none of the material in the paper has been published or is under consideration for publication elsewhere.

Mg(NO3)2·6H2O

Co(NO3) 2·6H2O

NH3·H2O

C6H8 O7·H2O

Precursor solution

Fe(NO3 )3·9H2O

3-4h (80℃) pH=7

Alcohol Blast Burner

Drying cabinet

Water bath stirring

Citric acid

3h (900℃)

2h (120℃)

Wet gel

Sintering

Dry gel

Silvery gray flocculent

Zn(NO3) 2·6H2O

Fig. 1. Flow chart for the synthesis of Mg0.2Co0.8-xZnxFe2O4 nanoferrite particles by sol-gel auto combustion process.

Fig. 2. Ferrites crystallize in the form of a cubic structure. Each corner of a ferrite unit cell consists of a ferrite molecule. (a) tetrahedral or A sites; (b) octahedral or B sites.

B 1 

r

f 2 P

qr

d

B

B

A

3 P

b

B

A β1 = 79°38′

P  c 4 qr

P

S

B e

r

A

A

B

β2 = 125°2′

P

β5

β5 = 154°34′

β4 = 125°9′

β3 = 90°

Fig. 3. Configuration of ion pairs in spinel ferrites with favourable distances.

(311) (111)

(220)

(311)

(b)

(440) (511) (422)

(400) (222)

x=0.8

x=0.8

Intensity in A.U

(a)

Intensity in A.U

x=0.6 x=0.4 x=0.2

x=0.6 x=0.4 x=0.2 x=0.0

x=0.0

10

20

40

30

50

60

34.0

80

70

34.5

35.0

35.5

36.0

36.5

37.0

2θ (degrees)

2θ (degrees)

Fig. 4. The XRD pattern of Mg0.2Co0.8-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8) nano ferrites along with the figure showing the shifting of the most intense (311) peak.

Lattice Constant (Å)

8.44

55

8.43

54

8.42

53

8.41

52

8.40

51

8.39

50

8.38

0.0

0.2

0.4

0.6

0.8

Zn Content x

48

56

49

Average Crystallite Size (D) in nm

Lattice Constant Average Crystallite Size

(a)

(b)

Porosity

44 40 Porosity (P%)

8.45

36 32 28 24 20

0.0

0.2

0.4

0.6

0.8

Zn Content x

Fig. 5. Variation of lattice constant, average crystallite size and porosity versus of Zn content (x).

1380 1630













% Transmittance

0.0 0.2 0.4 0.6 0.8 3420



1460

4000

3500 3000

2500

2000

1500 1000

Wave number cm-1 Fig. 6. FT-IR spectra of Mg0.2Co0.8-xZnxFe2-xO4 ferrite system.

1120

500

Fig. 7. Scanning electron microscopy micrographs of Mg0.2Co0.8-xZnxFe2O4 ferrite nano particles: (a) x=0.0, (b) x=0.2, (c) x=0.4, (d) x=0.6, (e) x=0.8.

50

30

(a)

40

20

30

Counts (

Counts (

(b)

25

20 10

15 10 5

50

60

70

80

90

100

110

120

130

70

140

75

80

85

50

(c)

Counts (

20

10

105

110

115

(d)

30 20 10

50 75

80

85

90

(e)

95

100

105

110

60

70

80

Particle size40 (nm)

Counts (

Counts (

100

40

30

70

95

Particle size (nm)

Particle size (nm) 40

90

90

30 20 10

70

100

Particle size (nm)

80

90

100

110

Particle size (nm)

120

130

140

110

120

130

Fig. 8. The particle size distribution (obtained from SEM) for Mg0.2Co0.8-xZnxFe2O4 ferrite nano particles: (a) x=0.0, (b) x=0.2, (c) x=0.4, (d) x=0.6, (e) x=0.8.

(a)

(b)

(c)

(d)

(e)

Fig. 9. EDX spectra of Mg0.2Co0.8-xZnxFe2O4 ferrite nano particles: (a) x=0.0, (b) x=0.2, (c) x=0.4, (d) x=0.6, (e) x=0.8.

Diamagnetism

Paramagnetism

Ferromagnetism

Fig. 10. Basic magnetic structures of five kinds of magnetism.

Antiferromagnetism

Ferrimagnetism

M Ferromagnetic materials Ferrimagnetic materials

Paramagnetic materials

0

Antiferromagnetic materials

H Diamagnetic materials

Fig. 11. Diagrams of magnetization curves of different magnets.

Magnetization (emu/g)

60 40 20

80

0.0 0.2 0.4 0.6 0.8

60 Magnetization (emu/g)

80

0 -20 -40 -60 -80 -20000

-10000

0

10000

40 20 0 -20 -40 -60 -80 -20000

20000

Magnetic Field (Oe) 80

Mg0.2Co0.6Zn0.2Fe2O4

60

60

40

40

20 0 -20 -40 -60 -10000

0

10000

20000

10000

20000

0 -40 -60 0

-10000

6

Mg0.2Co0.2Zn0.6Fe2O4

Mg0.2Zn0.8Fe2O4

4 Magnetization (emu/g)

Magnetization (emu/g)

10000

Mg0.2Co0.4Zn0.4Fe2O4

Magnetic Field (Oe)

20 10 0 -10 -20 -30 -40 -10000

0

10000

Magnetic Field (Oe)

Fig. 12. M-H hysteresis loops of Mg0.2Co0.8-xFe2O4 ferrite system.

20000

-20

-20000

20000

30

-20000

10000

20

Magnetic Field (Oe)

40

0

-80

-80 -20000

-10000

Magnetic Field (Oe)

Magnetization (emu/g)

Magnetization (emu/g)

80

Mg0.2Co0.8Fe2O4

20000

2 0 -2 -4 -6 -20000

-10000

0

Magnetic Field (Oe)

(a ) H = 0, M = 0

(b) H ≠ 0, M ≠ 0

Primitive demagnetization state

(c) H ≠ 0, M ≠ 0

Domain wall displacement process

H

Magnetic domain rotation

H

(d ) H ≠ 0, H = H s , M = M s

(e) H ≠ 0, H > H s , M > M s

Saturation magnetization state

Paramagnetic process

80

Saturation magnetization 4.0 Magnetic moment

(a)

3.5

70 60

2.5

50 40

2.0

30

1.5

20

1.0

10

0.5 0.0

0.2

0.4 Zn Content x

0.6

0.8

30

(b)

Remanent magnetization Squareness

25

0.4

20

0.3

15

0.2

10 0.1

5

0.0

0 0.0

0.2

0.4

0.6

0.8

Zn Content x

Fig. 14. Variation of saturation magnetization, magnetic moment, remanent magnetization and squareness with Zn content.

0.5

Squareness S (Mr/Ms)

3.0

Magnetic moment in μB

Saturation magnetization Ms (emu/g)

90

Remanent magnetization Mr (emu/g)

Fig. 13. Schematic diagram of three magnetization mechanisms of ferrites..

2Hm

x=0.0

140

dM/dH (10 emu/(g Oe))

dM/dH (10 emu/(g Oe))

50 40

-3

-3

30 20 10 0

120 100 80 60 40 20 0

-20

-10

0

10

20

-20

Magnetic field (kOe)

2Hm

x=0.4

140

140 120 100 80

-3

-3

dM/dH (10 emu/(g Oe))

160

-10

0

10

20

Magnetic field (kOe)

dM/dH (10 emu/(g Oe))

180

2Hm

x=0.2

60 40 20

2Hm

x=0.6

120 100 80 60 40 20 0

0 -20

-10

0

10

20

-20

Magnetic field (kOe)

-10

0

10

20

Magnetic field (kOe)

Fig. 15. Field dependence of dM/dH of different samples. 2Hm measures the magnetic field that separates two peaks.

Highlights 1. Zinc substituted magnesium-cobalt ferrite nanoparticles having the basic composition Mg0.2Co0.8-xZnxFe2O4 (where, x=0.0, 0.2, 0.4, 0.6, 0.8) were synthesized by sol-gel auto-combustion. This article uses sol-gel auto-combustion technology which can significantly save time and energy consumption and requires a lower sintering temperature, compared to traditional methods. 2. The structure, morphological and magnetic properties of the prepared Zn2+ ions substituted Mg-Co ferrites nanoparticles were characterized using XRD, FTIR, SEM, EDX and VSM. 3. For Zn-substituted Mg-Co ferrites, the magnetic properties decrease obviously with the increase of Zn content. This rapid decrease of magnetic properties reveals that the

magnetic properties of Zn-substituted magnesium-cobalt ferrite have realized the transition from hard magnetic to soft magnetic. Meanwhile, the coercivity is Hc = 21.26 Oe, the saturation magnetization is 4.56 emu/g and the residual magnetization is 0.03 emu/g for Mg0.2Zn0.8Fe2O4 sample. This also indicates that the ferrite samples prepared have a transition from ferromagnetic behavior to superparamagnetic behavior. The smaller coercivity value confirms that soft ferrite has been obtained. 4. Meanwhile, this transition from hard magnetism to soft magnetism can be used as a potential high frequency soft magnetic material.

Table 1. Parameters obtained from XRD data for Mg0.2Co0.8-xZnxFe2O4 nano ferrite particles. Inter planar

Lattice

Average

spacing

constant

crystallite size

‘d’ (Å)

‘a’ (Å)

‘D’ (nm)

0.0

2.5284

8.3858

52.46

589.70

3.63

0.2

2.5314

8.3957

50.24

591.79

3.96

0.4

2.5353

8.4086

49.63

594.53

4.06

0.6

2.5393

8.4219

51.46

597.35

3.78

0.8

2.5433

8.4352

54.13

600.19

3.41

Composition, x

Volume of unit cell

Dislocation line

‘a3’(Å3)

density ‘δ’(10-4 nm-2)

Table 2. Various structural parameters of Mg0.2Co0.8-xZnxFe2O4 nano ferrite particles. Composition, x

Sample

Molecular weight ‘M’ (g/mol)

X-ray density ’(Kg/m3)

‘ρx

Bulk density ’(Kg/m3)

‘ρb

Porosity ‘P’(%)

0.0

Mg0.2Co0.8Fe2O4

227.70

5131.45

3595.45

23.93

0.2

Mg0.2Co0.6Zn0.2Fe2O4

228.99

5142.10

3337.95

35.09

0.4

Mg0.2Co0.4Zn0.4Fe2O4

230.28

5147.26

3247.18

36.91

0.6

Mg0.2Co0.2Zn0.6Fe2O4

231.57

5151.69

3062.02

40.56

0.8

Mg0.2Zn0.8Fe2O4

232.86

5155.91

2935.71

43.06

Table 3. Distance between cation-anion (Me-O) and cation-cation (Me-Me).

Content,

Me-O/nm

x

p

Me-Me/nm q

r

s

b

c

d

e

f

rB /nm

u/nm

0.0

0.1025 0.3672 0.7031 0.4250 0.2965 0.3477 0.3631 0.5447 0.5135 0.0673 0.3778

0.2

0.1016 0.3694 0.7073 0.4261 0.2968 0.3481 0.3635 0.5453 0.5141 0.0665 0.3790

0.4

0.1006 0.3720 0.7123 0.4276 0.2973 0.3486 0.3641 0.5462 0.5149 0.0657 0.3804

0.6

0.0996 0.3745 0.7170 0.4287 0.2978 0.3492 0.3647 0.5470 0.5157 0.0649 0.3817

0.8

0.0987 0.3769 0.7218 0.4300 0.2982 0.3497 0.3653 0.5479 0.5165 0.0641 0.3830

Table 4.

Frequency bands υ1 of the samples.

Composition, x

Sample

υ1（cm-1）

0.0

Mg0.2Co0.8Fe2O4

590

0.2

Mg0.2Zn0.2Co0.6Fe2O4

588

0.4

Mg0.2Zn0.4Co0.4Fe2O4

586

0.6

Mg0.2Zn0.6Co0.2Fe2O4

577

0.8

Mg0.2Zn0.8Fe2O4

555

Table 5. Magnetic parameters of the prepared ferrite samples. Composition,

Remanent magnetization

Saturation magnetization

Coercivity

Squareness S

x

Mr (emu/g)

Ms (emu/g)

Hc (Oe)

(Mr/Ms )

0.0

28.45

66.40

948.49

0.428

0.2

19.83

77.09

166.05

0.257

0.4

8.02

71.99

50.29

0.111

0.6

0.56

36.33

2.65

0.015

0.8

0.03

4.56

21.26

0.006

Table 6. The values of magnetic moment, magnetic anisotropy, dM/dH and Hm calculated from M(H) data of Mg0.2Co0.8-xZnxFe2O4 (x=0.0, 0.2, 0.4, 0.6, 0.8) Composition,

Unit cell mag.

Anisotropy

Hm (Oe)

dM/dH (emu /g Oe)×10-3

x

mom µB(exp.)

constant K×102 (erg/g)

0.0

2.71

314.90

1048.7

18.19

50.50

0.2

3.16

64.00

198.3

84.99

133.43

0.4

2.97

18.10

59.6

134.18

164.17

0.6

1.51

0.481

19.5

132.78

133.16

0.8

0.19

0.485

1.1

0.30

0.30

H→0

H→Hm