Synthesis of novel hard/soft ferrite composites particles with improved magnetic properties and exchange coupling

SrFe12O19/Zn0.4Co0.2Ni0.4Fe2O4 hard/soft ferrite composite particles with 20, 40, 60 and 80 wt.% of soft phase were prepared by one-pot sol-gel auto-combustion and physical mixing methods. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and vibrating sample magnetometer (VSM) were used to characterize the structural and magnetic properties of the samples. XRD spectrum revealed the formation of mixed ferrite phases in the composite particles. The hysteresis loops of the samples showed the presence of exchange coupling between the hard and soft ferrites. The composite particles with 20 and 60 wt.% of the soft phase demonstrated the highest Mr/Ms ratio, i.e. 0.29 and 0.28, respectively. In addition, the highest Ms, Mr and Hc were achieved in the composite particles with 40, 60 and 20 wt.% of the soft phase, respectively. Compared to the physical mixing method (PM), the composite particles prepared by the sol-gel auto-combustion method (OP) demonstrated better magnetic properties. The exchange coupling interaction between the hard and soft ferrite phases was similar in both methods. These composite particles exhibited magnetically single phase behaviour, however, the saturation magnetization was lower in the physical mixing pared to that of the one-pot method.


I. Introduction
Magnetic composites are being used as magnetic fluids, microwave devices, biomedicines and permanent magnets in various applications [1].Composite with hard (SrFe 12 O 19 ) and soft (Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 ) phases can improve the magnetic properties because of the high exchange coupling of both phases.Consequently, with high exchange coupling between hard and soft phases, the high saturation magnetization of the soft phase and high coercivity of the hard phase can increase all magnetic properties rather than hard and soft phase itself [2].Ferrite composites composed of spinel soft and hard ferrites are good candidates for advanced permanent magnets, because of their low cost, excellent corrosion resistance, relatively high Curie temperature and high electrical resistivity [3].
In the present research, the ferrite composite particles of SrFe 12 O 19 as a hard phase and Zn 0.

Sample preparation
In the present work, SrFe In one-pot sol-gel auto-combustion method, the stoichiometric amounts of Sr and Fe nitrates were dissolved in deionized water at 80 °C to get a brown solution.Then citric acid was added to the solution, with the molar ratio of metal ions to citric acid of 1 : 1.5.In another beaker, stoichiometric amounts of Fe(NO 3 ) , ZnCl 2 and citric acid were mixed and dissolved in deionized water.After obtaining a light brown clear solution, the solution was added to the first beaker containing strontium hexaferrite salts.The molar ratio of soft metallic ions to citric acid was 1 : 1.After 2 h, ammonia solution was added dropwise to adjust pH at 7 and the mixed solution was heated at 120 °C.The solution became a viscous gel while evaporating.Then gel ignited in the microwave oven device and the black-coloured powder was prepared.The powders were calcined at 500 °C for 5 h and then at 1200 °C for 2 h to form SrFe Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 were synthesized separately by the sol-gel auto combustion method and then mixed with stoichiometric amount physically.The prepared samples were calcined at 1200 °C for 2 h (Fig. 1).

Characterization
X-ray diffraction measurements (XRD) were performed using a diffractometer (PW-1730, The Netherlands) with Cu Kα (1.54Å) radiation operated at 40 kV and 30 mA The morphology and composition of the composites were investigated by field emission scanning electron microscope (FE-SEM; TESCAN; Model MIRA3) and energy-dispersive spectroscopy spectrometer with 15 kV voltage.FTIR spectra were obtained by Fourier transform infrared spectrometer (NEXUS870).A vibrating sample magnetometer (VSM; ZVK, R&S) was used to measure the magnetic properties of the powders.phases, the average crystallite sizes were calculated to be about 71 nm and 214 nm, respectively (Table 1).By increasing the amount of the soft phase, the average crystallite size is reduced to 69 nm and 53 nm for hard and soft phases, respectively.Cobalt-nickel-zinc ferrite inhibits the growth of strontium ferrite and improves the growth of soft phase rather than hard phase [10]. .Also, the bands around 2350 cm -1 confirm the presence of carbonyl groups.The FTIR spectra of these compounds suggest that the hydroxyl stretching vibration of iron and strontium bonds with oxygen.In the FTIR spectra of all samples within the range from 800 to 400 cm -1 , the typical metal-oxygen absorption bonds confirm the formation of the hexagonal structure of strontium ferrite [21,24,25].

Structural characterization -FE-SEM results
The pure SrFe Comparing the microstructural features of the physically mixed samples (Figs.5b,d,f h) with the ones from one-pot (Figs.5a,c,e,g), it is obvious that the composite particles synthesized by one-pot method possess better homogeneous mixing of Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 and SrFe 12 O 19 phase than the composite particles prepared by physical mixing method.This shows that the processing method plays an important role in defining the microstructure of composite.In addition, it is known that magnetic particles could induce their agglomeration and, thus, contribute to microstructural changes [25].This difference in the microstructure of the samples may  play an important role in the magnetic and microwave absorption properties of the composites.
EDX analysis (Table 2) of the calcined composite particles indicated the presence of all the elements (e.g.Co, Ni, Zn, Fe, Sr, and O).EDX analysis showed that in the hard/soft composites, there was a good compromise with chemical stoichiometry.
The range of the exchange-coupling interaction between the grains of hard and soft magnetic phases, i.e., the exchange length, L ex , can be expressed as [20,24,26]: where A and K denote the exchange stiffness and the mean amplitude of the random effective anisotropy constant, respectively, and D is the diameter of the grain.So, larger grain size reduces the exchange length and consequently the coupled regions.Room temperature magnetic loops for the composite samples calcined at 1200 °C are given in Fig. 7.The M-H behaviour of the composite particles prepared by one- pot and physical mixing method shows a smooth curve without any step, meaning that the spinel and magnetoplumbite phase exchanged coupled with each other [27].The magnetization and demagnetization force have single ferrimagnetic property, because of the external magnetic force and soft magnetic moments rotate along with the hard phase.The difference in the demagnetization behaviour is the interplay of three types of spin interactions in the composite particles, i.e. between soft/soft phase, hard/hard phase and hard/soft phases.The interfacial interaction between the hard and soft phases should be dominant for exchange couple systems.Also, the grain size of the soft phase should be smaller than the domain wall width of the hard phase [10,24].The hard ferrite possesses a high magnetocrystalline anisotropy energy compared to the soft ferrite.
In the samples prepared by physical mixing and one-pot method, exchange coupling interaction between hard (SrFe 12 O 19 ) and soft (Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 ) ferrites can be helpful to both align the magnetization and arrange magnetic moments which are parallel to each other.This effect can improve the magnetic properties and get higher saturation magnetization [7].The magnetic parameters of the composite particles including saturation magnetization M s , remanent magnetization M r , coercivity H c and M r /M s ratio obtained from hysteresis loops are shown in Table 3.The high Ms of onepot composite particles is a consequence of interfacial coupling arising from the alignment of more magnetic moments [27].If there was no exchange coupling between the two phases, the saturation magnetization of the composite would be: where, M s,hard and M s,so f t are the saturation magnetization of hard and soft phase and f hard and f so f t are the weight fraction of the hard and soft phase [16].Due to the fact that M s of all composite particles are higher than the calculated M s from equation 2, consequently, in all composite particles prepared via one-pot sol-gel auto combustion and physical mixing methods, there is an exchange coupling between two phases.Coercivities of all the composite particles are lower than those of the pure hard phase or soft phase for onepot synthesized composite particles (Fig. 7).This means that by increasing the reverse field, the spinel domain walls move towards the interface between the soft and hard phases and exceed into the hard phase and cause magnetization reverse to that phase.So, the coercivity of the samples decreases in comparison to the hard phase region.At high values of the soft phase (60 and 80 wt.%), the exchange force on the spinel is reduced and the interaction among soft moments becomes important.This reduces the coercivity of composite [28].
Optimum exchange coupling is related to the grain size of the composite particles [2].Maximum exchange coupling was reached when the critical length of soft that the hard phase featured a hexagonal platelet-like morphology, while the soft phase was spherical with faceted edges.
The hard/soft ferrite composite particles exhibited good exchange coupling between hard and soft ferrite phases in the samples prepared by physical mixing and one-pot methods and possessed high saturation magnetization.The coercivity of the samples is reduced by increasing soft phase amount, which is related to the dominance of dipolar interaction in soft phase over exchange interaction.Our results show that the values of M r /M s of the composite particles are lower than 0.5, therefore; the particles interact magnetostatically.Thus, the prepared composite particles are good candidates for producing exchange-spring magnets.

4
Co 0.2 Ni 0.4 Fe 2 O 4 as a soft phase were synthesized to obtain the high exchange coupling.SrFe 12 O 19 is a hexagonal ferrite that has the highest coercivity among the other similar ferrites.CoFe 2 O 4 has high saturation magnetization (M s ∼ 80 emu/g) [22], and this value could be changed by Ni and Zn doping.SrFe 12 O 19 /Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 hard/soft composite ferrites with various amount of the soft phase (20, 40, 60 and 80 wt.%) have been synthesized by the one-pot sol-gel auto-combustion and physical mixing methods to investigate exchange coupling behaviour.

3. 1 .
Structural characterization -XRD analysis XRD patterns of SrFe 12 O 19 /Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 composite particles prepared by one-pot and physical mixing methods (calcined at 1200 °C) are presented in Figs.2a and 2b, respectively.There are only XRD peaks of the hard SrFe 12 O 19 phase (JCPDS card No. 01-084-1531) and soft Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 phase (JCPDS card No. 01-087-2336), without any impurity peak, such as NiO, ZnO, SrFe 2 O 4 or α-Fe 2 O 3 .Due to the fact that homogeneity of hard and soft phase is related to the synthesis method, the intensity of all peaks changes.

Figure 3 Figure 2 .Figure 3 .
Figure 2. XRD patterns of (SrFe 12 O 19 ) 1-x (Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 ) x (x = 0.2, 0.4, 0.6, 0.8) composites prepared by: a) one-pot sol-gel auto-combustion route and b) physical mixing method 12 O 19 and Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 are presented in Figs.4a and 4b, respectively.It can be seen from Fig. 4a that the pure hard phase calcined at 1200 °C has hexagonal platelet-like morphology.Figures 5a-h show FESEM micrographs of the hard/soft composites calcined at 1200 °C.The morphology of Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 particles is roughly spherical in shape.During one-step auto combustion process, ignition causes generation of huge amounts of gases that are released and highly porous nanoparticles are formed initially, but later only a few of them remain which grow in size at the cost of smaller ones.The larger grains correspond to the SrFe 12 O 19 , while smaller ones represent the Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 phase.Microstructure of the pure SrFe 12 O 19 (Fig. 4a) shows large elongated grains with submicrometer size.Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 grains (Fig. 4b) are nearly spherical in shape with an average size of ∼2 µm.

3. 4 .
Magnetic properties Room temperature magnetic loops for the pure SrFe 12 O 19 and Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 samples calcined at 1200 °C are given in Fig. 6.H c = 5813 Oe, M s = 46 emu/g and M r = 26 emu/g were measured for SrFe 12 O 19 powder.The high H c value confirms that SrFe 12 O 19 belongs to the hard magnetic materials.The saturation magnetization and coercivity of Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 are 67 emu/g and 64.7 Oe, respectively, confirming that Zn 0.4 Co 0.2 Ni 0.4 Fe 2 O 4 belongs to the soft magnetic materials.