3.1 Structural properties
Fig. 1 shows the XRD spectra of the CuAl and MF/CuAl samples. The XRD confirms a pure double spinel cubic structure for CuAl with MF without any undesired impurity peaks. It indicates that the seed/growth method success of preparation CuAl/MF structure. From the pattern of XRD, one can see that it is holding two spinel phases as follows: Double peaks side by side at particular positions as depicted in Fig. 1. Where the peaks located at the lower angles correspond to MF, but those at the higher correspond to CuAl, so the lattice constant of MF is higher than the CuAl as mentioned in Table 1. Moreover, a broadening of the peaks of all samples indicating that all samples have a small particle size. The structural parameters (lattice parameter a (Å), and crystallite size (D)) of MF and CuAl were compared with the Crystallography Open Database (COD) and tabulated in Table 1. In particularly, by considering the ionic radii of the metal ions Zn2+ (0.60 Å), Co2+ (0.58 Å), Mg2+ (0.57 Å) and Ni2+ (0.55 Å) [16], the lattice parameters of MF increased with the ionic radii as 8.443 Å for ZnF, 8.385 Å for CoF, 8.376 Å for MgF and 8.373 Å for NiF. From Table 2 notice that the lattice parameter of CuAl (8.074-8.089 Å) is varied according to MF, but it matches with the previous work [13]. Besides the lattice parameter of CoF, MgF, and ZnF is slightly decreased compared to the literature [17-20], but the NiF is increasing comparable to Ref. [19]. In other words, the lattice of ZnF, MgF, and CoF is compressed after mixing, but the NiF is expanded. It is indicated that there is an interaction force between the MF and CuAl affecting the crystal structure and the other properties of all nanocomposites under investigation. This behavior can be explained by the redistribution of the ions within the interstitial sites (A, and B-sites) according to the force between MF and CuAl, which caused changes in their properties [21]. The change in the lattice parameter of MF nanocomposites may be attributed to the degree of inversion [22].
The crystallites size obtained from Rietveld of all nanocomposites MF/CuAl was in the nanoscale (Table 1). Again, the value of the crystallites size of CuAl is dependent on the second phase, which differs from the previously reported (31 nm) [13]. Furthermore, it is noticed that the phase concentration (%) of all nanocomposites MF/CuAl was slightly compared to the preparation weight ratios.
3.2 Micro-Structural properties
Fig. 2 shows the TEM images of the MF/CuAl nanocomposites, with an inserted particle size histogram. The images confirm the formation of nanostructured composite with more than one grain size, indicating that there is more than one phase. The grains seem to be loose aggregates which are different slightly comparable to the particle size obtained by XRD. Although the XRD determines each lattice of MF/CuAl individually, the TEM recognizes the MF/CuAl as one compound. The average grain size (Z nm) of all nanocomposites was calculated and tabulated in Table 1. For the ZnF/CuAl nanocomposite, it is observed long nanotubes with a diameter of 12 nm besides some nanograins shown in Fig. 3. The obtained nanotube is dependent on the growth mechanism of the ZnFe2O4, as reported in the literature [23, 24].
3.3 FT-IR analysis
The FT-IR absorption spectroscopy was used to study the distribution of ions at the tetrahedral sites (nT) and octahedral sites (nO) of spinel structure [25]. The band (nT) and the band (nO) were obtained from the stretching vibration of the unit cell in the A-site, and the metal-oxygen vibration in the B-site, respectively [26]. In this work, the only band obtained at absorption range 550 - 520 cm-1 for the tetrahedral (nT), because the start point of the FT-IR spectroscopy was 400 cm-1, so the octahedral band (nO) was disappeared. Here, we will try to deconflict the interference between the peaks of CuAl and MF at (nT). Besides, the force constants of the ferrite systems will calculate. Fig. 4 shows the measured IR spectra of the MF/CuAl nanocomposite samples, and the absorption peak wavelengths are listed in Table 3. It is observed that the nt band of ZnF/CuAl recorded the lowest value in the wavenumber position while small shifts were observed with MgF/CuAl, CoF/CuAl, and NiF/CuAl towards higher wavenumbers. This indicates the strengthening of the metal-oxygen bonds at the tetrahedral sites due to the presence of Zn, the heaviest in mass, and the light shit is due to the presence of Fe ion in the others in the tetrahedral sites. This behavior agrees with the results of the Cu-Zn mixed ferrites [27].In other words, the changing in the mass ions on the tetrahedral (A) site causes a shift that increases the (A-O) bond length at the tetrahedral site of spinel structure (i.e. decrease in nt band).
The changing in the νT position of the MF/CuAl could be interpreted from four parameters: the 1st is the total mass of cations, the 2nd, and 3rd are the length and strength of the metal-oxygen bond respectively, and the 4th is the dimension of the unit cell. These parameters may compete with each other and cause a change in the ion position. All these parameters impact the position of the IR bands through the relation [28]:
(3)
where k is the force constant and m the atomic mass of the metal. The values of the force constant for tetrahedral (kT) sites are listed in Table 3.
3.4 Magnetic properties
The magnetic behavior of the MF/CuAl nanocomposites varies according to the magnetic moments of Ni2+, Co2+, Mg2+, and Zn2+ ions which have 2.3 mB, 3.9 mB, 0 mB, and 0 mB, respectively. To investigate the magnetic properties of the MF/CuAl nanocomposites, we measured the temperature dependence of magnetic susceptibility curves, i.e. c-T, and the applied magnetic field dependence of magnetization curves, i.e., M-H curve. Also, the heating efficiency (SAR) was determined.
3.4.1 Magnetic susceptibility
The Curie temperature (TC) is an essential magnetic property and is considered a compositional-dependent parameter. The measured M-T curve of the MF/CuAl nanocomposites is shown in Fig. 5. The TC is derived from the peak of dcm(T)/dT and the TC values are tabulated in Table 4. With increasing temperature, a smoothly drop-in magnetic susceptibility occurred. In other words, the smooth transition from ferrimagnetic to paramagnetic at the TC can be used as a measure of the degree of compositional homogeneity in all samples [29]. The TC for NiF/CuAl, CoF/CuAl, and the MgF/CuAl is 561 K, 462 K, and 438K respectively, which is lower than the NiF 0(865 K) [30], CoF (677 K) [22], and MgF (648K) [31]. This may be due to the influence of the grain size and the cation distribution of the A- and B-sites. The transition temperature can be explained by the finite scaling theory [30]. When the grain sizes are smaller, the transition temperature is lower. Furthermore, the mutual effect between MF and CuAl2O4 leads to a lower magnetic transition temperature. The same behavior was observed in the many compounds of nanoparticles [32-34].
Figure 6 shows the relation between the reciprocal magnetic susceptibility (cM-1) and the temperature (T) for NiF/CuAl, CoF/CuAl, and MgF/CuAl samples. The relation obeyed the well-known Curie-Weiss law in the paramagnetic region. So the experimental effective magnetic moments meff and the Curie Weiss constant (ϴ) values can be calculated from the plot of cM-1 versus T using the relation:
meff = 2.82787√C (4)
where C is the Curie constant equal to the slope of the line in the paramagnetic region and presented in Table 4. The Curie Weiss constant (ϴ) values are equal to the cross of the slope with the x-axis in Fig. 6 and listed in Table 4. The Curie-Weiss constant (ϴ) was found to possess large +ve values pointing to perfect ferrimagnetic character. The rise in ϴ values assures the enhanced ferrimagnetic order of the NiF/CuAl, CoF/CuAl, and MgF/CuAl. This indicates these samples have magnetic ferrimagnetic ordering in the temperature range 300-600K. Also, the values of the Curie Weiss constant (ϴ) for all samples are closed to the values of the Curie temperature of the magnetization (TC).
3.4.2 Magnetic hysteresis loops
Fig. 7 shows the hysteresis loops for the MF/CuAl nanocomposites at room temperature. The MF/CuAl nanocomposites exhibit a ferromagnetic behavior, except the ZnF/CuAl records a superparamagnetic one. It indicates the order of magnetic structure due to the presence of the ferrite material CoF, NiF, and MgF. The estimated values of saturation magnetization (Ms), remnant magnetization (Mr), and coercive field (Hc) of the nanocomposites were listed in Table 4. As expected, the values of (Ms) are different according to the multi-phase nanocomposite. One can notice that the magnetization values of NiF/CuAl and MgF/CuAl nanocomposites are less than that of CoF/CuAl nanocomposite, where the CoF nanoferrite is hard-magnetic, i.e., large magnetocrystalline anisotropy, while NiF and MgF are soft type nanoferrites. The magnetic properties are affected by the degree of magnetic phase dispersion in the Nanocomposites i.e., the mutual effect between MF and CuAl2O4; this effect appears in the magnetic properties. As shown in Table 4, the saturation magnetization of the MF/CuAl nanocomposites is lower than the standard value of MF nanoferrites; CoFe2O4 (Ms ≈ 52-54 emu/g) [8], NiFe2O4 (MS ≈ 50 emu/g) [12, 35], and MgFe2O4 (Ms ≈ 14.6 emu/g) [12, 36]. This result means that the effect of CuAl2O4 on the magnetic moment of CoF, NiF, and MgF is negative, i.e., the net magnetic moment of MF/CuAl reduced. But for ZnF/CuAl the situation is different, the ideal behavior of bulk ZnF ferrite recorded with normal spinel structure, where all Zn2+ ions occupy A sites and Fe3+ ions occupying B sites, it is antiferromagnetic. At the nanoscale, the ZnF has a partial inverse spinel structure and ferrimagnetic behavior (Ms ≈ 5.7 emu/g) [37, 38]. In our work, the ZnF/CuAl nanocomposite is superparamagnetic behavior as in the inset of Fig. 7, which agrees with the loops of ZnFe2O4 nanoparticles [39].
The decreasing or increasing in magnetization for any material can be attributed to the canting angle between the moments in the B-site, or and the total magnetic moments in the B-site [18]. So, the two factors may be to compete with each other and cause variation of Ms for all MF/CuAl nanocomposites. Also, the variation of Ms with the nanocomposites can be explained in light of the exchange interactions between the MF and CuAl. In other words, the combination of two different magnetic materials in which their different magnetic domains yield a new compound; the second domain may lead to a decrease or increase in the magnetization of the new compound, owing to interferences of the two magnetic domains [40].
The following equations for the determination of Bohr Magneton (mB), magnetocrystalline anisotropy constant (K), and initial permeability (mi) are used [41].
mB = [M × Ms] / [5585 × rXray] (5)
K = [Hc × Ms] / 0.96 (6)
mi = [
× D] / K (7)
where the M, Ms, Hc, and D are the molecular weight, magnetic saturation, coercivity, and grain size of the MF/CuAl nanocomposites. Table 4 listed the magnetic parameters such as Ms, Mr, Hc, Bohr Magneton, initial permeability, magnetic squareness, and anisotropy. The variation of Ms, Mr, Hc is due to the values of the magnetic moment of Ni2+, Co2+, Mg2+ Zn2+, Cu2+, Fe3+, and Al3+ ions, which are 2.3, 3.85, 0, 0, 1.73, 5, and 0 mB, respectively. Also, the variations in the magnetic moment of Ni2+, Co2+, Mg2+ Zn2+, Cu2+, Fe3+, and Al3+ ions are responsible for the variations in the magnetocrystalline anisotropy, Bohr Magneton, magnetic domain, and super exchange interactions. The coercive field generally decreases with particle size decrease because of lower anisotropy barriers [29]. A similar dependence of coercivity on the variations of the magnetic moment is reported in other works [42, 43].
3.3.3 Magnetic hyperthermia
The specific power absorption rate (SAR) values of the magnetic suspended solution were calculated using the Eq. [15]
SAR
(8)
where Ci and Mi are the specific heat and the mass component, i of the system, respectively, MFe the mass of the magnetic component and (DT/Dt) the initial heating rate. The (DT/Dt) was taken from the initial heating part of the curves in Fig. 8. The heating effect, due to the external magnetic field applied to the investigated samples, was measured with time starting from switching the power on the induction coil. The obtained values were represented by curves of temperature as a function of time. As seen from the figure, the NiF/CuAl has less time to reach the hyperthermia temperature as compared to the other samples.
Fig. 9 and Table 4 show the SAR values of MF/CuAl samples. The presented graphs revealed that the highest SAR effect was obtained for the NiF/CuAl sample while CoF/CuAl was the lowest. The MgF/CuAl and ZnF/CuAl are intermediate. Despite the CoF/CuAl sample is the highest value of magnetization, i.e., magnetic moment perturbation in the magnetic field, the value of the SAR is lower than that of NiF/CuAl and MgF/CuAl. These results can be explained according to Neel and Brownian relaxations [44, 45]. It is known that the magnetic anisotropy constant is directly proportional to magnetic coercivity. Hence, the heat generation process which governed by both Neel and Brownian relaxations rather than Ms. Thus, the SAR always decreases if Hc increase. The heating curves of the MF/CuAl, samples indicate that the NiF and MgF have a very significant effect on the SAR parameter.