Nickel ferrite is one of a group of common magnetic material that has an inverse spinel structure. The spinel ferrites involves two sublattices known as tetrahedral and octahedral which are denoted widely as A and B respectively. The magnetic properties are functions of the cations distribution on these sites. In this case, the cation distribution is another factor that affects the magnetic properties beside particle size in nanoferrite. Nickel ferrite (bulk or nanostructure) applications get expanded day by day, they include magnetic cores , catalysts , microwave applications , sensors , and even antibacterial . At bulk size scale, nickel ferrite has been studied widely from point of preparation conditions. Recently, when nanomaterial began to open a new era in material science, the role of preparation conditions takes up more attention.
Chemical routes are considered as the main methods for nanomaterial synthesizing. The size and shape controlling of nanoferrites is the main factor that affects their properties and subsequently their applications. The previous literature show fluctuations of the pH role in nanoferrite synthesizing, especially influencing the particle size [6, 7]. Sometimes it was found that increasing the pH lead to increasing the particle size [8, 9], while it is decreased in others . Therefore, the pH parameter can be considered as the key factor for controlling ferrites nanostructure and magnetic properties . The saturation magnetization as an example was increased with increasing pH, whereas other researchers find an optimal magnetic saturation and coercivity at pH value of 7.5 [12,13].
In this study the effect of pH on the structural properties of nano Ni ferrite prepared by hydrothermal method and subsequently on the magnetic properties were investigated. The XRD, SEM, and TEM was utilized to determine the structural properties and particles morphology. The moment-magnetic field hysteresis loop by the SQUID and ferromagnetic resonance FMR test were done.
2 Materials and Methods
Ferric chloride (FeCl3) and nickel chlorides (NiCl2·6H2O) in addition to NaOH were used as reactant materials to produce the Ni ferrite. It is believed that the ferrite formula NiFe2O4 was satisfied through the following chemical reaction: (1)
The quantity of NaOH in the above reaction is not constant and the mentioned value is just for balancing the equation, which experimentally gives a pH value of 1.5. Table (1) illustrates the calculated weights according to the above reaction to get 0.01 mole of ferrite.
Metal chlorides salts were dissolved in 200 ml distilled water. Then NaOH solution of 2.5 M was adding drop by drop until the required pH is reached under continuous stirring. During this step a dark brown precipitate was formed. This step was followed by transferring the suspension to a Pyrex flask (500 ml), which was placed inside a homemade autoclave and heated by a hotplate at 160 °C for three hours. The stirring process was kept continuous through hydrothermal operation and the pressure was fixed autogenously. It may be pointed out that the suspension before hydrothermal process did not respond to a magnet. After that, the suspension was washed three times to dismiss the produced salts and unreacted materials. Finally, filtering the products by tissue papers and drying them at 80 °C in an oven for four hours were undertaken.
Five samples were prepared at pH equal to 1.5, 4, 7, 10, and 13 with ±0.2 tolerance in the accuracy of measuring pH. Crystalline phase purity was verified by XRD for each sample under study by using X-pert Panalytical instrument with Cu-Kα radiation (λ=1.5418 Å) associated with High Score Plus sofware to analyze the produced patterns. The error in the measuring of 2θ is 0.001°. Depending on the fact that nearly all particles have a spherical shape, the crystallite size (t) was calculated by Scherrer-Debye formula as in Eq.2 : (2)
Where (β) is the linewidth at half maximum, (λ) is the X-ray wavelength and θ is the Bragg angle.
SEM and TEM microscopies utilized FEI LEO 1550 SEM and Philips CM 12 TEM instruments to determine the particle size and particle shapes. Composition analyses in Ni ferrite samples were checked by SEM-EDX and TEM-EDX.
The hysteresis loop and magnetization-temperature tests were done by SQUID by Quantum Design MPMS XL SQUID, with specification: field range: 5T, sensitivity: 1 x 10−8 emu at 2,500 Oe, accuracy: approximately 0.1%. BRUKER ESR E500 device was utilized to characterize the samples powders around the resonance frequency of 9.7 GHz, it is characterized by: tuning in the range (8-14 GHz), where the cavity resonator shorts the waveguide of microwaves radiation, The quality factors: up to 8000, field modulation: up to 100 kHz, signal-to-noise: 3000:1, The sliding magnetic field: up to 104 G with 1G peak to peak modulation amplitude, resolution of field axis: 1024 points. Powders of samples, which have weight of 3mg, were pressed in plastic capsule to carry out these tests.
3.1 XRD Test and Morphology Results
The XRD patterns of the five samples at the different pH values are shown in Fig.(1). The pattern of the sample at pH=1.5 show no spinel phase is existing and the pattern is belonging to α-Fe2O3 with JCPDS file No.24-0072. The low concentration of sodium hydroxide (low pH) leads the reaction (1) to produce α-Fe2O3 as: (3)
This behavior may be related to higher ion reactivity  and concentration of ferric chloride compared to nickel chloride. As pH value increases to 4, the sample pattern leaves α-Fe2O3 phase and begin to take the spinel phase. The pattern at pH=4 has also no good matching with spinel JCPDS cards. The extra increase in the pH value gives clearer spinel pattern, as a result of improving the reaction (1). The characteristic high intensive peaks at 30.8°, 35.7°, 63.3° which correspond to the planes (222), (310) and (440) of spinel structure began to appear when the pH value exceeds 4. The peaks around 46, 50 and others which are unmarked by miller indices are belonging to the presence of hematite and NaCl (halite phase) which support the occurrence of reaction(1) or (3). The variance of intensities around 31° and 36° is belongs to cation distribution on A and B sites. This variance in intensities and the low intensities itself are indications of the presence of nanosize especially at pH=4 and the presence of multiphases . The hemetite phase has a shift in peaks positions to higher values by 0.1 degree JCPDS 96-901-5965. The peaks located at 2θ higher than 40° have lower intensities than standard. The Ni ferrite peaks have also a shift to the higher value by 0.23° about the JCPDS 96-591-0065 ones.
The effect of pH on crystal size is linked to crystal growth. Table (2), illustrates the lattice constant (a) and crystallite size (t) variation with corresponding pH values. The lattice constant was minimum at pH=7, while it was maximum at pH=4. It is estimated that the tension stress, which is affected by the crystal size, is the reason behind the variation. The largest crystallite size was (64.5 nm) at pH=10. It is clear from Table (2) that the increase in pH lead to an increase crystallite size except for the case at pH=13. This behavior can be explained on the basis of crystal growth, where it thought that at pH=10 the growth rate is the largest which gives maximum crystallite size .
The predominant particle morphology was changed from nanorods to nanosemispheres when pH varied from 1.5 to 4 (or larger than 4) as seen in Fig.(2)a and b. The presence of nanorods may be attributed to the existence of α-Fe2O3 phase which is usually constructed under such condition , while spinel structure has spherical and low concentration of nanorods.
Besides that the sample of pH=1.5 had nanorods as a general particles, it also had a low concentration of nanosemispheres as shown in Fig.(3)a where an aggregation of nanorods appeared. The sample synthesis at pH values of more than 4 had a very low concentration of nanorods, this can be shown in Fig.(2)b.
The average particle sizes are 6.5 nm, 66.5 nm, 65.8 nm, 51 nm at pH=4, 7, 10, 13 respectively as shown in Fig. (2) and Fig.(3) and there is no large distribution in these sizes. It is considered that pH ˃10 acts as capping factor besides increasing nucleation centres because of reducing the path of species. Comparing the particle size with the crystallite size in Table (2), gives an indication that each particle contains just one or two crystals.
Nickel, iron and oxygen contents that checked by EDX techniques associated with SEM and TEM devices show good agreement when pH exceeds 7 with stoichiometric composition of samples.
The important benefits of using hydrothermal process for production nanostructures are narrow sized distribution, controlled shape and simplicity . For this study, it is believed that the controlling of size and shape particles is dependant on pH and other conditions such as solution concentration, the ratio of nickel chloride to ferric chloride and temperature.
Synthesizing Ni ferrite particle by solid state reaction  reported that the size was 6.16 nm and the lattice parameters was 8.337nm. The particle size is smaller but the lattice constant is nearly larger than the ones of this study, knowing that the bulk Ni ferrite has a=8.340 Å . The crystallite size of the prepared samples in general is lower than that prepared by hydrothermal method  but larger than ones synthesized by microwave hydrothermal . Of course these differences are related to preparation conditions which produce compressive or tension stresses in the lattices, as well as the starting material and differences in cations and oxygen concentration.
As in the present study, different nanostructures or particle shapes were also appeared in Zn ferrites and Ni ferrites prepared by co-precipitation . Particle size is the parameter that is very sensitive to preparation conditions and can be controlled by them. However the particles size of current study are asymptotic to that of other researchers  and other ferrites [26-29].
3.2 Magnetic Properties Results
Hysteresis loops of the prepared samples are presented in Fig.(4). The sample at pH=1.5 which has dominant α-Fe2O3 structure show paramagnetic like behavior with small hysteresis loop as magnified inside Fig.(4) which is probably because of the presence of a small amount of ferrite phase as shown by the expanded box. The sample at pH=4 shows more ferrimagnetic nature than sample of pH=1.5. When pH=7, the largest moment (or magnetization M) and initial permeability was produced as seen in Table (3). The parameter M reduced to lower values when pH was increased 13. Here, since there was no apparent saturation, the measured magnetization values are determined at 42000 Oe for comparison. The diference between the samples at pH=10 and 13 is that the latter has higher remanence (Mr) than former. Coercivity was increased as pH increased up to 7 then decreased with continuously increasing pH. These results are summarized in Table (3). One can say, pH and subsequently particle size influences both Mr and Hc intensively. The measured initial permeability (μin) is shown in Table (3). Again the higher value for μin is at pH=7 which of course is due to high magnetization and low coercivity.
Samples at pH equal to 7 and 10 had coercivity (Hc) values lower than single crystal Ni ferrite samples prepared by CVD  and higher than sol-gel Ni-ferrite . The saturation magnetization Ms cannot be compared to literature since there was no saturation behavior, but it is good to mention that Ms of bulk Ni ferrite is about 50 emu (3200 G).
The field cooling (FC) measurements were conducted at a magnetic field of 50 Oe through a temperature range (300 -10 K), whereas the zero field cooling (ZFC) measurements were done from 300 K to 50 K. The curves of FC and ZFC for the samples at pH=10 and 13 are not united below 350 K, as shown in Fig. (5). These curves refect the superparamagnetic behavior [34, 35] for the three samples with blocking temperature (Tb) higher than 350 K for the samples at pH=10 and 13 and around 350 K for the sample at pH=7. The curves uniting at pH=7 at lower temperature is believed to be due to smaller particle size. On the other hand, the reason behind the low decreasing rate in the FC curve is related to particles aggregations due to magnetic attraction and high surface area of nanoparticles. These aggregations are clearly seen in the Fig.(3) images. The particle aggregations promote magnetic interactions among these particles that lead to freezing them and not allowing them to be in the field direction at low field. For the ZFC curve, the particle magnetic moment is going to be zero when temperature is going to 0K.
Ferromagnetic resonance (FMR) analysis is an excellent tool to diagnose material effective magnetization, spin-lattice relaxation, and damping constant via determining linewidth and resonance field especially for microwave device application [37, 38]. FMR tests were done for samples prepared at pH=7, 10, 13 as shown in Fig.(6), whereas the other samples (pH=1.5 and 4) did not show noticeable FMR signals which is the derivative (dP/dH) of absorbed microwave power (P) with respect to the magnetic field (H)). The most intensive signal relates to pH=7 where the maximum magnetization resides as shown in Fig.(4). The rest signals are fairly less than the previous.
The complex permeability μrʺ which represents the intensity of absorbed microwave power that proportional to magnetic parameters as in Eq.4 : (4)
Where μo, K1 and αd are magnetic permeability, magnetic anisotropy and damping factor respectively, knowing the anisotropy field Ha=2K1/Ms. For single domain particle, Ha is equal Hc in certain conditions of measurements, which can be considered here for simplicity . It is clear from Eq.4 that the higher Ms the higher μrʺ and the higher Hc the lower μrʺ. The complex permeability result is shown in Fig.(5)b in arbitrary units that is an integration of Fig. (5)a. Where the higher absorption power is occurred for sample prepared under pH=7 and lower power is absorbed at pH=10 and pH=13.
The widest linewidth ΔHpp belongs to sample pH=7. This means that samples have the highest damping factor and the highest microwave resonance absorption. The FMR parameters extracted from Fig.(6) are written down in Table 4.
The resonance field Hres is shifted to a higher value when pH increases to 10 and 13. The resonance frequency fo (microwave frequency) given by Larmor relation fo= γ Hres/2π (γ is the gyromagnetic ratio) must involve the role of the internal field Hi besides the external one Hex, i.e. [41,42]: (5)
This field (Hi) is in turn attributed to: the exchange field, the anisotropy field, the demagnetization field, the porosity field due to the magnetic dipoles on pores. This will result that the internal field is altered somewhat from location to other in the sample. So Hres may be shifted a little if the tetrahedral (A site) and octahedral (B site) are not filled homogeneously or the sites occupancy is disorder. Subsequently. This might be happening in NiFe2O4 if Ni2+ on B sites and Fe3+ on both sites are distributed disorderly. These reasons of inhomogeneity will produce also a line broadening in FMR of ferrites. Regarding the line shape, one can see there is asymmetric in the signal at pH=7 which may be related to nonlinear effect at higher field values.
Finally, the increasing linewidth may be related to large particle size distribution which is associated with a slight resonance field shift to higher value. Also particle aggregation with respect to disperse one has a strong effect on the linewidth and resonance field where aggregation leads to broaden the line owing to particles in interaction and that will decrease the resonance field. It is believed that the both latter factors are responsible for the signal behaviors in Fig.(5). Most polycrystalline ferrites possess linewidth in range 10-100 Gauss and grow to several hundred for nanoferrite due to inhomogeneity and spin-orbit interaction . Literature [43-45] showed linewidth (500-1000 Oe) equivalent to samples of current study. The linewidth broadening is mainly related to conductivity of sample, containing relaxing ions (Ni2+), demagnetization and anisotropic broadening.
The role of pH in structural and magnetic properties involving hysteresis and FMR properties of nickel ferrite is affecting the particle size and/or crystallite size which is in turn affecting ferrites properties. The highest particle sizes is confined to pH values from 7 to 10, thus higher magnetization, lower coercivity, higher FMR linewidth exist in this pH range. Superparamagnetic behavior is shown for all samples due to low particle size in general. These parameter values encourage the application of these materials in biomedical applications like magnetic resonance imagining MRI, magneto-resistive sensors, antibacterial and catalyst.
This work was done in the University of Technology at Baghdad/ Iraq and University of Duisburg-Essen /Germany in the AG-Farle, for that we pleasure to take the opportunity and give our great thanks to prof. M. Farle and his group for their helps and supports.
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Published Online: 2017-03-23