Effect of heat treatment on particle growth and magnetic properties of electrospun Sr<Subscript>0.8</Subscript>La<Subscript>0.2</Subscript>Zn<Subscript>0.2</Subscript>Fe<Subscript>11.8</Subscript>O<Subscript>19</Subscript> nanofibers

Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉/poly(vinyl pyrrolidone) (PVP) composite fiber precursors were prepared by the sol–gel assisted electrospinning. Subsequently, the M-type ferrite Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers with diameters about 120 nm were obtained by calcination of these precursors at different heat treatment conditions. The precursor and resultant Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectrometer and vibrating sample magnetometer. With the calcination temperature increased up to 1,000 °C for 2 h or the holding time prolonged to 12 h at 900 °C, the Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ particles gradually grow into a hexagonal elongated plate-like morphology due to the dimensional control along the nanofiber length. These elongated plate-like particles will be linked one by one to form the nanofiber with a necklace-like morphology. The magnetic properties of the Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers are closely related to grain sizes, impurities and defects in the ferrite, which are influenced by the calcination temperature, holding time and heating rate. After calcined at 900 °C for 12 h with a heating rate of 3 °C/min, the optimized magnetic properties are achieved with the specific saturation magnetization 75.0 A m² kg⁻¹ and coercivity 426.3 kA m⁻¹ for the Sr_0.8La_0.2Zn_0.2Fe_11.8O₁₉ nanofibers.     

Effects of strontium silicate on structure and magnetic properties of electrospun strontium ferrite nanofibers

The composite nanofibers of xSrSiO₃/(100 − x)SrFe₁₂O₁₉ (x = 0–13 wt%) with diameters around 110 nm have been prepared by calcination of the electrospun SrSiO₃/SrFe₁₂O₁₉/poly (vinyl pyrrolidone) (PVP) composite fibers at 800–900 °C. The composite nanofibers were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer. After calcined at 800° the M-type strontium ferrite is formed and the strontium silicate exists as an amorphous state when the calcination temperature below about 950 °C. The addition of SrSiO₃ has an obvious suppression effect on the strontium ferrite grain growth and the ferrite grain size decreases from 66.9 to 33.5 nm corresponding SrSiO₃ content from 0 to 9 wt% in the composite. The specific saturation magnetization (Ms) of the xSrSiO₃/(100 − x)SrFe₁₂O₁₉ composite nanofibers exhibits a continuous reduction from 58.0 to 45.6 A m² kg⁻¹ with the increase of SrSiO₃ content from 0 to 13 wt%. With addition of SrSiO₃ from 0 to 13 wt%, the coercivity of the composite nanofibers obtained at 900 °C initially increases, reaching a maximum value 501.1 kA m⁻¹ at the silicate content 7 wt%, and then shows a reduction tendency with the strontium silicate content increase further up to 13 wt%. This influence on the coercivity by strontium silicate can be attributed mainly to the ferrite grain growth suppression and the non-magnetic phase barrier for the domains misalignment.     

Synthesis of electrospun BaSrTiO<Subscript>3</Subscript>/PVP nanofibers

Ba_1−x Sr _x TiO₃(x = 0–0.5, BST) nanofibers with diameters of 150–210 nm were prepared by using electrospun BST/polyvinylpyrrolidone (PVP) composite fibers by calcination for 2 h at temperatures in the range of 650–800 °C in air. The morphology and crystal structure of calcined BST/PVP nanofibers were characterized as functions of calcination temperature and Sr content with an aid of XRD, FT-IR, and TEM. Although several unknown XRD peaks were detected when the fibers were calcined at temperatures less than 750 °C, they disappeared with increasing the temperature (above 750 °C) due to its thermal decomposition and complete reaction in the formation of BST. In addition, the FT-IR studies of BST/PVP fibers revealed that the intensities of the O–H stretching vibration bands (at 3430 and 1425 cm⁻¹) became weaker with increasing the calcination temperature and a broad band at 540 cm⁻¹, Ti–O vibration, appeared sharper and narrower after calcination above 750 °C due to the formation of metal oxide bonds. However, no effect of Sr content on the crystal structure of the composites was detected.     

Thermal study on electrospun polyvinylpyrrolidone/ammonium metatungstate nanofibers: optimising the annealing conditions for obtaining WO<Subscript>3</Subscript> nanofibers

This article demonstrates how important it is to find the optimal heating conditions when electrospun organic/inorganic composite fibers are annealed to get ceramic nanofibers in appropriate quality (crystal structure, composition, and morphology) and to avoid their disintegration. Polyvinylpyrrolidone [PVP, (C₆H₉NO) _n ] and ammonium metatungstate [AMT, (NH₄)₆[H₂W₁₂O₄₀]·nH₂O] nanofibers were prepared by electrospinning aqueous solutions of PVP and AMT. The as-spun fibers and their annealing were characterized by TG/DTA-MS, XRD, SEM, Raman, and FTIR measurements. The 400–600 nm thick and tens of micrometer long PVP/AMT fibers decomposed thermally in air in four steps, and pure monoclinic WO₃ nanofibers formed between 500 and 600 °C. When a too high heating rate and heating temperature (10 °C min⁻¹, 600 °C) were used, the WO₃ nanofibers completely disintegrated. At lower heating rate but too high temperature (1 °C min⁻¹, 600 °C), the fibers broke into rods. If the heating rate was adequate, but the annealing temperature was too low (1 °C min⁻¹, 500 °C), the nanofiber morphology was excellent, but the sample was less crystalline. When the optimal heating rate and temperature (1 °C min⁻¹, 550 °C) were applied, WO₃ nanofibers with excellent morphology (250 nm thick and tens of micrometer long nanofibers, which consisted of 20–80 nm particles) and crystallinity (monoclinic WO₃) were obtained. The FTIR and Raman measurements confirmed that with these heating parameters the organic matter was effectively removed from the nanofibers and monoclinic WO₃ was present in a highly crystalline and ordered form.     

Effect of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on structure and magnetic properties of Ni<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanofibers

Ni_0.5Zn_0.5Fe₂O₄ nanofibers with addition of 0–5 wt% Bi₂O₃ were synthesized by calcination of the electrospun polyvinylpyrrolidone/inorganic composite nanofibers at the temperature below the melting point of Bi₂O₃. The effects of Bi₂O₃ addition on the phase structure, morphology and magnetic properties of the nanofibers were investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, selected area electron diffraction and vibrating sample magnetometer. It is found that the nanofiber diameter, crystallite size and magnetic parameters can be effectively tuned by simply adjusting the amount of Bi₂O₃ addition. The average diameter of Ni_0.5Zn_0.5Fe₂O₄ nanofibers doped with different contents of Bi₂O₃ ranges from 40 to 63 nm and gradually decreases with increasing Bi₂O₃ content. The addition of Bi₂O₃ does not induce the phase change and all the samples are a single-phase spinel structure. The amorphous Bi₂O₃ tends to concentrate on the nanoparticle surface and/or grain boundary and can retard the particles motion as well as the grain growth, resulting in a considerable reduction in grain size compared to the pristine sample. The specific saturation magnetization and coercivity of the nanofibers gradually decrease with the increase of Bi₂O₃ amount. Such behaviors are explained on the basis of chemical composition, surface effect, domain structure and crystal anisotropy.     

Synthesis of functional magnetic porous SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/P(St-DVB-MAA) microspheres by a novel suspension polymerization

In this study, a novel and effective suspension polymerization has been employed to prepare functional magnetic porous SrFe₁₂O₁₉/P(St-DVB-MAA) microspheres in the presence of bilayer surfactants (sodium dodecyl benzene sulfonate (SDBS) and oleic acid (OA)) coated on micro-size magnetic SrFe₁₂O₁₉. This was achieved by pre-polymerizing the organic phase, which contained co-monomers, porogens and treated magnetic particles, at 65°C for 0.5 h under ultrasound conditions. Aqueous solutions containing a dispersion agent were then added to effect suspension polymerization. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and magnetic property measurement system (MPMS) were used to characterize the functional magnetic porous microspheres. The results show that the microparticles are well shaped with a uniform size distribution of about 0.5 ∼ 0.7 mm and the surfaces of the microspheres have many micro-pores with an average diameter of 0.533 μm. There are carboxyl groups (−COOH) on the surface of the microspheres to the extent of 0.65 mmol g⁻¹, as determined by conductometric titration. According to the XRD spectra, iron oxide consists mainly of SrFe₁₂O₁₉ which reveals hexahedral structure. The content of magnetic SrFe₁₂O₁₉ reaches 17.81% (by mass), and the microspheres have good heat resistance. The magnetic porous microspheres are ferromagnetic with high residual magnetization and coercivity, 21.59 emu g⁻¹ and 4.13 kOe, respectively. The saturation magnetisation is around 42.85 emu g⁻¹.      

Non-isothermal decomposition kinetics of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles synthesized using egg white solution route

The thermal decomposition kinetics of nickel ferrite (NiFe₂O₄) precursor prepared using egg white solution route in dynamical air atmosphere was studied by means of TG with different heating rates. The activation energy (E _α) values of one reaction process were estimated using the methods of Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS), which were found to be consistent. The dependent activation energies on extent of conversions of the decomposition reaction indicate “multi-step” processes. XRD, SEM and FTIR showed that the synthesized NiFe₂O₄ precursor after calcination at 773 K has a pure spinel phase, having particle sizes of ~54 ± 29 nm.     

Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> powders by the complex polymerization method: synthesis,characterization and morphology

Lanthanum-modified bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂, BLTO) powders were prepared by the complex polymerization method. The structure and morphology of BLTO powders were investigated by X-ray diffraction and scanning electron microscopy. The complexation of citric acid with the metallic cations was detected by Fourier transformed infrared (FT-IR). The thermal analyses of obtained gels were investigated by differential thermal gravimetric (DTG). The pure and normally stoichiometric phase of BLTO powders could be obtained at relatively low temperature of 550–700 °C even if the bismuth content is not excess in the starting precursors, while the secondary phase could be detected at lower and higher calcination temperatures. The shape of the BLTO grains is similarly to platelet in Bi-layer structure and stoichiometry BLTO was detected by the analysis of energy dispersive spectrometry.     

Nonstoichiometric Zn Ferrite and ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> Composite Spheres: Preparation,Magnetic Properties,and Chromium Removal

Monodisperse and porous nonstoichiometric Zn ferrite can be prepared by a solvothermal method. Such non-Zn ferrite was used to be the precursor for synthesis of ZnFe₂O₄/Fe₂O₃ composite via calcination at 600°C for 3 h in air. X-ray powder diffractometer (XRD) and Energy Dispersive Spectrometer (EDS) proved the nonstoichiometry of Zn ferrite synthesized by solvothermal method and the formation of ZnFe₂O₄/Fe₂O₃ composite via calcination. TEM image showed that non-Zn ferrite spheres with wormlike nanopore structure were made of primary nanocrystals. BET surface area of non-Zn ferrite was much higher than that of ZnFe₂O₄/Fe₂O₃ composite. Saturation magnetization of non-Zn ferrites was significantly higher than that of ZnFe₂O₄/Fe₂O₃ composites. Calcination of non-Zn ferrite resulted in the formation of large amount of non-magnetic Fe₂O₃,which caused a low magnetization of composite. Because of higher BET surface area and higher saturation magnetization, non-Zn ferrite presented better Cr⁶⁺ adsorption property than ZnFe₂O₄/Fe₂O₃ composites.     

Synthesis of LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> nanoparticles by modified Pechini method and their enhanced rate capability

Layered LiNi_1/3Co_1/3Mn_1/3O₂ nanoparticles were prepared by modified Pechini method and used as cathode materials for Li-ion batteries. The pyrolytic behaviors of the foamed precursors were analyzed by use of simultaneous thermogravimetric and differential thermal analysis (TG-DTA). Structure, morphology and electrochemical performance characterization of the samples were investigated by X-ray diffraction (XRD), field emission scanning electron macroscopy(SEM), Brunauer-Emmett-Teller (BET) specific surface area and charge–discharge tests. The results showed that the samples prepared by modified Pechini method caclined at 900 °C for 10 h were indexed to pure LiNi_1/3Co_1/3Mn_1/3O₂ with well hexagonal structure. The particle size was in a range of 100–300 nm. The specific surface area was larger than that of the as-obtained sample by Pechini method. Initial discharge capacity of 163.8 mAh/g in the range 2.8–4.4 V (vs. Li/Li⁺) and at 0.1C for LiNi_1/3Co_1/3Mn_1/3O₂ prepared by modified Pechini method was obtained, higher than that of the sample prepared by Pechini method (143.5 mAh/g). Moreover, the comparison of electrochemical results at different current rates indicated that the sample prepared by modified Pechini method exhibited improved rate capability.     

Fabrication and magnetic properties of electrospun copper ferrite (CuFe2O4) nanofibers

Tetragonal copper ferrite (CuFe₂O₄) nanofibers were fabricated by electrospinning method using a solution that contained poly(vinyl pyrrolidone) (PVP) and Cu and Fe nitrates as alternative metal sources. The as-spun and calcined CuFe₂O₄/PVP composite samples were characterized by TG-DTA, X-ray diffraction, FT-IR, and SEM, respectively. After calcination of the as-spun CuFe₂O₄/PVP composite nanofibers (fiber size of 89 ± 12 nm in diameter) at 500 °C in air for 2 h, CuFe₂O₄ nanofibers of 66 ± 13 nm in diameter having well-developed tetragonal structure were successfully obtained. The crystal structure and morphology of the nanofibers were influenced by the calcination temperature. After calcination at 600 and 700 °C, the nature of nanofibers changed which was possibly due to the reorganization of the CuFe₂O₄ structure at high temperature, and a fiber structure of packed particles or crystallites was prominent. Crystallite size of the nanoparticles contained in nanofibers increases from 7.9 to 23.98 nm with increasing calcination temperature between 500 and 700 °C. Room temperature magnetization results showed a ferromagnetic behavior of the calcined CuFe₂O₄ samples, having their specific saturation magnetization (M_s) values of 17.73, 20.52, and 23.98 emu/g for the samples calcined at 500, 600, and 700 °C, respectively.     

Synthesis of high surface area transitional alumina from Al(OPh)<Subscript>3</Subscript>

Al(OPh)₃ involving sterically hindered phenyl groups on ultrasonic assisted micro hydrolysis yielded a mixture of boehmite and bayerite as deduced from the FTIR and powder X-ray diffraction pattern. In the thermogravimetric trace, the complete removal of decomposable moieties of the hydrolyzed gel occurred around 530 °C. Calcining the gel at temperatures 600, 700, 800 and 900 °C showed crystalline tetragonal δ-Al₂O₃ to be the product at 900 °C as deduced from FTIR, ²⁷Al NMR and PXRD techniques. δ-Al₂O₃ showed a surface area of 135 m²/g with rectangular bar like morphology with the sizes below 50 nm in the TEM images.     

Synthesis of Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> ceramic scintillators by the polymeric precursor method

Bismuth germanate ceramic powders were synthesized for the first time by the polymeric precursor method (Pechini’s method). Differential thermal analysis and thermogravimetric techniques were used to study the decomposition of the resin precursor, which indicated a suitable calcination temperature at 600 °C. It was observed that the mass loss occurs in two main stages that are associated with two exothermic reactions. The crystalline phases of the powders were inspected by the X-ray diffraction technique after thermal treatment between 300 and 600 °C. Single phase Bi₄Ge₃O₁₂ ceramic bodies were obtained after sintering at 840 °C for 10 h. The sintered ceramics presented a luminescence band emission centred at around 530 nm when excited with X-rays and UV radiation.     

Thermochemical properties of MnNb<Subscript>2</Subscript>O<Subscript>6</Subscript>

Homogeneous manganocolumbite (MnNb₂O₆) was synthesized from Nb₂O₅ and MnO oxides. Powder sample was orthorhombic with unit cell parameters: α = 0.5766 nm, b = 1.4439 nm, c = 0.5085 nm and V = 0.4234 nm³. Heat capacity over the temperature range of 313–1253 K was measured in an inert atmosphere with combined thermogravimetry and calorimetry using NETZSCH STA 449C Jupiter thermoanalyzer. Melting point was 1767 ± 3 K, enthalpy of melting was 144 ± 4 kJ mol⁻¹. Experimental heat capacity of MnNb₂O₆ is fitted to polynomial C _pm = 221.46 + 3.03 · 10⁻³ T + −39.79 · 10⁵ T ⁻² + 40.59 · 10⁻⁶ T ².     

Preparation and electrochemical performance of nano-structured Li<Subscript>2</Subscript>Mn<Subscript>4</Subscript>O<Subscript>9</Subscript> for supercapacitor

Nano-structured spinel Li₂Mn₄O₉ powder was prepared via a combustion method with hydrated lithium acetate (LiAc·2H₂O), manganese acetate (MnAc₂·4H₂O), and oxalic acid (C₂H₂O₄·2H₂O) as raw materials, followed by calcination of the precursor at 300 °C. The sample was characterized by X-ray diffraction, scanning electron microscope, and energy-dispersive X-ray spectroscopy techniques. Electrochemical performance of the nano-Li₂Mn₄O₉ material was studied using cyclic voltammetry, ac impedance, and galvanostatic charge/discharge methods in 2 mol L⁻¹ LiNO₃ aqueous electrolyte. The results indicated that the nano-Li₂Mn₄O₉ material exhibited excellent electrochemical performance in terms of specific capacity, cycle life, and charge/discharge stability, as evidenced by the charge/discharge results. For example, specific capacitance of the single Li₂Mn₄O₉ electrode reached 407 F g⁻¹ at the scan rates of 5 mV s⁻¹. The capacitor, which is composed of activated carbon negative electrode and Li₂Mn₄O₉ positive electrode, also exhibits an excellent cycling performance in potential range of 0–1.6 V and keeps over 98% of the maximum capacitance even after 4,000 cycles.     

Synthesis and optical property of NaTaO<Subscript>3</Subscript> nanofibers prepared by electrospinning

This paper describes a procedure of preparing sodium tantalite nanofibers for the first time. Sodium tantalite nanofibers were synthesised by electrospinning a sol–gel precursor solution of poly(vinyl pyrrolidone)/sodium tantalite, followed by careful sintering of the as-electrospun composite fibers at 550 °C for 3 h. The morphology, microstructure and crystal phase were investigated by transmission electron microcopy and X-ray diffraction. The optical property was characterized by ultraviolet–visible (UV–vis) spectrometer. Typical nanofibers were with diameter between 70 and 90 nm and length exceeding 0.1 mm. An unusual phenomenon, the red-shift of optical absorption band edge happened, indicated the fabricated NaTaO₃ nanofibers were potential good candidates for photocatalytic application. The experiment photodegradation of methylene blue by NaTaO₃ nanofibers under UV light irradiation was performed.     

Effect of the modifier ion on the properties of MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> and ZnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> pigments

Magnesium and zinc ferrites have been prepared by the polymeric precursor method. The organic material decomposition was studied by thermogravimetry (TG) and differential thermal analysis (DTA). The variation of crystalline phases and particle morphology with calcination temperature were investigated using X-ray diffraction (XRD) and scanning electronic microscopy (SEM), respectively. The colors of the ferrites were evaluated using colorimetry. Magnesium ferrite crystallizes above 800°C, presenting a yellow- orange color with a reflectance peak at the 600–650 nm range, while zinc ferrite crystallizes at 600°C, with a reflectance peak between 650–700 nm, corresponding to the red-brick color.     

Cellulose scaffolds modulated synthesis of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocrystals: preparation,characterization and properties

Magnetic Co₃O₄ nanoparticles were prepared by using microporous regenerated cellulose films as sacrificial scaffolds. The cellulose macromolecules and the porous structure of the films made them used as spatially confined reacting sites where Co(OH)₂ nanoparticles could be synthesized in situ. When the cellulose matrix was removed by sintering at 500 °C, Co₃O₄ nanoparticles were obtained. XRD and XPS indicated that the prepared nanoparticles were pure Co₃O₄ without any impurity. TEM and SEM images revealed that the particle size of the nanoparticles was smaller than 100 nm. The nanoparticles had weak ferromagnetic properties at 25 °C. Furthermore, the pronounced quantum confinement effects of the synthesized nanoparticles have been observed, the optical bandgap energies determined were about 1.92 ~ 2.12 and 2.74 ~ 2.76 eV for O²⁻ → Co³⁺ and O²⁻ → Co²⁺ charge-transfer processes, respectively. Furthermore, the resulted Co₃O₄ nanoparticles behaved stable electrochemical performance with promising applications in the electrode for lithium ion battery.     

Nanocrystalline SrCexFe12?xO19 (x?=?0.00, 0.02, 0.04, 0.06, 0.08) microfibers by sol–gel method

The SrCe_xFe_12?xO₁₉ (x????0.08) ferrite microfibers were prepared via sol?Cgel method from starting reagents of metal nitrate salts and citric acid. The obtained microfibers were characterized by TG-DSC, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The XRD patterns show that the main phase is M-type strontium hexaferrite without other impurity phases when calcined at 750?°C for 1?h, and the Ce³⁺ ions doping has not resulted in crystal structural changes of the Sr ferrite magnetoplumbite type structure. The SEM and TEM images indicate that samples with different calcination temperature and various values of x possess the diameters of below 10???m and homogeneous hexagonal plate-like morphology. The VSM results show that saturation magnetization (Ms) gradually increases with increase of calcination temperature while decreases with increase of addition of Ce³⁺ ions, and coercive force (Hc) reveals an increase at first and then decreases with increase of calcination temperature and Ce³⁺ ions content.     

The microstructure and magnetic properties of Ni<Subscript>0.4</Subscript>Zn<Subscript>0.6</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> films prepared by spin-coating method

Nickel zinc ferrite (Ni_0.4Zn_0.6Fe₂O₄) films on Si (100) substrate were synthesized using a spin-coating method. The crystallinity of the Ni_0.4Zn_0.6Fe₂O₄ films with the thickness of about 386 nm became better as the annealing temperature increased. The films have smooth surface, relatively good packing density and uniform thickness. The volatilization of Zn is serious at 900 °C. With the increase of annealing temperature, the saturation magnetization M _s increases in the temperature ranging from 400 to 700 °C, however, decreases above 700 °C, and the coercivity H _c increases in the temperature range 400–800 °C, decreases above 800 °C. After annealed at 700 °C for 2 h in air with the heating rate 2 °C/min, the film shows a maximum saturation magnetization M _s of 349 emu/cc and low coercivity H _c of 66 Oe. The M _s is higher than others which prepared by this method, however, the H _c is lower. The M _s of Ni_0.4Zn_0.6Fe₂O₄ films annealed at 700 °C increases with increasing annealing time and the H _c changes slightly.