Magnetic Properties Of Samples Research Articles

Effects of La-Co Co-Substitution and Magnetic Field Pressing on the Structural and Magnetic Properties of SrM Hexaferrites

We carefully investigated the effects of La-Co co-substitution and magnetic field pressing (MFP) on the structural and magnetic properties of SrM hexaferrites. Samples composed of Sr1−xLaxFe12−xCoxO19 were sintered at 1230 °C for 2 h in air with sintering additives composed of 0.7 wt% CaCO3 and 0.7 wt% SiO2. A single M-type phase was confirmed to exist up to x = 0.3. Rietveld refinement revealed a slight decrease in lattice parameter a and the unit cell volume (Vcell) with an increasing x, while parameter c showed a significant decrease. The saturation magnetization (Ms) values increased from 70.90 to 72.40 emu/g with an increasing x from 0.0 to 0.15 and then decreased to 71.38 emu/g with further increasing of x to 0.3, while the anisotropy field (Ha) increased from 17.7 to 25.9 kOe, leading to a continuous increase in the intrinsic coercivity (Hci), from 3.52 to 5.00 kOe, respectively. Using the MFP process, the c-axis of M-type hexaferrite grains could be effectively aligned to the applied field direction, which significantly affected the microstructures and, thus, magnetic properties of samples. Unlike non-MFP samples, exhibiting a significant increase in the average grain size (davg) but almost unaltered average thickness (tavg) with an increasing x from 0.0 to 0.3, MFP-processed samples exhibited almost unaltered davg values but a continuous decrease in tavg. Consequently, the variation in remanent flux density (Br) versus x followed that of Ms versus x and thus exhibited the highest Br of 4.05 kG for x = 0.15, leading to the highest maximum energy product {(BH)max} of 3.62 MGOe. With an increasing x from 0.0 to 0.3, the Hci values continuously increased from 3.14 to 3.84 kOe mainly due to a continuous increase in Ha, although they were significantly lowered in comparison with those of non-MFP samples because of a large increase in Br for a given composition x. A higher Mr/Ms ratio always resulted in a larger (BH)max in our samples regardless of x. A careful comparison of the microstructures and magnetic properties between MFP and non-MFP samples provided valuable insights into a broad area of permanent magnet optimization.

Structural and magnetic properties of sol-gel autocombustion and microwave synthesized nickel ferrite nanoparticles and their photocatalytic and anti-microbial applications

Nickel Ferrite was synthesized from its nitrate precursors via sol–gel auto combustion method and microwave synthesis with ethylene glycol as fuel. The phase identification was carried out by XRD and the crystalline size was calculated. The Williamson- Hall plot was drawn and lattice strain and crystallite size was determined. The spinel structure of ferrite was confirmed using FTIR spectra. The morphology of samples were studied from HR-TEM. The UV–Visible spectra of samples were obtained and the band gap was determined from Tauc plot. The magnetic properties of samples were studied using VSM. The antibacterial activity of samples on Escherichia coli and Staphylococcus aureus was studied by well diffusion method. Their photocatalytic degradation of Methyl Orange under UV light was carried out and studied using UV–Visible Spectrophotometry.

Magnetic properties of Sm-doped M-type barium ferrite by high-energy ball mill-assisted solid-phase reaction method

The primary objective of this study is to enhance the overall magnetic properties of pure barium ferrite through the addition of trace amounts of the light rare earth element Sm. This approach is more cost-effective compared to using heavy rare earth elements. The M-shaped barium ferrite, denoted as Ba1−xSmxFe12O19, was synthesized using a combination of solid-phase sintering method and high-energy ball milling. The high purity of samples was confirmed by determining their crystal structure using XRD. The internal structure of samples was examined using FTIR, and it was observed that the samples contained tetrahedral and octahedral clusters with corresponding bands at 599 cm−1 and 445 cm−1, respectively. The appearance of samples was examined using FESEM, and it was found that grains had hexagonal structures. EDS was used to characterize the elemental content of samples, and it was found that the content of Ba decreased with an increase in the content of Sm. The magnetic properties of samples were examined using VSM, and the coercivity reached a maximum value of 3096.22 Oe at a dopant fraction of 0.06, and the saturation magnetization reached a maximum value of 73.115 emu/g. The synthesized material has the potential for utilization in magnetic recording applications.

Exploring the enhancement of exchange bias in α-Fe2O3/g-C3N4 composites: Synthesis, magnetic properties and a first-order reversal curve analysis

α-Fe2O3/g-C3N4 composites were synthesized by loading different amounts of g-C3N4 on ordered α-Fe2O3 nanowires. The samples were characterized by XRD, SEM, TEM, EDS and N2 adsorption-desorption isotherms to study the composition, microstructure and morphology. It showed that g-C3N4 was uniformly dispersed around the α-Fe2O3 nanowires to form α-Fe2O3/g-C3N4 composites. The magnetic properties of samples were obtained by the ZFC-FC and hysteresis loops measurement. The enhancement of exchange bias field in α-Fe2O3/g-C3N4 composites was discovered. FCN-5 exhibited a higher HEB of 794 Oe at 10 K, which exceeded that of pure α-Fe2O3 of 720 Oe. The influence factors of exchange bias in α-Fe2O3/g-C3N4 composites were analyzed by FORC method. The results showed that the enhancement of HEB in α-Fe2O3/g-C3N4 composites was due to the RKKY interaction replacing the original coupling between the initial surface spin-disordered shell layer and the core in α-Fe2O3.

EPR Spectrum Analysis of DPPH and MnCl2

This paper presents the findings of electron paramagnetic resonance (EPR) spectroscopy experiments conducted on two different samples: 2,2-diphenyl-1-picrylhydrazyl (DPPH) and Manganese Chloride (MnCl2) dissolved in H2O. The experiments were conducted at Dr. D. Petasis EPR lab located in Allegheny College, Meadville, PA, USA. A Varian X-Band spectrometer was utilized for the analysis of the EPR absorption spectra. The primary objective was to determine the g-values and hyperfine structure of the two samples, which can be crucial in characterizing their electronic properties. The g-values obtained for both DPPH and MnCl2 were found to be remarkably close to the theoretically calculated values, suggesting the high accuracy and success of the experiments. This agreement validates the reliability of the EPR measurements and demonstrates the precision of the instruments and experimental procedures employed. For the MnCl2 sample, a hyperfine structure with 6 visible EPR lines was observed. By analyzing the number of EPR lines, the nuclear spin of the Mn(II) ion was determined to be I = 5/2, consistent with the expected value for Mn(II). The hyperfine constant (A) was also calculated from the field position differences between adjacent lines, providing further insights into the sample's magnetic properties. Comparisons were made between the obtained g-values and hyperfine constant with the expected values for Mn(II) in the literature, confirming the accuracy of the measurements and reinforcing the credibility of the experimental data. Overall, the EPR spectroscopy experiments conducted on DPPH and MnCl2 in H2O have provided valuable information about the electronic and magnetic properties of these paramagnetic materials. The results demonstrate the effectiveness of EPR spectroscopy as a powerful experimental technique for investigating various bio-chemical samples and understanding the effects of solvents on the EPR spectra of compounds. The close agreement between the experimental and theoretical values for the g-factors and hyperfine constants reinforces the reliability of the data obtained from the EPR experiments.

Zn- doped MnFe2O4 nanoparticles for magnetic hyperthermia and their cytotoxicity study in normal and cancer cell lines

To improve magnetic hyperthermia for the treatment of cancer, we prepared Mn1−xZnxFe2O4 (x = 0.1, 0.3, 0.4, & 0.5) magnetic nanoparticles (NPs) using the solvothermal reflux method. The preparation of NPs single phase was confirmed by X-ray diffraction (XRD) investigation. The prepared sample's magnetic properties were greatly improved by the addition of Zn, increasing its magnetization value. The specific loss power (SLP) values in DI water were used to assess the performance of the colloids. Zn0.1Mn0.9Fe2O4 NPs displayed the highest SLP value among the samples, at 1 mg/mL concentration (107.79 W/g) in an alternating magnetic field (AMF) of 35.2 kA/m and 316 kHz. This proved how effectively they could produce heat for magnetic hyperthermia therapy. Through cell cytotoxicity analysis, the cytotoxicity of the Zn-doped MnFe2O4 NPs was evaluated, and the results showed high biocompatibility and negligible cytotoxicity. This shows that the NPs are appropriate for biomedical applications, particularly in the treatment of cancer with magnetic hyperthermia. The better magnetic characteristics and effective heat generation for hyperthermia treatment were obtained through the synthesis of Zn-doped MnFe2O4 NPs. The NPs also showed great biocompatibility, making them viable options for biomedical applications, particularly in the treatment of cancer.

Selective Laser Melting of (Fe-Si-B)/Cu Composite: Structure and Magnetic Properties Study

A mixture of original 1CP powder and 10 wt.% of pure Cu-powder was prepared and 1CP-Cu composite samples were obtained by selective laser melting using different process parameters. Comparison of pure 1CP and composite samples showed that addition of Cu halved the porosity percentage of the obtained material. Distribution of Cu-phase in 1CP-matrix can be recognized as uniform in all the samples. X-ray diffraction of samples showed the presence of α-Fe solid solution, iron boride Fe2B, and crystal Cu. Scanning electron microscopy analysis also allowed to discover ordered solid solution Fe3Si in samples microstructure. Differential scanning calorimetry data showed that composite sample contains amorphous phase as opposed to pure 1CP sample manufactured using the same process parameters. Magnetic properties of samples were studied, and it was found that addition of 10 wt.% of Cu allowed to reduce magnetic field energy losses by approximately four times.

High dielectric constant and magnetic properties of Bi-In Co-substituted YIG ferrites

In this paper, the Bi-In co-substituted YIG ferrite materials composed of Y3−xBixFe4.55In0.45O12 (x = 0.0–1.0 with a step of 0.2) were synthesized by a conventional solid-state route. Samples with different Bi3+ ion contents were sintered at different temperatures. The phase formation, microstructure, dielectric and magnetic properties of samples were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), impedance analyzer, vibrating sample magnetometer (VSM), and ferromagnetic resonance (FMR). The results showed that Bi3+ ion substitution could reduce the sintering temperature, increase the bulk density (db), improve the dielectric constant (ε' = 22.26@100 MHz at × = 1.0) and keep a high saturation magnetization (4πMS = 1700–1850 Gs), but led to the increase of the FMR linewidth (ΔH = 316.21 Oe at × = 1.0).

Annealing process of Co-Fe-B based multilayers showing skyrmion Brownian motion

Skyrmions are topological spin textures that exhibit Brownian motion in solids. They have attracted increasing research interest in terms of realizing a device that utilizing stochastic behavior and investigating new physical phenomena. However, skyrmions that exhibit Brownian motion are sensitive to changes in magnetic properties and are easily affected by aging variation. For instance, although skyrmions appear in a sample immediately after fabrication, they sometimes disappear after few weeks. This characteristic prevents the reproducibility experiment and affects device stability. In this study, we demonstrated that aging variation can be suppressed by annealing in air for only 3 min, which is an easy and rapid method. We investigated the change in the magnetic properties by annealing and air exposure and found that the main mechanism of aging variation is oxidation of the sample surface. The magnetic properties of samples with Pt and thick SiO2 capping were analyzed, and we demonstrated that aging variation can be suppressed by avoiding surface oxidation. Our work accelerates the research of fundamental physics regarding skyrmion Brownian motion and of device applications utilizing stochastic system.

Magnetic and structural analysis of BaZnxZrxFe12–2xO19 (x = 0.1–0.7) hexaferrite samples for magnetic applications

Polycrystalline samples of M-type hexaferrite with BaZnxZrxFe12–2xO19 (x = 0.1–0.7) as chemical composition were synthesized successfully using solid state reaction technique. The structural, microstructural and magnetic properties of these samples were investigated. Rietveld analysis (goodness-of-fit ≈1.06–1.22) demonstrates the phase purity of the samples in the entire composition range. The `c/a′ ratio is < 3.98, for all samples implying that they are M-type hexaferrites. The XRD results also show a simultaneous decrease in crystallite size (149–69 nm) and an increase in cell volume (699.119–711.006 Å3) with increasing dopant concentration. FESEM micrographs reveal that the a) average grain size varies within 0.61–0.79 µm and b) sample porosity decreases with increasing concentration of Zn2+- Zr4+ dopants. EDX data reveals the presence of all the elements and Ba:Fe ratio ≈ 1:12. M-H studies reveal that a) Ms increases from 61.45 emu/g (at x = 0) to 72.87 emu/g (at x = 0.1) and decreases afterwards to 61.45 emu/g (at x = 0.7) and b) Hc decreases continuously from 4118 Oe (at x = 0.0) to 319 Oe (at x = 0.7). The excellent magnetic properties of these hexaferrite samples make them useful in magnetic data storage devices, devices for microwave absorption and in spintronic devices.

Regulation of Magnetocaloric Effect in Ni40Co10Mn40Sn10 Alloys by Using a Homemade Uniaxial Strain Pressure Cell

In this study, a homemade uniaxial strain pressure cell was designed to be directly used in the standard magnetometers whereby the magnetic properties of samples subjected to a uniaxial strain and magnetic field were characterized. Its feasibility has been demonstrated by the uniaxial strain control of the phase transition and magnetocaloric effect in Ni40Co10Mn40Sn10 (NCMS) alloys. With the assistance of a uniaxial strain of ~0.5%, the cooling temperature span of NCMS alloys is broadened by 2 K, and the refrigeration capacity under a 3 T magnetic field change increases from 246 to 277 J/kg. This research provides not only direct experimental assistance for the tuning of phase transition by the uniaxial strain but also possibilities for studying the coupled caloric effect in first-order phase transition materials under a combined uniaxial strain and magnetic field by the thermodynamic analysis.

Improvement of (BH)max in Ba-hexaferrite doped with La and Co

In this work we produce Ba-hexaferrite nanoparticles by the ionic coordination reaction method and study the influence of La and Co ions on the magnetic properties of Ba0.7La0.3Fe12-xCoxO19 for x = (0.0, 0.25, 0.50, 0.75 and 1.0) nanoparticles. The crystalline and morphological properties were studied by X-ray diffraction and scanning electron microscopy techniques. The magnetic properties of samples were studied in a vibrating sample magnetometer under an applied field of ±6 T. The Fe occupancy in the three octahedral (12k, 2a, and 4f2), one tetrahedral (4f1) and one trigonal bipyramidal (2b) sites was studied by Mössbauer spectroscopy. The samples are single domain nanoparticles with crystallite sizes in the range 71.5-50.1 nm, decreasing with the Co doping. The results suggest that the sample with x = 0.25 had the largest (BH)max of 6.98 kJ/m3, this effect is mostly due to the occupancy of 4f2 sites by Co2+. For x > 0.25, the Co2+ occupy mainly both the 4f2 and 12k sites, the presence of oxygen vacancies is discussed.

Room temperature luminescence and ferromagnetism from transition metal ions: Mn-, Co- and Ni-doped ZnS nanoparticles

ZnS nanoparticles (NPs) and transition metal ions (TMIs) doped ZnS NPs were synthesized by the chemical precipitation method facile via aqueous CTAB micelles. The structural properties, crystallography, morphology, composition analysis, photoluminescence (PL), and magnetic properties of samples were characterized by Fourier transform infrared spectrum (FTIR), X-ray diffraction (XRD) analysis, Field emission scanning electron microscope (FESEM), Energy dispersive X-ray (EDX) spectroscopy, Fluorescence spectroscopy, and Vibrating sample magnetometer (VSM). All the synthesized NPs exhibited a cubic zinc-blende single-phase crystal structure with crystallite size in 2–8 nm range, which indicates the TMIs ions substituted the lattice site of Zn2+ ions, due to almost similar ionic radii of Zn2+ and TMIs (Mn2+, Co2+, and Ni2+), i.e., 0.074, 0.083, 0.069, and 0.065 nm, respectively. PL peak intensity enhanced with TMIs doping causes an excited state to the ZnS NPs, resulting in PL spectra turning into the visible region. ZnS NPs show mixed dominant diamagnetic and weak ferromagnetic behavior. TMIs ions doped ZnS NPs show the well-defined ferromagnetic behavior at room temperature (RT).

Experimental investigation of the phase equilibria in Co–Fe-Sm system with special attention to the effect of Fe substitution in structure and magnetic properties of intermetallic phases

Phase equilibria in the Sm–Co, Sm–Fe and Co–Fe-Sm systems were investigated in detail using DTA, X-ray diffraction, SEM and EPMA. Based on the experimental data from the present work, the liquidus and solidus projections, the isothermal section at 1000 °C and the Scheil reaction scheme for the Co–Fe-Sm system over the whole concentration range were constructed. The phase diagram of the Sm–Co system was revised. The Sm-rich region of the Sm–Fe system was studied for the first time. The stability domains of the intermetallic phases (binary phases and their extension inside ternary) were defined. Three continuous solid solutions Sm2(Co,Fe)17, Sm(Co,Fe)3 and Sm(Co,Fe)2 were found in the Co–Fe-Sm system at the solidus temperatures. In contrast to a previous analysis of the phase diagram for the system, wide homogeneity regions were revealed for the SmCo5 and Sm2Co7 phases and alternative invariant four-phase equilibria. The magnetic properties of samples containing Sm2(Co,Fe)17 phase were measured. The values of the Curie temperature and the magnetic moment of these samples were quite close to those earlier presented in the literature.

Low dielectric loss and narrow FMR linewidth of Ca-Ge co-substituted YInIG ferrites for microwave device application

Recently, the yttrium iron garnet (Y3Fe5O12, YIG) ferrite material is widely applied in the microwave devices due to its good frequency stability and loss magnetic loss. In this work, Y3−xCaxIn0.6GexFe4.4−xO12 (x = 0.0–0.6 with a step of 0.1) materials were synthesized by the conventional solid-state reaction method. The effects of Ca-Ge co-substitution on the microstructural, dielectric and magnetic properties of YInIG materials were investigated. Ca-Ge co-substitution did not change the phase formation of YInIG ferrite, and enhanced dielectric and magnetic properties of samples. The high saturation magnetization (4πMs), low dielectric constant (εr), low dielectric loss (tanδε), small coercivity (Hc) and narrow ferromagnetic resonance (FMR) linewidth (ΔH) were obtained for high frequency microwave device application. When substituted content x = 0.4, Y2.6Ca0.4In0.6Ge0.4Fe4O12 has the optimum microwave properties: εr = 13.884@10.8 GHz, tanδε = 3.053 × 10–4, Hc= 22.75 Oe, 4πMs = 1226.83 Gs, ΔH= 19.3 Oe. The narrow FMR linewidth and low loss dielectric loss of the investigated samples suggested the possible use of these materials in high frequency applications.

Remarks on Selecting Length of Cylindrical Sample to Determine Magnetic Properties of its Material

When experimentally studying magnetic properties of the ferromagnetic materials, preferences are often given to a more convenient method involving the use of sufficiently long cylindrical samples with the L length and the D diameter placed in the solenoid field as an alternative to the method based on using classical samples of the toroidal shape (to exclude manifestations of the demagnetizing factor). Currently, required justification is practically missing for specific values of the relative L/D length, which would indicate such [L/D] values (for L/D ≥ [L/D]), at which magnetic properties of a sample (already long enough) correspond to magnetic properties of its material. While the existing recommendations such as [L/D] = 50 are postulated, let us note that relevant experimental studies of magnetic properties of the cylindrical samples with the L/D parameter targeted variation were not made. An attempt was made to fill this gap. For cylindrical steel samples with the different L/D values (from 1 to 50), families of the B magnetic induction and of the μ permeability field dependencies were obtained experimentally using the ballistic method. The sought [L/D] values were determined from the B and μ dependencies on the L/D by the junction abscissa of the ascending and self-similar branches. It was established that in the accepted field strength in the range of H = 0.94--54.2 kА/m magnetizing field, the [L/D] parameter is a variable substantially depending on H (and/or μ). It varies from [L/D] = 10--15 at H = 54.2 kA/m (μ = 30) to [L/D] = 50--60 at H = 4.7 kA/m (μ = 270). And at H &lt; 4.7 kA/m, [L/D] &gt; 50--60, i.e., more than is commonly believed. Thus, it was stated that the data on magnetic properties obtained when using even long samples (L/D = 50) and declared as data on the magnetic properties of the corresponding material, are only close to those with H &lt; 4.7 kA/m. Phenomenological expressions were obtained for the [L/D] determination: exponential with the H argument and logarithmic with the μ argument

SYNTHESIS OF REDUCED GRAPHENE OXIDE - Cu0.5Ni0.5Fe2O4 - PRUSSIAN BLUE NANOCOMPOSITE MATERIALS FOR CESIUM ADSORPTION FROM AQUEOUS SOLUTION

Adsorbents composed of reduced graphene oxide, Cu0.5Ni0.5Fe2O4 ferrite and prussian blue (RGO-CNF-PB nanocomposites) were fabricated for the adsorption of cesium and rapid magnetic separation of absorbent from contaminated water. The morphology, structure and magnetic properties of samples were characterized by SEM, XRD, FTIR, VSM. The effect of pH, contact time and adsorption isotherms were conducted in batch experiments. It was found that reduced graphene oxide was exfoliated and decorated homogeneously with ferrite nanoparticles. Cu0.5Ni0.5Fe2O4 has the average particle diameter of about 15 nm and prussian blue has been covered smoothly onto RGO-CNF surfaces. The remanences (Mr) and coercive forces (Hc) are near to zero, indicating that obtained material is superparamagnetic. The adsorption of cesium could be suitably described by the pseudosecond- order and the Langmuir models. The highest adsorption capacity of the composites for cesium was evaluated to be 125 mg/g at pH = 7 and 25°C.

Preparation of fully dense and magnetically controllable CaO-Al2O3-SiO2-Na2O-Fe3O4 glass ceramics by hot pressing

Fully dense and magnetically controllable glass ceramics was successfully synthesized by method of hot pressing using CaO-Al2O3-SiO2-Na2O glass powder and Fe3O4 powder as raw materials. The influences of sintering temperature and time, content and particle size of Fe3O4, as well as particle size of glass powder on the densification and magnetic properties of samples were investigated. It was found that the saturation magnetization gradually increased with increasing magnetite content. In addition, the samples containing smaller size magnetite particles had a higher coercivity. However, for samples using smaller size glass powder, magnetite particles could partially dissolve into the glass matrix, which led to the decrease of saturation magnetization and the increase of coercivity. It was also concluded that the precipitation of crystalline phase from smaller size glass powder caused the decrease of degree of densification, and after decreasing the sintering temperature, the degree of densification of product was enhanced.

Electromagnetic Approach to Cavity Spintronics

The fields of cavity quantum electrodynamics and magnetism have recently merged into cavity spintronics, investigating a quasiparticle that emerges from the strong coupling between standing electromagnetic waves confined in a microwave cavity resonator and the quanta of spin waves, magnons. This phenomenon is now expected to be employed in a variety of devices for applications ranging from quantum communication to dark matter detection. To be successful, most of these applications require a vast control of the coupling strength, resulting in intensive efforts to understanding coupling by a variety of different approaches. Here, the electromagnetic properties of both resonator and magnetic samples are investigated to provide a comprehensive understanding of the coupling between these two systems. Because the coupling is a consequence of the excitation vector fields, which directly interact with magnetization dynamics, a highly accurate electromagnetic perturbation theory is employed that predicts the resonant hybrid mode frequencies for any field configuration within the cavity resonator. The coupling is shown to be strongly dependent not only on the excitation vector fields and sample's magnetic properties but also on the sample's shape. These findings are illustrated by applying the theoretical framework to two distinct experiments: a magnetic sphere placed in a three-dimensional resonator and a rectangular, magnetic prism placed in a two-dimensional resonator. The theory provides comprehensive understanding of the overall behavior of strongly coupled systems and it can be easily modified for a variety of other systems.

Role of the fuel stoichiometry and post-treatment temperature on the spinel inversion and magnetic properties of NiFe2O4 nanoparticles produced by solution combustion synthesis

Soft-magnetic nickel ferrite (NiFe2O4) nanoparticles were synthesized via solution combustion synthesis (SCS). The influence of the fuel concentration and the post-treatment temperature on the structural and magnetic properties of the samples were studied. The samples produced in a fuel-short regime presented the purest NiFe2O4 nanoparticles, with an average diameter of 32 nm. Spinel inversion degree, composition, crystallite size, and surface area were evaluated regarding their influence on the magnetic behavior of the nanoparticles. Interestingly, the magnetic properties of pure NiFe2O4 samples remained unaffected even after heat-treated above 900 °C. This result is promising for industrial applications that require magnetic stability over temperature changes. NiFe2O4 presented a critical size of 40 nm for single-domain to multi-domain transition. Below 40 nm, the discharge of Ni and α-Fe2O3 from the NiFe2O4 structure is the main factor influencing the magnetism. Pure NiFe2O4 samples treated at different temperatures presented a similar spinel inversion and consequently similar coercivities, approximately 170 Oe.