Mn Substituted Cobalt Ferrite Research Articles

Tailoring the Physicochemical and Magnetic Properties of an Mn Substituted Cobalt Ferrite System

An Mn substituted cobalt ferrite system was prepared using a simple, low temperature auto-combustion method followed by heating at 600 °C for 3 h. In this method, the thermochemical decomposition of amino acids such as glycine at elevated temperatures in the presence of salt nitrates, especially iron nitrate, produced a ferrite system. The as-prepared Mn-Co ferrite was characterized by XRD, SEM, IR and EDX techniques. These techniques revealed that the employed preparation method led to formation of a single phase of nanocrystalline spinel Mn0.5Co0.5Fe2O4 powder. However, various structural and morphological properties of the as-synthesized ferrite were determined. Porous and agglomerated morphology of the bulk sample was displayed in the scanning electron microscopy image. The VSM technique displayed the saturation magnetism of Mn0.5Co0.5Fe2O4 as 51.53 emu/g.

Correlation between AC and DC transport properties of Mn substituted cobalt ferrite

The CoFe2-xMnxO4 compound is prepared by following the sol gel technique. The structural analysis through XRD and Rietveld has been confirmed for the single cubic phase having Fd3¯m space group for CoFe2-xMnxO4 and also verified it through Raman spectroscopy measurements. The tetrahedral site observed to be red shifted with increase in Mn concentration in cobalt ferrite. All the XRD patterns have been analyzed by employing the Rietveld refinement technique. The particle size was found to be in the range of 30–40 nm. The electrical properties of polycrystalline CoFe2-xMnxO4 for x = 0.00, 0.10, 0.15, and 0.2, spinel ferrite was investigated by impedance spectroscopy. The influence of doping, frequency and temperature on the electrical transport properties of the CoFe2-xMnxO4 for x = 0.00, 0.10, 0.15, and 0.20 were investigated. The magnitude of Z′ and Z″ decreases with increase in temperature. Only one semicircle is observed in each Cole Cole plot which reveals that ac conductivity is dominated by grains. The grain resistance and grain boundary resistance both were found to decrease as a function of temperature. Temperature variation of DC electrical conductivity follows the Arrhenius relationship. A detailed analysis of electrical parameters provides assistance in connecting information regarding the conduction mechanism as well as determination of both dielectric and magnetic transition temperatures in the substituted cobalt ferrite. Detailed analysis of ac impedance and DC resistivity measurement reveals that, the magnetic ordering temperature in the Mn substituted cobalt ferrite does not respond to the frequency of ac electrical signal; however, it responds to the DC resistivity. The correlation between ac impedance and DC resistivity has been established.

Magnetic, magnetostrictive, and AC impedance properties of manganese substituted cobalt ferrites

In this study, we investigated the effects of substituting Mn3+ for some Fe3+ in spinel lattice on the structure, magnetic properties, magnetostriction behavior, and AC impedance characteristics of cobalt ferrites. The manganese substituted cobalt ferrites (Co–Mn ferrites), CoMnxFe2−xO4, with x varied from 0 to 0.3 in 0.1 increments, were prepared by solid-state reaction. XRD examination confirmed that all sintered Co-based ferrites had a single-phase spinel structure. The average grain size, obtained from SEM micrographs, increased from 8.2μm to 12.5μm as the Mn content (x) increased from 0 to 0.3. Both the Curie temperature and coercivity of Co-based ferrites decreased with greater amounts of Mn, while the maximum magnetization (at H=6kOe) of Mn-substituted cobalt ferrites was larger than that of the pure Co-ferrite. Magnetostrictive properties revealed that the pure Co-ferrite had the largest saturation magnetostriction (λS), about −167ppm, and the CoMn0.2Fe1.8O4 sample exhibited the highest strain sensitivity (|dλ⊥/dH|m) of 2.23×10−9A−1m among all as-prepared Co-based ferrites. In addition, AC impedance spectra analysis revealed that the real part (Z′) of the complex impedance of Co–Mn ferrites was lower than that of pure Co-ferrite in the low frequency region, and the Co-based ferrites exhibited semiconductor-like behavior.

Mn substituted cobalt ferrites (CoMnxFe2−xO4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0)): As magnetically separable heterogeneous nanocatalyst for the reduction of nitrophenols

Manganese substituted cobalt ferrite nanoparticles with composition CoMnxFe2−xO4 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were synthesized using sol–gel technology and characterized using the Fourier transform infrared spectroscopy, high resolution transmission electron microscopy and X-ray diffraction techniques to confirm their formation. The prepared ferrite samples were explored as catalysts for the reduction of nitrophenols in the presence of NaBH4 as reducing agent. Pure cobalt ferrite was found to be inactive. However, catalytic efficiency enhanced dramatically with the introduction of Mn ions into the catalytically active surface sites (octahedral sites) of the cobalt ferrite lattice. This could be due to the presence of synergistic effect between the Co3+, Mn3+ and Fe3+ ions present in the octahedral sites. CoMn02Fe1.8O4 ferrite was observed to have the best catalytic activity for the reduction of nitrophenols because of the highest Fe3+/Mn3+ and Co3+/Mn3+ ionic ratio at the catalytically active octahedral sites. The kinetics of reduction was studied and the reduction reaction followed pseudo first order kinetics. The rates of reduction of the three isomers of nitrophenols followed the order – 2-nitrophenol>4-nitrophenol>3-nitrophenol.

Effect of manganese substitution on magnetoimpedance and magnetostriction of cobalt ferrites

Influence of manganese substitution on the magnetoimpedance ratio and magnetostrictive constant of cobalt ferrite samples has been studied in the present investigation. Magnetoimpedance was found to vary with manganese content and frequency under the action of applied axial magnetic field. Maximum value of magnetoimpedance ratio was obtained for the composition Co1.1Mn0.1Fe1.8O4 at a low frequency of 102Hz. In order to understand magnetoimpedance behavior of the samples Cole–Cole plot of Mn substituted cobalt ferrites was studied. It was also noted that magnetostrictive constant under applied dc magnetic field changes with manganese content. The magnetic field required for maximum magnetostriction decreases with substitution of cobalt by manganese in Co1.2−xMnxFe1.8O4. The strain sensitivity ratio dλ/dH was maximum for Co1.1Mn0.1Fe1.8O4, whereas value of magnetostriction was maximum for CoMn0.2Fe1.8O4 sample.

Structural, morphological, dielectrical and magnetic properties of Mn substituted cobalt ferrite

The Co1−xMnxFe2O4 (0 ≤ x ≤ 0.5) ferrite system is synthesized by using an auto combustion technique using metal nitrates. The influence of Mn substitution on the structural, electrical, impedance and magnetic properties of cobalt ferrite is reported. X-ray diffraction patterns of the prepared samples confirm that the Bragg's peak belongs to a spinel cubic crystal structure. The lattice constant of cobalt ferrite increases with the increase in Mn content. The microstructural study is carried out by using the SEM technique and the average grain size continues to increase with increasing manganese content. AC conductivity analysis suggests that the conduction is due to small polaron hopping. DC electrical resistivity decreases with increasing temperature for a Co1−xMnxFe2O4 system showing semiconducting behavior. The activation energy is found to be higher in the paramagnetic region than the ferromagnetic region. Curie temperature decreases with Mn substitution in the host ferrite system. Dielectric dispersion having Maxwell—Wagner-type interfacial polarization has been observed for cobalt ferrite samples. Magnetic properties have been studied by measuring M—H plots. The saturation and remanent magnetization increases with Mn substitution.

High magnetostriction coefficient of Mn substituted cobalt ferrite sintered from nanocrystalline powders and after magnetic field annealing

Magnetostriction characteristics of Mn substituted cobalt ferrite, CoFe2−xMnxO4 (0 ≤ x ≤ 0.3), sintered from nanocrystalline powders of average particle size of ∼4 nm have been studied. Larger value of magnetostriction at lower magnetic field is achieved after substitution of Mn for Fe. The maximum value of magnetostriction coefficient is not much affected and the slope of the magnetostriction is increased with increasing Mn content. Higher maximum value of magnetostriction coefficient (λ) of 234 ppm comparable to that of the unsubstituted composition with larger strain derivative (dλ/dH) is obtained for x = 0.2 in CoFe2−xMnxO4. The magnetostriction coefficient is increased to 262 ppm with further enhancement in the strain derivative after annealing the sintered compact at 300 °C in a magnetic field of 400 kA/m for 30 min.

Effect of Mn substitution on the cation distribution and temperature dependence of magnetic anisotropy constant in Co1−xMnxFe2O4 (0.0≤x≤0.4) ferrites

Abstract The effect of Mn substitution on temperature dependent magnetic properties of Mn substituted cobalt ferrite, i.e., Co1−xMnxFe2O4 (x=0.0–0.4), prepared by a ceramic method has been investigated. X-ray diffraction (XRD) analysis reveals that all samples posses a single phase cubic spinel structure. The lattice constant determined from XRD increases with Mn substitution whereas the bulk density of the samples decreases. Mossbauer results reveal that Co, Fe and Mn ions are distributed over the tetrahedral (A) and octahedral (B) sites for the prepared samples. Hysteresis loops yield a saturation magnetization (Ms) and coercive field (Hc) that vary significantly with temperature and Mn content (x). The temperature dependence of the magnetization obtained for μoH=5 T presents a maximum at 175 K which is also dependent on the value of x. The high field regimes of the hysteresis loops are modeled using the Law of Approach to Saturation (LAS) to determine the first-order cubic anisotropy coefficient (K1). It has been found that the anisotropy of these materials increases significantly with decreasing temperature. However, below 175 K, the shape of the anisotropy energy function changes significantly causing a first-order magnetization process (FOMP) at higher fields, which also prevents the magnetization to saturate even under a maximum applied field of 5 T. In general, the anisotropy coefficient decreases with increasing Mn substitution at a given temperature, which could be explained in terms of the site occupancy of the Mn2+ substituent in the cubic spinel lattice.

Influence of manganese substitution on the microstructure and magnetostrictive properties of Co1−xMnxFe2O4 (x = 0.0–0.4) ferrite

Manganese substituted cobalt ferrite, Co1−xMnxFe2O4 (x = 0.0–0.4), was prepared by a ceramic method. The heat-treated powders were pressed at hydrostatic pressure of 167 MPa, and were annealed at 1350 °C for 24 h. These samples present a single-phase cubic spinel structure and the compositional mass ratios are close to the empirical formula. The lattice constant determined from XRD increases with the increase of Mn content, whereas SEM study reveals that Mn substitution changes the microstructure and cause pores within the grains, which reduces the bulk density of the samples. The magnetocrystalline anisotropy constant, coercive field, and magnetostriction were observed to decrease with increasing Mn substitution; however, the strain derivative (dλ/dH) reaches a maximum value for x = 0.3. The observed variation in strain derivative in the Mn substituted cobalt ferrite is correlated to the microstructure whereas the reduced anisotropy of the system plays only a minor role.

Stress effects on complex permeability spectra of Mn-substituted cobalt ferrite

The effects of axial applied stresses on the magnetization processes in Mn-substituted cobalt ferrite (CoMnxFe2-xO4 with x = 0 to 0.4) toroids have been studied by measuring the circumferential complex permeability spectra μ* = μ′ − iμ″ from 1 kHz to 15 MHz. The μ* spectra were described using a dispersion equation to determine the relative susceptibilities due to domain wall motion (χD0) and magnetization rotation (χS0). Mn-substitution into cobalt ferrite was found to reduce the magnetocrystalline anisotropy and hence the susceptibility is increased. Under axial compressive stresses, χD0 and χS0 of all samples decrease. The Mn-substituted samples with x = 0.2 and 0.3 show larger reductions in χD0 and χS0 than the pure cobalt ferrite. The stress effects on χS0 and the dependence of the magnetomechanical effect on Mn content were reproduced in model calculations by taking into account the combined effects of stress-induced and magnetocrystalline anisotropies on magnetization rotation.

Magnetic and magnetoelastic properties of Cr-substituted cobalt ferrite

We have investigated the magnetic and magnetoelastic properties of a series of Cr-substituted cobalt ferrite CoCrxFe2−xO4 (x=0.0–0.79) samples. Substitution of Cr for some of the Fe in cobalt ferrite reduced the Curie temperature, and the effect is more pronounced than that observed in Mn-substituted cobalt ferrite samples. Cr substitution also caused the maximum magnetostriction to decrease at a greater rate than substitution of the same amount of Mn. The maximum of the strain derivative, dλ∕dH, was reached for x=0.38. The behavior of the Curie temperature of Cr-substituted and Mn-substituted cobalt ferrites has been analyzed using the Néel molecular field model and compared with recent Mössbauer spectroscopy results.

Temperature dependence of magnetic anisotropy in Mn-substituted cobalt ferrite

The temperature variation of magnetic anisotropy and coercive field of magnetoelastic manganese-substituted cobalt ferrites (CoMnxFe2−xO4 with 0⩽x⩽0.6) was investigated. Major magnetic hysteresis loops were measured for each sample at temperatures over the range 10–400 K, using a superconducting quantum interference device magnetometer. The high-field regimes of the hysteresis loops were modeled using the law of approach to saturation equation, based on the assumption that at sufficiently high field only rotational processes remain, with an additional forced magnetization term that was linear with applied field. The cubic anisotropy constant K1 was calculated from the fitting of the data to the theoretical equation. It was found that anisotropy increases substantially with decreasing temperature from 400 to 150 K, and decreases with increasing Mn content. Below 150 K, it appears that even under a maximum applied field of 5 T, the anisotropy of CoFe2O4 and CoMn0.2Fe1.8O4 is so high as to prevent complete approach to saturation, thereby making the use of the law of approach questionable in these cases.