Monocrystalline high-saturation magnetization ferrites for video recording head application: II. MnZn ferrites with large FeII contents

Monocrystalline Fe^II-rich MnZn ferrites are considered as candidate materials for video recording heads, to be used for both writing and reading when magnetic tapes of high coercivity are applied. It is examined how saturation magnetization and magnetostriction constants vary with chemical composition within relevant areas of the ternary composition diagram MnFe₂O₄-ZnFe₂O₄-Fe^IIO₄. From the results and data known for the magnetocrystalline anisotropy a composition is selected which offers a fairly good compromise between high-saturation magnetization and suitable soft magnetic properties. A study is presented of magnetic and electrical properties of monocrystals of this composition, relevant to their use as video recording head materials. The present paper is the second in a series of three dealing with Fe^II-rich ferrites for video recording head application.     

Monocrystalline high-saturation magnetization ferrites for video recording head application: III. CoII doped MnZn ferrites with large FeII contents

It is shown that substitution of Co^II into Fe^II-rich MnZn ferrites makes it possible to enlarge the Fe^II content permissible for combined write/read video recording heads. This results in an increase of the saturation magnetization M_s, which is important when high-coercivity magnetic tapes are used. A properly chosen Co^II content compensates the magnetocrystalline anisotropy of the ferrites in such a way that the temperature of the maximum in the magnetic permeability μ′ versus temperature curve, caused by anisotropy compensation by Fe^II, is lowered to the level at which a recording head operates. We report on monocrystalline ferrites with suitable Co^II-Fe^II concentration combinations, showing M_s values of up to 0.660 T and a maximum in μ′(T) at 40°C of 600–700 at 4.5 MHz. We show that with Co^II substitutions, the enlargement of the Fe^II content does not give rise to larger magnetic disaccommodation effects. The present paper is the third in a series of three dealing with Fe^II-rich ferrites for video recording head application.     

Spectral studies of Co substituted Ni-Zn ferrites

The spinel ferrites Zn_0.35Ni_0.65−xCo_xFe₂O₄, 0≤x≤1, have been prepared using the standard ceramic technique. Room temperature Mössbauer, X-ray and infrared IR spectra were used for carrying out this study. X-ray patterns reveal that all the samples have single-phase cubic spinel structure. The Mössbauer spectra of the samples show a paramagnetic phase for x=0 and a six-line magnetic pattern and a central paramagnetic phase for x≥0.1. They are analyzed and attributed to two magnetic subpatterns and two quadrupole doublets due to Fe³⁺ ions at the tetrahedral A-sites and octahedral B-sites. Four absorption bands are observed in IR spectra. They confirm the spinel structure of the samples and existence of Fe³⁺ ions in the sample sublattices. The deduced hyperfine interactions, lattice parameters, absorption band positions and intensities and force constant are found to be dependent on the substitution factor x, where the cation distribution is estimated. The hyperfine magnetic fields, magnetization and lattice resonant frequency are found to be dependent on the interionic distance.     

Analysis of magnetic disaccommodation in La-Co-substituted strontium ferrites

M-type strontium ferrites substituted by La³⁺-Co²⁺(Sr_1−xLa_xFe_12−xCo_xO₁₉) were prepared by ceramic process. Effects of the substituted amount of La³⁺ and Co²⁺ on structure and magnetic properties of Sr_1−xLa_xFe_12−xCo_xO₁₉ compounds have systematically been investigated by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and magnetic disaccommodation. In the measurement range from 80 to 500 K, the magnetic disaccommodation is represented by means of isochronal curves. It is well known that magnetic disaccommodation cannot be obviously found in the M-type of pure strontium ferrites. However, three peaks were observed in Sr_1−xLa_xFe_12−xCo_xO₁₉, and this behavior is explained in terms of the presence of Fe²⁺ cation and to the site occupation by the magnetic Co²⁺ ionic within the hexagonal structure.     

Zn-substituted brownmillerite-type strontium ferrites

New La/Zn substituted strontium ferrites Sr_{(2 ?x)}La _x [Fe_{(2 ? x)}Zn _x ]O₅, (0 ≤ x ≤ 0.3) with brownmillerite- type structure are obtained and studied via X-ray phase analysis and Mössbauer spectroscopy. Depending on the conditions of synthesis, substituting Zn²⁺ cations can either mainly occupy tetrahedral positions in a brownmillerite structure, or be uniformly distributed between the tetrahedral and octahedral positions. It is shown that they are reversibly oxidized by atmospheric oxygen at elevated temperatures.     

Impacts of neodymium on structural,spectral and dielectric properties of LiNi0.5Fe2O4 nanocrystalline ferrites fabricated via micro-emulsion technique

Soft ferrites are technologically advanced smart materials and their properties can be tailored by controlling the chemical composition and judicial choice of the metal elements. In this article we discussed the effect of rare earth neodymium (Nd³⁺) on various properties of LiNi_0.5Nd_xFe_2−xO₄ spinel ferrites. These ferrites have been synthesized by facile micro-emulsion route and characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), a.c. electrical conductivity and thermal analysis. The influence of Nd³⁺ doping on structural and electrical parameters has been investigated. XRD analysis revealed the formation of single cubic spinel structure for x≤0.07. Few traces of secondary phase (NdFeO₃) were found for x≥0.105. The secondary phase induced owing to the solubility limit of Nd³⁺ cations in these ferrites. The lattice parameter (a) and crystallite size (D) both exhibit non-linear relation. The values of “a” and “D” were found in the range 8.322–8.329 Å and 25–32 nm respectively. These variations were attributed to the larger ionic radius of Nd³⁺ cations as compared to the host cations and lattice strain produced in these ferrites. The dielectric parameters were studied in the range 1 MHz to 3 GHz and these parameters were damped by Nd³⁺ incorporation and also by increasing the frequency. The reduced dielectric parameters observed in wide frequency range proposed that these nanocrystalline ferrites are potential candidates for fabricating the devices which are required to operate at GHz frequencies.     

NMR investigations of ion distribution in real structure of manganese-zinc ferrites

Using the nuclear magnetic resonance, X-ray and electromagnetic methods we investigated structure and properties of the single- and polycrystalline manganous and manganese-zinc ferrites: MnFe₂O₄, Mn_0.8Zn_0.2Fe₂O₄, Mn_0.6−xCu_xZn_0.3Fe_2.1O₄ (0 ≤x ≤ 0.3). The real structure of the manganous and manganese-zinc ferrites is found to contain simultaneously vacancies in cationic and anionic sublattices. The analysis of the NMR spectra of⁵⁵Mn,⁵⁷Fe,^63,65Cu and⁶⁷Zn ions allowed us to find the magnetic and valence states of all ions and their distribution in the real spinel structure. It was shown that the Fe³⁺ paramagnetic ions, Zn²⁺ diamagnetic ions and cationic vacancies in tetrapositions are distributed statistically. The Mn³⁺ and Cu²⁺ Jahn-Teller ions and presumably Fe²⁺ in octahedral positions are distributed nonstatistically forming cluster-type mesoscopic inhomogeneities. The correlation between distribution of ions and vacancies in the real structure of manganese-zinc ferrites and their functional electromagnetic properties was found.     

Influence of magnetic ordering on the structural properties of dilute copper ferrites

The x-ray structural properties of samples in the CuGa_xAl_xFe_2?2x O₄ (x = 0?0.7) and CuGa_xAl_2x Fe_2?3x O₄ (x = 0?0.5) systems are studied. It is found that magnetic ordering in dilute copper ferrites affects their structural properties. It is concluded that the frustration of magnetic coupling leads to suppression of the cooperative Jahn-Teller effect in dilute copper ferrites with a frustrated magnetic structure.     

Magnetic interactions in Cu-substituted manganese ferrites

A series of Mn_1−xCu_xFe₂O₄, with x=0, 0.25, 0.50, 0.75 and 1.0, spinel ferrites were prepared by standard ceramic method, to study the effect of compositional variation on magnetic susceptibility, saturation magnetization (M_s), Curie temperature (T_c) and magnetic moments (μ_B). The Curie temperatures have been evaluated by measuring the ac susceptibility using the mutual inductance technique. On increasing Cu contents from 0.0 to 0.50, the saturation magnetization increases while the Curie temperature decreases. On further increase in Cu contents, x>0.50 a decreasing trend in M_s is exhibited while T_c continues to decrease. This effect can be partially related to the low magnetic moments of Cu⁺² ions. The dominant interaction in all ferrite samples is A-B interaction which is due to the negative values of the characteristic temperature θ(K) showing that the magnetic ordering is antiferromagnetic. The Y-K angle increases gradually with increasing copper contents and extrapolates to 90° for CuFe₂O₄. From the computation of Y-K angles for Mn_1−xCu_xFe₂O₄, it can be concluded that the mixed copper ferrites exhibit a non-collinearity of the Y-K type while MnFe₂O₄ shows a Neel type of ordering.     

Effect of Mn substitution on the magnetic properties of MgCuZn ferrites

A series of Mn substituted MgCuZn ferrites (Mg_0.2Cu_0.2Zn_0.6O) (Fe_2−xMn_xO₃)_0.97 with x=0.00,0.01,0.03,0.05,0.07 were prepared with nanosized precursor powders synthesized by a sol–gel auto-combustion method. All the ceramic samples can be sintered at low temperature (930°C) (below the melt point of Ag (961°C)). The effect of Mn content on microstructures and magnetic properties were investigated. Experiment shows that low temperature sintered MgCuZn ferrites doped with Mn possess higher initial permeability and better grain structure than that of low temperature sintered NiCuZn ferrites prepared by the same method. Therefor, Mn doped MgCuZn ferrites should be ideal materials for high inductance multilayer chip inductor. It is thought that the variation of initial permeability of MgCuZn ferrites with the Mn substitution was attributed to the decrease of magnetostriction constant.     

Influences of La3+ substitution on the structure and magnetic properties of M-type strontium ferrites

M-type strontium ferrites with substitution of Sr²⁺ by rare-earth La³⁺, according to the formula Sr_1−xLa_xFe₁₂O₁₉, are prepared by the ceramic process. Influences of the substituted amount of La³⁺ on structure and magnetic properties of Sr_1−xLa_xFe₁₂O₁₉ compounds have systematically been investigated by XRD, VSM and Mössbauer spectrum. When the substituted amount x is below 0.30, X-ray diffraction shows that the samples are single M-type hexagonal ferrites. It is found that the suitable amount of La³⁺ substitution may remarkably increase saturation magnetization σ_s and intrinsic coercivity H_cJ. With the La³⁺ addition for the same sintering temperature, σ_s and H_cJ increase at first, then decrease gradually. However, the substituted amount x at the maximum value of H_cJ is bigger than that of σ_s. Mössbauer spectroscopy of ⁵⁷Fe in Sr_1−xLa_xFe₁₂O₁₉ has shown that the substitution of Sr²⁺ by La³⁺ is associated with a valence change of Fe³⁺ to Fe²⁺ at 2a or 4f₂ site. The magnetic properties are reflected in the Mössbauer spectra which indicate that the magnetic hyperfine field (H_hf) is detected with the highest value at x=0.20. The different exchange paths between the iron sublattices are discussed according to the increased hyperfine fields of the 12k- and 2b-site. The Curie temperature of Sr_1−xLa_xFe₁₂O₁₉ decreases linearly with increasing La³⁺ substitution.     

Effects of MnO2 on the electromagnetic properties of NiCuZn ferrites prepared by sol-gel auto-combustion

Mn-doped NiCuZn ferrites with compositions of (Ni_0.2Cu_0.2Zn_0.6)O(Fe_2−x,Mn_xO₃)_0.98 (x=0, 0.02, 0.04, 0.06) were prepared by a novel sol–gel auto-combustion process. The synthesized nano-sized ferrite powders can be sintered at 900°C, and the sintered ferrites are characterized by fine-grained microstructural feature and high permeability. Mn content in formulations largely affects the grain size and main electromagnetic properties of sintered NiCuZn ferrites. With increasing Mn content, the initial permeability is significantly increased, while the electrical resistivity and quality factor are decreased. The dielectric constant and dissipation factor are also affected by the incorporation of MnO₂. The possible mechanism for the influence of MnO₂ on the electromagnetic properties was discussed.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Infrared and structural studies of Mg1-xZnxFe2O4 ferrites

Compositions of polycrystalline Mg-Zn mixed ferrites with the general formula Mg_1−xZn_xFe₂O₄ (0≤x≤1) were prepared by the standard double sintering ceramic method. The structural properties of these ferrites have been investigated using X-ray diffraction and infrared absorption spectroscopy. The lattice parameter, particle size, bonds length, force constants, density, porosity, shrinkage and cation distribution of these samples have been estimated and compared with those predicted theoretically. Most of these values were found to increase with increasing Zn content. The energy dispersive (EDS) analysis confirmed the proposed sample composition. The scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographs showed aggregates of stacked crystallites of about 200-800 nm in diameter. Far infrared absorption spectra showed two significant absorption bands. The wave number of the first band, ν₁, decreases with increasing Zn content, while the band, ν₂ shifts linearly towards higher wave numbers with Zn contents, over the whole composition range. The room temperature electrical resistivity was found to decrease as Zn-content increases. Values of the vacancy model parameters showed that the packing factors P_a and P_b decrease, the fulfillment coefficient, α, remains almost constant and the vacancy parameter, β, strongly increases with increasing Zn content in the sample. The small values of P_a, P_b, α and the strong increase of the vacancy parameter, β, indicate the presence of cation or anion vacancies and the partial participation of the Zn²⁺ vacancies in the improvement of the electrical conductivity in the Mg-Zn ferrites.     

Mössbauer effect and NMR studies of copper-cadmium ferrites

Hyperfine magnetic field distributions have been observed at room temperature in Cd _x Cu_1?x Fe₂O₄ (0≤x≤0.5) ferrites by the Mössbauer and NMR methods and are explained by statistical distributions of metallic ions separately within A and within B crystal sublattices.     

Electromagnetic properties of lithium zinc ferrites doped with aluminum

We present the results of the effect of Al substitution on the magnetic and electrical properties of Li_0.2Zn_0.6Fe_2.2−xAl_xO₄ ferrites (0.0≤x≤0.5) prepared by the standard ceramic technique. The characterization has been performed using XRD, SEM, magnetic and dielectric response in frequency. XRD analysis confirms that the system exhibits polycrystalline single phase cubic spinel structure only for low dopant content. Doping decreases the dielectric loss tangent and the ferrite conductivity in more than two orders of magnitude in the whole analyzed frequency range. Attenuation has a maximum intensity (86 dB) near 90 MHz for x=0.4. The wider bandwidth at 20 dB (94.6 MHz) is for x=0.3.     

Zn- and Cu-substituted Co2Y hexagonal ferrites: Sintering behavior and permeability

Y-type polycrystalline hexagonal ferrites Ba₂Co_2−x−yZn_xCu_yFe₁₂O₂₂ with 0≤x≤2 and 0≤y≤0.8 were prepared by the mixed-oxide route. Single phase Y-type ferrite powders were obtained after calcinations at 1000 °C. Samples sintered at 1200 °C show a permeability that increases with the substitution of Zn for Co and display maximum permeability of μ′=35 at 1 MHz for x=1.6 and y=0.4. A resonance frequency f_r=500 MHz is observed for Zn-rich ferrites with y=0 and 0.4. The saturation magnetization increases with substitution of Zn for Co. Addition of Bi₂O₃ shifts the temperature of maximum shrinkage down to T≤950 °C. Moreover, an increase of the Cu-concentration further lowers the sintering temperature to T≤900 °C, enabling co-firing of the ferrites with Ag metallization for multilayer technologies. However, low-temperature firing reduces the permeability to μ′=10 and the resonance frequency is shifted to 1 GHz. Thus substituted hexagonal Y-type ferrites can be used as soft magnetic materials for multilayer inductors for high frequency applications.     

Mössbauer study of Zn2+ and Sn4+ additives in Nickel ferrites

The Mössbauer study of Fe⁵⁷ in polycrystalline solid solutions of Ni_{1 + x ? y}Zn_ySn_xFe_{2 ? 2x}O₄ (y = 0.1, 0.3, 0.5 and x = 0.1 to 0.5 with x varying in steps of 0.1) has been made. The present work has been aimed at investigating the variation of isomer shift, nuclear magnetic field at Fe⁵⁷ nuclei and ionic distribution at tetrahedral and octahedral sites when the concentration of Zn²⁺ and Sn²⁺ ions is successively increased in nickel ferrites. The ionic distribution in samples having low tin concentration has been explained on the basis of Neel's molecular field model whereas the ionic distribution of samples having high tin and zinc concentration has been explained on the basis of formation of isolated superparamagnetic clusters at the octahedral site. It has been inferred that a larger substitution of zinc and tin ions enhances relaxation effects.     

Effects of In-substitution on the structure and magnetic properties of multi-doped YIG ferrites with low saturation magnetizations

Multi-doped YIG ferrites {Y_1.7Gd_0.5Ca_0.8}[Fe_2−xIn_x](Fe_2.15V_0.4Mn_0.05Al_0.4)O₁₂ (x=0, 0.3, 0.6, 0.7, 0.8 and 0.9) with low saturation magnetizations (4πM_s=400-600 G at 298 K) were prepared by a conventional ceramic technology and the effects of In³⁺-substitution on their structures and magnetic properties were systematically investigated using XRD, SEM and VSM. It has been found that as-synthesized powders and sintered ferrites showed a single-phase of garnet structure with a cell parameter (a) that increased linearly with increase in In³⁺ concentration from x=0 up to 0.9. Apparent relative densities of sintered samples were all over 98%, but no remarkable influences of In³⁺-substitution were observed by SEM on the refinement of crystal grains and the enhancement of sintering of ferrites. In addition, the Curie temperature T_c decreased almost linearly as In³⁺concentration increased, while the corresponding saturation magnetization at room temperature presented a variation characterized by a gradual increase first and then a rapid plunge. On the basis of quantitative analysis of XRD data and the theory on super-exchange interactions, it has been established that the incorporated In³⁺ ions via doping were exclusively located at the sites with octahedral coordinations in the crystal structure and the aforementioned magnetic properties can be simply attributed to weakening super-exchange interactions between neighboring magnetic ions through oxygen ions due to the “dilution effect” of added non-magnetic In³⁺ ions.     

Effect of Pr doping on magnetic and dielectric properties of Ni-Zn ferrites by “one-step synthesis”

Pr³⁺-doped Ni-Zn ferrites with a nominal composition of Ni_0.5Zn_0.5Pr_xFe_2−xO₄ (where x=0-0.08) were prepared by a one-step synthesis. The magnetic and dielectric properties of the as-prepared Ni-Zn ferrites were investigated. X-ray diffraction data indicated that, after doping, all samples consisted of the main spinel phase in combination of a small amount of a foreign PrFeO₃ phase. The lattice constants of the ferrites initially increased after Pr³⁺ doping, but then became smaller with additional Pr³⁺ doping. The addition of Pr³⁺ resulted in a reduction of grain size and an increase of density and densification of the as-prepared samples. Magnetic measurement revealed that the saturation magnetization of the as-prepared ferrites, M_s, decreased, while the coercivity, H_c, increased with increasing substitution level, x, and the Curie temperature, T_c, kept a rather high value, fluctuating between 308 and 320 °C. Both the real and imaginary parts of permeability of the ferrites decreased slightly after Pr³⁺ doping. However, the natural resonance frequency shifted towards higher frequency from 13.07 to 36.17 MHz after the addition of Pr³⁺, driving the magnetic permeability to much higher frequency, reaching the highest value (36.17 MHz) when x=0.04. Introduction of Pr³⁺ ions into the Ni-Zn ferrite reduced the values of the dielectric loss tangent, especially in the frequency range of 1-400 MHz. However, the magnitude of dielectric loss of the samples doped with different amounts of Pr³⁺ raised little.