Monocrystalline high-saturation magnetization ferrites for video recording head application: I. Zn ferrous ferrites

Monocrystalline Zn ferrous ferrites Zn_xFe_3-xO₄, with x ? 0.4, are considered as candidate materials for video recording heads, to be used for writing on magnetic tapes of high coercivity. The saturation magnetization of these ferrites can be as high as 0.7 T at 20°C. We show that because of the small dimensions of modern video recording heads, the relatively high electrical conductivity of the Zn ferrous ferrites is not an obstacle to their use at video frequencies. Measurements are reported of magnetic and electrical parameters relevant to recording head application. It is shown that some of the magnetic parameters can be influenced positively by Co^II additions. The present paper is the first of 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.     

Synthesis of (Mg0.476Mn0.448Zn0.007)(Fe1.997Ti0.002)O4 powder and sintered ferrites by high energy ball-milling process

(Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder prepared by high energy ball-milling process were consolidated by microwave and conventional sintering processes. Phases, microstructure and magnetic properties of the ferrites prepared by different processes were investigated. The (Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder could be prepared by high energy ball-milling process of raw Fe₃O₄, MnO₂, ZnO, TiO₂ and MgO powders. Prefired and microwave sintered ferrites could achieve the maximum density (4.86 g/cm⁻³), the average grain size (15 μm) was larger than that (10 μm) prepared by prefired and conventionally sintered ferrites with pure ferrite phase, and the saturation magnetization (66.77 emu/g) was lower than that of prefired and conventionally sintered ferrites (88.25 emu/g), the remanent magnetization (0.7367 emu/g) was higher than that of prefired and conventionally sintered ferrites (0.0731 emu/g). Although the microwave sintering process could increase the density of ferrites, the saturation magnetization of ferrites was decreased and the remanent magnetization of ferrites was also increased.     

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.     

Structural,dielectric and magnetic properties of cobalt based spinel ferrites

CoYb_xFe_2-xO₄ (x = 0.00, 0.025, 0.05, 0.075, 0.10) spinel ferrites were synthesized by co-precipitation technique. Structural, dielectric and magnetic properties were measured. X-ray diffraction analysis showed that all the prepared spinel ferrites possessed cubic spinel structure. Dielectric constant, AC conductivity and dielectric loss decreased with the addition of rare earth ions. The impedance analysis explained the role of grains and grain boundaries with in prepared samples. Cole-Cole plots helped to measure the values of grains and grain boundary's resistance. The magnetic properties proved the soft nature of these ferrites. Saturation magnetization and remanence decreased while coercivity was enhanced with the addition of ytterbium concentration. All these parameters suggested that these prepared samples might be suitable for high frequency applications.     

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.     

Tailoring the temperature characteristics of the magnetic permeability of NiZn ferrites

The effect of cobalt addition on the temperature characteristics of the magnetic permeability of NiZn ferrites was studied and a comparison to the respective behaviour of cobalt in NiCuZn ferrites was examined. Cobalt-doped NiZn and NiCuZn ferrites were manufactured by the ceramic route and sintered under various atmosphere profiles. The chemical and morphological characteristics were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The evaluation of the magnetic performance of the sintered ferrites showed that N₂-rich atmosphere profiles during the top temperature and cooling time of the sintering process favour the temperature stability of the permeability in the case of NiZn ferrites, while preserving the losses at low levels. Two mechanisms that take place at the same time are proposed: the change of the Fe²⁺/Fe³⁺ ratio due to the reduction-promoting atmosphere of N₂ in combination with an increase in magnetocrystalline anisotropy and magnetostriction due to the presence of Co²⁺ suggest a useful method to tailor the temperature factor α_F of NiZn ferrites. However, the method cannot be applied in NiCuZn ferrites, as the reduction Cu²⁺-Cu⁺ taking place under N₂-rich atmospheres enhances secondary re-crystallization phenomena, causing a dramatic increase in losses.     

Influences of Fe-deficiency on electromagnetic properties of low-temperature-fired NiCuZn ferrites

Low-temperature-fired NiCuZn ferrites with the formula Ni_0.45Cu_0.2Zn_0.35Fe_2−xO_4−3/2x with x values ranging from 0.00 to 0.25 in steps of 0.05 and sintered at 900 °C have been investigated in the present work. It was found that the content of Fe-deficiency could obviously influence the microstructure, sintering behavior, saturation magnetization, permeability and permittivity spectra properties of the ferrites. The variations were much different from those of the high-temperature-fired NiZn ferrites. And the corresponding mechanisms involved were discussed in detail. All-around consideration, the NiCuZn ferrite with 0.10 Fe-deficiency in composition had the best performances on sintering behavior and electromagnetic properties.     

Mössbauer effect study of nonstoichiometric CoNiZn ferrites

The existence of fast electron exchange between pairs of Fe³⁺?Fe²⁺ ions was detected in Mössbauer spectra of non-stoichiometric CoNiZn ferrites containing an excess of Fe₂O₃. Analysis of spectra from a number of samples of various preparations showed correlation of bulk magnetic properties with hyperfine parameters of ferric ions at octahedral sites in the spinel structure.     

Effects of composition and sintering temperature on properties of NiZn and NiCuZn ferrites

Effects of composition and sintering temperature on grain size, porosity and magnetic properties of the NiZn and NiCuZn ferrites were investigated. It was found that the lowest power loss could be obtained with the equimolar composition for both NiZn and NiCuZn ferrites, which could be attributed to the lowest porosity. A slight deficiency or excess of Fe₂O₃ content had no pronounced influence on saturation magnetic flux density (B_s) in our testing range. However, a slight excess of Fe₂O₃ was effective to improve the initial permeability, which could be attributed to decrease of the magnetocrystalline anisotropy. With the increase of sintering temperature, the initial permeability and power loss of the NiZn and NiCuZn ferrites had different development trend, which could be explained by the different variation trend of the grain size and porosity. Power losses of the NiCuZn ferrite samples were lower than that of the NiZn ferrite samples at any sintering temperature. Synthetically, the NiCuZn ferrites had a better performance than the NiZn ferrites in power field use.     

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.     

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 properties of rare earth ion (Sm) added nanocrystalline Mg-Cd ferrites, prepared by oxalate co-precipitation method

Nanocrystalline ferrite powder having the general formula Mg_1−xCd_xFe₂O₄+5% Sm³⁺ (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) was synthesized by chemical oxalate co-precipitation technique. The synthesized powder was characterized by X-ray, IR and SEM techniques. The XRD analysis confirms cubic spinel phase with orthoferrite secondary phase. The lattice constant increases with increase in Cd²⁺ content (x). It is smaller than that for pure Mg-Cd ferrites. The average crystallite size lies in the range 28.69-32.66 nm. Saturation magnetization and magnetic moment increase with cadmium content up to x=0.4 and decrease thereafter. This is attributed to the existence of localized canted spin. The decrease in saturation magnetization and magnetic moment beyond x=0.4 is due to the presence of triangular spin arrangement on B-site. Coercivity and remanent magnetization decrease while Y-K angles increase with Cd²⁺ content. The Sm³⁺ addition improves the magnetic properties.     

Distribution of copper ions in some copper-manganese ferrites

The determination of the distribution of copper ions among tetrahedral and octahedral sites in two copper-manganese ferrites was attempted by means of X-ray diffraction and saturation magnetization measurements. The amounts of copper in octahedral positions for the ferrites Cu_0·5.Fe_2·5O₄and Cu_0·5Mn_0·5Fe₂O₄ were found to be 0·28±0·02 and 0· 47±0·03 respectively.The authors are greatly indebted to Mrs. A. Dobrovodská for technical assistance and Mr. A. Novák for the careful chemical analysis.     

Anomalous magnetic properties of ferrites of the CuFe2−x CrxO4 system with high Cr3+ content

The paper reports on a comprehensive study of magnetic properties of CuFe_2−x Cr_xO₄ ferrites (x=0.0, 0.2, 0.3, 1.0, 1.4, 1.6, and 2.0). The curves of magnetization versus temperature, σ _s(T), of the ferrites with x=1.0, 1.4, and 1.6 have anomalous shapes: the magnetization begins to fall off at lower temperatures than the Curie point TC. The experimental results and analysis of exchange interactions suggest that in ferrites with high Cr³⁺ content, magnetic transitions to either a frustrated magnetic structure or a clustered spin glass can take place. Zh. éksp. Teor. Fiz. 111, 1732–1737 (May 1997)     

Impact of zinc substitution on the structural and magnetic properties of chemically derived nanosized manganese zinc mixed ferrites

Mn_1−xZn_xFe₂O₄ nanoparticles (x=0-1) were synthesized by wet chemical co-precipitation techniques. X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy were effectively utilized to investigate the different structural parameters. The elemental analysis was conducted using energy-dispersive spectrum and inductively coupled plasma analysis. The magnetic properties such as magnetization and coercivity were measured using vibrating sample magnetometer. The observed magnetization values of the nanoparticles were found to be lower compared to the bulk counterpart. The magnetization showed a gradual decrease with zinc substitution except for a small increase from x=0.2 to 0.3. The Curie temperature was found to be enhanced in the case of ferrites in the nanoregime. The variation in lattice constant, reduced magnetization values, variation of magnetization with zinc substitution, the presence of a net magnetic moment for the zinc ferrite and the enhancement in Curie temperature in Mn_1−xZn_xFe₂O₄ all provide evidence to the existence of a metastable cation distribution together with possible surface effects at the nanoregime.     

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.     

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.     

Structure and magnetic properties of Ni0.5Zn0.5Mn0.5-xMoxFe1.5O4 ferrites prepared by sol-gel auto-combustion method

A series of Mo doped Ni-Mn-Zn ferrites compounds with the formula Ni_0.5Zn_0.5Mn_0.5-xMo_xFe_1.5O₄ (x = 0, 0.025, 0.05, 0.075 and 0.1) were first synthesized by sol-gel auto-combustion method. The X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), and vibrating sample magnetometer (VSM) analysis were carried out to characterize the microstructural and magnetic properties of ferrites. Rietveld refinement of X-ray diffraction data confirmed the formation of cubic spinel structure and the emergence of FeMoO₄ phase with the substitution of Mo⁶⁺ contents. The grain size increased remarkably due to the formation of the liquid phase. The saturation magnetization (M_s) increased while the coercivity (H_c) decreased from 67.3 to 12.1 Oe due to the decrease of magneto-crystalline anisotropy constant. The initial permeability (μ_i) increased significantly from 34 (x = 0) to 114 (x = 0.075) and later decreased for x = 0.1. In our experiment, Ni_0.5Zn_0.5Mn_0.425Mo_0.075Fe_1.5O₄ ferrite presented the best microstructure and soft magnetic properties.