On the thermal stability of Co2Z hexagonal ferrites for low-temperature ceramic cofiring technologies

Co₂Z hexaferrite Ba₃Co₂Fe₂₄O₄₁ was prepared by a mixed oxalate co-precipitation route and the standard ceramic technology. XRD studies show that at T<1300 °C different ferrite phases coexist with the M-type hexaferrite as majority phase between 1000 and 1100 °C and the Y-type ferrite at 1230 °C. The Z-type material has its stability interval between 1300 and 1350 °C. Both synthesis routes result in almost single-phase Z-type ferrites after calcination at 1330 °C, intermediate grinding and sintering at 1330 °C. The permeability of Co₂Z-type ferrite of about μ=20 is stable up to several 100 MHz, with maximum losses μ′′ around 700 MHz. Addition of 3 wt% Bi₂O₃ as sintering aid shifts the temperature of maximum shrinkage down to 950 °C and enables sintering of Z-type ferrite powders at 950 °C. However, the permeability is reduced to μ=3. It is shown here for the first time that Co₂Z ferrite is not stable under these conditions; partial thermal decomposition into other hexagonal ferrites is found by XRD studies. This is accompanied by a significant decrease of permeability. This shows that Co₂Z hexagonal ferrite is not suitable for the fabrication of multilayer inductors for high-frequency applications via the low-temperature ceramic cofiring technology since the material is not compatible with the typical process cofiring temperature of 950 °C.     

Electromagnetic properties and microwave absorption of W-type hexagonal ferrites doped with La

W-type barium hexaferrites with compositions of Ba₁Co_0.9Zn_1.1Fe₁₆O₂₇ and Ba_0.8La_0.2Co_0.9Zn_1.1Fe₁₆O₂₇ were synthesized by the sol-gel method. The electromagnetic properties and microwave absorption behavior of these two ferrites were studied in the 2-18 GHz frequency range. The microstructure and morphology of the ferrites were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques. The complex permittivity spectra, the complex permeability spectra and microwave reflection loss were measured by a microwave vector network analyzer. The XRD patterns show that the main phase of the Co₂W ferrite forms without other intermediate phases when calcined at 1200 °C. The SEM images indicate that flake-like hexagonal crystals distribute uniformly in the materials. Both the magnetic and dielectric losses are significantly enhanced by partial substitution of La³⁺ for Ba²⁺ in the W-type barium hexaferrites. The microwave absorption property of the La³⁺ doping W-type hexaferrite sample is enhanced with the bandwidth below −10 dB around 8 GHz and the peak value of reflection loss about −39.6 dB at the layer thickness of 2 mm.     

A study on the heat-treatments of nanocrystalline nickel substituted BaW hexaferrite produced by low combustion synthesis method

The novel low temperature combustion synthesis (LCS) method for the preparation of nanocrystalline W-type BaW hexaferrite i.e. BaNi₂Fe₁₆O₂₇ has been carried out by citrate precursor using the sol-to-gel (S–G) followed by gel-to-nanocrystalline (G–N) conversion. Decomposition behaviors and the phases associated therein are investigated by means of thermal analysis (DTA/DTG/TG) and XRD, respectively. Atomic absorption spectroscopy (AAS) has been used to determine the elemental analysis in different conditions. Surface morphology of the nonporous ultra fine particles have been examined by SEM. The TEM micrographs show that the particles of the size of 10 nm were seemed to be agglomerated in the ‘as synthesized’ condition. Room temperature Fe-57 Mossbauer spectrum, MS has showed doublet of ‘as synthesized’ nanocrystalline powder that indicates the superparamagnetic behavior of the material. This effect is further confirmed by vibrating sample magnetometer (VSM) wherein it was noticed that the magnetic field (10 KG max) did not have any effect on the material. The material was annealed at 400, 700 and 1000 °C in the furnace for 4 h. The grain size is found to increase from 10 to 70 nm after annealing at 1000 °C for 4 h. MS after annealing at 700–1000 °C for 4 h, showed that the doublets of ‘as synthesized’ is further resolved into broad sextets due to the presence of both superparamagnetic and ferrimagnetic particles, in the wide size range from 10 to 70 nm. Only slight increase in particle size (from 10 to 15 nm) is noticed after the heat-treatment for 1–3 and 5 min in microwave oven (2.45 GHz with 760 W) but with predominant phase changes. TEM after the heat treatment revealed the presence of microcrystalline nature of grains of the size ∼70 nm. The transformation of the magnetic properties i.e. from superparamagnetic to ferrimagnetic behaviour after heating in microwave oven has been revealed by hysteresis loops under VSM study. The saturation magnetisation, M_s after heat treatment has been seen to increase from 26.7 to 44.5 emu/gm. Remanence and coercivity have also increased four and seven times, respectively. M_s of the as synthesised hexaferrite nano powder and heat-treated powder in microwave oven for 5 min show doublets, confirming the presence of superparamagnetic relaxation in the nano particles as only slight increase in the particle size is associated with the heat treatment.     

Magnetic properties of cobalt substituted M-type barium hexaferrite prepared by co-precipitation

The co-precipitation and solid state methods were used in the synthesis of barium hexaferrite (BaM). Phase pure BaM was obtained with 1, 2, 3, 5, 10, 15, 20 and 30 wt% cobalt oxide (Co₃O₄). The addition of Co^2+/3+ ions to the BaM increased the permeability and magnetic loss tangent to a value of 3.5 at 5% and reduced to 1 at 30% doping. With increased Co doping, M_s was reduced from 87-58 emu/g, M_r increased from 11 to 40 emu/g with 3–5 wt% Co and 9 emu/g for 30% doping. H_c sharply increased from 540 to 2200 Oe with a reduction to 280 Oe at 10 K with increasing temperature to 300 K. T_c increased from 740 to 750 K for 30% Co doping. DTA–TGA studies of green body showed decarboxilation to occur at around 825 °C and the transformation of residual Co₃O₄ to Co₂O₃ at around 577 °C. The XRD data confirmed the Co ions substituting into Fe sites until a 10–15% doping level where the structure altered to W-type hexaferrite. The densities of the compounds varied with doping to a maximum of 4.45 g/cm³.     

Composition and magnetic studies of ultrafine Al-substituted Sr hexaferrite particles prepared by citrate sol-gel method

Ultrafine aluminum-substituted strontium hexaferrite particles have been prepared via citrate sol-gel route. Gels were synthesized with molar ratios [Al³⁺]:[Fe³⁺] of 0.4:11.6, 1:11, 1.5:10.5 and 2:10 and the ferrite particles were obtained by annealing the gels at 950 °C for 2 and 24 h. Electron energy dispersive X-ray spectroscopy (EDX) verified the presence of Al in the substituted samples. X-ray diffraction (XRD) confirmed the formation of the M-type hexaferrite phase in the samples with some indication of α-Fe₂O₃. Scanning electron microscope showed that the hexaferrite powder consists hexagonal crystals with average diameter D_av (80-186 nm) that decreases with increasing Al content and increases with increasing annealing time. Magnetic properties were determined using a pulsed-field magnetometer and a vibrating sample magnetometer (VSM). The saturation magnetization at room temperature and the Curie temperature were found to decrease while the coercivity increases with increasing the Al content. The highest coercivity of 10.1 kOe was achieved for the sample with the molar ratio [Al³⁺]:[Fe³⁺]=2:10 annealed for 24 h. The influences of the particle size, composition and impurity on the magnetic properties were discussed.     

Magnetic properties of xNi0.53Cu0.12Zn0.35Fe1.88O4+(1−x)BaTiO3 nanocomposites

The nanocrystalline Ni_0.53Cu_0.12Zn_0.35Fe_1.88O₄ and BaTiO₃ powders were prepared using Microwave-Hydrothermal (M-H) method at 160 °C/45 min. The as synthesized powders were characterized using the X-ray diffraction (XRD) and Transmission Electron Microscope (TEM). The size of the powders that were synthesized using M-H system was found to be ∼30 and ∼50 nm for ferrite phase and ferroelectric phases, respectively. The powders were densified using microwave sintering method at 900 °C/30 min. The ferrite and ferroelectric phases were observed from XRD and morphology of the composites was observed with the Scanning Electron Microscope (SEM).The magnetic hysteresis loops were recorded using the Vibrating Sample Magnetometer (VSM).The frequency dependence of real (μ′) and imaginary (μ″) parts of permeability was measured in the range of 1 MHz-1.8 GHz. The permeability decreases with an increase of BaTiO₃ content at 1 MHz. The transition temperature (T_C) of ferrite was found to be 245 °C. The T_C of composite materials decreases with an increase in BaTiO₃ content.     

Perpendicular anisotropy of Co3Pt deposited on the NiFeMo/Ru buffer

The multilayer of Ta/NiFeMo/Ru/Co₃Pt was sputter deposited on the Si (1 0 0) wafer. Using the NiFeMo buffer layer greatly enhanced the texture of Co₃Pt layer. The enhanced texture increased the perpendicular magnetic anisotropy of Co₃Pt. According to the VSM and XRD results, only the 5 nm of NiFeMo was good enough to produce the texture and perpendicular anisotropy in Co₃Pt layer. The perpendicular anisotropy was attributed to the existence of short-range-ordered HCP structure of Co₃Pt.     

Preparation of substituted barium ferrite BaFe12−x(Ti0.5Co0.5)xO19 by citrate precursor method and compositional dependence of their magnetic properties

As a possible candidate for the left-handed metamaterial with negative permeability, a series of Ti, Co-substituted M-type barium hexaferrite BaFe_12−x(Ti_0.5Co_0.5)_xO₁₉ (x=0, 1, 2, 3, 4 and 5) was prepared by citrate precursor method. The formation processes of the substituted barium hexaferrite compounds from the precursors were followed by the measurements of powder X-ray diffraction (XRD), Infrared absorption spectra (FT-IR), and thermogravimetry and differential thermal analysis (TG/DTA) coupled with mass spectroscopy (MS). In the case of the non-substituted sample, the formation of the barium hexaferrite is regulated by the thermal decomposition of BaCO₃ and the solid-state reactions of BaO and Fe₂O₃ in the temperature range from 800 to 1100 K. The formation temperature of the substituted BaFe_12−x(Ti_0.5Co_0.5)_xO₁₉ is higher than that for the non-substituted sample and increases with the value of x, due to the effects of carbonate ions incorporated by the partial substitution of Fe³⁺ by (Ti_0.5Co_0.5)³⁺. On heating up to ca. 1200 K, all the substituted samples transform into the magenetoplumbite phase as is the non-substituted sample. The compositional dependence of the magnetic properties of the substituted barium hexaferrite was investigated by the magnetization measurement. The decrease in the magnetic anisotropy was confirmed by the change in the magnetization curve and coercivity H_C with the composition x. A negative permeability spectrum was observed in the BaFe₉(Ti_0.5Co_0.5)₃O₁₉ in the frequency range from 2 to 4 GHz.     

Effect of nanocrystalline particles on the magnetic properties of Z-type hexaferrites

In order to improve the magnetic properties of Z-type hexaferrites such as high initial permeability and high-quality factor, the nanocrystalline hexaferrite particles (NHPs) with the same chemical composition were introduced. The influence of NHPs on the densification, microstructures and magnetic properties of the ceramics prepared by a combined method was investigated. The results show that these NHPs, which spread around the micron-sized hexaferrite particles (MHPs), enhance the densification by increasing the inter-diffusion of the particles due to the increase of contact area; simultaneously, the grain growth in the direction of c-plane is dominant. Due to the special microstructure, high sintering density, and no addition sintering aids with different chemical composition, relatively high initial permeability and Q-factor than those of the samples with 1.0 wt% Bi₂O₃ were obtained in the samples with proper nanocrystalline particles sintered at 900 °C.     

Synthesis and magnetic characterization of cobalt-substituted ferrite (CoxFe3−xO4) nanoparticles

Cobalt-substituted ferrite nanoparticles were synthesized with a narrow size distribution using reverse micelles formed in the system water/AOT/isooctane. Fe:Co ratios of 3:1, 4:1, and 5:1 were used in the synthesis, obtaining cobalt-substituted ferrites (Co_xFe_3−xO₄) and some indication of γ-Fe₃O₄ when 4:1 and 5:1 Fe:Co ratios were used. Inductively coupled plasma mass spectroscopy (ICP-MS) verified the presence of cobalt in all samples. Fourier transform infrared (FTIR) showed bands at ∼560 and ∼400 cm⁻¹, characteristic of the metal–oxygen bond in ferrites. Transmission electron microscopy showed that the number median diameter of the particles was ∼3 nm with a geometric deviation of ∼0.2. X-ray diffraction (XRD) confirmed the inverse spinel structure typical of ferrites with a lattice parameter of a=8.388 Å for Co_0.61Fe_0.39O₄, which is near that of CoFe₂O₄ (a=8.394 Å). Magnetic properties were determined using a superconducting quantum interference device (SQUID). Coercivities higher than 8 kOe were observed at 5 K, whereas at 300 K the particles showed superparamagnetic behavior. The anisotropy constant was determined based on the Debye model for a magnetic dipole in an oscillating field and an expression relating χ′ and the temperature of the in-phase susceptibility peak. Anisotropy constant values in the order of ∼10⁶ erg/cm³ were determined using the Debye model, whereas anisotropy constants in the order of ∼10⁷ erg/cm³ were calculated assuming Ωτ=1 at the temperature peak of the in-phase component of the susceptibility curve as commonly done in the literature. Our analysis demonstrates that the assumption Ωτ=1 at the temperature peak of χ′ is rigorously incorrect.     

Magnetic properties of Co2.4Al0.6O4/SiO2 nanocomposite obtained via sol-gel method

Nanocomposite made of 10 wt% of Co_2.4Al_0.6O₄ particles dispersed in an amorphous SiO₂ matrix has been synthesized by a sol-gel method. X-ray diffraction, transmission electron microscopy and magnetic measurements have been used to characterize the properties of nanocomposite. Most of the particles are well crystallized and have an average diameter below 100 nm. Smaller particles with size below 10 nm have also been observed. A large value of the effective magnetic moment per Co²⁺ ion of 5.08 μ_B and negative and the low Curie-Weiss paramagnetic temperature Θ∼−6 K, obtained from the high-temperature susceptibility data, indicate a possible mixing of Co²⁺ and Co³⁺ ions between tetrahedral and octahedral sites of the spinel crystal lattice. The measurements of static and dynamic magnetic susceptibilities have shown that Co_2.4Al_0.6O₄ particles in SiO₂ matrix display a spin glass behavior at low temperatures.     

Glass additive influence on the sintering behavior, microstructure and microwave magnetic properties of Cu-Bi-Zn co-doped Co2Z ferrites

The Bi₂O₃-B₂O₃-ZnO-SiO₂ (BB35SZ) glass effects on the sintering behavior and microwave magnetic properties of Cu-Bi-Zn co-doped Co₂Z ferrites were investigated to develop low-temperature-fired ferrites. The glass wetting characteristics on the Co₂Z ferrite surface, X-ray diffractometer, scanning electron microscopy and a dilatometer were used to examine the BB35SZ glass effect on Co₂Z ferrite densification and the chemical reaction between the glass and Co₂Z ferrites. The results indicate that BB35SZ glass can be used as a sintering aid to reduce the densification temperature of Co₂Z ferrites from 1300 to 900 °C. 3(Ba_0.9Bi_0.1O)·2(Co_0.8Cu_0.2O)·12(Fe_1.975Zn_0.025O₃) ferrite with 2 wt% BB35SZ glass can be densified below 900 °C, exhibiting an initial permeability of 3.4. This process provides a promising candidate for multilayer chip magnetic devices for microwave applications.     

The grain size effect on the magnetic properties in NiZn ferrite and the quality factor of the inductor

Polycrystalline ferrite materials with the chemical composition of Ni_0.49Zn_0.49Co_0.02Fe_1.90O_x have been fabricated using the conventional ceramic sintering method. Grain sizes have been adjusted from ∼2.2 to ∼13.5 μm with changing sintering conditions. From the measurements of the complex permeability, it is suggested that the permeability is dominated only by the spin rotation at mono-domain state and both domain wall and spin rotation contribute at multi-domain state. At mono-domain state, the core loss has been drastically decreased similar to the other work. The measurement result for the loss angle indicates that the low loss state can be maintained up to the higher magnetic field with smaller grain size in spite of the mono-domain state. The simplified wire-wounded type inductors have been also fabricated and characterized. The results have shown that the inductor fabricated with the smaller grain size has a better performance in the quality value under relatively higher current.     

Effect of Co underlayer on soft magnetic properties and microstructure of FeCo thin films

High saturation magnetization soft magnetic FeCo (=Fe₆₅Co₃₅) films were prepared using a thin Co underlayer. The FeCo/Co films exhibited a well-defined in-plane uniaxial anisotropy with easy axis coercivity (H_ce) of 10 Oe and hard axis coercivity (H_ch) of 3 Oe, and a half reduction of H_c with H_ce=4.8 Oe and H_ch=1.0 Oe was obtained when the composition was adjusted to 25 at% Co. The effective permeability of the films remains flat around 250 to 800 MHz. The saturation magnetostriction was 5.2×10⁻⁵ and the intrinsic stress was 0.8 GPa in FeCo single layer, both were slightly reduced by Co underlayer. The Co underlayer changed the preferred orientation of the FeCo films from (2 0 0) to (1 1 0) but more significantly, reduced the average grain size from ∼74 to ∼8.2 nm. It also reduced the surface roughness from 2.351 to 0.751 nm. The initial stage and interface diffusion properties were examined by TEM and XPS.     

Influence of Co doping content on its valence state in Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films

Zn_1−xCo_xO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol-gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn_1−xCo_xO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet-visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn_1−xCo_xO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn_1−xCo_xO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn_1−xCo_xO (0 ≤ x ≤ 0.05), Zn_0.90Co_0.10O and Zn_0.85Co_0.15O are CoO, Co₃O₄ and Co₂O₃, with an increase of Co content, respectively.     

Preparation and magnetic properties of anisotropic (Sm,Pr)Co5/Co composite particles

Anisotropic (Sm,Pr)Co₅/Co nanocomposite particles have been fabricated by chemical coating the 2 h ball milled (Sm,Pr)Co₅ flakes with Co nanoparticles. The Co nanoparticles were synthesized with mean particle sizes in the range of 20-50 nm. The nanocomposite particles present [0 0 1] out-of-plane texture and improved magnetic properties, e.g., an enhanced remanent magnetization of 72 emu/g for (Sm,Pr)Co₅/Co and 66 emu/g for (Sm,Pr)Co₅. In addition, by using the 8 h ball milled powders (much smaller than the 2 h ball milled powders) as the starting materials, Co nanoparticles can also be successfully coated on the surface of the flakes. A plausible mechanism for the formation of Co nanoparticles on the surface of (Sm,Pr)Co₅ flakes is discussed.     

Interparticle interaction effects on magnetic behaviors of hematite (α-Fe2O3) nanoparticles

The interparticle magnetic interactions of hematite (α-Fe₂O₃) nanoparticles were investigated by temperature and magnetic field dependent magnetization curves. The synthesis were done in two steps; milling metallic iron (Fe) powders in pure water (H₂O), known as mechanical milling technique, and annealing at 600 °C. The crystal and molecular structure of prepared samples were determined by X-ray powder diffraction (XRD) spectra and Fourier transform infrared (FTIR) spectra results. The average particle sizes and the size distributions were figured out using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The magnetic behaviors of α-Fe₂O₃ nanoparticles were analyzed with a vibrating sample magnetometer (VSM). As a result of the analysis, it was observed that the prepared α-Fe₂O₃ nanoparticles did not perform a sharp Morin transition (the characteristic transition of α-Fe₂O₃) due to lack of unique particle size distribution. However, the transition can be observed in the wide temperature range as “a continuously transition”. Additionally, the effect of interparticle interaction on magnetic behavior was determined from the magnetization versus applied field (σ(M)) curves for 26±2 nm particles, dispersed in sodium oxalate matrix under ratios of 200:1, 300:1, 500:1 and 1000:1. The interparticle interaction fields, recorded at 5 K to avoid the thermal interactions, were found as ∼1082 Oe for 26±2 nm particles.     

Complex permittivity, complex permeability and microwave absorption properties of ferrite-paraffin polymer composites

We have investigated the electromagnetic (EM) characteristics of Co_xMn_1−xFe₂O₄ spinel ferrite (where x=0.0, 0.5 and 1.0) nanoparticles (NPs)/paraffin nanocomposite material at 8-20 GHz. Co_xMn_1−xFe₂O₄ NPs have been synthesized by cetyltrimethylammonium assisted hydrothermal route using NaOH. A variation in complex dielectric permittivity and magnetic permeability at room temperature with frequency in the range 8-20 GHz has been studied. Particles showed phase purity and crystallinity in powder X-ray diffraction (XRD) analysis. At the same time, Co_xMn_1−xFe₂O₄ NPs demonstrated a spinel cubic structure from XRD results. A reflection loss of −46.60 dB was found at 10.5 GHz for an absorber thickness of 2 mm. Co_xMn_1−xFe₂O₄ may be attractive candidates for EM wave absorption materials.     

Magnetic and Microwave Absorbing Properties of Electrospun Ba(1−x)LaxFe12O19 Nanofibers

Ba_(1−x)La_xFe₁₂O₁₉ (0.00≤x≤0.10) nanofibers were fabricated via the electrospinning technique followed by heat treatment at different temperatures for 2 h. Various characterization methods including scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and microwave vector network analyzer were employed to investigate the morphologies, crystalline phases, magnetic properties, and complex electromagnetic parameters of nanofibers. The SEM images indicate that samples with various values of x are of a continuous fiber-like morphology with an average diameter of 110±20 nm. The XRD patterns show that the main phase is M-type barium hexaferrite without other impurity phases when calcined at 1100 °C. The VSM results show that coercive force (H_c) decreases first and then increases, while saturation magnetization (M_s) reveals an increase at first and then decreases with La³⁺ ions content increase. Both the magnetic and dielectric losses are significantly enhanced by partial substitution of La³⁺ for Ba²⁺ in the M-type barium hexaferrites. The microwave absorption performance of Ba_0.95La_0.05Fe₁₂O₁₉ nanofibers gets significant improvement: The bandwidth below −10 dB expands from 0 GHz to 12.6 GHz, and the peak value of reflection loss decreases from −9.65 dB to −23.02 dB with the layer thickness of 2.0 mm.     

XRD, HRTEM, magnetic and Mössbauer studies on chemically prepared Fe-doped nanoparticles of cerium oxide

Nanocrystalline samples of Fe-doped cerium oxide (Ce_0.90Fe_0.1O₂) are prepared by sol-gel method. The precursor materials used for the synthesis are ferric nitrate and cerium nitrate. The as-prepared samples is annealed at different temperatures to obtain the sample with different particle sizes. The crystallographic phases of the nanocrystalline materials have been confirmed by X-ray diffractograms (XRD). The sizes of the nanoparticles estimated from the peaks of the XRD patterns using Debye-Scherrer equation are in the range 6-58 nm. Results extracted from the high-resolution transmission electron microscopy (HRTEM) are in agreement with the findings obtained from XRD. The average magnetic susceptibilities of all the samples with different particle sizes are measured in the temperature range 300-14 K. The average susceptibilities of the samples annealed below ∼740 °C show paramagnetic behaviour. The susceptibilities of the samples annealed at and above ∼740 °C sharply decrease at ∼240 K and this sharp transition is quite likely due to the anti-parallel alignment of Fe³⁺ spins and is attributed to Morin transition of α-Fe₂O₃. Mössbauer spectra of the samples annealed at and above ∼740 °C give sextet patterns indicating the presence of exchange interaction among the Fe³⁺ ions of these samples and these sextets are also of typical nature of the α-Fe₂O₃ phase. The Mössbauer spectra of the samples annealed below ∼740 °C are doublets which may be attributed to either superparamagnetic and/or paramagnetic type nanoparticles.