Microwave ferrites, part 1: fundamental properties

Ferrimagnets having low RF loss are used in passive microwave components such as isolators, circulators, phase shifters, and miniature antennas operating in a wide range of frequencies (1–100 GHz) and as magnetic recording media owing to their novel physical properties. Frequency tuning of these components has so far been obtained by external magnetic fields provided by a permanent magnet or by passing current through coils. However, for high frequency operation the permanent part of magnetic bias should be as high as possible, which requires large permanent magnets resulting in relatively large size and high cost microwave passive components. A promising approach to circumvent this problem is to use hexaferrites, such as BaFe₁₂O₁₉ and SrFe₁₂O₁₉, which have high effective internal magnetic anisotropy that also contributes to the permanent bias. Such a self-biased material remains magnetized even after removing the external applied magnetic field, and thus, may not even require an external permanent magnet. In garnet and spinel ferrites, such as Y₃Fe₅O₁₂ (YIG) and MgFe₂O₄, however, the uniaxial anisotropy is much smaller, and one would need to apply huge magnetic fields to achieve such high frequencies. In Part 1 of this review of microwave ferrites a brief discussion of fundamentals of magnetism, particularly ferrimagnetism, and chemical, structural, and magnetic properties of ferrites of interest as they pertain to net magnetization, especially to self biasing, are presented. Operational principles of microwave passive components and electrical tuning of magnetization using magnetoelectric coupling are discussed in Part 2.     

Structural, electrical and magnetic properties of cadmium substituted nickel-copper ferrites

Polycrystalline ferrites with the general formula Ni_0.95−xCd_xCu_0.05Fe₂O₄ in which x varies from 0.1 to 0.3 were synthesized by standard double sintering ceramic technique. The existence of single phase cubic spinel structure of ferrites was confirmed from XRD measurement. Surface morphology and compositional features were studied by SEM and EDX measurements. Absorption bands observed in FTIR spectra at 600 cm⁻¹ (υ₁) and 410 cm⁻¹ (υ₂) corresponds to vibrations of tetrahedral and octahedral complexes respectively. In the dc conductivity measurements the decrease of dc resistivity with increase of temperature indicates the semiconducting nature of ferrites. The dielectric measurement of the samples at room temperature studied in the frequency range 20 Hz to 1 MHz shows dispersion in the low frequency region and remains constant at high frequency region. However, the small polaron hoping type of conduction mechanism was inferred from the linear increase of ac conductivity. The magnetic properties of ferrites such as saturation magnetization, magnetic moment and Y-K angles was estimated as a function of cadmium content by VSM technique. The smaller value of Mr/Ms reveals the existence of multidomain (MD) particles in the ferrite samples.     

Microwave hydrothermal synthesis of nanosize Ta2O5 added Mg-Cu-Zn ferrites

Tantalum oxide added MgCuZn ferrite powders were synthesized by a co-precipitation method using NaOH in a microwave-hydrothermal (M-H) apparatus. The phase identification of the prepared samples was done by X-ray diffraction and crystal size and morphology were determined by transmission electron microscopy (TEM). Nano-phase ferrites with high surface area were synthesized at 160°C after a treatment time of 1 hour. The M-H synthesized powders were conventionally sintered at a temperature of 900°C/4h. The variations of the sintered density, initial permeability and electrical resistivity as a function of additive concentration at room temperature have been investigated.     

Magnetic and electrical properties of hot-pressed Ni-Zn-Li ferrites

The magnetic properties of Ni-Zn ferrites have been upgraded by preparing hot-pressed Ni_0.4Zn_0.6–2xLi_xFe_2+xO₄ ferrites wherein 2xZn²⁺ ions have been substituted byxLi¹⁺ andxFe³⁺ ions. This results in an increase of saturation magnetization, Curie temperature and dielectric constant, whereas resistivity and initial permeability are reduced. The values of saturation magnetization, Curie temperature and dielectric constant are improved due to hot pressing in which grain growth and densification are controlled simultaneously. The variations of saturation magnetization and Curie temperature can be explained by the preferential site occupancy of Li¹⁺ and Ni²⁺ ions at the tetrahedral site, sublattice magnetization with canted spin structure, and magnetic exchange interactions. The inferences drawn from the bulk magnetic properties of these ferrites conform with the conclusions drawn from variations of internal magnetic field, obtained from Mössbauer studies of these samples. The decrease in d.c. resistivity due to substitution of Li¹⁺ ions is attributed to increased hopping due to formation of Fe²⁺ and Ni³⁺ ions.     

Synthesis,electrical and dielectric characterization of cerium doped nano copper ferrites

The nanosized CuFe_2−xCe_xO₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8) ferrites doped with cerium are synthesized by chemical co-precipitation method. The synthesized materials are characterized by XRD, FTIR, TGA and SEM. XRD analysis of cerium substituted copper ferrites confirms the cubic spinel structure. The average crystallite size calculated by using Scherrer's formula ranges from 37 to 53 nm. The values of cell constant and cell volume vary with the dopant concentration. These variations can be explained in terms of their ionic radii. The DC electrical resistivity, measured by two point probe method, increases with increase in dopant concentration while it decreases with rise in temperature exhibiting semiconductor behaviour. Energy of activation of these ferrites is calculated by using Arrhenius type resistivity plots. Dielectric measurements of the synthesized compounds show exponential decrease in dielectric constant and dielectric loss factor with increase in frequency. This indicates the normal dielectric behaviour of ferrites.     

Structural,electrical and magnetic properties of copper-cadmium ferrites prepared from metal oxalates

Seven samples of the system Cu_1–xCd_xFe₂O₄ were prepared by thermal decomposition of mixed metal oxalates with x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0. The formation of the ferrispinel was studied by XRD, FT-IR, Mössbauer spectroscopy, electrical conductivity and magnetic susceptibility measurements. XRD showed single cubic spinel phase for all the samples except for CuFe₂O₄ sample which shows a tetragonal structure. An increase in the lattice parameter of the ferrispinel was observed with increasing the cadmium content. The temperature variation of ac conductivity showed four definite regions with three transitions which attribute to the change in the conduction mechanism with increasing temperature. Magnetic properties of the sample with x 0.5 showed a decrease in the effective magnetic moment with the increase in the cadmium content which means that the ferromagnetics are widely separated and enclosed by non-magnetic cadmium ions. A well defined hyperfine Zeeman spectrum is observed for samples with x 0.3 at room temperature and resolved into two sextets corresponding to the octahedral and tetrahedral sites. The propable ionic configuration of the system proposed is (Cd_xCu_yFe_1–x–y)[Cu_1–x–yFe_1+x+y]O₄.     

The effect of Al-substitution on structure and electrical properties of Mn-Ni-Zn ferrites

X-ray diffraction, Infra-red spectroscopy and scanning electron microscopy have been used to investigate the Mn_0.5Ni_0.1Zn_0.4Al_xFe_{2 − x}O₄ system (0 ≤ x ≤ 0.15 in steps of 0.025). The analysis of IR spectra indicates the distribution of Al-ions between both A and B-sites. The electrical resistivity and thermoelectric power as a function of temperature and Al-content have been investigated. It is observed that the resistivity increases with increasing the Al-content. It is also found that the temperature variation of resistivity exhibits two breaks, each break is associated with a change in the activation energy. Measurement of the thermoelectric power reveals n-type conduction for all samples. The activation energies of all samples in the paramagnetic region are found to be greater than those in the ferrimagnetic region. The results are explained on the basis of the formation of spin polarons in the paramagnetic region.     

Crystallographic and electrical study of the chromium substituted ferrous zinc copper ferrites

The structural and electrical properties of the oxidic spinel Zn_0·5Cu_0·5Fe _x Cr_2−x O₄ (x=0·8, 0·9, 1·0, 1·1) have been investigated through X-ray diffraction, electrical resistivity, and thermoelectric measurements. X-ray diffraction data showed formation of single spinel phase with cubic structure. D.C. resistivity measurements from room temperature to 850 K was carried out and activation energy for all the compositions evaluated. Thermoelectric measurements showedp-type semiconducting nature in all the samples.     

Hydrothermal synthesized bismuth ferrites particles: thermodynamic, structural, and magnetic properties

A family of bismuth ferrites (BFO), including Bi2Fe4O9, BiFeO3, and Bi25FeO39 with different morphologies, has been prepared by the hydrothermal method assisted by different alkaline mineralizers. X-ray diffraction refinement calculations are carried out to study the crystal structures of bismuth ferrites. A thermodynamic calculation based on the dissolution-precipitation model was carried out to analyze the hydrothermal synthesis of BFO powders. Magnetic measurements of the obtained bismuth ferrites show different magnetic properties from 5 K to 350 K.     

Synthesis, sintering behavior and magnetic properties of Cu-substituted Co2Z hexagonal ferrites

Cu-substituted Z-type Ba₃Co_2?x Cu _x Fe₂₄O₄₁ hexagonal ferrites (0 ?? x ?? 1) were prepared by the standard oxide route. Almost single phase Z-type ferrite powders were observed after calcination at 1,260 °C. The permeability of samples sintered at 1,260 °C varies with the Cu content; a maximum permeability at 10 MHz of ??? = 17 was found for x = 0.6. It is shown that the permeability behavior is determined by the concentration of Z-type ferrite in the sample and the microstructure. Addition of 3 wt% Bi₂O₃ leads to almost complete densification of Co₂Z samples under low temperature ceramic cofiring (LTCC) conditions at 900 °C. However, the permeability of samples sintered at 900 °C is reduced to ??? = 5 with resonance frequency at 2 GHz. It is shown that Cu-substituted Co₂Z ferrites are not stable at T < 1,260 °C. Partial decomposition into other hexagonal ferrites causes a reduction of the permeability. In spite of this severe drawback, Co₂Z hexagonal ferrites with Cu modifications are suitable materials for multilayer inductors for high frequency applications.     

An analysis of the field failure of passive and active components

This paper briefly describes work that is in hand on establishing and running an organization for the collection and analysis of field failure data on passive and active components in electronic equipment. Data have now been collected from all the sources for nearly two and a half years as of June 1989 and the results obtained are presented. Analysis covers a wide range of electronic component categories.     

Antibacterial performance of polydopamine-modified polymer surfaces containing passive and active components

A growing number of device-related nosocomial infections, elevated hospitalization costs, and patient morbidity necessitate the development of novel antibacterial strategies for clinical devices. We have previously demonstrated a simple, aqueous polydopamine dip-coating method to functionalize surfaces for a wide variety of uses. Here, we extend this strategy with the goal of imparting antifouling and antimicrobial properties to substrates, exploiting the ability of polydopamine to immobilize polymers and induce metal nanoparticle formation. Polydopamine was deposited as a thin adherent film of 4 nm thickness from alkaline aqueous solution onto polycarbonate substrates, followed by grafting of antifouling polymer polyethylene glycol and in situ deposition of silver nanoparticles onto the polydopamine coated polycarbonate substrates. Elemental and morphological surface analyses confirmed successful grafting of polyethylene glycol brushes onto polydopamine-coated substrates, as well as spontaneous silver nanoparticle formation for polydopamine-coated substrates incubated in silver-nitrate solutions. Sustained silver release was observed over at least 7 days from silver-coated substrates, and the release kinetics could be modulated via additional polydopamine overlayers. In vitro functional assays employing gram negative and positive strains demonstrated dual fouling resistance and antibacterial properties of the coatings due to the fouling resistance of grafted polyethylene glycol and antibacterial effect of silver, respectively. Polycarbonate substrates coated only with silver using a method similar to existing commercial coatings provided an antibacterial effect but failed to inhibit bacterial attachment. Taking into account the previously demonstrated substrate versatility of polydopamine coatings, our findings suggest that this strategy could be implemented on a variety of substrate materials to simultaneously improve antifouling and antimicrobial performance.     

Influence of mechanical milling and thermal annealing on electrical and magnetic properties of nanostructured Ni-Zn and cobalt ferrites

The present article reports some of the interesting and important electrical and magnetic properties of nanostructured spinel ferrites such as Ni_0.5Zn_0.5Fe₂O₄ and CoFe₂O₄. In the case of Ni_0.5Zn_0.5Fe₂O₄, d.c. electrical conductivity increases upon milling, and it is attributed to oxygen vacancies created by high energy mechanical milling. The real part of dielectric constant (?′) for the milled sample is found to be about an order of magnitude smaller than that of the bulk nickel zinc ferrite. The increase in Néel temperature from 538 K in the bulk state to 611 K on the reduction of grain size upon milling has been explained based on the change in the cation distribution. The dielectric constant is smaller by an order of magnitude and the dielectric loss is three orders of magnitude smaller for the milled sample compared to that of the bulk. In the case of cobalt ferrite, the observed decrease in conductivity, when the grain size is increased from 8–92 nm upon thermal annealing is clearly due to the predominant effect of migration of some of the Fe³⁺ ions from octahedral to tetra-hedral sites, as is evident from in-field Mössbauer and EXAFS measurements. The dielectric loss (tan δ) is an order of magnitude smaller for the nano sized particles compared to that of the bulk counterpart.     

Crystallographic and d.c. electrical resistivity study of Cd-Co and Cr3+ substituted Cd-Co ferrites

Polycrystalline compounds of the series Cd_xCo_1-xFe_2-yCr_yO₄ (x = 0, 0.25, 0.50, 0.75 and 1.00; y = 0, 0.15 and 0.30) were prepared by a standard ceramic technique. The crystallographic data were obtained using X-ray diffraction. All the compounds were found to have f.c.c. symmetry. The ionic radii on A and B sites (r_A and r_B, respectively) and the bond lengths on A and B sites (A–O and B–O, respectively) were calculated. The values of r_B and B–O were found to be greater than r_A and A–O, except for the Cd and Cr³⁺ substituted Cd ferrites. The d.c. electrical resistivity of pelletized samples was calculated by measurements of voltage and current using a two-probe method at various temperatures. The values of Curie temperatures (T_c) observed in d.c. resistivity measurements, for all the composition under investigation, were found to be in good agreement with those observed in susceptibility measurements and by the Loria–Sinha technique. The activation energies ( E) were found to be higher in the para-region than in the ferri-region. The resistivity of the samples is found to be dependent on the saturation magnetic moments (n) of the samples. The resistivity of Co ferrite is found to be higher than that of Cd ferrite at 475 K.     

Transport,magnetic and spectroscopic studies of nickel-gallium ferrites

X-ray, electrical conductivity, magnetic hysteresis and infrared spectroscopic studies for the system NiGa_2−2xFe_2xO₄ were carried out. All the compounds, 0 x⩽1 showed cubic symmetry. The activation energy, thermoelectric coefficient values decreased with increasing Fe³⁺ concentrations but with increasing number of Fe³⁺ ions, the values of lattice constant and magnetic moment increased. Compounds with x⩽0.5 are p-type while those with x⩾0.75 are n-type semiconductors. Magnetic hysteresis loops indicated that the compounds with x⩾0.25 are ferrimagnetic while the compound NiGa₂O₄ (x=0.0) is anti-ferromagnetic. Constant and low values of coercive force indicated that the compounds with x⩾0.50 exhibit multidomain behaviour. X-ray intensity calculations, electrical conductivity, magnetic hysteresis and infrared studies indicate the presence of Ga³⁺ ions at the tetrahedral site, Ni²⁺ ions at the octahedral site and Fe³⁺ ions are present in both tetrahedral and octahedral sites. The probable ionic configuration for the system NiGa_2−2xFe_2x O₄ is suggested to be Ga _1−x ³⁺ Fe _x ³⁺ [Ni²⁺ Fe³⁺ O ₄ ²⁻ .     

Precipitation hardening of Cu-Fe-Cr alloys part I Mechanical and electrical properties

This research is part of a project whose scope was to investigate the engineering properties of new non-commercial alloy formulations based on the Cu rich corner of the Cu-Fe-Cr ternary system with the primary aim of exploring the development of a new cost-effective high-strength, high-conductivity copper alloy. The literature indicated that Cu rich Cu-Cr and Cu-Fe alloys have been thoroughly investigated. A number of commercial alloys have been developed and these are used for a variety of applications requiring combinations of high-strength, high-conductivity and resistance to softening. Little evidence was found in the literature that the Cu rich corner of the Cu-Fe-Cr system had previously been investigated for the purpose of developing high-strength, high-conductivity copper alloys resistant to softening. The aim of these present investigations was to explore the possibility that new alloys could be developed that combined the properties of both sets of alloys, ie large precipitation hardening response combined with the ability to stabilise cold worked microstructures to high temperatures while at the same maintain high electrical conductivity. To assess thefeasibility of this goal the following alloys were chosen for investigation: Cu-0.7wt%Cr-0.3wt%Fe, Cu-0.7wt%Cr-0.8wt%Fe, Cu-0.7wt%Cr-2.0wt%Fe. This paper reports on the mechanical property investigation which indicated that the Cu-0.7wt%Cr-0.3wt%Fe, and Cu-0.7wt%Cr-2.0wt%Fe alloys were worthy of further investigation.