Growth of ultra-fine cobalt ferrite particles by a sol–gel method and their magnetic properties

Ultra-fine CoFe₂O₄ particles are fabricated by a sol–gel method and magnetic and structural properties of powders are investigated. Cobalt ferrite powders fired at and above 450 °C have only a single-phase spinel structure and behave ferrimagnetically. Powders annealed at 350 °C have a typical spinel structure and are of the paramagnetic and ferrimagnetic nature, simultaneously. With X-ray diffraction and Mossbauer spectroscopy measurements, the formation of nano-crystallized particles is confirmed when cobalt ferrite is annealed at 200 °C. In addition, the transition from the paramagnetic to the ferrimagnetic state is observed in samples fired at 200 °C as the measuring temperature decreases from the room to liquid nitrogen temperature. The magnetic behaviour of CoFe₂O₄ powders fired at and above 350 °C shows that an increase of the annealing temperature yields a decrease in the coercivity and, in contrast, an increase in the saturation magnetization. The maximum coercivity and the saturation magnetization of cobalt ferrite powders prepared by the sol–gel method are 2020 Oe and 76.5 e.m.u. g^–1, respectively.     

Synthesis and properties of Fe3C film by r.f. magnetron sputtering

Fe₃C film, which is a promising new magnetic recording material, can be synthesized by r.f. magnetron sputtering. Several graphite plates were attached to an iron plate to adjust the area of a composite target for the control of film composition. The crystalline phases in a film changed from Fe-C solid solution to Fe₃C with increasing substrate temperature from 350 °C and above. Sputtering at an argon pressure of 5 Pa was favourable for the formation of crystalline Fe₃C film. All Fe₃C films showed in-plane magnetization. The saturation magnetization of the film was around 100–120 e.m.u. g^–1 regardless of the deposition conditions. The coercivity of the films increased from 1 Oe to 250 Oe with increasing substrate temperature, and the coercivity remained constant at 250 Oe at 350 °C, regardless of argon pressure.     

Magnetic properties of Nd–Fe–B/FeMn multilayer films

[Nd–Fe–B(x nm)/FeMn(d nm)]_n thin films were deposited by magnetron sputtering on Si (100) substrates heated at 650 °C. The influence of the composition and thickness of FeMn layer on the structure and magnetic properties of Nd–Fe–B films are investigated. The Nd–Fe–B/FeMn multilayer films present an enhanced coercivity and a reduced saturation magnetization, in comparison with those of a Nd–Fe–B single layer. The coercivity of [Nd–Fe–B(x nm)/FeMn(5 nm)]_n films increases with increasing the period number of FeMn layer for the same thickness of magnetic layer, while the coercivity in [Nd–Fe–B(50 nm)/FeMn(5 nm)]_n films increases with decreasing the period number of Nd–Fe–B/FeMn bilayers. The coercivity H_c of about 17.2 kOe is achieved in the Nd–Fe–B(50 nm)/FeMn(5 nm) film.     

Extraordinary role of Ce–Ni elements on the electrical and magnetic properties of Sr–Ba M-type hexaferrites

The structural, electrical and magnetic behavior of Sr_0.5Ba_0.5−xCe_xFe_12−yNi_yO₁₉ (where x = 0.00–0.10; y = 0.00–1.00) hexaferrite nanomaterials are reported in this paper. The structural analysis indicates that the Ce–Ni doped Sr–Ba M-type hexaferrite samples synthesized by the co-precipitation method are stoichiometric, single magnetoplumbite phase with crystallite sizes in the range of 35–48 nm. The dc-electrical resistivity of the pure Sr–Ba hexaferrite is enhanced to almost 10² times by doping with Ce–Ni contents of x = 0.06; y = 0.60. The dielectric constant and dielectric loss tangent decrease to values 14 and <0.2, respectively, by increasing the frequency up to 1 MHz. Small relaxation peaks at frequencies >10⁵ Hz are observed for the samples with Ce content of x > 0.06. The values of saturation magnetization increase from 66.32 to 84.33 emu/g and remanance magnetization from 42.64 to 56.01 emu/g but coercivity decreases from 2.85 to 1.59 kOe by substitution of Ce–Ni. Sharp ferri-paramagnetic transition is observed in the samples, which is confirmed by DSC results. Ce–Ni substitution acts to reduce the electron-hopping between Fe²⁺/Fe³⁺ ions and also improves the magnetic properties. These characteristics are desirable for their possible use in microwave and chip devices.     

The influence of pH and bath composition on the properties of Ni–Co coatings synthesized by electrodeposition

Ni–Co coatings were produced on Cu substrates by electrodeposition from electrolytes with different pH values and different Co²⁺ concentration. The current efficiency increases from 52.1% to 81.2% with the pH increasing from 2.0 to 5.4. It is clearly observed that the content of cobalt in the deposited coatings gradually increases from 9.4% to 19.6% as the pH value varies from 2.0 to 5.4. The Co content in the deposited coatings increases from 16.5% to 72.7% as the molar ratio of CoSO₄/NiSO₄ varying from 1:5 to 1:2 in electrolyte. XRD patterns reveal that the structure of the coatings strongly depends on the Co content in the binary coatings. Both granular and dendritic crystals were investigated by SEM and the different crystallization behaviors were illustrated. The saturation magnetization of the coatings goes up from 96.36 kAm⁻¹ to 136.08 kAm⁻¹ with the pH value increasing from 2.0 to 5.4. The saturation magnetization (M_s) and coercivity (H_c) move up from 144.84 kAm⁻¹ and 15.27 kAm⁻¹ to 175.13 kAm⁻¹ and 125.20 kAm⁻¹ with the increase of Co in the electrolyte, respectively.     

The influence of pH and bath composition on the properties of Ni-Co coatings synthesized by electrodeposition

Ni-Co coatings were produced on Cu substrates by electrodeposition from electrolytes with different pH values and different Co²⁺ concentration. The current efficiency increases from 52.1% to 81.2% with the pH increasing from 2.0 to 5.4. It is clearly observed that the content of cobalt in the deposited coatings gradually increases from 9.4% to 19.6% as the pH value varies from 2.0 to 5.4. The Co content in the deposited coatings increases from 16.5% to 72.7% as the molar ratio of CoSO₄/NiSO₄ varying from 1:5 to 1:2 in electrolyte. XRD patterns reveal that the structure of the coatings strongly depends on the Co content in the binary coatings. Both granular and dendritic crystals were investigated by SEM and the different crystallization behaviors were illustrated. The saturation magnetization of the coatings goes up from 96.36 kAm⁻¹ to 136.08 kAm⁻¹ with the pH value increasing from 2.0 to 5.4. The saturation magnetization (M_s) and coercivity (H_c) move up from 144.84 kAm⁻¹ and 15.27 kAm⁻¹ to 175.13 kAm⁻¹ and 125.20 kAm⁻¹ with the increase of Co in the electrolyte, respectively.     

Formation of ternary carbide Fe3Mo3C by mechanical activation and subsequent heat treatment

Ternary carbide, Fe₃Mo₃C was prepared from the powder mixture of Fe/Mo/C = 1/1/1 which was ground for 3 h in a planetary ball mill and subsequently heated at a temperature as low as 700°C, its amount increased with heating temperature. In contrast, when the 1 h-ground and unground samples were heated at 700–1000°C, Mo₂C formed. From the results obtained about the effect of mixing ratio, grinding time and heating temperature of Fe/Mo/C samples on the formation of Fe₃Mo₃C, it was found that the formation of Fe₃Mo₃C strongly depends on the mixing homogeneity and the activated state of the particles of Fe, Mo and C components induced by mechanical grinding. Fe₃Mo₃C obtained belongs to a hard magnet, having saturation magnetization of 0.4 emu g^–1, remanence of 0.13 emu g^–1 and coercivity of 200 Oe.     

Effect of hydrothermal synthesis environment on the particle morphology,chemistry and magnetic properties of barium hexaferrite

Barium nitrate and iron nitrate have been used as precursors in the hydrothermal synthesis of barium hydroxide, iron oxide and barium hexaferrite sols under specified standard synthesis conditions (temperature, time, stirring, alkali concentration, amount of water and heating rate) as a function of the base species used during synthesis. The hydrothermal synthesis of barium hydroxide and iron oxide has been used to develop an understanding of the hydrothermal synthesis of barium hexaferrite from a mixture of their precursors. The investigation has shown that the nucleation and growth behaviour as well as the phase composition, thermal behaviour, particle size, particle-size distribution and magnetic properties are strong functions of the base species used. The electrostatic potential difference between the barium hydroxide and the iron oxide decreases with increasing cation size in the order NaOH, KOH, (C₂H₅)₄NOH and NH₄OH. Note the potential difference between the two sol species determines their tendency to coagulate into clusters; hence, the heterocoagulation will be greater when using NaOH or KOH than (C₂H₅)₄NOH or NH₄OH. Under the standard synthesis conditions, only NaOH and KOH are able to facilitate the formation of plate-like particles of barium hexaferrite. In contrast, ultrafine particles of iron oxide (10–20 nm) together with only a small amount of barium hexaferrite are produced when either NH₄OH or (C₂H₅)₄NOH base is used. The samples synthesized in the presence of the NaOH and KOH exhibit relatively higher saturation magnetization (i.e. 258 mT (39 e.m.u.g^–1) and 215 mT (32 e.m.u.g^–1), respectively) than those samples synthesized in the presence of NH₄OH or (C₂H₅)₄NOH which exhibit negligible saturation magnetization owing to the small amount of magnetic phase (BaFe₁₂O₁₉) present.     

Magnetic properties of nanocrystalline CoFe2O4 synthesized by thermal plasma in large scale

The paper reports the large scale synthesis of nanoparticles of CoFe₂O₄ using thermal plasma reactor by gas phase condensation method. The yield of formation was found to be around 15 g h⁻¹. The magnetic properties of CoFe₂O₄, synthesized at different reactor powers, were investigated in view of studying the effect of operating parameters of plasma reactor on the structural reorganization leading to the different cation distribution. The values of saturation magnetization, coercivity and remanent magnetization were found to be influenced by input power in thermal plasma. Although the increase in saturation magnetization was marginal (61 emu g⁻¹ to 70 emu g⁻¹) with increasing plasma power; a significant increase in the coercivity (552 Oe to 849 Oe) and remanent magnetization (16 emu g⁻¹ to 26 emu g⁻¹) were also noticed. The Mössbauer spectra showed mixed spinel structure and canted spin order for the as synthesized nanoparticles. The detailed analysis of cation distribution using the Mössbauer spectroscopy and X-ray photoelectron spectroscopy leads to the conclusion that the sample synthesized at an optimized power shows the different site selective states.     

Magnetic properties of Ni–Co doped barium strontium hexaferrite

A series of Ni–Co substituted barium strontium hexaferrite materials, Ba_0.5Sr_0.5Ni _x Co _x Fe_12–2x O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8 mol%) was synthesized by the sol–gel method. X-ray diffraction analysis has shown that the Ni–Co substitutions maintain in a single hexagonal magnetoplumbite phase. The room temperature magnetic properties and the cation site preferences of Ni–Co substituted ferrite were investigated by VSM. Substitutions led to decrease in coercivity while saturation magnetization remains the almost same. It indicates that the saturation magnetization (52.81–59.8 Am²/kg) and coercivity (69.83–804.97 Oe) of barium strontium hexaferrite samples can be varied over a very wide range by an appropriate amount of Ni–Co doping contents.     

Field-dependent spin-lattice relaxation of Cr3+ in Al2O3

The variation of spin-lattice relaxation time (T ₁) with magnetic field (B) for the Cr³⁺ ion in ruby has been measured under non-Kramers conditions for fields between 0.3 and 2.6 tesla, corresponding to microwave frequencies in the range 9 to 71 GHz. Below about 0.8 tesla there is a slow variation, approximately asT ₁, B ^–0.4; at higher fields the variation becomes more rapid, approachingT ₁, B ^–2.0 at 2.6 tesla. This behaviour is explained in terms of the non-linear divergence of the energy levels with field.     

A study of the evolution of the constituent phases and magnetic properties of hydrogen-treated Sr-hexaferrite during calcination

A hydrogen treatment followed by calcination, has been developed in order to enhance the intrinsic coercivity of Sr-hexaferrite (SrFe₁₂O₁₉). Fully hydrogen-treated Sr-hexaferrite consists of a mixture of 73%, by weight, of Fe and 27% of Sr₇Fe₁₀O₂₂ phases. Calcination of this material to reform the SrFe₁₂O₁₉ phase occurs in two stages. Between room temperature and 600°C, oxygen was absorbed resulting in a large increase in weight with the formation of a mixture of SrFeO_3–x and Fe₂O₃( and ). During the second stage, the intermediate phases reacted to form SrFe₁₂O₁₉ at a temperature of between 700 and 800°C. A partial desorption of oxygen occurred until calcination reached completion at 1000°C. The magnetization at 1100 kA m^–1 and the remanence were similar to those of the untreated material, but, because of a much refined grain size, the intrinsic coercivity was considerably larger, with values around 400 kA m^–1. Grain growth occurs at temperatures > 1000°C, resulting in a decrease in the intrinsic coercivity.     

Preparation and magnetic properties of the CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> thin films on Si substrate by sol-gel technique

A stable precursor for CoFe₂O₄ thin film was prepared by sol-gel technique from the aqueous solution of FeCl₃·6H₂O and CoCl₂·6H₂O. Sol was deposited on a naturally oxidized silicon-substrate by spinning technique (2000 rpm) and heat treated at different temperatures ranging from 700 to 1100 °C. Thickness of the films was controlled in the range of 400–500 nm and all the films were characterized by using XRD and SEM. The effects of temperature and the composition on the formation of CoFe₂O₄ thin film were also studied. Films obtained at relatively lower temperature showed multi-phases of α-Fe₂O₃, CoFe₂O₄ and CoO while the formation of CoFe₂O₄ phase increases with increasing temperature. Furthermore, the composition of the solution in mol% has great role on the formation of CoFe₂O₄ films and the film containing 50 mol% of Co²⁺ exhibited CoFe₂O₄ mono-phase. Surface morphology of the films was studied by scanning electron microscope (SEM). Magnetic properties of the films, studied by using vibrating sample magnetometer (VSM), showed relatively high saturation magnetization (8.04–22.21 kWb/m²) as well as high coercivity (44.59–63.30 kA/m). Saturation magnetization also increases with increasing heat treatment temperature.     

Tunable magnetostatic waves bandpass filters with scandium doped yttrium iron garnet films

This paper reports results of theoretical and experimental studies of microwave bandpass tuned filters based on magnetostatic surface wave (MSSW) propagation in yttrium iron garnet (YIG) films. Filter transmission and input impedance characteristics for saturation magnetization 139 kAm^–1 of YIG films and 32 kAm^–1 of scandium-doped YIG films have been calculated and measured experimentally. Structures of a planar, multilayer metal-dielectric-YIG geometry are used to model the filters. Parameters of the structures have been optimized with the help of a computer. Scandium-doped YIG films of thickness near 100 m and saturation magnetization 32 kAm^–1 have been found potentially useful in the manufacture of tunable bandpass filters.     

Taguchi design for fabrication of high-performance as-deposited Fe-Sm-O thin films

The Taguchi experimental design was used in this study to obtain optimal conditions for which as-deposited Fe-Sm-O thin films with both good soft magnetic properties and high electrical resistivity could be fabricated by r.f. magnetron sputtering method. The factors considered were the number of Sm₂O₃ chips, sputtering power and time, base vacuum, Ar work pressure, and O₂ partial pressure. The results showed that the optimal conditions were as follows: four Sm₂O₃ chips sputtering power of 350 W, sputtering time of 10 min, base vacuum of 2.6×10^-4Pa, work pressure of 0.2 Pa and O₂ content in Ar of 5%. The thin film fabricated at those conditions had the composition of Fe_75.3Sm_4.3O_20.4. The properties of as-deposited Fe_75.3Sm_4.3O_20.4 thin film were: saturation magnetization of 16.3 kG, coercivity of 0.9 Oe, effective permeability of 2200 in the range of 0.5–100 MHz, and electrical resistivity of 190 cm. The percentage contribution of each factor to electrical resistivity, soft magnetic properties such as magnetic permeability, coercivity and saturation magnetization were also calculated.     

A study of the immobilization of strontium over crystalline titania

The immobilization of strontium over titania was carried out by the method of coprecipitation. From a preliminary study, an appreciable uptake (52.8%) of⁹⁰Sr was observed over preformed titania material. A weighable amount of strontium was coprecipitated with Ti(IV) hydroxide and a maximum of 34 wt% Sr was found to be adsorbed. The leachability of the mixed materials prepared by the addition of 250 and 400 mg Sr²⁺, calcined at 1000 °C, by soxhlet apparatus refluxing at 97 °C and repeated seven times at intervals of 24 h, was found to be of the order of 10^–9 and 10^–5 g cm^–2 d^–1, respectively. X-ray powder diffraction analysis revealed that strontium was immobilized in the crystalline matrix of rutile, which suffered some structural changes with the formation of new phases, SrTiO₃+Sr₂TiO₄ and SrTiO₃, respectively.     

The effects of carbon on the magnetic properties of nanocrystalline Fe-based alloys

We investigate the magnetic properties of nanocrystalline Fe_73.5Cu₁M₃Si_13.5B₉ (M=Nb or Mo) alloys when C is substituted for B up to 2 at%. It is found that the permeability and coercivity deteriorate with the content of C in the case of both M=Nb and Mo. The saturation magnetization also deteriorates as C is substituted for B in the case of M=Mo but it improves linearly with the C content in the case of M=Nb. This increase in the saturation magnetization of the Fe-Cu-Nb-Si-B alloy with C addition can provide an opportunity to overcome one of the main disadvantages, low magnetic flux density, of the alloy. In the latter part of the work we also investigate the magnetic properties of Fe_76.5–y Cu₁Nb_y (Si_0.5B_0.4C_0.1)_22.5 (0y3) alloys, particular emphasis being given to the role of Nb in the presence of C. It is found that C may help Nb to suppress the growth of -Fe grains in the alloy.     

Soft magnetic properties of NiFeTa thin films for biasing film applications in magnetoresistive elements

The effects of Ta addition on the magnetic properties of permalloy thin films have been investigated. The alloy compositions on a weight basis are (Ni₈₁Fe₁₉)_1-x Ta _x with 0 x 0.105, and the films are sputtered onto a glass substrate at between room temperature and 300 C. The saturation magnetization and anisotropic magnetic field decrease with increasing Ta content. The saturation magnetization is 0.75 T at 5 wt % Ta. The coercivity remains constant at 125 Am^-1. The electrical resistivity increases linearly with increasing Ta content, then saturates at approximately 7.5 wt% Ta. The saturation resistivity is approximately 1.00 m. The magnetoresistivity ratio (/) decreases with increasing Ta content, mainly due to increased electrical resistivity (). The magnetostriction changes from negative to positive with increasing Ta content and reaches nearly zero at 2 wt% Ta. The NiFeTa films containing 5–6 wt% Ta have potential for use as the soft-biasing film in magnetoresistive elements.     

Production of flaky amorphous powders in Fe-Zr-B system by a supercooled liquid quenching method,and their magnetic properties

By using a supercooled liquid (two-stage) quenching method, in which supercooled liquid droplets produced by high-pressure gas atomization are flattened at high kinetic energies on a rapidly rotating wheel, flaky amorphous powders with a thickness of 1–3 m and a large aspect ratio of 20–300 were produced for an Fe₈₅Zr₈B₆Cu₁ alloy. This is in contrast to the result that spherical powders produced only by high-pressure gas atomization consist of amorphous, bcc and compound, even in the particle size range below 25 m. The amorphous powder changes to a mostly single bcc phase by annealing for 3.6 ks at 873 K, and the further rise in annealing temperature causes the mixed structure of -Fe + Fe₃(Zr,B). The annealed flaky powders with the bcc phase exhibit soft magnetic properties of 140 emu g^–1 for saturation magnetization _10k and 0.6 Oe for coercivity, H _c. The composite made from the bcc flaky powders and phenol resin at a weight ratio of 91 has a high degree of laminated structure. The composite also exhibits soft magnetic properties of 5.3 kG for D₁₀₀, 150 for _max and 1.1 Oe for H _c. The B ₁₀₀ value is 2.1 times as high as that for the composite of amorphous Co-Fe-Si-B flaky powders and resin. Thus the present composite is expected to be used in applications which require both high saturation magnetization and soft magnetic properties, which cannot be obtained for the composites made from amorphous Co- and Fe-based flaky powders.