Hard Ferrites Research Articles

A study on the extent of exchange coupling between (Ba0.5Sr0.5Fe12O19)1−x(CoFe2O4)x magnetic nanocomposites synthesized by solgel combustion method

One step citrate gel combustion method followed by high temperature annealing was employed for preparing (Ba0.5Sr0.5Fe12O19)1−x(CoFe2O4)x (x=0.1, 0.2, and 0.3) composite ferrite powders. The powders were subjected to annealing at 800°C in order to decisively study the phase evolution of the combined hard and soft ferrites. Thermogravitry (TGA)/differential scanning calorimetry (DSC) analysis exhibited three stages of decomposition in the precursor gels combined with an exothermic peak at 210°C. X-ray diffraction (XRD) analysis confirmed that the diffraction peaks were perfectly indexed to the hexagonal magnetoplumbite structure of Ba0.5Sr0.5Fe12O19 and the cubic spinel structure of CoFe2O4. Fourier transform infrared spectroscopy (FT-IR) analysis for the samples showed a Co–O stretching vibration accompanied with Co–O–Co or Fe–O–Fe bands at 1220cm−1. The morphology of the samples were examined by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The crystallinity of a selected sample was evaluated by using the high resolution transmission electron microscope (HR-TEM) and selected area electron diffraction (SAED) pattern. It confirmed the presence of planes comprising the hard and soft phases in the synthesized nanocomposites. The magnetic parameters like saturation magnetization MS, remanent magnetization MR, squareness ratio SR, coercivity HC and magnetic moment µB were evaluated using hysteresis by employing vibrating sample magnetometer (VSM). Maximum HC of 4.7kOe and MS of 60.4emu/g were obtained for (Ba0.5Sr0.5Fe12O19)0.9(CoFe2O4)0.1. Switching field distribution curves were analysed by using the demagnetization curve. The exchange coupling between the hard and soft phases were analysed by the dM/dH plots and it indicated the exchange coupling first increased with the increase in the concentration of spinels and then decreased. The possible comparison of exchange coupling between the hard and soft phases has been discussed in detail.

Response of iron oxide on hetero-nanostructures of soft and hard ferrites

Core-shell nanostructures of Zinc and Cobalt ferrites synthesized by solution evaporation route are observed to perform novel magnetic response. The structural and morphological analyses of ZnFe2O4−core/Fe3O4−shell and CoFe2O4−core/Fe3O4−shell nanoparticles (NPs) are confirmed with X-ray diffractometer (XRD) and high resolution transmission electron microscopy (HRTEM). Physical properties measurement system (PPMS) investigations at room temperature reveal that the synthesized core-shell NPs (∼8 nm in diameter) are in super-paramagnetic state. The coercivity of ZnFe2O4/Fe3O4 is much lower than that of CoFe2O4/Fe3O4 NPs (e.g., at 5 K: Hc=0.6kOe for ZnFe2O4/Fe3O4 and Hc = 18kOe for CoFe2O4/Fe3O4. Fe3O4 is preferable to reduce magnetic hardness for Zinc ferrite and enhance for Cobalt Ferrite.

Exchange spring magnetic behavior in BaFe12O19/Fe3O4 nanocomposites

We report the investigation on exchange spring coupling behavior of BaFe12O19/Fe3O4 nanocomposite synthesized by simple mixing followed by heat treatment of individual ferrites. Morphologically tuned, well crystalline hard and soft ferrites were synthesized by simple chemical method and the phase composition, crystallinity, surface morphology and magnetic properties of the as prepared ferrites as well as the nanocomposites were studied by using XRD, FESEM and VSM respectively. Exchange coupling behavior is observed in the nanocomposite samples heated at 600°C with simultaneous enhancements of (BH)max and remanence.

Characteristic and magnetic properties of Ni1–xZnxFe2O4 ferrites

Ferrites are magnetic ceramic materials which have additional metallic ion in ferrous oxide compounds. Ferrites are usually classified as soft or hard ferrites. In this study, characteristics and magnetic properties of magnetic materials having NiO1–xZnOxFe2O4 structure were investigated. Mechanical mixing of high purity NiO, ZnO and Fe2O3 powders were done to obtain homogenous NiO1–xZnOxFe2O4 powder mixture for x = 0.15, x = 0.50 and x = 0.85. These powder mixtures were pressed using hydraulic press machine and then subjected to sintering at same temperatures of 1000 °C for 1 hour. Obtained specimens were analyzed with scanning electron microscopy (SEM) imaging and energy dispersive X‐ray fluorescence (EDXRF) technique for the investigation of structural analysis; magnetic properties were determined using vibrating sample magnetometer (VSM). However, effects of composition, specimens and Zn% element in magnetic materials after energy dispersive X‐ray fluorescence on maximum magnetic moment (Ms) were analyzed using Taguchi orthogonal array design of experiments technique. The study indicates that Zn% element is the main process parameter that has the highest statistical influence on maximum magnetic moment. However, another parameter, composition, also has a significant effect on maximum magnetic moment. Then, Zn‐content was found to have a significant influence on the magnetic properties of the system.

A study of the physical properties of single crystalline Fe5B2P

Single crystals of Fe5B2P were grown by self-flux growth technique. Structural and electrical and magnetic anisotropic properties are studied. The Curie temperature of Fe5B2P is determined to be 655±2K. The saturation magnetization is determined to be 1.72μB/Fe at 2K. The temperature variation of the anisotropy constant K1 is determined for the first time, reaching ∼0.50MJ/m3 at 2K, and it is comparable to that of hard ferrites. The saturation magnetization is found to be larger than the hard ferrites. The first principle calculations of saturation magnetization and anisotropy constant are found to be consistent with the experimental results.

Structural phases and Maxwell–Wagner relaxation in magnetically soft-ZnFe2O4 and hard-Sr2Cu2Fe12O22 nanocomposites

In the present investigation magnetically soft-ZnFe2O4 (S-spinel) and hard-Sr2Cu2Fe12O22 (Y type) mixed ferrite nano-composites (S:Y as 1:0, 1:0, 1:1, 1:2, 1:4, 1:8 and 0:1) were synthesized by using chemical co-precipitation technique. X-ray diffraction analysis revealed the formation of mixed ferrite phases containing Y-type hexaferrite and spinel ferrite phases. The particle size of the prepared samples was estimated from transmission electron microscopy and it lies within the nanometer range. Fourier transform infrared spectroscopy was employed to determine the local symmetry in the crystalline solids and to shed light on the ordering phenomenon of ferrite nano-composite. Curie temperature of all the samples was evaluated using AC susceptibility measurements. Dielectric constant and loss factor was studied as a function of frequency and it is found to be minimum for pure soft ferrite and increased with increase in the concentration of hard ferrite. The results were interpreted in terms of the two-layer model that conducting grains partitioned from each other by poorly conducting grain boundaries. This dielectric dispersion was well explained in terms of Maxwell–Wagner relaxation.

Nanometer-size hard magnetic ferrite exhibiting high optical-transparency and nonlinear optical-magnetoelectric effect.

Development of nanometer-sized magnetic particles exhibiting a large coercive field (Hc) is in high demand for densification of magnetic recording. Herein, we report a single-nanosize (i.e., less than ten nanometers across) hard magnetic ferrite. This magnetic ferrite is composed of ε-Fe2O3, with a sufficiently high Hc value for magnetic recording systems and a remarkably high magnetic anisotropy constant of 7.7 × 106 erg cm−3. For example, 8.2-nm nanoparticles have an Hc value of 5.2 kOe at room temperature. A colloidal solution of these nanoparticles possesses a light orange color due to a wide band gap of 2.9 eV (430 nm), indicating a possibility of transparent magnetic pigments. Additionally, we have observed magnetization-induced second harmonic generation (MSHG). The nonlinear optical-magnetoelectric effect of the present polar magnetic nanocrystal was quite strong. These findings have been demonstrated in a simple iron oxide, which is highly significant from the viewpoints of economic cost and mass production.

자전 연소 전구체로 합성한 나노 크기 경/연 복합페라이트의 자기 특성

The goal of this research is the create novel magnets with no rare-earth contents, with larger energy product by comparison with currently used ferrites. For this purpose we developed nano-sized hard-type/soft-type composite ferrite in which high remanent magnetization (Mr) and high coercivity (Hc). Nano-sized Ba-ferrite, Ni-Zn ferrite and BaFe12O19/ Ni0.5Zn0.5Fe2O4 composite ferrites were prepared by sol-gel combustion method by use of glicine-nitrate and citric acid. Nanocomposite ferrites were calcined at temperature range 700-900 o C for 1h. According to the X-ray diffraction patterns and FT-IR spectra, single phase of NiZn-ferrite and Ba-ferrite were detected and hard/soft nanocomposite ferrite was indicated to the coexistence of the magnetoplumbite-structural BaFe12O19 and spinel-structural Ni0.5Zn0.5Fe2O4 that agreed with the standard JCPDS 10-0325 data. The particle size of nanocomposite turn out to be less than 120 nm. The nanocomposite ferrite shows a single-phase magnetization behavior, implying that the hard magnetic phase and soft magnetic phase were well exchange-coupled. The specific saturation magnetization (Ms) of the nanocomposite ferrite is located between hard (BaFe12O19) and soft ferrite(Ni0.5Zn0.5Fe2O4). The remanence (Mr) of nanocomposite ferrite is much higher than that of the individual BaFe12O19 and Ni0.5Zn0.5Fe2O4 ferrite, and (BH)max is increased slightly.

Facile preparation and microwave absorption properties of porous hollow BaFe12O19/CoFe2O4 composite microrods

The porous hollow BaFe12O19/CoFe2O4 microrods were prepared by an easy and reproducible in situ synthetic method. Cotton cellulose was used as the sacrificial bio-template. The as-prepared hard/soft ferrite composites were homogeneously distributed on the surface of the microrods. It was demonstrated that the porous hollow microrods-like structure and the exchange-coupled interaction between hard and soft ferrites were beneficial to improve the microwave absorption properties. The weight content of the hard/soft ferrite composites was low to 60 wt%, whereas reflection loss could reach to −10 dB in the frequency range of 8.2–16.3 GHz. The fabrication of porous hollow BaFe12O19/CoFe2O4 microrods was an efficient approach to solve the heavy mass problem of the conventional hard/soft ferrite matrix microwave absorption materials.

Exchange spring behaviour in SrFe12O19-CoFe2O4 nanocomposites

Nanocomposites of hard (SrFe12O19) and soft ferrite (CoFe2O4) are prepared by mixing individual ferrite components at appropriate weight ratio and subsequent heat treatment. The magnetization of the composites showed hysteresis loop that is characteristic of the exchange spring system. The variation of Jr/Jr(∞) vs. Jd/ Jr(∞) for these nanocomposites are investigated to understand the presence of both the interacting field and the disorder in the system. This is further corroborated with the First Order Reversal Curve analysis (FORC) on the nanocomposites of 1:4 (Cobalt Ferrite: Strontium Ferrite) and 1:16 (Cobalt Ferrite: Strontium Ferrite). The FORC distribution reveals that the pinning mechanism is stronger in the nanocomposite of 1:4 compared to 1:16. However, the nanocomposite of 1:16 exhibit superior exchange coupling strength in contrast to 1:4. The asymmetric nature of the FORC distribution at Hc = 0 Oe for both the nanocomposites validates the intercoupling between the reversible and irreversible magnetization.

Enhanced remanence and (BH)max of BaFe12O19/CoFe2O4 composite ceramics prepared by the microwave sintering method

As promising candidate materials for permanent magnets, (1 − x)BaFe12O19/xCoFe2O4 composite ceramics (with x = 10 wt%, 20 wt%, 30 wt%, 40 wt%) were prepared by the microwave sintering (MS) techniques. The phase composition and surface morphology of the composites were investigated using XRD and SEM, respectively. The magnetic properties of the composites were also studied. The results show that all the composite ceramics indicate an excellent exchange coupling behavior. The simultaneous enhancements of 12.84% in remanence and 32.20% in (BH)max, compared with the parent hard ferrite (BaFe12O19) has been achieved. The exchange coupling between the hard and soft grains can be confirmed by the switching field distribution curves.

Observation of longitudinal spin-Seebeck effect in cobalt-ferrite epitaxial thin films

The longitudinal spin-Seebeck effect (LSSE) has been investigated in cobalt ferrite (CFO), an exceptionally hard magnetic spinel ferrite. A bilayer of a polycrystalline Pt and an epitaxially-strained CFO(110) exhibiting an in-plane uniaxial anisotropy was prepared by reactive rf sputtering technique. Thermally generated spin voltage in the CFO layer was measured via the inverse spin-Hall effect in the Pt layer. External-magnetic-field (H) dependence of the LSSE voltage (VLSSE) in the Pt/CFO(110) sample with H ∥ [001] was found to exhibit a hysteresis loop with a high squareness ratio and high coercivity, while that with H∥[11̄0] shows a nearly closed loop, reflecting the different anisotropies induced by the epitaxial strain. The magnitude of VLSSE has a linear relationship with the temperature difference (ΔT), giving the relatively large VLSSE /ΔT of about 3 μV/K for CFO(110) which was kept even at zero external field.

Exchange spring behavior in Co0.6Zn0.4Fe2O4/SrFe10.5O16.75 nanocomposites

In this article we have observed the exchange spring behavior in the soft (Co0.6Zn0.4Fe2O4)–hard (SrFe10.5O16.75) ferrite nanocomposites by tailoring the particle size of the individual phases and by suitable thermal treatment of the composites. Two phases were prepared with sol–gel method and under appropriate stoichiometry that improved magnetic properties of these two phases. XRD analysis indicates formation of pure phase for each hard, soft and nanocomposite magnetic phases. The magnetization curves for the nanocomposites heated at different temperatures show the presence of exchange spring behavior in the composite heated at 800°C and an enhancement of 53% in (BH)max compared to the parent hard ferrite (SrFe10.5O16.75).

Study of hard-soft magnetic ferrite films prepared by pulsed laser deposition

Soft magnetic Mg0.1Ni0.3Zn0.6Fe2O4 and hard magnetic BaFe12O19 bulk nanocrystalline ferrites were synthesized using the sol-gel auto-combustion method, and were used as targets to deposit soft-hard thin films by the pulsed laser deposition (PLD) method. Various soft-hard thin films with different preparation conditions were deposited on Si (100) substrate, which can be effectively utilized to get better magnetic properties. The prepared films were characterized by the X-ray diffraction (XRD), atomic force microscopy (AFM) and magnetic measurements. XRD confirms the presence of soft and hard phases in the thin films. Coercivity of the prepared films ranges from 1.67 to 2.66 kA/m. AFM images show clustering of grains at the film surface with a characteristic columnar growth.

Development of a novel one-pot synthetic method for the preparation of (Mn0.2Ni0.4Zn0.4Fe2O4)x–(BaFe12O19)1−xnanocomposites and the study of their microwave absorption and magnetic properties

The development of a simple yet novel aqueous solution based ‘one-pot’ method has been reported for the preparation of nanocomposites composed of soft ferrite (Mn0.2Ni0.4Zn0.4Fe2O4) and hard ferrite (BaFe12O19) phases. A physical mixing method has also been employed to prepare nanocomposites having the same compositions. The effects of synthetic methodologies on the microstructures of the nanocomposites as well as their magnetic and microwave absorption properties have been evaluated. The crystal structures and microstructures of these composites have been investigated using X-ray diffraction, transmission electron microscopy and scanning electron microscopy. In the nanocomposites prepared by both methods, the presence of nanocrystalline Mn0.2Ni0.4Zn0.4Fe2O4 and BaFe12O19 phases were detected. However, nanocomposites prepared by the one-pot method possessed better homogeneous distribution of hard and soft ferrite phases than the nanocomposites prepared by the physical mixing method. Very good spring exchange coupling interaction between the hard and soft ferrite phases was observed for the nanocomposites prepared by the one-pot method and these composites exhibited magnetically single phase behaviour. The spring exchange coupling interaction enhanced the magnetic properties (high saturation magnetization and coercivity) and microwave absorption properties of the nanocomposites prepared by the one-pot method, in comparison with the nanocomposites prepared by the physical mixing method as well as pure Mn0.2Ni0.4Zn0.4Fe2O4 and BaFe12O19 nanoparticles. The minimum reflection loss of the composites was found to be ∼−25 dB (i.e. >99% absorption) at 8.2 GHz with an absorber thickness of 3.5 mm.

BaFe12O19/Ni0.5Zn0.5Fe2O4나노복합체 Ferrite의 Exchange-coupling 상호 작용과 자기 특성

Nano-sized Ba-ferrite, Ni-Zn ferrite and nanocomposite ferrite were prepared by sol-gel combustion method. Nanocomposite was calcined at temperature range of for 1 h. According to the diffraction patterns, hard/soft nanocomposite was indicated to the coexistence of the magnetoplumbite structural and spinel and agree with the standard data (JCPDS 10-0325). The particle size of nanocomposite turn out to be less than 90 nm. The nanocomposite ferrite shows a single-phase magnetization behavior, implying that the hard magnetic phase and soft magnetic phase were well exchange-coupled. The specific saturation magnetization () of the nanocomposite is located between hard () and soft ferrite (). The remanence (Mr) of nanocomposite ferrite is much higher than that for the individual and ferrite. is increased, generally.

Synthesis and electromagnetic absorption properties of polypyrrole/BaFe12O19–Ni0.8Zn0.2Fe2O4/multi-walled carbon nanotube composites

The nanocomposite of hard ferrite (BaFe12O19)/soft ferrite (Ni0.8Zn0.2 Fe2O4) was prepared by using a sol–gel method, and then the polypyrrole/nanocomposite ferrite/multi-walled carbon nanotube composites were fabricated by an in situ polymerization process. Experimental results, such as X-ray diffraction (XRD), transmission electron microscope (TEM), fourier transform infrared (FTIR) and Raman spectra analyses indicated the formation of PPy and BaFe12O19–Ni0.8Zn0.2Fe2O4 on the surface of carbon nanotubes. The conductivities of the composites were enhanced with the increasing of MWCNTs׳ content and the value reached 19.514S/cm in the presence of 30% MWCNTs. However, the magnetic properties did not show any obvious changes. The electromagnetic properties of MWCNTs (5%) composites exceeding −10dB ranged from 8.5 to 10.3GHz and the maximum loss value was −25.46dB at 8.9GHz. The introduction of MWCNTs had an important influence on the electrical and microwave absorbing properties of the nanocomposite.

A Study of Mold Friction and Wear in Injection Molding of Plastic-Bonded Hard Ferrite

In injection molding of highly filled polymers, such as plastic-bonded hard ferrite, mold life decreases significantly due to wear and surface damage. This deterioration process leads to lower accuracy concerning the surface finish and the dimensional and geometric tolerance of the molded bonded magnets. In this work, the wear of a steel mold caused by the molding of a plastic-bonded ferrite magnet has been studied with the aim of determining the main wear factors. Furthermore, two different surface treatments have been tested, namely gas nitriding and DLC superlattice coating. Friction and wear induced by part de-molding was investigated by means of tribological pin-on-disc tests. The results of such experiments were analyzed in terms of loads, surface roughness evolution and surface investigations by means of Scanning Electron Microscope observations.

Heat treatment of Ni–Mn–Cu cast iron

Examined was the influence of chemical composition defined by the nickel equivalent, as well as time and temperature of soaking on course of decomposition process of metallic matrix in the austenitic Ni–Mn–Cu cast iron. In alloys with the equivalent value ranging from 16.0% to 23.0%, with properly selected heat-treatment parameters, possible is controlled, partial transition of austenite to hard acicular ferrite. Obtained is the structure morphologically corresponding to that of ausferrite, characteristic for cast iron quench-hardened with isothermal transformation (e.g. CGI or ADI). Replacing the quench-hardening operation by soaking permits, among others, obtaining homogeneous structure in the entire cross-section of solid thick-walled castings or elimination of quenching stresses.

Exchange bias of [formula omitted] and [formula omitted] core/shell nanoparticles

We compare here exchange bias (EB) properties of chemically synthesized core–shell nanoparticles (NPs), based either on a core of soft ferrite (MnFe2O4) or hard ferrite (CoFe2O4) protected by a maghemite shell (γ-Fe2O3). These NPs dispersed in acidic solutions are electrostatically stabilized, yielding to stable colloidal dispersions with a strong interparticle repulsion and negligible dipolar interactions in the probed range of temperatures. Field cooled (FC) magnetic hysteresis loops of non-textured frozen dispersions (with magnetic anisotropy axis of NPs distributed at random) and those of a powder based on the same NPs present a shift along the H-axis, expressing the coupling between the spin-ordered cores and the disordered surface layer of the NPs. The bias field is found to present a maximum, larger for NPs based on harder ferrite core. It is obtained for a cooling field of the order of one half of the anisotropy field, which is much larger for the CoFe2O4 cores than for MnFe2O4 ones. In powders, particles are in contact leading to an interparticle exchange which is not present in the dilute solutions where exchange bias properties are only due to an intraparticle exchange between core and surface. The thermal dependence of the bias field is well described by a reduced exponential behavior with a characteristic freezing temperature of about 8K.