Substituted Strontium Hexaferrite Research Articles

Impact of yttrium on the physical, electro-magnetic and dielectric properties of auto-combustion synthesized nanocrystalline strontium hexaferrite

Yttrium substituted strontium hexaferrite, SrAl4YxFe8−xO19 with x = 0.0, 0.05, 0.10, 0.20, and 0.30, are prepared by the auto combustion method. The ferrites have been examined using XRD, SEM, FTIR, and VSM. The structural study confirms that the Y3+ ions reorganize themselves without troubling the parent lattice. In FTIR, the low wavenumber absorption bands confirm the hexagonal ferrites. Saturation (Ms) and remanence (Mr) magnetization is observed to decrease whereas, coercivity (Hc) and Curie temperature (Tc) is improved with Y3+ content. The variation in the dielectric constant $$( \in )$$ and dielectric loss factor (tanδ) is discussed in the frequency range of 20Hz–20 MHz. The value of dielectric constant $$( \in )$$ and dielectric loss factor (tanδ) is decreased with Y3+ content. The optimized magnetic properties are obtained from x = 0.20 compositions having Ms ~ 4.45 emu/g, Mr ~ 1.91 emu/g, Hc ~ 36 kOe, (BH)max ~ 36.64 MGOe and Tc ~ 614 °C. This material is very suitable for longitudinal recording media.

Magnetic and dielectric characteristics of Nd and Nd-Mg substituted strontium hexaferrite

A systematic investigation on the phase formation, magnetic, dielectric and impedance properties of strontium hexaferrites doped with Nd and Nd-Mg was performed. All ferrite samples were prepared by the citrate-precursor method and characterized with a combination ofX-ray diffraction, Vibrating sample magnetometer and impedance analyser. XRD analysis confirms the magnetoplumbite structure with space groupp63/mmcwithout any secondary phase. The lattice parameter ratioc/alies in the range of 3.92 to 3.94 and shows that the prepared material exhibits M-type hexagonal structure. An increase in coercivity and a decrease in magnetization were observed for all the samples. Large value of coercivity suggests that these materials are useful in longitudinal recording media. Further, it was found that the value of magnetocrystalline anisotropy constantK1decreases with the substitution of Nd and Nd-Mg. Measurement of dielectric loss and dielectric constant were performed as a function of temperature and frequency. Dielectric constant and ac conductivity of Sr0.95Nd0.05Mg0.05Fe11.95O19is more as compared to Sr0.95Nd0.05Fe12O19and SrFe12O19for all frequencies. The value of grain boundary resistance (Rgb) of Sr0.95Nd0.05Fe12O19is less as compared to SrFe12O19and Sr0.95Nd0.05Mg0.05Fe11.95O19and vice versa trend is observed in the value of capacitance of the grain boundary (Cgb) and in the values of relaxation time (τgb).

Tuning the ferromagnetic resonance by doping strontium hexa-ferrite nanopowders

Mg–Ti substituted strontium hexa-ferrites nanopowders (SrFe12−x(MgTi)x/2O19, x = 0–3) were prepared by the sol–gel method. The morphology, structure and composition of the nanostructures were examined by field emission scanning electron microscopy (FESEM) and X-ray diffraction. The effect of Mg–Ti doping on the magnetic properties of the powders was investigated by vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) at ambient temperature. Experimental results showed that the materials exhibit hexagonal structures with tunable magnetic properties. The saturation magnetization and the coercive field (Hc) decreased through the Mg and Ti substitution. FMR proved that by incorporation of Mg and Ti in strontium ferrite lattice, crystalline anisotropy, and microwave absorption can be tuned. SrFe12−x(MgTi)x/2O19 ferrites are good candidate for applications at X-band microwave frequencies. A low field absorption signal was observed with the same phase as the FMR absorption in all doped ferrites.

57Fe Mössbauer and Co Kβ x-ray emission spectroscopic investigations of La-Co and La substituted strontium hexaferrite

In the ferrite magnets, the magnetoplumbite-type strontium hexaferrite (SrFe12O19) is an important base material with a relatively high Curie temperature. The La-Co substitution in the Sr hexaferrites ((Sr,La)(Fe,Co)12O19) significantly improves magnetic coercivity. We have investigated the electronic states of Sr1−xLaxFe12−yCoyO19 by 57Fe Mössbauer and Co Kβ x-ray emission spectroscopies using the single-crystalline samples. The Co Kβ x-ray emission spectrum shows that all Co2+ ions are in the high-spin state. The hyperfine parameters extracted from the observed 57Fe Mössbauer spectra clearly show the Co and La substitution dependences at two Fe sites of the five crystallographically nonequivalent Fe sites, revealing that most Co2+ ions occupy the 4f1 and 2a Fe sites in Sr1−xLaxFe12−yCoyO19. These Co site occupations are consistent with the higher magnetic coercivity without a decrease in remanent magnetization in the La-Co substituted Sr hexaferrites.

Effect of ionic substitutions on the magnetic properties of strontium hexaferrite: A first principles study

We investigated the effect of substitution of various ions at the Fe sites on magnetic properties of strontium hexaferrite (SrFe12O19) using first principles method based on density functional theory. The site occupancies of substituted atoms were estimated by calculating the substitution energies of different configurations. The formation probabilities of configurations were used to calculate magnetic properties of substituted strontium hexaferrite. A total of 21 elements (M) were screened for their possible substitution in strontium hexaferrite, SrFe12−xMxO19 with x = 0.5. In each case the site preference of the substituted atom and the magnetic properties were calculated. We found that Bi, Sb, Sn, and Sc can effectively increase the magnetization and P, Co, Al, Ga, and Ti can enhance the anisotropy field when substituted into strontium hexaferrite.

Microwave Absorption Properties of Cobalt-Zirconium Doped Strontium Hexaferrites in Ku-Frequency Band

In this article, microwave electromagnetic properties of doped strontium hexaferrites have been investigated in Ku microwave frequency band (12.4–18 GHz), along with their performance as microwave absorbing materials. Vector network analyzer has been used for characterization of cobalt-zirconium substituted strontium hexaferrites Sr(CoZr)xFe(12−2x)O19 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0), synthesized using sol-gel method. Real parts of permittivity and permeability have shown very small variations up to 16 GHz. There has been a dip in spectrum of real part of permittivity and a peak in spectrum of real part of permeability at the same frequency in 16–18 GHz range for every composition. Regarding the electromagnetic wave absorption, all the six prepared compositions have potential (≥90% absorption) for usage in suppressing electromagnetic pollution. The composition with x = 1.0 has achieved the highest −10 dB absorption bandwidth from 15 to 18 GHz frequency. The realization of absorbing materials with maximum absorption is related to the phenomenon of impedance matching and high electromagnetic losses. The correlation of absorption peaks with attenuation constant, wave impedance and complex thickness has also been studied.

Preparation, magnetic properties and microwave absorption of Zr–Zn–Co substituted strontium hexaferrite and its nanocomposite with polyaniline

M-type strontium hexaferrite substituted by Zr4+, Zn2+, and Co2+ was firstly synthesized by a sol-gel auto combustion method. Then the polyaniline/hexaferrite nanocomposite with 20 wt % hexaferrite was prepared by in situ polymerization method. The structure, morphology, and composition of substituted hexaferrite, polyaniline and their composition were investigated by using XRD, FTIR, TGA-DSC, UV-Vis spectroscopy, and FESEM techniques. The results showed that M-type hexaferrite nanoparticles were formed and nanoparticles were effectively coated by polyaniline chains. The magnetic properties of samples were measured by the vibrating sample magnetometer (VSM). The saturation magnetization (M s), magnetic remanence (M r), and coercivity (Hc) of composite are lower than those of pure hexaferrite. The microwave absorption properties of samples were measured by vector network analyzer (VNA). The results indicated that the minimum reflection loss (RL) of composite is–20 dB at 10.1 GHz and its bandwidth with more than 90% absorption is up to 3.5 GHz which has covered the overall X-band.

Origin of absorption peaks in reflection loss spectrum in Ku- frequency band of Co-Zr substituted strontium hexaferrites prepared using sucrose precursor

This study presents the detailed explanation of the factors, contributing towards the absorption peaks in reflection loss spectrum of hexaferrites. Cobalt-Zirconium substituted strontium hexaferrites, synthesized using sucrose precursor sol-gel technique, were analyzed in 12.4–18GHz frequency range. The concepts of impedance matching through quarter wavelength condition, complex thickness, dielectric phase angle and attenuation constant have been used to determine the location as well as intensity of absorption peaks. This study also demonstrates the potential application of three compositions of this series with doping content (x)==0.0, 0.6 and 0.8 as an effective microwave absorbers in Ku-frequency band.

M-Zn (M = Sb, V, and Nb) Substituted Strontium Hexaferrites with Enhanced Saturation Magnetization for Permanent Magnet Applications

M-Zn (M = Sb, V, Nb) substituted M-type strontium hexaferrites were prepared by a ceramic method. The phase composition, morphology and magnetic properties were studied by x-ray diffractometry, scanning electron microscopy and vibrating sample magnetometry. Saturation magnetization increases with a substitution up to 75.0 emu/g (2.5 % higher compared to unsubstituted hexaferrite) and then decreases with a further substitution. A coercive field of substituted hexaferrite powders with highest saturation magnetization is more than 3 kOe. Substituted strontium hexaferrite powders prepared in this work are a rare example of high <TEX>$M_S$</TEX> compositions without doping rare-earth elements and would be a promising candidate for a permanent magnet application.

Structural, magnetic and microwave absorption behavior of Co-Zr substituted strontium hexaferrites prepared using tartaric acid fuel for electromagnetic interference suppression

Strontium hexaferrites, doped with varying Co-Zr content (x) have been synthesized by sol-gel auto-combustion route using tartaric acid as fuel at 800°C. X-ray diffraction and Fourier transform Infra-red have been carried out to confirm the phase formation, particle size (average 21.9–36.8nm) and the bond formation respectively. Magnetic properties are scrutinized using vibrating sample magnetometer. Techniques like scanning electron microscopy, transmission electron microscopy and energy dispersive scattering have been employed to explore the surface morphology, particle size and composition of the nano-powders. Electromagnetic characterization of the prepared ferrites has been done using Vector Network Anlyzer in 12.4–18GHz frequency range. The effect of calcination temperature (500–1000°C) on the structure, morphology and magnetic properties has also been studied for x=0.2 and 800°C has been found to be the most suitable temperature with the best magnetic properties. Increase in doping has resulted in resonance peaks in dielectric and magnetic loss spectra, leading to microwave absorption peaks. Ferrites with x=0.2, 0.8 and 1.0 have appropriate reflection loss less than −10dB and bandwidth in Ku-band, hence can be used as effective absorbers in suppression of electromagnetic interference (EMI). The governance of impedance matching in deciding the absorption properties has been proved by using input impedance calculations.

Structural and optical absorption studies of cobalt substituted strontium ferrites, SrCoxFe12−xO19 (x = 0.1, 0.2 and 0.3)

Cobalt substituted strontium ferrites SrCoxFe12−xO19 (x = 0.1, 0.2 and 0.3) were synthesized via sol–gel method and the dried gel obtained was annealed at 800 °C. The powder X-ray diffraction studies helped in the determination of the crystallite size that measured ≈ 12–14 nm. The optical properties of the powdered nanoparticles were determined by means of the UV–Vis absorption spectra of their dispersed solutions in liquid media. Despite these measurements, it was difficult to determine their band gap (Eg) precisely. However, the Kubelka–Munk treatment on the diffuse reflectance spectra of the powdered nanoparticles was used in order to extract their Eg unambiguously. The Co substituted strontium hexaferrites are used for optical studies. The energy band gap for all the ferrite compositions was found to be ≈ 1.46–1.78 eV. The study made on the dielectric behaviour of the substituted SrFe12O19 is also discussed in this paper.

Structure and magnetic properties of Zr–Mn substituted strontium hexaferrite Sr(Zr,Mn) x Fe12−2x O19 nanoparticles synthesized by sol–gel auto-combustion method

In this research, nano-sized powders of Zr–Mn substituted strontium hexaferrite (Sr(Zr,Mn) x Fe12−2x O19 (x = 0, 2, 2.5, 3)) were synthesized by sol–gel auto-combustion route using subsequent heat treatment. The samples were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), Mössbauer spectroscopy and vibration sample magnetometer (VSM) techniques. XRD and Mössbauer spectroscopy results revealed formation of Sr(Zr,Mn) x Fe12−2x O19 accompanied with Mn δ Fe2−δ O4 lateral phase in the samples. Also, FTIR and XRD results demonstrated presence of SrO impurity phase. FESEM micrographs show particle size reduction and presence of two distinct powder morphologies with different brightness levels with Zr4+ and Mn2+ substitutions which approves existence of lateral phases in the substituted samples. TEM micrographs show nanometric particles with sizes smaller than 100 nm with high crystallinity. Mössbauer results showed that at low level of substitution, Zr4+ ions prefer to occupy both 4f1 and 2b however, at higher level of substitution, they prefer exclusively 4f1 site. While, Mn2+ ions distributed approximately equally between 12k and 2a sites. The presence of nonmagnetic Zr4+ cation leads to decrease in exchange interaction, especially at 12k and 2a sites. VSM results showed decrement of coercivity force ( i H c ) from 5593.60 to 3282.46 Oe and maximum magnetization from 62.60 to 46.15 emu g−1, respectively, by increment of Zr–Mn substitution values. Variations in maximum magnetization magnitude have been explained on the basis of occupation of the substituted cations at different iron sites.

Influence of Al–Cr co-substitution on physical properties of strontium hexaferrite nanoparticles synthesized by sol–gel auto combustion method

Substituted strontium hexaferrite ceramics SrCr x Al x Fe12−2x O19 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by sol–gel auto-combustion method. X-ray diffraction (XRD) study revealed the M-type hexagonal structure of the synthesized nanoferrites with some additional peaks of Fe2O3. The lattice constants (a and c), unit cell volume (V), X-ray density (ρ x ), bulk density (ρ m ), porosity (P) and average crystallite size (t) values changes when Al–Cr ions are co-substituted in SrFe12O19 lattice, resulting in the structural variation. The surface morphology of the grains was examined by scanning electron microscopy (FESEM). Fourier transform infrared spectroscopy (FTIR) confirmed the formation of hexagonal ferrite structure. Ferromagnetic nature confirmed by recording M-H curves exhibited typical hysteresis loop at room temperature using pulse field hysteresis loop tracer technique. The large coercivity (H c ) values indicate the nanocrystalline nature of the present samples. The coercivity (H c ), saturation magnetization (M s ), remanence magnetization (M r ) and magneton number (n B ) decreases with increase in Al–Cr content x. The DC electrical resistivity studies of the prepared samples were carried out in the temperature range of 300–873 K using a standard two-probe technique. Curie temperature (Tc) i.e. ferrimagnetic to paramagnetic transition temperature for all samples was obtained from resistivity data. The Curie temperature decreases linearly as the concentration of Al–Cr content is increased. The activation energy below and above Tc was calculated. The dielectric parameters such as dielectric constant (ɛ′), dielectric loss (ɛ″) and loss tangent (tan δ) were measured in the frequency range 50 Hz–5 MHz at room temperature. All the dielectrical parameters show compositional as a function of frequency dependences. At lower frequencies, it is observed that the dielectric constant (ɛ′), dielectric loss (ɛ″) and loss tangent (tan δ) are high.

Investigation of Structural, Magnetic and Dielectric Properties of Terbium Doped Strontium Hexaferrite for High Frequency Applications

Background/Objectives: Microwave absorbing materials can be used to reduce electro- magnetic interference. M-type hexaferrites (SrFe12O19 and BaFe12O19) have attracted intensive attention on account of their important applications in microwave absorbing materials. Methods/Statistical Analysis: M-type hexaferrite nanoparticles with a composition of TbxSr1-xFe12O19 (x = 0-0.1) were amalgamted by a chemical method known as co-precipitation method. A successive solid state reaction at different calcination temperatures at fixed holding time has lead to the formation of pure and Tb substituted strontium hexaferrite. The reaction of swaping of Tb cations on the magnetic, structural, and electrical properties of nanoparticles was characterized by X-ray diffraction XRD, FTIR,TEM and VSM. Findings: Microstructure by TEM showed that grains are of extremely fine phase. Identification by XRD confers the single-phase material of nanometeric size. Evaluation on magnetic characteristics for the material showed that coercivity decreased progressively as number of Tb3+ ion increases. Although this was followed by a significant increase in total magnetization. Reflection loss and reflection coefficient and loss tangent for a range of 1-5 KHz were derived which confirms that the material can be considered for high frequency applications.

Magnetic, dielectric and complex impedance properties of lanthanum and magnesium substituted strontium hexaferrite

Lanthanum and magnesium substituted strontium hexaferrites, having the general formula Sr1−xLaxMgyFe12−yO19 (x, y = 0, 0.05, 0.10, 0.15, 0.20), have been synthesized by citrate precursor method. X-ray diffraction confirms all the samples exhibit single hexagonal phase with space group P63/mmc. The infrared spectrum shows two prominent peaks in the range of 430–590 cm−1 indicating the formation M-type hexagonal ferrite. Magnetic study shows that the value of saturation magnetization and retentivity decreases while as coercivity increases with the concentration of doping. Temperature dependent magnetization presents a sharp peak just below the Curie temperature, showing super-paramagnetic behaviour of the prepared material. The dielectric parameters: dielectric constant, loss, ac conductivity were studied as a function of frequency. The impedance spectroscopy method has been used to analyze the effect of grain and grain boundary on the electrical properties of the prepared material. The Cole–Cole diagram suggests that conduction is mainly due to grain boundaries. La–Mg substitution into strontium ferrite increases the magnetic and dielectric properties which could ensure a higher memory storage capability.

Complex permittivity, permeability and microwave absorbing properties of Co–Ti substituted strontium hexaferrite

AbstractM-type strontium ferrite with compositions SrFe(12-2x)CoxTixO19(x = 0.0, 0.3, 0.5, 0.7, 1.0), were prepared by two route ceramic method. The effects of Co–Ti substitution on their microstructure, electromagnetic properties, and microwave absorptive behavior were analyzed. The complex permittivity (∊′-j∊″) and complex permeability (μ′-jμ″) have been measured from 8.2 to 12.4 GHz using a network analyzer. Scanning electron microscope was used to analyze the grain size distribution and porosity of the ferrite. X-ray diffraction confirmed the M-type structure of the doped strontium ferrite. Vibrating sample magnetometer was used to study hysteresis loop of the ferrite. This study suggests that the control of grain size, decrease in coercivity and enhanced values of dielectric constant and loss are effective means to improve microwave absorption. The dielectric constant and loss were enhanced in comparison to the permeability constant and loss over the entire frequency range.

Polyaniline coated magnetic particles of manganese titanium substituted strontium hexaferrites composites as microwave absorbers

Polyaniline were synthesized through the oxidative chemical polymerization of aniline in the presence of ammonium peroxydisulfate and used to form coated MnTi-substituted Sr hexaferrites composites. The structure and morphologies of the products were characterized using scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction and ultraviolet–visible spectroscopy. In the magnetization of the PANI/Sr(MnTi)Fe10O19 composites (PSMTFO), saturation magnetization (Ms) and coercivity (Hc) decreased after PANI coating. Under an applied magnetic field, the composites exhibited hysteresis loops of the ferromagnetic behavior, such as Ms (1.92 emu/g) and Hc (0.919 kG). Finally, the PSMTFO composites and a thermal plastic resin absorbed microwave bandwidth because of reflection losses from −23 to −35 dB at frequencies between 25 and 35 GHz, as observed by a high-frequency network analyzer. The microwave absorption of the composites was strongly dependent on the increased amounts of PANI coating.

Preparation and Magnetic Properties of La-Substituted Strontium Hexaferrite by Microwave-Assisted Sol-Gel Method

Sr1−x La x Fe12O19 (x = 0, 0.15, 0.25, 0.5) hexaferrites were prepared by microwave-assisted sol-gel method. The thermal decomposition process, structural, and magnetic properties of the products were studied by thermal differential scanning calorimeter (DSC), thermogravimetry (TG), X-ray diffraction (XRD), and vibrating sample magnetometer (VSM). The phase of α-Fe2O3 appeared at x = 0.25 and x = 0.5. The coercivity of La3+-substituted strontium hexaferrites is improved to 5960.2 Oe at x = 0.25.

Self-Biased Y-Junction Circulators Using Lanthanum- and Cobalt-Substituted Strontium Hexaferrites

In this paper, we propose the application of polycrystalline lanthanum- and cobalt-substituted strontium hexaferrites in the realization of self-biased circulators. These materials present a high anisotropy field, dependent on the substitution rate, which makes it possible to reach operating frequencies in the millimeter-wave range. A first demonstrator was successfully designed and realized using a 20% rate of substitution $(\hbox {Sr}_{0.8}\hbox {La}_{0.2}\hbox {Fe}_{11.8}\hbox {Co}_{0.2}\hbox {O}_{19})$ . This circulator showed insertion losses of 1.79 dB and an isolation level of 28.1 dB at 41.4 GHz without magnets. Performances can be significantly improved by applying a low magnetic field $(\hbox {H}_{\rm app}=2100 \hbox {Oe})$ . According to the literature, increasing the substitution rate makes it possible to increase the anisotropy field, and thus, the internal field. Consequently, a 30% substituted strontium hexaferrite was tested. It appears that the anisotropy field was not higher in this case. However, magnetic losses are much lower and enabled us to halve insertion losses of the self-biased circulator (0.87 dB at 41 GHz).

Thickness dependence of magnetic properties in La–Co substituted strontium hexaferrite films with perpendicular anisotropy

The thickness dependence of magnetization reversal and coercivity behavior for La–Co substituted strontium hexaferrite (Sr-M) films was investigated. It is found that perpendicular anisotropy appears only when film thickness (t) is above 110nm. With increasing t, perpendicular anisotropy energy (Ku⊥) increases gradually to its maximum of 1.76×106erg/cm3 at t=300nm, but turns to decrease when t>300nm. Moreover, when t>110nm, those films exhibit domains pinning or Stoner–Wohlfarth reversal model, present large Ku⊥ values and a rapid increase in Hc⊥. However, while t≤110nm, Sr-M films show nucleation model of magnetization reversal and perform low coercivity. The origin of the coercivity varying with thickness should be correlated with the grain size and preferred orientations in Sr-M films.