Natural Ferromagnetic Resonance Frequency Research Articles

Modifiable natural ferromagnetic resonance frequency and strong microwave absorption in BaFe12-y-2xAlySnxMnxO19 M-type hexaferrite

The site preferences of the ions in M-type BaFe12-y-2xAlySnxMnxO19 were determined from the XRD patterns refined with the Fullprof program. Due to the preferential occupation of the nonmagnetic Sn4+ ions, the doping Sn4+ and Mn2+/Mn3+ ions decreased the saturation magnetization. The doping also decreased the anisotropic field and the coercivity as well. BaFe12-y-2xAlySnxMnxO19 had strong natural ferromagnetic resonances at 15.37, 9.96, and 6.96 GHz with x = 0.7, 0.9 and 1.1, respectively. The frequency of the natural ferromagnetic resonance fell in the frequency range between 2 and 18 GHz was closely related to the decreased anisotropic field with the Sn4+ and Mn2+/Mn3+ doping. Accompanied with the ferromagnetic resonance, a high attenuation factor was obtained. The highest attenuation factor was 240 in x = 0.7. The minimum reflection loss was at the frequency with the impedance matching being close to 1, and at the thickness satisfying the quarter wavelength mechanism. The impedance matching was good around the natural resonance frequency in each compound. These led to a strong microwave absorption in the x = 0.7, 0.9 and 1.1 compounds. As the absorber’s thickness was as thin as 1.8 mm, the x = 0.7 compound had an effective bandwidth of over 4.1 GHz in the range from 13.9 to higher than 18 GHz. At thin thickness of 2.5––2.9 mm, the x = 0.9 compound had a broad absorption bandwidth of 8–14.8 GHz, which covered the full X band of 8––12 GHz. By varying the doping content ×, the microwave absorption band covered the range of 6.1 to 18 GHz with the absorber thickness thinner than 2.9 mm. The quarter wavelength mechanism played an important role on the strong microwave absorption.

Synthesis, structure and electromagnetic properties of FeCoCu/C nanocomposites

FeCoCu ternary nanoparticles distributed and stabilized in the carbon matrix of FeCoCu/C metal-carbon nanocomposites have been synthesized using controlled IR pyrolysis of precursors consisting of the “polymer / iron acetylacetate / cobalt and copper acetates” type system obtained by joint dissolution of components followed by solvent removal. The effect of the synthesis temperature on the structure, composition and electromagnetic properties of the nanocomposites has been studied. By XRD was shown that the formation of the FeCoCu ternary nanoparticles occurs due to the interaction of Fe3С with the nanoparticles of the CoCu solid solution. An increase in the synthesis temperature leads to an increase in the size of the metal nanoparticles due to their agglomeration and coalescence as a result of matrix reconstruction. Furthermore, ternary alloy nanoparticles having a variable composition may form depending on the synthesis temperature and the content ratio of the metals. Raman spectroscopy has shown that the crystallinity of the carbon matrix of the nanocomposites increases with the synthesis temperature. The frequency responses of the relative permittivity and permeability of the nanocomposites have been studied at 3–13 GHz. It has been shown that a change in the content ratio of the metals noticeably increases both the dielectric and the magnetic losses. The former loss is caused by the formation of a complex nanostructure of the nanocomposite carbon matrix while the latter one originates from an increase in the size of the nanoparticles and a shift of the natural ferromagnetic resonance frequency to the low-frequency region. The reflection loss has been calculated using a standard method from the experimental data on the frequency responses of the relative permittivity and permeability. It has been shown that the frequency range and the absorption of electromagnetic waves (from –20 to –52 dB) can be controlled by varying the content ratio of the metals in the precursor. The nanocomposites obtained as a result of the experiment deliver better results in comparison with FeCo/C nanocomposites synthesized under similar conditions.

Tuning the microstructure, magnetostatic and magnetodynamic properties of highly Al-substituted M-type Sr/Ca hexaferrites prepared by citrate-nitrate auto-combustion method

Double substitution of strontium hexaferrite by calcium and aluminum leads to a tremendous rise of hard magnetic properties, such as coercivity and natural ferromagnetic resonance frequency (NFMR). However, the properties are also inextricably linked to the material microstructure (especially, particle size), to the solid solution inhomogeneity as well as aluminum ions distribution among iron sites in crystal structure. In this work, we obtained M-type hexaferrite particles Sr1-x/12Cax/12Fe12-xAlxO19 (x = 4–6) via a facile citrate-nitrate auto-combustion method and studied the influence of the annealing temperature in a broad range on the microstructure, features of crystal structure and hard magnetic properties. At low annealing temperatures (900–1000°С) hexaferrite nanoparticles with 90% of nominal Al content and a wide chemical distribution are formed. Next, with an increase in the annealing temperature the distribution significantly narrows, chemical composition becomes close to the nominal one and particles size transfer firstly to submicron, then to micron range. The aluminum distribution over iron sites is independent distinctly on the annealing temperature. For all the compositions single domain particles with the maximum coercivity values between 22.8 and 36 kOe are obtained at 1200 °C. At 900–1000 °C the samples demonstrate coercivities up to 25 kOe, while above 1300 °C, the crystallites begin to pass into a polydomain state with a reduced coercivity. The hexaferrites with narrow chemical distribution reveal resonance absorption in sub-terahertz band. The highest NFMR frequency of 270 GHz was observed for x = 5.5 sample annealed at 1400 °C.

Synthesis, structure and electromagnetic properties of FeCoCu/C nanocomposites

FeCoCu ternary nanoparticles distributed and stabilized in the carbon matrix of FeCoCu/C metal-carbon nanocomposites have been synthesized using controlled IR pyrolysis of precursors consisting of the “polymer / iron acetylacetate / cobalt and copper acetates” type system obtained by joint dissolution of components followed by solvent removal. The effect of the synthesis temperature on the structure, composition and electromagnetic properties of the nanocomposites has been studied. By XRD was shown that the formation of the FeCoCu ternary nanoparticles occurs due to the interaction of Fe3С with the nanoparticles of the CoCu solid solution. An increase in the synthesis temperature leads to an increase in the size of the metal nanoparticles due to their agglomeration and coalescence as a result of matrix reconstruction. Furthermore, ternary alloy nanoparticles having a variable composition may form depending on the synthesis temperature and the content ratio of the metals. Raman spectroscopy has shown that the crystallinity of the carbon matrix of the nanocomposites increases with the synthesis temperature. The frequency responses of the relative permittivity and permeability of the nanocomposites have been studied at 3–13 GHz. It has been shown that a change in the content ratio of the metals noticeably increases both the dielectric and the magnetic losses. The former loss is caused by the formation of a complex nanostructure of the nanocomposite carbon matrix while the latter one originates from an increase in the size of the nanoparticles and a shift of the natural ferromagnetic resonance frequency to the low-frequency region. The reflection loss has been calculated using a standard method from the experimental data on the frequency responses of the relative permittivity and permeability. It has been shown that the frequency range and the absorption of electromagnetic waves (from –20 to –52 dB) can be controlled by varying the content ratio of the metals in the precursor. The nanocomposites obtained as a result of the experiment deliver better results in comparison with FeCo/C nanocomposites synthesized under similar conditions.

Radar absorbing and shielding characteristics in ferrite-polymer composites Mn-Zn ferrite/P (TFE-VDF)

The article discusses the electromagnetic absorbing and shielding properties of ferrite-polymer composites of the composition Mn-Zn ferrite/fluoroplast-42, obtained by pressing a mixture of powders with heating. The measurement of the complex magnetic and dielectric permittivity spectra, as well as the reflection coefficient spectra was carried out in the frequency range 0.1-7 GHz. Using the obtained spectra, a comprehensive analysis of the absorbing characteristics of the composites was carried out, and the factors responsible for the absorption were determined. Fitting of the composites magnetic permeability spectra show that the process of natural ferromagnetic resonance prevails over the resonance of domain walls, and a decrease in the concentration of ferrite inclusions leads to a significant shift in the frequency of natural ferromagnetic resonance to high frequencies. It was found that for composites with a thickness of 5-10 mm, compositions with a mass fraction of ferrite ≤0.4 show radio-absorbing properties, while compositions with a fraction of ≥0.6 show shielding properties. Keywords: polymer composite, magnetic properties, radio absorption, feromagnetic resonance, polyvinylidene fluoride, manganese-zinc ferrite.

Радиопоглощающие и радиоэкранирующие характеристики феррит-полимерных композитов Mn-Zn феррит/П (ТФЭ-ВДФ)

The article discusses the electromagnetic absorbing and shielding properties of ferrite-polymer composites of the composition Mn-Zn ferrite/fluoroplast-42, obtained by pressing a mixture of powders with heating. The measurement of the complex magnetic and dielectric permittivity spectra, as well as the reflection coefficient spectra was carried out in the frequency range 0.1 - 7 GHz. Using the obtained spectra, a comprehensive analysis of the absorbing characteristics of the composites was carried out, and the factors responsible for the absorption were determined. Fitting of the composites magnetic permeability spectra show that the process of natural ferromagnetic resonance prevails over the resonance of domain walls, and a decrease in the concentration of ferrite inclusions leads to a significant shift in the frequency of natural ferromagnetic resonance to high frequencies. It was found that for composites with a thickness of 5 - 10 mm, compositions with a mass fraction of ferrite ≤ 0.4 show radio-absorbing properties, while compositions with a fraction of ≥ 0.6 show shielding properties.

Electromagnetic Properties of Carbon Nanotube/BaFe12-xGaxO19/Epoxy Composites with Random and Oriented Filler Distributions.

The microwave properties of epoxy composites filled with 30 wt.% of BaFe12–xGaxO19 (0.1 ≤ x ≤ 1.2) and with 1 wt.% of multi-walled carbon nanotubes (CNTs) were investigated in the frequency range 36–55 GHz. A sufficient increase in the microwave shielding efficiency was found for ternary 1 wt.%CNT/30 wt.% BaFe12–xGaxO19/epoxy composites compared with binary 1% CNT/epoxy and 30 wt.% BaFe12–xGaxO19/epoxy due to the complementary contributions of dielectric and magnetic losses. Thus, the addition of only 1 wt.% of CNTs along with 30 wt.% of barium hexaferrite into epoxy resin increased the frequency range where electromagnetic radiation is intensely attenuated. A correlation between the cation Ga3+ concentration in the BaFe12–xGaxO19 filler and amplitude–frequency characteristics of the natural ferromagnetic resonance (NFMR) in 1 wt.%CNT/30 wt.% BaFe12–xGaxO19/epoxy composites was determined. Higher values of the resonance frequency (51.8–52.4 GHz) and weaker dependence of on the Ga3+ concentration were observed compared with pressed polycrystalline BaFe12–xGaxO19 ( = 49.6–50.4 GHz). An increase in the NFMR amplitude on the applied magnetic field for both random and aligned 1 wt.% CNT/30 wt.% BaFe12–xGaxO19/epoxy composites was found. The frequency of NFMR was approximately constant in the range of the applied magnetic field, H = 0–5 kOe, for the random 1 wt.% CNT/30 wt.% BaFe12–xGaxO19/epoxy composite, and it slightly increased for the aligned 1 wt.% CNT/30 wt.% BaFe12–xGaxO19/epoxy composite.

Tuning the particle size, natural ferromagnetic resonance frequency and magnetic properties of ε-Fe2O3nanoparticles prepared by a rapid sol–gel method

A fast method for the synthesis of ε-Fe2O3, yielding 100% pure material with a variable FMR frequency, is proposed.

Ferromagnetic Resonance in Cast Microwires and its Application for The Non-Contact Diagnostics

The natural ferromagnetic resonance reveals large residual stresses appearing in the microwire core in the course of casting. These stresses, together with the magnetostriction, determine the magnetoelastic anisotropy. Beside the residual internal stresses, the natural ferromagnetic resonance frequency is influenced by external stresses applied to the microwire or to the composite containing the latter (the so-called stress effect).

Highly Efficient Wideband Microwave Absorbers Based on Zero-Valent Fe@γ-Fe2O3 and Fe/Co/Ni Carbon-Protected Alloy Nanoparticles Supported on Reduced Graphene Oxide.

Electronic systems and telecommunication devices based on low-power microwaves, ranging from 2 to 40 GHz, have massively developed in the last decades. Their extensive use has contributed to the emergence of diverse electromagnetic interference (EMI) phenomena. Consequently, EMI shielding has become a ubiquitous necessity and, in certain countries, a legal requirement. Broadband absorption is considered the only convincing EMI shielding solution when the complete disappearance of the unwanted microwave is required. In this study, a new type of microwave absorber materials (MAMs) based on reduced graphene oxide (rGO) decorated with zero-valent Fe@γ-Fe2O3 and Fe/Co/Ni carbon-protected alloy nanoparticles (NPs) were synthesized using the Pechini sol-gel method. Synthetic parameters were varied to determine their influence on the deposited NPs size and spatial distribution. The deposited superparamagnetic nanoparticles were found to induce a ferromagnetic resonance (FMR) absorption process in all cases. Furthermore, a direct relationship between the nanocomposites’ natural FMR frequency and their composition-dependent saturation magnetization (Ms) was established. Finally, the microwave absorption efficiency (0.4 MHz to 20 GHz) of these new materials was found to range from 60% to 100%, depending on the nature of the metallic particles grafted onto rGO.

Effect of gallium doping on electromagnetic properties of barium hexaferrite

The BaFe12-xGaxO19 (x ≤ 1.2) hexaferrites were synthesized by the usual ceramic technology. It was been established that with the x increase the unit cell and magnetic parameters monotonically decrease. The frequency of natural ferromagnetic resonance firstly decreases from 49.6 GHz down to 49.1 GHz when x = 0.6 and then it increases up to 50.5 GHz. The line width monotonically increases from 3.5 GHz up to 5 GHz. The peak amplitude of resonant curve changes slightly with the exception for the x = 0.9 when it reaches −16 dB. The 1.3 GHz/kOe frequency shift in bias field is more intensive for small x = 0.3. The decreasing of the magnetic parameters is a result of the dilution of the Fe3+ - O2− - Fe3+ superexchange interactions. The behavior of the amplitude-frequency characteristics is largely determined by the reduction of the uniaxial exchange anisotropy. The prospects of the Ga-substituted hexaferrites as a material effectively absorbing the high-frequency electromagnetic radiation are shown.

Epitaxially stabilized thin films of \u03b5-Fe2O3 (001) grown on YSZ (100)

Epsilon ferrite (ε-Fe2O3) is a metastable phase of iron(III) oxide, intermediate between maghemite and hematite. It has recently attracted interest because of its magnetocrystalline anisotropy, which distinguishes it from the other polymorphs, and results in a gigantic coercive field and a natural ferromagnetic resonance frequency in the THz range. Moreover, it possesses a polar crystal structure, making it a potential ferroelectric, hence a potential multiferroic. Due to the need of size confinement to stabilize the metastable phase, ε-Fe2O3 has been synthesized mainly as nanoparticles. However, to favor integration in devices, and take advantage of its unique functional properties, synthesis as epitaxial thin films is desirable. In this paper, we report the growth of ε-Fe2O3 as epitaxial thin films on (100)-oriented yttrium-stabilized zirconia substrates. Structural characterization outlined the formation of multiple in-plane twins, with two different epitaxial relations to the substrate. Transmission electron microscopy showed how such twins develop in a pillar-like structure from the interface to the surface. Magnetic characterization confirmed the high magnetocrystalline anisotropy of our film and revealed the presence of a secondary phase which was identified as the well-known magnetite. Finally, angular analysis of the magnetic properties revealed how the presence of twins impacts their azimuthal dependence.

Ferromagnetic resonance with long Josephson junction

In this work we propose a hybrid device based on a long Josephson junction (JJ) coupled inductively to an external ferromagnetic (FM) layer. The long JJ in a zero-field operation mode induces a localized AC magnetic field in the FM layer and enables a synchronized magnetostatic standing wave. The magnetostatic wave induces additional dissipation for soliton propagation in the junction and also enables a phase locking (resonant soliton synchronization) at a frequency of natural ferromagnetic resonance. The later manifests itself as an additional constant voltage step on the current–voltage characteristics at the corresponding voltage. The proposed device allows to study magnetization dynamics of individual micro-scaled FM samples using just DC technique, and also it provides additional phase locking frequency in the junction, determined exclusively by characteristics of the ferromagnet.

Crystal structure, magnetic, and microwave properties of solid solutions BaFe12–x Ga x O19 (0.1 ≤ x ≤ 1.2)

The crystal structure of solid solutions of an M-type hexagonal barium ferrite BaFe1–xGaxO19 (x = 0.1–1.2) with the isostructural diamagnetic substitution of Ga3+ ions has been studied by X-ray diffraction. The unit cell parameters have been calculated for all compositions. The field and temperature dependences of the specific magnetization of these solid solutions have been measured by vibrational magnetometry. The microwave properties of BaFe12–xGaxO19 (x = 0.1–1.2) have been studied in a bias field. It has been shown that the frequency of natural ferromagnetic resonance decreases as the Ga concentration increases from x = 0.1 to x = 0.6 and it increases once again as the Ga concentration increases to x = 1.2. As the Ga concentration increases, the natural ferromagnetic resonance lines broaden. This indicates an increase in the frequency range of strong absorption of electromagnetic radiation. In this case, the amplitude of the resonance curve peak changes insignificantly. The frequency shift of the natural ferromagnetic resonance in an external magnetic field increases as the gallium ion concentration in the sample decreases.

Magnetic and absorbing properties of M-type substituted hexaferrites BaFe12–x Ga x O19 (0.1 < x < 1.2)

X-ray powder diffraction is used to determine the unit cell parameters and to refine the crystal structure of the solid solutions of M-type hexagonal barium ferrite BaFe12–xGaxO19 (x = 0.1–1.2) with isostructural diamagnetic cation Ga3+ substitution at T = 300 K. As the level of substitution increases, the unit cell parameters are shown to decrease monotonically. The temperature (300 K ≤ T ≤ 750 K, H = 8.6 kOe) and field (T = 300 K,–20 kOe ≤ H ≤ 20 kOe) dependences of the saturation magnetization of these solid solutions are studied with a vibrating-sample magnetometer. The concentration dependences of the Curie temperature TC, the specific spontaneous magnetization, and the coercive force are plotted. The magnetic parameters are found to decrease with increasing substitution. The microwave properties of the solid solutions are analyzed in an external magnetic field (0 ≤ H ≤ 4 kOe). As the cation Ga3+ concentration increases from x = 0.1 to 0.6, the natural ferromagnetic resonance (NFMR) frequency decreases; as the concentration increases further to x = 1.2, this frequency again increases. As the cation Ga3+ concentration increases, the NFMR line width increases, which indicates a widening of the frequency range where electromagnetic radiation is intensely absorbed. Here, the resonance curve peak amplitude changes insignificantly. The shift of the NFMR frequency in an applied magnetic field is more pronounced for samples with low cation Ga3+ concentrations. The role of diamagnetic substitution is revealed, and the prospects and advantages of Ga-substituted beryllium hexaferrite as the material absorbing high-frequency electromagnetic radiation are demonstrated.

Influence of Cu Underlayer on the High-Frequency Magnetic Properties of FeCoSiO Thin Films

A series of soft magnetic FeCoSiO films was fabricated on Si (100) substrates using Cu as the underlayer by radio frequency magnetron sputtering. The influence of Cu underlayers on the dynamic magnetic properties of FeCoSiO thin films was studied in detail. The results show that the Cu underlayers have a significant influence on the high-frequency properties and the damping factor of as-sputtered FeCoSiO films, while the static magnetic properties of sputtered films do not demonstrate obvious improvement. In the absence of Cu underlayers, the optimal films with the desired properties of low coercivity, $H_{c} \sim $ 2.5 Oe; high saturation magnetization, $4\pi M_{s} \sim $ 11.6 kGs; and high natural ferromagnetic resonant frequency, $f_{r} \sim $ 4.05 GHz, were obtained. With the increase of the underlayer’s thickness, the complex permeability of FeCoSiO films has a significant change. The damping factor, which can be determined by curve fitting, based on the theoretical equation changed from 0.04 to 0.19 and reaches its maximum value when the Cu thickness is 2 nm. The increasing extent of damping factor is much larger than that in the previous reports. The reason is suggested to be related to change the film’s stress, surface, and interface roughness by the underlayer. It shows that using a Cu underlayer is an effective way for tuning the high-frequency properties and damping factor of soft magnetic thin films. It also implies that the effect of underlayer must be considered when the magnetic films are used in the gigahertz band.

Microwave and millimeter wave dielectric permittivity and magnetic permeability of epsilon-gallium-iron-oxide nano-powders

In millimeter wave frequency range, hexagonal ferrites with high uniaxial anisotropic magnetic fields are used as absorbers. These ferrites include M-type barium ferrite (BaFe12O19) and strontium ferrite (SrFe12O19), which have natural ferromagnetic resonant frequency range from 40 GHz to 60 GHz. However, the higher frequency range lacks suitable materials that support the higher frequency ferromagnetic resonance. A series of gallium-substituted ε-iron oxides (ε-GaxFe2−xO3) are synthesized, which have ferromagnetic resonant frequencies appearing over the frequency range of 30 GHz to 150 GHz. The ε-GaxFe2−xO3 is synthesized by the sol-gel method. The particle sizes are observed to be smaller than 100 nm. In this paper, in-waveguide transmission and reflection method and the free space magneto-optical approach have been employed to study these newly developed ε-GaxFe2−xO3 particles in millimeter waves. These techniques enable to obtain precise transmission spectra to determine the dielectric and magnetic properties of both isotropic and anisotropic ferrites in the microwave and millimeter wave frequency range from single set of direct measurements. The complex dielectric permittivity and magnetic permeability spectra of ε-GaxFe2−xO3 are shown in this paper. Strong ferromagnetic resonances at different frequencies determined by the x parameter are found.

Current-driven non-linear magnetodynamics in exchange-biased spin valves

This work investigates the excitation of parametric resonance in exchange-biased spin valves (EBSVs). Using a mechanical point contact, high density dc and microwave currents were injected into the EBSV sample. Observing the reflected microwave power and the small rectification voltage that develops across the contact allows detecting the current-driven magnetodynamics not only in the bulk sample but originating exclusively from the small contact region. In addition to ferromagnetic resonance (FMR), parametric resonance at twice the natural FMR frequency was observed. In contrast to FMR, this non-linear resonance was excited only in the vicinity of the point contact where current densities are high. Power-dependent measurements displayed a typical threshold-like behavior of parametric resonance and a broadening of the instability region with increasing power. Parametric resonance showed a linear shift as a function of applied dc bias which is consistent with the field-like spin-transfer torque induced by current on magnetic moments in EBSV.

Tuning of soft magnetic properties in FeCoHfO thin films by various sputtering power and pressure

FeCoHfO thin films with soft magnetic properties were deposited on Si (100) substrates using radio frequency reactive magnetron sputtering in Ar+O2 gases. During the deposition process, the partial pressure of oxygen was fixed at 3%. Sputtering power and sputtering pressure varied from 50 to 350 W and from 0·2 to 1 Pa respectively. In the condition of stationary sputtering gases pressure of 0·4 Pa, it was found that the film electrical resistivity ρ decreases steeply with the increasing sputtering power, and the natural ferromagnetic resonant frequency fr and static anisotropy field Hk increase with increase in power from 50 to 250 W, and then decrease when power exceeded 250 W. The coercivity of films had a contrary trend compared with fr. At 250 W sputtering powers, the optimal properties with fr≈3·02 GHz, saturation magnetisation 4πMs≈19·6 kGs, coercivity Hc≈2 Oe and ρ≈2200 μΩ cm were obtained. In the condition of fixed sputtering power of 250 W, fr, 4πMs and Hk decreased with the increases in sputtering pressure; Hc and ρ had an opposite trend. The physical nature of the effect was suggested to be related to the structure and composition changes in films.

Magnetic and high frequency properties of nanogranular CoFe-yttrium-doped zirconia films

Soft magnetic nanogranular FeCo-Yttrium-doped Zirconia thin films were fabricated using RF magnetron sputtering at different sputtering power. It was found that film electrical resistivity (ρ) decreased steeply with the increase of sputtering power, while both saturation magnetization (4πMs) and natural ferromagnetic resonant frequency (ƒr) increased with the sputtering power ascending from 100 W to 200 W, but decreased when sputtering power exceeded 200 W. X-ray diffraction analysis and transmission electron microscopy confirmed that the films were nanocrystalline/amorphous composites. A saturation magnetization as high as 15.4 kGs and a ferromagnetic resonance frequency above 3 GHz were obtained.