Automatic technique for measuring the Polder resonance in ferritespheres

An automatic technique for measuring the parameters of Polder resonance in polycrystalline ferrite spheres is proposed. The diagonal elements of the external susceptibility tensor versus DC magnetic field are calculated from the changes in resonance frequency and unloaded Q factor of a microwave cavity by perturbation theory. From these diagonal elements, all the elements of the intrinsic permeability tensor can be obtained. By fitting the theoretical curves to experimental data, the parameters of Polder resonance, ΔH M_s, and the g factor, are simultaneously calculated. The whole measurement procedure is controlled by a desktop computer. An accuracy of 5% is obtained in ΔH and M _s, and of 0.1% in the g factor     

Microwave properties of planar hexagonal ferrites

A large number of samples of hexagonal ferrite, with planar anisotropy incorporating various metal ions, were synthesized and tested for FMR (ferromagnetic resonance). Measurements were made at room temperature in transmission cavities at frequencies between 10 and 35 Gc/s on magnetically oriented polycrystalline compounds. Of the possible crystal forms for hexagonal ferrite, those designated asW, Y, andZwere investigated. TheYcompounds were found to have the most useful microwave properties. The resonance line width was found to vary between 100 and 1200 Oe, and the anisotropy field varied from zero to 40000 Oe. TheWandZcompounds have planar or uniaxial anisotropy, depending on the composition. The region of zero anisotropy was determined. The magnetically oriented polycrystalline materials resemble single crystals in having easy and hard directions of magnetization which require different magnetic field values for FMR. Qualitative information about how well the crystallites are aligned with each other can be obtained from the examination of two FMR absorption curves made with the sample turned so that its easy and hard directions, respectively, are aligned with the magnetic field. Curves are shown to illustrate the difference between nonoriented, partially oriented, and well-oriented materials.     

Ionic Gel Modulation of RKKY Interactions in Synthetic Anti‐Ferromagnetic Nanostructures for Low Power Wearable Spintronic Devices

To meet the demand of developing compatible and energy‐efficient flexible spintronics, voltage manipulation of magnetism on soft substrates is in demand. Here, a voltage tunable flexible field‐effect transistor structure by ionic gel (IG) gating in perpendicular synthetic anti‐ferromagnetic nanostructure is demonstrated. As a result, the interlayer Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction can be tuned electrically at room temperature. With a circuit gating voltage, anti‐ferromagnetic (AFM) ordering is enhanced or converted into an AFM–ferromagnetic (FM) intermediate state, accompanying with the dynamic domain switching. This IG gating process can be repeated stably at different curvatures, confirming an excellent mechanical property. The IG‐induced modification of interlayer exchange coupling is related to the change of Fermi level aroused by the disturbance of itinerant electrons. The voltage modulation of RKKY interaction with excellent flexibility proposes an application potential for wearable spintronic devices with energy efficiency and ultralow operation voltage.     

Solvothermal synthesis and characterization of functionalized graphene sheets (FGSs)/magnetite hybrids

Novel functionalized graphene sheets (FGSs)/Fe₃O₄ hybrids were synthesized through a facile one-step solvothermal method using FeCl₃ as iron source, ethylene glycol as the reducing agent and graphene nanosheets as templates. The morphology, composition and phase structure of as-prepared hybrid materials were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). These results showed that denseness, size and crystallinity of magnetite can be altered by controlling the reaction parameters. Magnetization measurement indicated that both coercivity and saturation magnetization increased linearly with increasing magnetite concentration in hybrid materials. The measured relative complex permittivity indicated that a high resistivity existed in the FGSs/Fe₃O₄ inorganic hybrids. The magnetic loss was caused mainly by ferromagnetic natural resonance, which is in agreement with the Kittel equation. The novel inorganic hybrid materials are believed to have potential applications in the microwave absorbing performances.     

Garnet ferrite films with two thermostable frequencies of ferromagnetic resonance

The problem of thermal stabilization of the frequency of magnetization autooscillations in anisotropic cubic ferrite films is considered. The variable parameters are the magnetic field direction and the crystal lattice orientation relative to the film plane. It is shown that both parameters can be chosen so as to provide the thermal stability of two ferromagnetic resonance frequencies.     

Electronic structure and magnetic studies of V-doped ZnO: <Emphasis Type="BoldItalic">ab initio</Emphasis> and experimental investigations

In this study, the electronic structure of V-doped ZnO system is studied by means of density functional theory. Different concentrations of V and rising of Fermi level increase the relative occupation of majority/minority spin of 3d state and also induce strong spin-splitting. The existence of three different states of V spin moment has been confirmed and is found to be concentration dependent. We found that O p-orbitals are responsible for the origin of the magnetic moment. Ruderman–Kittel–Kasuya–Yosida mechanism and the atomic spin polarization of V are the key factors for the appearance of ferromagnetism in V-doped ZnO system. The synthesized nanoparticles exhibit hexagonal wurtzite crystal structure, where both crystallite size and lattice parameters vary with V content. Magnetic measurements at room temperature confirm the ferromagnetic behaviour of V-doped ZnO system.     

横向常磁场中铁磁圆板的主共振特性与静载效应

针对横向常磁场中铁磁圆板的主共振问题进行研究。基于电磁基本理论,得到薄板在常磁场中所受的磁体力和洛伦兹力,应用哈密顿变分原理,推导出磁场中铁磁圆板磁弹性耦合横向振动微分方程。常磁场中铁磁圆板受到的磁体力为静载荷,根据伽辽金法得到周边夹支边界条件下铁磁圆板在静载荷作用下的初挠度,进一步应用多尺度法对周期载荷作用下的非线性方程进行一阶和二阶近似求解,得到主共振下系统幅频响应方程。通过算例,给出了系统幅频特性曲线图、振幅随磁场强度和激励力变化的曲线图,分析了板厚、磁场强度、激励力对系统共振振幅的影响,并对比了一阶近似和二阶近似计算结果的不同。结果表明,共振区域内振幅显著增加,磁场强度较小时一阶近似与二阶近似计算结果相近,而磁场强度较大时,二阶近似计算结果更加准确。     

Vacuum field correlations and three-body Casimir–Polder potential with one excited atom

The three-body Casimir–Polder potential between one excited and two ground-state atoms is evaluated. A physical model based on the dressed field correlations of vacuum fluctuations is used, generalizing a model previously introduced for three ground-state atoms. Although the three-body potential with one excited atom is already known in the literature, our model gives new insights on the nature of non-additive Casimir–Polder forces with one or more excited atoms.     

Preparation of iron boride-silica core-shell nanoparticles with soft ferromagnetic properties

A one-pot aqueous chemical synthesis for silica-passivated ferromagnetic nanoparticles is presented. The average size of these particles is 84 ± 20?nm. The x-ray and electron diffraction experiments revealed that the nanoparticles are mainly composed of polycrystalline iron boride. The broad x-ray diffraction peak leads to an average crystallite size of 1.8?nm, which is much smaller than the overall size of the particles, and is consistent with the polycrystalline nature of the samples. M?ssbauer spectroscopy and magnetization experiments were used to establish the room temperature magnetic properties as well as the chemical nature of the particles. Fe(2)B dominates the composition of the nanoparticles, having a hyperfine field broadly distributed in the 10-33?T range. Alpha iron, the second ferromagnetic material identified in the particles, amounts to 4.6% of the composition. Finally, a paramagnetic phase accounting for approximately 14.6% of the material of the particles was also detected. These nanoparticles contain a core with soft ferromagnetic properties surrounded by a passivating silica layer, and are suitable for magnetically targeted drug delivery and electromagnetic induction heating applications.     

Electrically controlled frequency bands of nonreciprocal passage of microwaves in metastructures

Measurements of the transmission coefficients of linearly polarized waves in a rectangular waveguide along a metastructure formed by a transversely magnetized ferrite plate and double-split rings with varactors indicated the presence—in addition to the ferromagnetic resonance—of a resonance region of nonreciprocal passage, which is controlled, in contrast to the ferromagnetic resonance, by the electric field. This effect manifests itself in magnetic fields substantially lower than the field exciting the ferromagnetic resonance at these frequencies. Electrically controlled nonreciprocal passage of microwaves cannot be achieved by means of known natural materials or traditional ferromagnets.     

NiMnGa铁磁性形状记忆合金颗粒/树脂智能复合材料的研究进展*

介绍了NiMnGa铁磁性形状记忆合金颗粒/树脂智能复合材料的制备、温度场和磁场响应特性以及阻尼性能的最新研究进展.与NiMnGa多晶材料相比,NiMnGa铁磁性形状记忆合金颗粒/树脂智能复合材料具有很好的加工成型性能,克服了NiMnGa多晶材料的脆性,同时表现出良好的温度场响应特性和阻尼性能,是一种很有发展前景的新型驱动器材料和阻尼材料.在磁场响应方面,新型铁磁性形状记忆合金(NiCoMnIn、NiCoMnSb、NiCoMnGa)/树脂智能复合材料将是未来研究的重点.     

Magnetic relaxation in rare earth garnets

An attempt has been made to review the ferromagnetic resonance and relaxation data on rare earth iron garnets and in the light of it, to analyze the data on Sm and Eu thin films and Gd and Dy bulk polycrystalline samples obtained in our laboratory. A phenomenological approach adopted to explain the temperature and composition dependence of the line width andg _eff appears to explain the complex spectra in most cases.     

Investigation of Magnetic Properties of Multilayer Thin Films with Spin-Wave Resonance

A theory has been developed to simulate spin-wave resonance (SWR) modes in the multilayer systems consisting of alternate magnetic and nonmagnetic layers. An equation of motion of magnetization with Gilbert-type damping parameter for simulating SWR modes was used. It has been realized that the theory developed for the magnetic multilayer films is suitable to study the spin dynamics and extract various magnetic parameters. It has been shown that SWR modes strongly depend on an effective magnetic anisotropy constant (K _eff), interlayer exchange coupling constant (A ₁₂) and effective magnetization (M _eff). The nature of the effective magnetic anisotropy and interlayer exchange coupling constants has been investigated by using the developed SWR theory in detail. The separation between optic and acoustic modes strongly depends on the magnitude of the interlayer exchange coupling constant, whereas the relative position of the acoustic and optic modes depends on the sign of \(A_{12}\). With increasing the interlayer exchange coupling constant, the resonance field of the optic mode decreases (increases) for ferromagnetic (antiferromagnetic) coupling. When the effective magnetic anisotropy constant increases, the resonance field of the acoustic and optic modes increases for both the ferromagnetic and antiferromagnetic couplings. The increasing of the effective magnetization results in decreasing of the resonance field of SWR mode at parallel geometry, whereas that of SWR mode increases at the perpendicular geometry. The results are compatible to the other theories and experimental results.     

Interactions between spin waves and elastic waves in one-domain ferromagnetic insulators

The interaction between spin waves and the elastic waves is theoretically investigated in the ferromagnetic insulators which are made to behave like one-domain bodies by means of an applied magnetic field. The free energy of the material, that is, the potential of stresses, is derived by continuum approximations of a microscopic model, and the physical meaning of various contributions in the free energy is discussed. The dipole-dipole interaction which may be considered to be a long-range interaction is approximated by a nearest neighbor interaction. As an application of the theory, interactions of spin waves and elastic waves are studied. A correspondence between the microscopic and the continuum theories of ferromagnetic resonance is presented.     

Microwave transmission through ferromagnetic metals

Transmission of microwave radiation through a metal ferromagnet is enhanced at the field corresponding to ferromagnetic antiresonance. This phenomenon can be used to measure the magnetic parameters of a specimen. It is a particularly useful and sensitive tool for studying the damping processes in metals. This technique is compared with that of ferromagnetic resonance absorption (FMR). It is pointed out that the FMAR transmission method is complimentary to the FMR method when non-local (i.e. wave-number dependent) effects are important, because the spatial variation of the RF magnetization and fields is much greater at FMR than at FMAR. This point is illustrated using experiments on nickel for which the damping parameter measured using FMAR is larger than that measured using FMR at temperatures less than 300K.     

One-pot solvothermal synthesis of sandwich-like graphene nanosheets/Fe3O4 hybrid material and its microwave electromagnetic properties

A novel sandwich-like graphene nanosheets (GNs)/Fe₃O₄ hybrid material was synthesized through a facile one-pot solvothermal method using FeCl₃ as iron source, ethylene glycol as the reducing agent and graphene nanosheets as templates. The Fe₃O₄ nanoparticles, with the average diameters of ca. 40 nm, were self-assembled on the graphene nanosheets through electrostatic attraction and formed sandwich-like nanostructure. The ferromagnetic signature emerged with the saturated magnetization of ~ 72.3 emu g^− 1, and the coercive force of ~ 196.1 Oe at 300 K. The magnetic loss was caused mainly by natural resonance which is in agreement with the Kittel equation. The novel electromagnetic hybrid material is believed to have potential applications in the microwave absorbing performances.     

Structural and Magnetic Properties of Transition Metal-Adsorbed MoS<Subscript>2</Subscript> Monolayer

The electronic and magnetic properties of transition metal (TM) atoms (TM = Co, Cu, Mn Fe, and Ni) adsorbed on a MoS₂ monolayer are investigated by density functional theory (DFT). Magnetism appears in the case of Co, Mn, and Fe. Among the three magnetic cases, the Co-adsorbed system has the most stable structure. Therefore, we further study the interaction in the two-Co-adsorbed system. Our results show that the interaction between the two Co atoms is always ferromagnetic (FM) and the p–d hybridization mechanism results in such ferromagnetic states. However, the FM interaction is obviously suppressed by increasing the Co–Co distance, which could be well explained by the Zener–Ruderman–Kittel–Kasuya–Yosida (RKKY) theory. Moreover, similar magnetic behavior is observed in the two-Mn-adsorbed system and a longrange FM state is shown. Such interesting phenomena suggest promising applications of TM-adsorbed MoS₂ monolayer in the future.     

Nonreciprocal transmission spectrum of a ferrite plate-resonant element grating metasandwich with split resonance

It is established that, when microwaves propagate in a structure comprising a ferrite plate and a grating of conducting resonant elements (situated on or close to the plate), the nonreciprocal wave transmission is accompanied by a nonreciprocal splitting of the microwave resonance at the grating frequency and by an expansion of the resonance bandwidth upon application of a constant magnetic field with a magnitude about one order smaller than that of the field exciting the ferromagnetic resonance (FMR). The sign of the nonreciprocity changes when the ferrite is arranged on the opposite side of the grating and/or when the applied constant magnetic field reaches a level at which the difference between the FMR and the grating resonance frequencies changes sign.     

A phenomenological theory of hysteresis damping of vibrations in ferromagnetic materials

The hysteresis damping of vibrations in magnetostrictive polycrystalline ferromagnetic materials is investigated theoretically. The constitutive relation for applied moment and twisted angle in torsion is derived directly from the stress-strain constitutive relation. Precise damping characteristics of torsional vibrations of ferromagnetic wires are obtained in small amplitudes and in large amplitudes after the saturation of magnetostriction.     

Non-destructive evaluation of defects in ferromagnetic plates using a sensitive magnetic sensor based on second harmonic response of superconducting Bi1.6Pb0.4Sr4Ca2Cu3O10+δ pellet

The characteristics of a magnetic sensor, based on the non-linear electromagnetic response of the weak links present in polycrystalline BPSCCO superconductor are reported. The second harmonic response of the sensor in an alternating magnetic field at 40 kHz and at 77 K being a strong linear function of low d.c. field is utilized for magnetic field sensing. The noise limited resolution of the sensor is found to be 3.16 x 10⁻⁹ T/√Hz forH _a.c.= 16 G and frequency 40 kHz. The magnetic sensor has been applied for nondestructive detection of various types of flaws in ferromagnetic plates and also for detection of small magnetic inclusions in a non-magnetic matrix. Our results suggest that the 2f response based BPSCCO superconductive magnetometer has potential for its application in the area of non-destructive evaluation of defects in ferromagnetic materials.