Comparison between the Kittel and Polder resonances inpolycrystalline ferrites

Kittel and Polder resonance parameters in polycrystalline ferrite spheres have been determined simultaneously using the perturbation theory. Measurements were made with the same sample in order to establish a proper comparison between the Kittel and Polder parameters. The meaning of the shift of the ferromagnetic resonance field and the different parameters used to characterize the resonance losses in polycrystalline ferrites are discussed. Discrepancies between the Kittel and Polder parameters are explained by studying the dependence of the linewidth and the ferromagnetic resonance field on the applied field. There is good agreement between the experimental results and the theoretical predictions     

Exchange coupled nanocomposite hard magnetic alloys

Abstract

The processing, structures and phase constitutions and the magnetic properties of nanocomposite hard magnetic alloys are reviewed. The emphasis is on rare earth (RE)–iron–boron alloys in which the hard magnetic phase RE₂Fe₁₄B is intermixed with one or more soft magnetic phases. Processing–structure–property relationships are the principal focus, in particular, the role of the hard and soft nanocrystallite dimensions in promoting intergrain ferromagnetic exchange coupling and the consequent enhancement of remanent magnetisation and the technologically important maximum energy density. The powder processing, chill block melt spinning, mechanical alloying and thin film deposition routes to develop nanocrystalline and nanocomposite structures are reviewed. The coercivity mechanism in ultrafine grained alloys and the influence of crystallite dimensions are discussed, as are the effects on intrinsic and extrinsic properties of RE substitutions, replacement of iron by other transition metals and enrichment of the boron content. Exchange enhancements in Sm–Co based nanocomposite bulk alloys and in nanoscale FePt/α-Fe composite thin films are briefly considered, together with thin film materials involving exchange coupling between ferromagnetic and antiferromagnetic phases, in core–shell type structures of transition metal compounds surrounded by oxides and in mechanically alloyed materials. The processing and magnetic properties of bonded magnets based on nanocrystalline/nanocomposite REFeB alloys are discussed. The possibility of producing anisotropic hard/soft composites with properties approaching the theoretical maximum is considered and the extent to which this goal has been realised for fully dense alloys identified.     

The refined theory of magnetoelastic rectangular beams

Summary. The problem of deducing a one-dimensional theory from a three-dimensional theory for a soft ferromagnetic elastic isotropic body is investigated. Based on the linear magnetoelasticity, the refined theory of magnetoelastic beams is presented by using the general solution for the soft ferromagnetic elastic solids and the Lure method. Based on the refined theory of magnetoelastic beams, the exact equations and solutions for the homogeneous beams are derived and the equations can be decomposed into three governing differential equations: the fourth-order equation, the transcendental equation and the magnetic equation. Moreover, the approximate equations and solutions for the beam under transverse loadings and magnetic field perturbations are derived directly from the refined beam theory. By omitting higher order terms and coupling effects, the refined beam theory can be degenerated into other well-known elastic and magnetoelastic theoretical models.     

Effect of Substitutions of Zn for Mn on Size and Magnetic Properties of Mn–Zn Ferrite Nanoparticles

In this study Mn?CZn ferrite nanoparticles (Mn_(1?x)Zn _x Fe₂O₄, x=0, 0.3 and 0.5) were produced by a chemical co-precipitation method. The structure and size of the Mn?CZn ferrite nanoparticles were characterized using X-ray diffraction (XRD) and Transmission electron microscopy (TEM). Results show that the ferrite nanoparticles have the spinel structure. It was found that the size of Mn?CZn ferrite nanoparticles decreases by increasing of the Zn concentration. The magnetic properties of Mn?CZn ferrite nanoparticles were investigated with a vibrational sample magnetometer (VSM) and it was observed that Mn_0.7Zn_0.3Fe₂O₃ ferrite nanoparticles have the maximum saturation magnetization and that the initial susceptibility decreases with the increase in Zn concentration.     

Room-temperature ferromagnetism in doped face-centered cubic fe nanoparticles

The magnetism of Fe and its alloys has been at the center of scientific and technological interest for decades. Along with the ferromagnetic nature of body-centered cubic Fe, the magnetic properties of face-centered cubic (fcc) Fe have attracted much attention. It is well known that fcc Fe is thermodynamically unstable at ambient conditions and not ferromagnetic. Contrary to what is known, we report that elongated nanoparticles of fcc Fe, grown within graphitic nanotubes, remain structurally stable and appear ferromagnetic at room temperature. The magnetic moment (2+/-0.5 microB) in these nanoparticles and the hyperfine fields for two different components of 57Fe (33 and 21 T), measured by M?ssbauer spectroscopy, are explained by carbon interstitials in the expanded fcc Fe lattice, that is, FeC(x) where x approximately 0.10, which result in the formation of a dominant Fe4C stoichiometry. First-principles calculations suggest that the ferromagnetism observed in the fcc Fe is related to both lattice expansion and charge transfer between iron and carbon. The understanding of strain- and dopant-induced ferromagnetism in the fcc Fe could lead to the development of new fcc Fe-based alloys for magnetic applications.     

Review Modern magnetic materials in data storage

The current status of the technology of magnetic recording as used in disk drives is reviewed. The emphasis is on the magnetic materials used in the application and on some of the technical problems that may limit the increase in areal density. The new technology of magnetic random access memory (MRAM), which has evolved from the magnetic recording application, is also reviewed. A wide range of magnetic materials is essential for the advance of magnetic recording and the MRAM technology. For the magnetic-recording application the requirements are for high-magnetization, soft magnetic materials for write heads, new antiferromagnetic alloys with high blocking temperatures, large coupling to ferromagnetic films and low susceptibility to corrosion for pinning films in giant magnetoresistive sensors, and for the MRAM application, the requirement is for new ferromagnetic alloys with large values of tunneling polarization ratio. A significant limitation to magnetic recording is found to be the inconsistent demands on media thickness: small media thicknesses are required for large values of signal-to-noise ratio, while large values of thickness are required to reduce the impact of the superparamagnetic effect, which results in the potential for data loss over time. Both of these requirements are discussed. Multilayer ferromagnetic films for recording surfaces are shown to allow both large signal-to-noise ratio and adequate resistance to data loss.     

Methods for measuring the Q-factor (or resistance) of induction coils at audio frequencies

Conclusions The above methods, especially those with the T-type circuits, may prove useful in studying coils, in testing, etc., for instance, in certifying coils used for checking the corresponding parameters of bridges (or Q-meters). They can also be used for evaluating losses in ferrites, ferromagnetic materials and other substances.     

Effect of magnetostriction on fracture of a soft ferromagnetic medium with a crack-like flaw

The magnetic‐induction field in the vicinity of an elliptical inclusion embedded in an infinite soft ferromagnetic medium is determined based on complex potential theory. By using a constitutive relation of magnetostriction for isotropic materials, the stress field in the vicinity of an elliptical flaw is obtained. Furthermore, the stress field at the tip of a slender elliptical crack is determined for the case in which only an external magnetic field perpendicular to the major axis of the ellipse is applied at infinity. The results indicate that the stress field in the neighbourhood of the tip is governed by the magnetostriction and permeability of the soft ferromagnetic material. The induction magnetostrictive modulus is a key parameter in determining which of the two mechanisms, i.e., magnetostriction and magnetic‐force‐induced deformation, is dominant in determining the stress field in the neighbourhood of the tip of a crack‐like flaw. With regard to the influence of the magnetic field on the apparent toughness of a soft ferromagnetic body with a crack‐like flaw, soft ferromagnetic materials can be roughly divided into two categories: one possesses a large induction magnetostrictive modulus and the other has a small modulus. An approximate criterion for categorizing the materials is presented. For the benefit of engineering design, the expressions of the stress‐intensity factor for these two categories of soft ferromagnetic materials are presented. The results show that the stress‐intensity factor is affected not only by the flaw geometry, but also by the permeability of the medium inside the flaw.     

锰锌、镍锌铁氧体的研究现状及最新进展

对常用的锰锌、镍锌软磁铁氧体材料的应用及其性能进行了介绍,着重从配方要求、添加剂的作用等方面综合介绍了国内外的研究情况及最新进展,指出了今后软磁铁氧体研究的主要方向及所要达到的性能要求.研究表明,配方是决定铁氧体材料性能好坏的决定性因素,加入添加剂是改善铁氧体材料性能的有效方法之一,烧结工艺是制备高性能铁氧体的关键.今后软磁铁氧体发展的重点是高频低功耗、高磁导率材料和片式化的表面贴装元件,还应开展纳米软磁铁氧体的研究.     

High-frequency MnZn soft magnetic ferrite by engineering grain boundaries with multiple-ion doping

MnZn soft magnetic ferrites have been widely utilized in power electronics,owing to the combined merits of high permeability and low energy loss.However,their deployment would result in a drastic increase in power dissipation at ＞3 MHz,thus limiting the scope extent of miniaturization,together with their efficiency.Here,we report a high-performance MnZn ferrite by doping multiple ions (La,Ti,Si,Ca) at grain boundaries,achieving the most optimized power loss of 267 kW/m3 at 5 MHz (10 m T,100 ℃) and initial permeability of 644,which is much better than the previously reported results and commercial products.Such an improvement is attributed to weakened magnetic exchange coupling at grain-boundary regions,associated with a significant transition from the multi-to mono-domain structures,originating physi-cally from large crystallographic mis-orientations (＞25°).The present study bears important significance in understanding the intrinsic correlation between the crystallographic mis-orientation and magnetic domain structure,and provides an alternative way for optimizing high-frequency soft magnetic ferrites.     

Spin Turbulence Made by the Oscillating Magnetic Field in a Spin-1 Spinor Bose-Einstein Condensate

We numerically study spin turbulence made by the oscillating magnetic field in a two-dimensional homogeneous spin-1 spinor Bose-Einstein condensate. We confine ourselves to the case of the ferromagnetic interaction, where the ground state is ferromagnetic. By the oscillating magnetic field along some direction makes the system unstable. At first, the spin density vector revolves in the plane perpendicular to the magnetic field, exciting long-wavelength modes. Secondly, appear some vector components along the magnetic field. Finally, the system becomes spin turbulence. In terms of the energy spectrum of the spin-dependent interaction energy, the peak appears first at low wave number region. Gradually, the peak shifts from low to high wave number region. Eventually, the spectrum exhibits the ?7/3 power law. The ?7/3 power law is confirmed by the scaling analysis using the hydrodynamic equation of the spinor Bose-Einstein condensate.     

The Magnetic Properties of Mn x Fe1−x NiSi (x=0,0.25,0.5,0.75,1) Alloys

The magnetic properties of Mn _x Fe_1?x NiSi (x=0,0.25,0.5,0.75,1) alloys are studied using density functional theory and the WIEN2k package. The exchange correlation potential is treated by generalized gradient approximation (GGA). The total energy calculations of these alloys confirm the stability of the ferromagnetic phase as compared to a nonmagnetic phase. The total magnetic moment is not a linear function of x. By increasing x, it increases and then decreases. The peak position of the magnetic moment is near x=0.75.     

A Review on the Regulation of Magnetic Transitions and the Related Magnetocaloric Properties in Ni-Mn-Co-Sn Alloys

In Ni-Mn-X(X=In,Sn,Sb) ferromagnetic shape memory alloys,a ferromagnetic transition from paramagnetic to ferromagnetic austenite and a martensitic transformation from ferromagnetic austenite to weak magnetic martensite occur in some particular composition ranges,in which abundant physical properties have been observed by the abrupt change of magnetization and resistivity around their transition temperatures in these alloys.Therefore,tuning the martensitic transformation temperature(TM) and enlarging the workingtemperature interval for Ni-Mn-X(X=In,Sn,Sb) alloys,are of great importance.In the present paper,we will focus on the effect of external factors,including pre-deformation,annealing,and high pressure annealing,on the magnetic transitions and the related magnetocaloric properties in Ni-Mn-Co-Sn ferromagnetic shape memory alloys.Our approaches and the main results in this particular field will be reviewed.     

Microstructure and Magnetic Properties of Bulk FeCo Alloys Fabricated from Mechanically Alloying and Chemically Synthesized Powders

Three different methods of fabrication of FeCo soft magnetic material using ferromagnetic powders are compared: (i) simple sintering of elemental powders of Fe and Co, (ii) sintering of mechanically alloying FeCo powder, and (iii) sintering of chemically synthesized FeCo powder. The microstructure of ferromagnetic powders and bulk sintered alloys is characterized using scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis. The best magnetic properties with high saturation magnetization, M _s = 211.3 emu/g, are obtained for bulk FeCo alloy sintered from chemically synthesized powder. It consists of nearly spherical FeCo particles with diameters from 5 to 15 μ m. The mean particle size of chemically synthesized FeCo powder can be controlled by changing the melt composition, temperature, and process duration. The relatively large size of FeCo particles reduces the influence of surface oxidation on the particle magnetic properties. The low-cost chemical technology developed is promising for a large-scale production of small FeCo magnetic components of arbitrary shapes with high-dimensional precision.     

First-Principle Calculations of Structural,Electronic, and Magnetic Properties of Cubic Al1−x TM x N (TM = V,Cr, Mn,Fe)

We investigate the structural, electronic, and magnetic properties of zinc-blende diluted magnetic semiconductors (DMS) Al _1?x TM _x N for transition metals (TM) = V, Cr, Mn, and Fe at x = 0.125 and 0.25, using first-principle calculations with the full-potential linearized augmented plane wave (FP-LAPW) method within the density functional theory and local spin-density approximation, in an aim to predict properties of Al _1?x TM _x N compounds. We have analysed the reliance of structural parameter values on the composition x in the range of x= 0.125?0.25. Results of calculated electronic structures reveal that Al _1?x TM _x N have stable ferromagnetic ground state and they are ideal half-metallic (HM) ferromagnetic at their equilibrium lattice constants. We have also studied the magnetic moment of Al _1?x TM _x N by increasing the concentration of TM atom; this is the most important source of the total magnetic moment in these alloys, while the contributions from Al, N, and interstitial moments are minor.     

Development of P/M Fe–P soft magnetic materials

Phosphorous is treated as an impurity in conventional steels owing to segregation of phosphorous and formation of brittle phosphides along the grain boundaries. It is responsible for cold and hot shortness in wrought steels. In conventional powder metallurgy, involving compaction and sintering, high phosphorous content (up to 0·7%) in Fe-based alloys exhibit attractive set of mechanical and magnetic properties. These powder-processed alloys suffer from increasing volumetric shrinkage during sintering as phosphorous is increased beyond 0·6%. Thus both cast as well as conventional powder metallurgy routes have their own limitations in dealing with iron?Cphosphorous alloys. Hot-powder forging was used in the present investigation for the development of high-density soft magnetic materials containing 0·3?C0·8% phosphorous to overcome these difficulties. It was observed that phosphorous addition improves the final density of the resulting product. It was further observed that hot-forged iron?Cphosphorous alloys have excellent hot/cold workability and could be easily shaped to thin strips (0·5?C1·0?mm thick) and wires (0·5?C1·0?mm diameter). The powder hot-forged alloys were characterized in terms of microstructure, porosity content/densification, hardness, soft magnetic properties and electrical resistivity. Magnetic properties such as coercivity 0·35?C1·24?Oe, saturation magnetization 14145?C17490 G and retentivity 6402?C10836 G were observed. The obtained results were discussed based on the microstructures evolved.     

Synthesis and Study of the Lanthanum Substitution in the Lead M-Type Hexaferrite

In this work we investigated the best conditions to obtain the lanthanum-substituted lead M-type hexaferrite using the ceramic method. In order to survey the solubility of lanthanum in the structure of the hexaferrite, we varied the molar fraction from 0.1 to 0.9. We performed a structural characterization using X-ray diffraction and used the Rietveld method to refine the crystal structure. The magnetic properties were obtained using vibrating sample magnetometry, and the microstructural characteristics were analyzed using scanning electron microscopy. The results indicate that the crystal structure of the hexaferrite is preserved with a substitution up to 70 %?mol of lanthanum. Moreover, the magnetic characterization exhibits a complicated behavior of parameters when lanthanum concentration increases, which can be attributed to the change of valence state of some iron cations that arises from lanthanum substitution in the hexaferrite; even more, the coercivity presents a drastic fall leading to consider these hexagonal ferrites as magnetically soft.     

Effect of oxygen nonstoichiometry on the structure, 55Mn and 57Fe NMR,electromagnetic properties,and magnetoresistance of manganese zinc ferrites

Detailed studies of manganese zinc ferrites for electronic applications are used to assess the effect of oxygen nonstoichiometry on their structural, electrical, and magnetic properties. The optimal synthesis conditions and nonstoichiometry are determined which ensure the preparation of high-permeability manganese zinc ferrites with low electromagnetic energy losses. The manganese zinc ferrites are shown to be attractive as materials for high-performance magnetoresistive electric current and magnetic field sensors and spintronic devices.     

The Electronic and Magnetic Properties of Half-metal Type MnFe x Co2?x Si (with x=0, 0.25, 0.5, 0.75 ,1, 1.25, 1.5, 1.75, and 2) Alloys

The electronic and magnetic properties of MnFe _x Co_2?x Si alloys have been calculated using density functional theory by Wien2k package, and a ferromagnetic ground state structure has been found for these alloys. The half-metallicity of the MnFe_0.25Co_1.75Si and MnFe_1.75Co_0.25Si is discussed in the light of changes in the orbital hybridization as a result of Fe and Co doping in MnCo₂Si and MnFe₂Si, respectively. The calculated magnetic moment of MnFe _x Co_2?x Si alloys shows that the effect of Si atom on magnetic properties of these compounds is negligible compared to Mn, Fe, and Co atoms. The variation of magnetic moment versus x has been investigated. By fitting the nonlinear variation of the calculated magnetic moment versus concentration with third- order polynomials, the magnetic moment bowing factor has been calculated.     

A general expression of magnetic force for soft ferromagnetic plates in complex magnetic fields

The experiments of ferromagnetic plates in different magnetic environments exhibit two distinct phenomena, i.e. the magnetoelastic instability of a ferromagnetic plate in transverse magnetic fields, and the increase of natural frequency of a ferromagnetic plate with low susceptibility in an inplane magnetic field. Although these two typical phenomena can be predicted separately by two kinds of theoretical models in which the magnetic forces are formulated by totally different expressions, no theoretical model has been found to commonly describe them. This makes it difficult to predict theoretically magnetoelastic interaction of a ferromagnetic structure in complex magnetic environment. A variational principle, here, is proposed to establish the governing equations of magnetoelastic interaction for soft ferromagnetic thin plate structures under complex magnetic fields. The functional is chosen as the summation of the magnetic energy and the strain energy as well as the external work from applied magnetic fields. From manipulations of the variational principle, the governing equations of the magnetic field and mechanical deformation together with an expression of equivalent magnetic force exerted on the ferromagnetic plates are obtained. It is shown that this theoretical model can commonly characterize the experimental phenomena of the magnetoelastic interaction aforementioned.