Influence of extreme dilutions on the magnetic properties of single-domain particle aggregates

Various magnetic properties of samples of single-domain particles dispersed in a nonmagnetic matrix are examined as functions of the packing fractionp. The range of variability ofpis from 0.0003 to 0.20. The squareness ratio and the rotational and alternating hysteresis integrals change withp, while the coercive field, the initial anhysteretic susceptibility, and the areas between the remanence curves are not dependent onp. These results are interpretated as a consequence of the formation of agglomerates interacting with each other, rather than as a consequence of analogous interactions among the single-domain particles which are in the aggregate.     

Rotational hysteresis in polycrystalline alloys

Rotational hysteresis has been studied at very low rotational speeds in nearly isotropic polycrystalline materials. Measurements were made on disc shaped samples of pure iron, 1.2% C steel and mild steel. The magnitude and direction of the moment were measured as the disc was rotated starting from an initial magnetization parallel to the external field. For large angles of rotation the measurements give the usual hysteresis loss measured in continually rotating specimens. However it was also possible to measure the approach to the steady state and the response of the moment to small changes in the direction of the external field. The results are explained in terms of two models, one appropriate to low fields, below the maximum in the rotational loss curve, the other to the high field regime. At low fields the losses are due to domain wall motion, and the model predicts that the ratio of alternating to rotational loss ispisqrt{2}. At high fields the loss is thought to be due to a mechanism not present in alternating fields. This is the unstable rotation of the magnetization in each grain past its direction of difficult magnetization. To test the model Barkhausen noise measurements were made under alternating and rotating conditions. The results are compared with numerical calculations on the behaviour of 25 interacting dipoles.     

高能球磨工艺对B4C/Al复合粉体结构演变及分布均匀性的影响

制备B4C增强Al基复合材料存在的难点主要是B4C颗粒在Al基体中的均匀分布及界面结合。本研究采用卧式搅拌高能球磨法制备了B4C/Al复合粉体,研究了搅拌轴转速和球磨时间对B4C/Al复合粉体结构演变及分布均匀性的影响。结果表明,随搅拌轴转速的提高,复合粉体受磨球碰撞时所获能量增大,增强体颗粒瞬间被破碎同时使Al粉发生较大的塑性变形,随球磨时间的延长,破碎的B4C颗粒逐渐在Al基体中分散均匀并与基体焊合,利于粉体实现均匀分布和良好的界面结合。球磨过程中B4C沿颗粒棱边脆性断裂,在Al粉的冷焊变形过程中被嵌入,形成一种片状化的Al粉基体包裹B4C增强相的复合粉体。在搅拌轴转速为600/800r/min(交变转速,交变频率为1min),球磨时间为2h时,B4C/Al复合粉体的粒度得到细化,B4C颗粒在Al基体中分布均匀、界面结合紧密。     

Rotational and alternating hysteresis losses in 4% silicon-iron single crystal with the {110} surface

The rotational and the alternating hysteresis losses have been measured on small discs of 4% silicon-iron with the {110} surface, and the results have been explained on the basis of the changes in domain-structure observed under various conditions. It has been clarified that the magnetization under the rotating field or the alternating field in the <110> direction proceeds by forming finely divided 90° domain-structure and that the greater part of the hysteresis loss under such conditions can be attributed to the energy dissipation due to the annihilation of domain walls. The thickness dependence of the hysteresis losses has been explained from the changes in domain-wall spacing.     

An investigation of magnetic oxides' defects and conflict of anisotropies for diluted powders

Magnetostatic measurements and rotational hysteresis were carried out on extremely diluted samples of pure, surface modified, bulk doped iron oxides and CrO2. It is found that in all these materials but pure γ-Fe2O3a conflict of anisotropy is active. As a result, the values of remanence ratio jrcan be either lower or higher than the theoretical value of ½ for strictly uniaxial particles depending on the angle between the two anisotropy axis and the relative value of the anisotropy constants. It is therefore found that whilej_{r} < 1/2for acicular magnetite and CrO2, the introduction of cobalt always produces an increase of jr: the extreme effect of such conflict can be found in high remanence isotropic particles. Such a conflict is minimized in Surface Modified Materials. From rotational hysteresis, Hcvs. angle with applied field and, over all, from CF and IFF parameters, it is suggested that the postreatment of the iron oxides with the Co-containing solutions, beside the increase of Hc, has a beneficial effect on the morphology of the particles.     

Surface impedance of superconducting Nb3Sn

Microwave cavities with a resonant frequency of 8 GHz are coated with Nb3Sn by the vapour deposition technique. The surface resistance and the change of the penetration depth were determinded by measuring the quality factor and the shift of the resonant frequency of the cavity in the temperature range from 2 K to 20K. The temperature dependence of the surface resistance can be described well by the BCS-theory in the temperature rangeT < 0.5 T_{c}, however, the value of the reduced energy gapDelta_{0}/kT_{c}has to be increased from 1.76 to 2.15. The temperature dependence of the penetration depth shows significant deviations from the predictions of the BCS-theory for temperatureT < 0.5 T_{c}. The increase of the reduced energy gap is not sufficient to fit the data but one has to treat the effects of strong electron-phonon coupling consistently. Therefore, we calculated the surface impedance for strong coupling superconductors using an Einstein model for the phonon density of states. The absolute value and the temperature dependence of the surface impedance in the whole temperature rangeT < T_{c}are discussed and a comparison with the experimental data is given.     

Fe73.5Cu1Nb3Si13.5B9软磁粉/硅橡胶力敏复合材料的逾渗特性

以三元嵌段共聚硅橡胶为基体,Fe_73.5Cu₁Nb₃Si_13.5B₉软磁粉为应力敏感填充粒子,采用机械共混和精密铸压热成型方法制备出系列粉体含量的Fe_73.5Cu₁Nb₃Si_13.5B₉软磁粉/硅橡胶接触应力敏感复合材料,重点研究了其交流阻抗、直流电阻、粒子间及粒子与硅橡胶间相互作用程度等性能的逾渗特性。研究表明,该柔性力敏复合材料具有较显著的交流阻抗双逾渗特性。分析发现,“第一逾渗区”内材料的交流阻抗值由粒子的隧道跃迁主导,“第二逾渗区”内材料的交流阻抗大小取决于Fe_73.5Cu₁Nb₃Si_13.5B₉粒子链间的接触电阻值。     

High-field magnetism in non-polar γ-Fe2O3recording particles

Fine γ-Fe2O3particles produced by a process which involves hydrothermal conversion of iron hydroxides to α-Fe2O3have been investigated. Such particles appear to lack pores or dentrites which cause internal magnetic poles, and exhibit superior properties for magnetic recording. Mössbauer spectroscopy, x-ray diffraction, and transmission electron-microscopy were used. The particles were of length 250-500 nm and width 35-50 nm; some samples had cobalt adsorbed onto the surface. Mössbauer spectra were collected at temperatures from 4.2 to 300 K, and in zero and 5 T applied magnetic fields. In general the patterns are typical of bulk γ-Fe2O3. The 300 K spectra of Co-doped samples show sub-patterns of weak intensity associated with at least two distinct additional components with reduced splitting. One of these has Bhf= 45.2(2) T; its origin is suggested to be iron-atoms lying in or close to the cobalt-modified surface. In a 5 T field, the iron-atom moments deviate from the applied field direction by average angles in the range 13-15°. Because the angle for conventional similarly sized γ-Fe2O3issim 13deg, it is concluded that the improved particle morphology does not lead to an improvement in the high-field alignment.     

添加Ta2O5对MnZn功率铁氧体性能的影响

采用氧化物陶瓷工艺制备MnZn功率铁氧体,研究了不同Ta₂O₅含量对MnZn功率铁氧体微观结构和磁性能的影响. 结果表明：少量Ta₂O₅的加入可以使铁氧体烧结样品的晶粒尺寸增大,气孔率下降,起始磁导率、饱和磁感应强度和电阻率升高,损耗降低. 而加入过多的Ta₂O₅会导致异常晶粒长大,气孔率升高,起始磁导率、饱和磁感应强度和电阻率降低,损耗增大. 当Ta₂O₅含量为0.04wt%时,铁氧体烧结样品的晶粒尺寸大小均匀,气孔率最低,起始磁导率、饱和磁感应强度和电阻率达到最大值,损耗最低.     

High performances boron doped γ-Fe2O3particles prepared from oxalic precursors

γ-Fe₂O₃ particles (L≈0.15 to 0.2 μm, L/D≈2 to 5) with coercivity up to 400 Oe have been prepared from oxalic precursors. To preserve the particle form, the oxalates were doped with boron. The effects due to this element was studied during the different thermal treatments which effect the pseudomorphous transformation of oxalate particles into spinel ferrite. ESCA spectroscopy, and X-ray measurements could point out the presence of a Fe₃BO₅ phase at the surface of γ-Fe₂O₃ particles. In spite of the important mass losses (H₂O, CO, CO₂) and the important crystallographic structure modifications occurring during the transformation of oxalate into magnetic oxide, γ-Fe₂O ₃ particles obtained by this method have very good textural characteristics. Boron-doped γ-Fe₂O₃ particles were used to make 3.5-in diskettes with standard formulation. In these conditions, media properties were comparable to trading diskettes properties     

Propriétés des ferrites de lithium partiellement substitués par l'aluminum destinés aux dispositifs micro-ondes

The properties of lithium-alumina substituted ferrites are intended for microwave devices. It is possible to distinguish two main cases. In the first, the materials are magnetically saturated and in the second, the materials work in the hysteresis loop region. Data on lithium-alumina polycristalline materials from the family Li2O; 5(1-x) [(1-y)Fe2O3,yAl2O3];zMnO2; are given,xandzbeing chosen to optimize the dielectric properties. At room temperature, asyis varied from 0 to 0.20, the properties of these materials vary in the following manner: saturation magnetization 4π Ms, from 3770 to 1340 G; Curie temperature θcfrom 590 to 430 °C. The coercitive forceHc, the squareness ratioR, and linewidthDeltaHdepend upon the firing conditions and technology chosen. Measured values are in theHc= 1.1- to 2.5-Oe range andR= 0.75 to 0.92 range. These properties appear satisfactory for microwave devices and, in particular, for latching devices aboveS-band frequencies.     

ΔE effect and magnetomechanical coupling factor in Fe80B20and Fe78Si10B12glassy ribbons

Field annealing experiments on amorphous Fe80B20ribbon (Metglas 2605) and amorphous Fe78Si10B12ribbon (Tsuya, Tohoku University) are reported. The maximum magnetomechanical coupling constant,k, and change in Young's modulus, ΔE/E0found in Fe80B20were 0.64 and 0.87, respectively, for an anneal of 376° for 6 min in a field of 2.0T. The maximumkfor Fe78Si10B12was found to be 0.82, following a similar anneal at 373°C. This is higher than 0.75 reported by Tsuya and Arai for Fe78Si10B12, previously the highest known for any material.     

Magnetic properties of amorphous (fe, ni)80B20, (fe, ni)80B19si1, and (fe, ni)80P14B6alloys

Magnetic measurements were performed on amorphous (Fe, Ni)80B20, (Fe, Ni)80B19Si1, and (Fe, Ni)80P14B6alloy ribbons to yield the values of the magnetic moment per transition metal atom at 0 K and the Curie temperature. A close fit to the moment data, obtained by allowing not only the moments on Ni and Fe atoms to vary with Fe concentrationx, as revealed by neutron diffraction in crystalline alloys, but also the number of electrons per atom npand nbthat P and B atoms, respectively, donate to the transition metaldbands as described by the relationsn_{p} = 1.74 + 0.75 (1 - x)andn_{b} = 0.58 + 1.20 (1 - x), suggests a moment of 0 andsim0.13 mu_{B}per Ni atom in amorphous Ni80P14B6and Ni80B20alloys, respectively. The functional dependence of the Curie temperature onx, when followed closely for low Fe concentrations, gives the critical concentration xcabove which ferromagnetism appears in amorphous FexNi80-xB20and FexNi80-xP14B6alloy series asx_{c} cong 5and 8 at % Fe, respectively. The results of a theory based on coherent-potential approximation have been fitted to the measured values of Curie temperature in order to arrive at the Fe concentration dependence of the exchange interaction (JNiNi) between Ni-Ni pairs. The physical significance of the variation of JNiNiwithxobtained thereby has been discussed.     

不同TiB2颗粒粒径的TiB2/Cu复合材料耐电弧侵蚀行为

采用放电等离子烧结法（SPS）制备了不同TiB₂颗粒粒径的3wt% TiB₂/Cu复合材料,研究了3wt% TiB₂/Cu复合材料致密度、导电率、硬度和耐电弧侵蚀性能随TiB₂颗粒粒径的变化规律,重点分析了不同TiB₂颗粒粒径的3wt% TiB₂/Cu复合材料耐电弧侵蚀行为。结果表明:3wt% TiB₂/Cu复合材料致密度和硬度随TiB₂颗粒粒径的增大而略有降低;TiB₂颗粒粒径越小,TiB₂/Cu复合材料的综合性能越好。随着TiB₂颗粒粒径的增大,3wt% TiB₂/Cu复合材料耐蚀稳定性降低,3wt% TiB₂/Cu阴极材料的损耗量明显增加;当TiB₂颗粒粒径为10 μm时,3wt% TiB₂/Cu复合材料的耐电弧侵蚀性能最佳。电弧蚀形貌观察表明:不同TiB₂颗粒粒径的3wt% TiB₂/Cu复合材料经电弧侵蚀后,3wt% TiB₂/Cu复合材料均由阴极向阳极发生转移;随着TiB₂颗粒粒径的增大,阴极质量损耗逐渐增加,触头表面电弧侵蚀面积增加;而在Cu基体中引入较小的TiB₂颗粒,有利于减弱电接触实验过程中TiB₂/Cu复合材料的喷溅现象。      

Magnetic and magnetooptical properties of sputtered Fe5Si3films

Polycrystalline films of Fe5Si3have been prepared by RF sputtering from a sintered target composed of 62.5 at% Fe and 37.5 at% Si. The Faraday rotation and optical absorption were measured for these films in the visible and near infrared. At a wavelength of 6328Å, room-temperature absorption (corrected for reflection) and specific Faraday rotation are 3.7 × 10⁵cm^-1and 1.3 × 10⁵deg/cm, respectively. The rotation decreases with rising temperature and vanishes around 120°C, the Curie temperature of the films. From polar and in-plane hysteresis loop measurements the spontaneous magnetization is found to lie in the film plane. Square hysteresis loops are observed when the external field is applied in the plane of the film, and the in-plane coercive force decreases monotonically with temperature at an average rate of -2 Oe/°C as Tcis approached.     

Temperature dependence of ac-losses in multifilamentary Bi-2223 tapes between 4.2 K and Tc

The ac-losses of twisted multifilamentary Bi-2223/Ag tapes were measured in the temperature range between 4.2 K and the critical temperature T_c∼110 K. Stacks of tapes in perpendicular and in parallel field were investigated. The loss measurement is performed by the standard magnetisation technique, the absolute loss value is obtained by a calorimetric calibration measurement. For a fixed ac-field amplitude the energy loss per cycle Q is determined, at a given temperature, as a function of frequency f. The extrapolation to f→0 gives the hysteresis loss. Coupling losses are obtained from the slope of the loss curve Q(f) in the low frequency range ωτ?1. The measured total losses as a function of temperature show, at small field amplitudes, a minimum around 50 K. This is explained by the different temperature dependence of hysteresis and coupling losses. While coupling losses decrease with increasing temperature, hysteresis losses increase for field amplitudes below the penetration field. Coupling losses show a much weaker temperature dependence than the silver conductivity, which is explained by the existence of an interfacial resistance between filaments and silver matrix. Measured hysteresis losses were compared with available theoretical models and good agreement was found in parallel field.     

苯胺-SrTi(C2O4)2颗粒制备及其电流变性能

用沉淀法制备了苯胺-草酸氧钛锶（SrTi（C₂O₄）₂）颗粒。FTIR分析表明,苯胺已引入到SrTi（C₂O₄）₂颗粒的表面;XRD分析表明,苯胺-SrTi（C₂O₄）₂颗粒为含少量结晶的无定形态。采用SEM观察颗粒的形貌,结果表明,随反应体系中苯胺与钛原子物质的量比n_aniline/n_Ti的增大,苯胺-SrTi（C₂O₄）₂颗粒由近似球状（n_aniline/n_Ti =0）变为多面体状（n_aniline/n_Ti =2）,而后变为棒状与团簇状颗粒的混合体系（n_aniline/n_Ti =3）。以苯胺-SrTi（C₂O₄）₂颗粒为分散相制备颗粒质量分数为66.7%电流变液,电流变性能测试结果表明,在n_aniline/n_Ti =2时,电流变液具有较高的剪切应力和剪切屈服强度、较大的漏电流密度。苯胺在颗粒制备过程中起控制颗粒形貌的作用,在电流变液体系中起极性分子的作用,其对苯胺-SrTi（C₂O₄）₂电流变液性能的影响是两种效应综合作用的结果。     

The influence of applied tensile stress on power loss in Co-rich amorphous Co-Fe-Si-B ribbons with induced magnetic anisotropy

The influence on power loss PTof applied tensile stress σ in amorphous (Co0.89Fe0.11)72Mo3Si15B10(lambda_{s} > 0) and Co73Mo2Si15B10(lambda_{s} < 0) ribbons with different induced magnetic anisotropy Kuis reported. The losses are measured under sinusoidal flux conditions atf = 50Hz,J_{max} = 0.57T and atf = 400Hz,J_{max} = 0.10T. Measurements are carried out on samples in a stress-relieved state and with magnetic anisotropies induced by stress or field annealing. Atf = 50Hz, a minimum m Ptversus σ is observed. The σ-value (sigma_{min}) corresponding to the minimum PTincreases with increasing |Ku|. Atf = 400Hz, a minimum in PTversus σ is observed in the samples with induced magnetic anisotropy, whereassigma_{min} = 0in the stress-relieved samiales. However, no correlation between andsigma_{min}and Kuis possible from the present data.     

Temperature Stability and Flux Losses Over Time in Sintered Nd–Fe–B Permanent Magnets

Demagnetization occurring with time in a constant temperature is often assumed to be negligible, provided that the operating point of the magnet is above the knee of the $Bhbox{–}H$ curve. However, because it is important to ensure that no permanent flux losses occur in permanent magnets during their use in industrial applications, there is a clear need to quantify these time-dependent losses. We measured losses over time for four commercial sintered Nd–Fe–B magnet grades at five different temperatures (23$,^{circ}$ C–150$,^{circ}$ C). We tested samples with three different permeance coefficient (Pc) values for each material. The time-dependent losses measured fitted the logarithmic law of magnetic viscosity well. We demonstrated that the total flux loss in a lifetime of 30 years can be estimated according to the temperature, coercivity of the material, and the permeance coefficient of the magnet. With proper selection of the magnet material, in accordance with the designed Pc of the application, the total flux loss in 30 years can be minimized almost to zero even at 150$,^{circ}$ C.      

Microstructure, Electrical, and Magnetic Properties of ZrO$_{2}$ Added MnZn Ferrites

We investigated the influence of ZrO$_{2}$ on the microstructure and electromagnetic properties of MnZn ferrites by characterizing fracture surface micrographs, magnetic properties, and dc resistivity. Powders of Mn $_{0.68}$Zn $_{0.25}$Fe $_{2.07}$O $_{4}$ composition were prepared by the conventional ceramic technique. Toroidal cores were sintered at 1350 $^{circ}$C for 4 h in N$_{2}$/O$_{2}$ atmosphere with 4% oxygen. The results show that the lattice constant and average grain size increase with ZrO$_{2}$ concentration, but excessive ZrO $_{2}$ concentration will result in exaggerated grain growth and porosity increase. The dc resistivity, activation energy, saturation magnetic flux density, and initial magnetic permeability increase monotonically when the ZrO$_{2}$ concentration is not more than 0.04 wt% and then decrease with further increase of ZrO$_{2}$ concentration. On the other hand, the porosity, drift mobility, resonance frequency, and core loss decrease initially and then increase with the increase of ZrO$_{2}$ concentration.