Magnetic behaviour of cobalt sponges in cemented carbides

The measurement and analysis of magnetisation curves, including anhysteretic and initial curves of heterogeneous and anisotropic materials (sintered tungsten carbides of constant grain size containing 3–25 wt% Co and Co-rich alloys) are presented. Néel's law of approach to saturation magnetisation for ferromagnetics containing non-magnetic inclusions was compared with the results from an empirical Lamont-Frölich-Kennelly equation and the standard law of approach to saturation magnetisation. Internal demagnetising factors N_if, where f refers to the magnetic fraction obtained from inverse anhysteretic susceptibilities (H ≈ 0) X_af^-1 were compared with those from high-field inverse susceptibilities A_f (LFK equati on) and calculated high-field demagnetising factors a/M_sf (Néel theory); all obtained as a function of volume fraction V of WC. A_f and a/M_sf both increase to a maximum at V ≈ 0.9 and then decrease. A_f is twice the magnitude of a/M_sf at the maximum, having a smaller difference at lower V. A_f is also twice the magnitude of N_if being an increasing function of applied field and is interpreted to be a high-field demagnetising factor. Assuming the Néel equation to account for voids, the LFK equation is interpreted to measure the effects of voids, of demagnetisation by hard directions in hcp Co (at low V) and of demagnetising fields in Bloch walls in very thin Co films at high V. The Néel equation was also fitted directly to the results, showing that the theory derived for isotropic materials is inadequate in dealing with large magnetocrystalline anisotropies. A relationship between H_c, j_r (remanence ratios) and N_if for presintered (pressed powder) and sintered specimens with a range of Co contents and WC grain sizes was established.     

Quantitative analysis of WC grain shape in sintered WC-Co cemented carbides

The morphology of WC grains is examined by ab initio calculations, and by transmission electron microscopy in a WC-Co cemented carbide. Two compositions are studied to determine the effect of carbon chemical potential on the shape development. The WC grains are found to be truncated triangles, and the grain morphology is adequately reproduced by the ab initio calculations. The energy difference between the two types of grain facets is shown to be due to the formation of stronger bonds with more covalent character between low coordinated W and Co atoms at one type of facet.     

Methods for atom probe analysis of microgradients in functionally graded cemented carbides

Methods to prepare needle-shaped specimens for atom probe field ion microscopy from near surface regions have been developed. The material used was a cemented carbide with a composition gradient towards the surface, but the method is equally applicable for other materials. The preparation technique involves dimple grinding, electropolishing and focused ion beam (FIB) milling. The use of FIB milling allows for specimen preparation of materials which due to the preferential etching of different phases are difficult to electropolish. The technique also allows for preparation of specimens at well defined depth from the sample surface, selection of phase to be analysed, and to sharpen and re-use already analysed specimens.

Atom probe analyses of the near surface zone region in a gradient sintered WC–Ti(C,N)–TaC–Co cemented carbide are presented.     

Femtosecond laser ablation of cemented carbides: properties and tribological applications

Laser ablation with fs laser pulses was performed in air on cobalt cemented tungsten carbide by means of a Ti : sapphire laser (800 nm, 100 fs). Small and moderate fluences (2, 5, 10 J/cm²) and up to 5×10⁴ pulses per irradiated spot were used to drill holes with aspect ratios up to 10. Cross-section cuts from laser-irradiated samples were produced and they were analysed with optical microscopy and SEM. EDX analyses were carried out on selected zones. Quasi-cylindrical holes were found for 2 J/cm², whereas for 5 and 10 J/cm² irregular shapes (lobes, bottoms wider than hole entrances) were found to occur after a given number of incident pulses. Layers with modified structure were evidenced at pore walls. SEM revealed a denser structure, while EDX analyses showed uniform and almost similar contents of W, C, and Co in these layers. As a direct application, patterning of coated WC-Co was carried out with 2 J/cm² and 100 pulses per pore. The resulted surfaces were tribologically tested and these tests revealed an improved friction and wear behaviour. PACS 42.65.Cs; 79.60.Ds     

Experimental investigations and DICTRA simulations on formation of diffusion-controlled fcc-rich surface layers on cemented carbides

Wear resistant fcc-rich surface layers were produced on cemented carbides by nitridation of W-Ti-Ta-Nb-Co-C compositions at 1400 °C in nitrogen atmosphere. A 15 ± 3 μm thick (Ti,Ta,Nb,W)(C,N) top-layer formed on the surface of the cemented carbides. The driving force for formation of the fcc-rich layers was the difference in nitrogen activity between the sintering atmosphere and the cemented carbide bulk, which promoted in-diffusion of nitrogen and out-diffusion of Ti, Ta and Nb. The diffusion-controlled process was modeled by DICTRA considering that all diffusion occurred in the liquid binder phase of a dispersed system model with a labyrinth factor of λ(f) = f. Good agreement between experimental and simulations regarding layer thickness, phase fraction distribution and element profiles was obtained for the presented model.     

Possible negative refraction in two dimensional ferromagnetic wire lattice

In this paper, we present theoretical and experimental results for the indication of negative refraction in ferromagnetic metallic wire lattice. We have studied microwave transmission through a two dimensional wire lattice made of ferromagnetic metallic wires under the applied static magnetic field. We have found that, the microwave transmission were significantly changed at ferromagnetic resonance frequency region. Thus the magnetic permeability can be tuned by external dc magnetic field. Since the dielectric permittivity of metallic wire lattice is negative and can take a value close to unity then the crystal exhibits negative index of refraction at microwave region under the external magnetic field.     

关于两个量子统计模型中铁磁态平均场理论的讨论

利用平均场理论讨论了零温下具有长程库仑势的电子气体和短程相互作用的费米原子气体的铁磁相变问题,比较了两个模型中铁磁转变的不同性质.     

具有反铁磁相互作用的双铁磁链中的非线性激发

采用双子格模型和相干态表示,研究了链与链之间存在反铁磁相互作用的双铁磁链系统中的非线性激发的性质,得到了孤子的宽度,能量及有效质量。结果表明在不同情况下的各向异性双铁磁链中可以激发拓扑性孤子或非拓性孤子或非拓生反孤子。     

The effect of two ferromagnetic metal stripes on valley polarization of electrons in a graphene

As the key factor for designing the valleytronic devices is to well understand the valley-dependent transport mechanism in graphene, we investigate, in this work, the effect of two ferromagnetic (FM) metal stripes on the valley polarization in a graphene nanostructure with a strain. The nearly 100% valley polarization is observed at certain energy windows and it can be easily controlled through changing the width and the position of the FM stripe as well as the strength of the magnetic field induced by the FM stripe. Our interesting findings reveal the valley-dependent transport mechanism of electrons and promote the realization of the new types of valleytronic devices modulated by the FM stripe and the strain.     

Preparation and properties of ferromagnetic Z-type hexaferrite from wet milled mixtures of intermediates

Almost pure Z-type phases of hexaferrites were synthesized by firing preliminarily milled M- and Y-type phases intermediates at 1250 °C. These phases were obtained by calcining the stoichiometric powder mixture precursors at 1080 °C, followed by wet milling in a planetary mill for 1 h and subsequent heating at 1250 °C that increased the fractional crystallization of the Z-phase up to 96%. Addition of 0.2 wt% SiO₂ to the intermediates reduced the milling time necessary for the sintered density required for practical permeability measurement. Z-phase hexaferrite sintered at 1250 °C for 2 h exhibited fairly good high-frequency properties, i.e. an initial real permeability of up to 19.3 below 100 MHz and 8–10 at around 1 GHz.     

Forced ferromagnetic ordering in systems with magnetic ions of two types

The distribution of magnetization in domain walls of ferromagnetic (FM) crystals with two order parameters interacting with each other—F_1z and F_2z, is considered. In one case the interaction is represented as (F_1zF_2z) and in another—(F_1z²F_2z²). It is shown that in the former case the magnetization distribution has space amplitude modulation, and in latter case appears space frequency modulation. Changes of the paramagnetic-ferromagnetic phase transition temperature in view of interaction of two FM subsystems have been studied.     

两种气泡混合的声空化

将非线性声波方程和改进的Rayleigh-Plesset方程联立可以描述空化环境中的声场及相应的气泡动力学特征. 用时域有限差分方法模拟了圆柱形容器内两种气泡相互混合时的空化情况. 在烧杯内的稳态背景声场形成过程中, 瓶壁耗散吸收扮演了重要的角色. 在稳态背景声场的基础上, 分析了混合气泡与声场的相互作用、气泡之间的相互作用、混合情况下的频谱特性. 结果表明: 两种气泡平衡半径都不太大时, 气泡与声场的相互作用不强, 声场及气泡的行为也比较规律; 相反, 当其中一种气泡平衡半径相对比较大时, 声场与气泡具有较强的非线性相互作用, 声场及气泡的行为表现出复杂的特性.     

The Nernst-Ettingshausen effect in ferromagnetic metals

A formula is given which relates the Nernst-Ettingshausen coefficient to the Hall effect and the thermoelectric power, assuming the “side jump” mechanism for the Hall effect to be dominant. Using this formula, the experimental data can be analysed to give the variation of the Hall effect with electron energy. Results of this analysis are consistent with observed behaviour of the Hall effect on alloying.     

The two dimensional classical anisotropic Heisenberg ferromagnetic model with nearest- and next-nearest neighbor interactions

We use the self-consistent harmonic approximation (SCHA) to study the two-dimensional classical Heisenberg anisotropic (easy-plane) ferromagnetic model including nearest- and next-nearest neighbor exchange interactions. For temperatures much lower than the Kosterlitz-Thouless phase transition temperature T _KT, spin waves must be the most relevant excitations in the system and the SCHA must account for its behavior. However, for temperatures near T _KT, we should expect vortex pairs to be quite important. The effect of these vortex excitations on the phase transition temperature is included in our theory as a renormalization of the exchange interactions. Then, combining the SCHA theory to the renormalization effect due to vortex pairs, we calculate the dependence of T _KT as a function of the easy-plane anisotropies and exchange interactions. Received 3 April 2001 and Received in final form 20 September 2001     

Nuclear spin ferromagnetic phase transition in an interacting two dimensional electron gas

Electrons in a two-dimensional semiconducting heterostructure interact with nuclear spins via the hyperfine interaction. Using a a Kondo lattice formulation of the electron-nuclear-spin interaction, we show that the nuclear-spin system within an interacting two-dimensional electron gas undergoes a ferromagnetic phase transition at finite temperatures. We find that electron-electron interactions and non-Fermi liquid behavior substantially enhance the nuclear-spin Curie temperature into the mK range with decreasing electron density.     

First order phase transition in the plane rotator ferromagnetic model in two dimensions

We show that the two-dimensional isotropic ferromagnetic rotator model exhibits a first order phase transition if the interaction decays asr ^– with 2<<4.Work supported by Fonds National Suisse de la Recherche Scientifique.     

The theory of ferromagnetic resonance in metal whiskers

The paper describes the dependence of surface impedance on the external static field of a longitudinally magnetized cylinder. In agreement with the experimental arrangement, the whisker is located in the centre of a wave-guide, at the point of maximum electric component of the h.f. field. In conclusion, the asymptotic solution, i.e. when the cylindrical problem becomes a plane problem, is discussed.In conclusion, the author thanks Dr. Z. Frait, Dr. J. Kaczér and Dr. M. Ondris for valuable advice and remarks.     

铁磁/半导体(绝缘体)/铁磁异质结中渡越时间与两铁磁层磁矩夹角变化的关系

在群速度概念的基础上, 研究了自旋极化电子隧穿通过铁磁体/半导体(绝缘体)/铁磁体异质结时, 渡越时间随两端铁磁层中磁矩夹角变化的关系. 研究结果表明: 当中间层为半导体层时, 由于半导体层中的Rashba自旋轨道耦合强度的影响, 自旋向上电子和自旋向下电子的渡越时间差会在两铁磁层相对磁矩夹角为π/2和3π/2附近出现一个极小值. 当中间层为绝缘体层时, 势垒高度的变化会导致不同取向的自旋极化电子渡越时间差的变化, 并当势垒高度超过一临界值时发生翻转. 关键词： 铁磁体/半导体(绝缘体)/铁磁体异质结 Rashba自旋轨道耦合强度 渡越时间 磁矩