Angle-dependent magnetoresistance oscillations and Fermi surface reordering at high magnetic fields is α-(ET)2KHg(SCN)4

Angle dependent magnetoresistance oscillations (AMRO) have been studied in the charge transfer salt α-(ET)₂KHg(SCN)₄ for magnetic fields in the range 0 - 30T. This salt exhibits the onset of antiferromagnetic order at temperatures T_N 8-10 K and the presence below this temperature of a region of sharp negative magnetoresistance at a field around 22 T known as the 'kink'. AMRO have been measured in this salt for a wide range of applied fields since the period, amplitude, and nature of the oscillations can be used to directly infer the character of the Fermi surface (FS) as a function of field. The data indicate that a profound change in the band structure occurs at this kink transition; the high field phase is characterised by quasi-2D oscillations from a closed cylindrical FS which is elongated in the c direction; the low field phase appears to be a spin density wave groundstate, with a FS consisting of a sheet (which is quasi-1D in character and tilted at an angle of 21° to the b^*c plane) and small closed 2D pockets. It is suggested that the breakdown orbits between the pockets and the 1D sheets are able to account for the various Shubnikov-de Haas frequencies observed below the kink.     

Revisiting the superconducting phase diagram of(TMTSF)2ClO4

We present results of a search for evidence of reentrance at high magnetic field of the superconducting state in the quasi-one dimensional organic superconductor (TMTSF)₂ClO₄, for a specific orientation of the field at low temperature. This search was motivated by the theoretical work of Lebed' and of Dupuis and Montambaux, and in part by that of Te anovic and coworkers, who predict novel magnetic field-temperature phase diagrams in quasi-1-D and quasi-2-D superconductors in a strong magnetic field. In the quasi-1-D case, a field directed both normal to the chains and along the layers is proposed to induce exotic oscillations of the critical temperature at high field, while for the 2-D case, quantum effects associated with Landau quantization lead to similar predictions. We have investigated the superconducting transition in a quasi-1-D candidate material, (TMTSF)₂ClO₄, to 0.06 K with field oriented along the b′-axis (± 0.02°). From mostly constant field temperature sweeps, we find positive curvature in H_c2(T) below about T_c/2. In addition, a resistivity decrease is observed to persist to 7 T, the highest field employed, with a slightly positive slope ∂H/∂T above 5T. The possibility of these features being representative of high field superconductivity is discussed.     

Magnetic field studies of the peculiar electronic state in the α-(BEDT-TTF)2MHg(SCN)4 family

Magnetoresistance anomalies exhibited by the members of the α-(BEDT-TTF)₂MHg(SCN)₄ family with M = Tl, K and Rb are reviewed in connection with possible modification of their electronic structure at low temperatures. Special attention is paid to angular magnetoresistance oscillations (AMRO). The analysis of the AMRO behavior at various temperatures and magnetic fields, both below and above the phase transition temperature, enables us to get an important information about changes in the electronic band structure at the phase transition. Other peculiarities of both semi-classical magnetoresistance and quantum oscillations are discussed within the proposed model of the low-temperature state of these compounds.     

Effect of weak and high magnetic fields in longitudinal and transverse configurations on magneto-thermoelectric properties of quantum Bi wires

We report on the magneto-thermopower of single-crystal Bi nanowires with diameters 75 and 250 nm in a longitudinal and transverse magnetic fields of 0–14 T. The temperature range is 1.5–300 K. Bi nanowires in a glass capillary have been prepared by the high frequency liquid phase casting. The temperature dependence R(T) shows the transition from metallic to semiconducting behavior due to quantum size effect, where the Bi-wire diameter is reduced to less than 80 nm. It is for the first time that the effect of the negative magneto-resistance in a transverse magnetic field, due to the quantum size effect on 75 nm Bi wires, has been observed. The thermopower is very sensitive to the wire diameter, up to a change in the sign from negative to positive at low temperatures, and to a significant extent in a weak longitudinal magnetic field. The field dependences of longitudinal and transverse magneto-resistance have features characteristic of the occurrence of the quantum size effect and galvanomagnetic size effect, and provide information on the parameters of the energy spectrum and charge carrier mobility. The enhancement of the thermoelectric figure of merit for Bi nanowires is discussed. We also discuss the power factor α²σ and its dependence on the diameter, magnetic field and temperature.     

Anomalous damping effects of magneto-quantum oscillations in the extremely 2D electronic system κ-(BEDT-TTF)2I3

The electronic system of the organic superconductor κ-(BEDT-TTF)₂I₃ (BEDT-TTF = bis(ethylenedithiolo)tetrathiafulvalene) is identified as extremely two-dimensional (2D). The topology of the Fermi surface (FS) was investigated by means of Shubnikov-de Haas (SdH) as well as de Haas-van Alphen (dHvA) experiments focusing on the verification of the 2D character of the system. This two-dimensionality specially takes effect as soon as the magnetic field is oriented exactly perpendicular to the conducting planes (i.e. Θ = 0°). Under such conditions strong anomalous damping effects in the field and temperature dependence of quantum oscillation amplitudes are observed. These anomalous damping effects are discussed in terms of the occurrence of quasi-particle excitations with fractional statistics (QPFS) which may only occur in extremely 2D systems at high magnetic fields and low temperatures (i.e., only when ħω_C ≫ k_BT). Taking up these requirements, the aim of this work is to quantify the extreme two-dimensionality of the electronic system of κ-(BEDT-TTF)₂I₃ and to show that the observed damping effects in fact are determined by the ratio ħω_C/k_BT. These facts may support the interpretation of the observed damping effects of quantum oscillation amplitudes at high magnetic fields, low temperatures and Θ = 0° as generated by the possible occurrence of such QPFS.     

THEORETICAL STUDY OF GIANT MAGNETO-IMPEDANCE IN AMORPHOUS FILMS WITH LONGITUDINAL MAGNETIC ANISOTROPY

1. IntroductionGiant magneto-impedance (MI) effect has been observed in ferromagnetic wires, ribbonsand filmsll--7]. Considering the potential applications of films in micromagnetic sensors forhigh frequencies, a large amount of work on experiments and theories has been done, somesuccessful results have been obtained. FOr example, MI ratio reaches as high as 700% at20MHz in CoSiB/SiOZ/Cu/SiOZ/CoSiB multilayered film.14]. The great sensitivity of thiseffect at very low external magnetic…     

Landau理论研究TiNi顺磁合金热/强磁场耦合下的马氏体相变

利用改进后的Landau理论模型研究了顺磁TiNi合金在热/强磁场耦合作用下的马氏体相变行为.利用第一原理计算了TiNi合金在不同相变剪切应变(序参量)下的Flermi面态密度,得到相变过程中磁化率和磁自由能的变化,把磁场效应引入到Landau模型中.结果表明,稳恒强磁场可使顺磁材料的相变温度(M6和T0)出现突变性的升高,这主要是因为相变驱动力随磁场增强而呈二次曲线上升规律.另外,因强磁场下马氏体变体之间的自由能差急剧增大,导致磁场对变体的促进和抑制作用变得明显,出现取向生长现象.在5 T稳恒强磁场下的TiNi合金相变实验中,利用TEM观察到了一些变体以相互垂直的二变体形式出现(即取向生长),证实了模型的结果.     

碳化硅复合材料与碳钢钎焊接头的抗剪强度及微观结构

采用磁控溅射镀膜技术对碳/碳化硅复合材料(C/SiC)表面进行镀Ti金属化,以AgCu₂₈为钎料,无氧铜为中间层与碳钢进行钎焊连接. 研究无氧铜中间层、Ti膜厚度和钎焊温度对接头组织形貌和力学性能的影响. 结果表明,采用无氧铜中间层可有效降低接头的残余应力,提高接头强度,并阻挡C/SiC复合材料中的Si元素在钎焊过程中扩散至碳钢侧,防止了碳钢界面FeSi_x恶性反应层的形成. 在试验范围内,钛膜厚度和钎焊温度与接头抗剪强度之间均存在峰值关系. 860 ℃,3 μm Ti膜接头平均抗剪强度最高,达到25.5 MPa. 由剪切试样碳钢侧断口,可观察到大量平行断口方向的碳纤维和碳纤维脱粘坑. 断裂发生在C/SiC复合材料内部距界面约300 μm处. C/SiC界面反应产物以Ti₅Si₃为主,含少量TiC. 钎缝中有TiCuSi相生成.     

3D Simulation of magnetic field distribution in electromagnetic forming systems with field-shaper

Impulse electromagnetic forming (IEMF) is an effective and powerful technique widely used for joining and shaping metals and field-shaper is a main part of the electromagnetic forming which has important effect on the distribution of magnetic field. In this technique, a metal work-piece is pushed to a die and formed by a pressure created using an intensive, transient magnetic field. This magnetic field is produced by passing a pulse of electric current through a forming coil in a pulsed power circuit. The produced transient magnetic field induces eddy currents in the surface of work-piece. Induced eddy currents in work-piece produce a magnetic field with reverse direction of initial magnetic field; this results in a mutual repulsion between coil and work-piece and in this way the work-piece is thrown toward the die. In this process created magnetic forces applied to work-piece are much like uniform, but in real applications, some regions of a work-piece have to be more deformed and therefore a much greater pressure has to be applied to these regions. The task of concentration of magnetic forces to some desired regions can be accomplished using field-shapers. Yu et al. [Yu, H., Li, C., Zhao, Z., Li, Z., 2005. Effects of field-shaper on magnetic pressure in electromagnetic forming. J. Mater. Process. Technol. 168, 245–249] have recently shown the effect of field-shaper on the distribution of the magnetic fields in electromagnetic forming, but because of the nature of 2D simulations some edge effects in real geometries could not be taken into consideration. In this paper, a 3D simulation using the FEA software MAXWELL has been applied to study the magnetic field distribution during an impulse electromagnetic forming process. Comparison of the 3D and 2D simulation results indicates that the maximum magnetic fields achieved in front of nodules of the field-shaper are about 15% stronger than those expected by 2D simulations.By changing the geometry of the field-shaper, the influence of the shape of the field-shaper on the distribution of the applied forces on the work-piece has been studied. Based on these simulations, some simple guidelines to design the field-shaper have been derived.     

The role of anisotropic thermal expansion of shape memory alloys in their functional properties

The functional properties of a ferromagnetic shape memory alloy, which undergoes martensitic transformation of a cubic-tetragonal type, have been analyzed using the Landau theory of phase transitions. A key role of the temperature dependency of the crystal lattice parameters in the formation of the elastic and magnetic properties of the martensitic phase has been determined. To this end the temperature dependencies of the shear elastic moduli and magnetic anisotropy energy densities (MAED) of two quasi-stoichiometric Ni–Mn–Ga alloys have been computed from the experimental temperature dependencies of their lattice parameters. It turned out that the differences in the temperature dependencies of the lattice parameters of these alloys gave rise to drastic differences in their shear moduli and MAED as a function of temperature. The possibility of the spontaneous reorientation of the magnetization vector on cooling of the alloy and a right-angle realignment of this vector by a low magnetic field have been predicted.     

Resonance modes of two-layer exchange-coupled ferromagnetic film

Ferromagnetic and spin-wave resonances in two-layer exchange-coupled ferromagnetic films have been investigated numerically at different intensities of a magnetic field when it is directed in parallel or perpendicular to the film plane. Layers of the film have finite thicknesses and possess anisotropy of the easy-plane and easyaxis types. It has been shown that at a nonzero parameter of interlayer exchange coupling the dynamic component of magnetization upon ferromagnetic resonance is distributed nonuniformly across the film thickness. Its change has been described when the external magnetic field decreases from the saturating field to zero.     

Terahertz Magneto-Optical Absorption by Cylindrical Metallic Nanowires

运用Kubo公式对圆柱形金属纳米线的磁光响应特性进行了理论研究。结果发现：与具有抛物线型电子限制势的低维半导体量子线不同,金属纳米线的磁光吸收谱以及相位延迟响应都敏感地依赖于光的偏振,这主要是由于不同的偏振光具有不同的磁光选择定则所致。另外,电子在不同子能带间磁光跃迁所产生的光吸收在太赫兹频率范围,可以通过改变纳米线半径以及调节磁场强度来有效调节金属纳米线在太赫兹频段的共振磁光吸收特性。这些理论结果揭示了金属纳米线的基本磁光响应特性     

High-frequency properties of two-layer exchange-coupled ferromagnetic structures

A numerical study of peculiarities of ferromagnetic resonance (FMR) in a two-layer exchange-coupled ferromagnetic structure has been performed at various magnitudes and directions of an external magnetic field. The layers have a finite thickness and are characterized by an anisotropy of the easy-plane or easy-axis type. The FMR frequencies have been found to decrease relative to the frequencies calculated in the model of an infinitely thin film. It has been demonstrated that with an increase in the layer thickness the direction of the displacement of the lower FMR mode depends on the magnitude and direction of the external magnetic field and that the frequency of the higher mode decreases at any direction of the external field. The amplitudes of the resonance curves decrease with increasing magnetic field, with the ratio of the half-width of the resonance curve to the resonance frequency decreasing up to the saturation point, after which this parameter varies only slightly. The FMR frequencies have been studied as a function of the constant of the interlayer exchange interaction for fields located in the saturation region.     

Magnetooptical and magnetic properties of Fe/ZnTe/Fe nanoheterostructures: Manifestation of interlayer exchange coupling

Magnetooptical and magnetic properties of Fe/ZnTe bilayers and Fe/ZnTe/Fe sandwiches with a variable thickness of the semiconductor layer t _ZnTe = 6–24 Å prepared by magnetron sputtering on Si(100) substrates at room temperature have been studied. In the sandwiches, two ranges of thicknesses, t _ZnTe = 8–12 and 20–23 Å, were found at which the magnitude of the magnetorefractive effect (MRE) approximately twofold exceeds the magnitude of the MRE in the bilayers. In the field dependences of the transverse Kerr effect at ZnTe thicknesses corresponding to the greatest values of the MRE, two features have been found in the form of kinks (steps) which are absent in the case of bilayers. At the same thicknesses of the semiconductor layer, the magnetic measurements of trilayers revealed a sharp decrease in the imaginary component of the ac susceptibility with increasing temperature from 10 to 300 K. A conclusion is drawn on the appearance of a weak interlayer exchange coupling of the antiferromagnetic type in the Fe/ZnTe/Fe system with increasing temperature.     

Anisotropic ac field screening of superconducting κ-(BEDT-TTF)2CU[N(CN)2]X (X = Br, Cl)

We measured the ac field screening of title compounds in the superconducting mixed state under dc magnetic field for two different configurations with respect to an ac field and the conducting plane. The signal exhibits anisotropic behavior for both salts, which is explained in terms of the suppression of the phase coherence among superconducting layers above the interlayer decoupling temperature T_d.     

MAGNETORESISTANCE EFFECT OBSERVED IN Fe／Mo MULTILAYERS PREPARED BY ELECTRON BEAM EVAPORATION

The Fe/Mo multilayers were prepared by electron beam evaporation, the micro structure and magnetic properties of the multilayers were studied by X-ray diffraction, vibrating-sample magnetometer (VSM) et al. The experimental results revealed that the Fe/Mo multilayers in our experimental conditions behaved magnetoresistance effect with a sharp peak on magnetoresistance (MR) ratio curve, and magnetoresistance is easily saturated at low applied magnetic fields. For [Fe(1.5nm)/Mo(1.0nm)]4,2 multilayers, MR ratio could arrive to 0.1%. The antiferromagnetic interlayer coupling could be observed in some films at room temperature. The strength of the antiferromagnetic interlayer coupling J in the films is low because of the low saturation field Hs. The relationship between magnetic properties and micro structure was also discussed in this paper.     

Metal (3d)–organic (2p)-hybrid magnets made of Mn(II) ions with tris(aminoxyl) radicals (R’s) as bridging ligands: 2D complexes [{Mn(hfac)2}3·R2]

The magnetic properties of three 2D-network complexes [{Mn(hfac)₂}₃·R₂] have been studied. Tris(aminoxyl) radicals R’s have quartet ground states and contribute not only to the formation of the extended network structures but essentially to the overall magnetism. Antiferromagnetic interactions between Mn(II) and the aminoxyl radicals and ferromagnetic intraradical aminoxyl–aminoxyl interactions both within the layer are responsible for the development of the characteristic magnetic properties in these heterospin systems. Depending on the nature of interlayer interactions, they show either ferro/ferrimagnetic or antiferromagnetic long-range order. The hierarchy of the different exchange interactions is established and the Mn–aminoxyl exchange integrals were evaluated from analysis of the temperature dependence of their paramagnetic susceptibility. With increasing intraradical exchange interaction, the complexes exhibit a clearer 2D behavior.     

Zeeman study of electronic Raman transitions in Nd2CuO4

We have measured electronic Raman transitions involving the ⁴I_9/2 and ⁴I_13/2 multiplets of Nd³⁺ in Nd₂CuO₄ single crystals as a function of magnetic field up to 14 T. To account for the main features of 4f excitations observed we have used a single-ion Hamiltonian including the crystal field potential, the exchange interaction Nd–Cu and a Zeeman term as well as standard second-order theory of Raman intensities.     

Single-atom spintronics

Recent work on magnetic quantum point contacts （QPCs） was discussed. Complete magnetoresistance loops across Co QPCs as small as a single atom was measured. The remarkable feature of these QPCs is the rapid oscillatory decay in magnetoresistance with the increase of contact size. In addition, stepwise or quantum magnetoresistance loops are observed, resulting from varying transmission probability of the available discrete conductance channels because the sample is cycled between the ferromagnetic （F） and antiferromagnetic （AF） aligned states. Quantized conductance combined with spin dependent transmission of electron waves gives rise to a multi-channel system with a quantum domain wall acting as a valve, i.e., a quantum spin-valve. Behavior of a few-atom QPC is built on the behavior of a single-atom QPC and hence the summarization of results as ＇single-atom spintronics＇. An evolutionary trace of spin-dependent electron transmission from a single atom to bulk is provided, the requisite hallmarks of artefact-flee magnetoresistance is established across a QPC - stepwise or quantum magnetoresistance loops and size dependent oscillatory magnetoresistance.