Structural and magnetic properties of electrodeposited Cobalt nanowire arrays

Ordered magnetic nanowires have tremendous potential in future magnetic storage and high frequency magnetic logic devices. Here, we present the fabrication of ordered arrays of Cobalt nanowires by electrodeposition through porous polycarbonate membranes. Vertically and horizontally aligned nanowires were produced in presence of an external bias field during post deposition etching of the polycarbonate membrane. Structural and compositional analyses have been carried out to establish the material and structural purity. The magneto-optical Kerr effect was employed to measure the magnetic hysteresis loops for the nanowires assembled in the substrate plane. A good magneto-optical signal to noise ratio is observed with clean ferromagnetic hysteresis loops. The loops measured with external magnetic field applied parallel and perpendicular to the axis of the nanowires show a clear difference in the shape and the coercive field, indicating the effect of shape anisotropy in these samples. Micromagnetic simulations were performed to understand the experimental results and to obtain insight to the magnetization reversal mechanism in magnetic nanowires.     

Magnetization reversal dynamics in Co nanowires with competing magnetic anisotropies

We present the magnetization reversal dynamics of Co nanowires with competing magnetic anisotropies. The aspect ratio (R) of the nanowires is varied between 2.5 and 60, and we observe a cross-over of the directions of the magnetic easy and hard axes at R=6.8. Micromagnetic simulations qualitatively reproduce the observed cross-over and give detailed insight into the reversal mechanisms associated with the cross-over. The reversal mechanism for a field applied along the long axis of the nanowire exhibits a quasi-coherent rotation mode and a corkscrew-like mode, respectively, above and below the cross-over, with the formation of a Bloch domain near the cross-over region. For a field applied along the short axis, the reversal occurs by nucleation and propagation of reversed domains from the two ends of the nanowires for very high values of the aspect ratio down to the cross-over region, but it transforms into quasi-coherent rotation mode for smaller aspect ratios (below the cross-over region).     

Experimental determination technique for magnetic anisotropy of individual nanowires

We present an experimental technique to determine the magnetic anisotropy of ferromagnetic nanowires. In the technique, the magnetization state is monitored by measuring the anisotropic magnetoresistance with rotating the external magnetic field. The measured magnetoresistance curves exhibit basically the same curves typically appeared in the torque magnetometric measurements, which are then readily analyzed based on the Stoner–Wohlfarth theory with a single fitting parameter – the magnetic anisotropy. By applying the present technique to Permalloy nanowires, it is shown that the shape anisotropy in real nanowires is significantly influenced by the edge roughness.     

Ultrafine Fe-Co nanowires: Fabrication and heat treatment influence on the structure and magnetic properties

Ultrafine nanowires of Fe-Co with a diameter around 15 nm have been fabricated by electrodeposition method using anodic porous alumina as a template. The alloy nanowires were in the form of arrays and consisting of polycrystalline structures. They showed obvious shape anisotropy parallel to the axis of nanowires and the perpendicular coercivity (Hc_⊥) was found to be 2576.8 Oe which is higher than any coercivity value reported in the literature. The effects of critical factors such as heat treatment and temperature of annealing on the structure and magnetic properties of the ultrafine nanowire arrays were studied and discussed.     

Angular dependence of the coercivity and remanence of ordered arrays of Co nanowires

The angular dependence of the coercivity and remanence of ordered hexagonal arrays of Co nanowires prepared using anodic aluminum oxide templates was investigated. The experimental evolution of coercivity as a function of the angle, in which the external field is applied, is interpreted considering micromagnetic simulations. Depending on the angle between the axis of the wire and the applied magnetic field direction our results show that the magnetization reversal mode changes from vortex to a transverse domain wall. Besides, we observed that the dipolar interactions cause a reduction in coercive fields, mainly in the direction of easy magnetization of the nanowires. Good agreement between numerical and experimental data is obtained.     

消磁场对纳米铁磁线磁畴壁动力学行为的影响

为了实现基于磁畴壁运动的自旋电子学装置, 掌握磁畴壁动力学行为是重要争论之一.研究了在外磁场驱动下L-型纳米铁磁线磁畴壁的动力学行为. 通过微磁学模拟,在各种外磁场的驱动下考察了纳米铁磁线磁畴壁的动力学特性; 在较强外磁场的驱动下, 在不同厚度纳米线上考察了纳米线表面消磁场对磁畴壁动力学行为的影响. 为了进一步证实消磁场对磁畴壁动力学的影响, 在垂直于纳米线表面的外磁场辅助下分析了磁畴壁的动力学行为变化. 结果表明, 随着纳米线厚度和外驱动磁场强度的增加, 增强了纳米线表面的消磁场的形成, 使得磁畴壁内部自旋结构发生周期性变化, 导致磁畴壁在纳米线上传播时出现Walker崩溃现象. 在垂直于纳米线表面的外磁场辅助下, 发现辅助磁场可以调节消磁场的强度和方向. 这意味着利用辅助磁场可以有效地控制纳米铁磁线磁畴壁的动力学行为.     

Study of low concentrated ionic ferrrofluid stability in magnetic field by ultrasound spectroscopy

Ultrasound spectroscopy has been used to investigate the effect of magnetic field on particle (cluster) size distribution in ionic ferrofluid. The method applied is non-destructive, fast and sensitive to structural changes of a suspension as it is based on measurements of ultrasonic attenuation. Changes in the ultrasound attenuation induced by an external magnetic field have been measured for different frequencies of the acoustic wave. According to the Harker-Temple theory, the aggregation process has been analysed on the basis of the cluster size distribution determined for different magnetic field intensities.     

(In, Mn)As nanowires with ultrahigh Mn concentration: Growth, morphology and magnetic anisotropy

(In,Mn)As nanowires with ultrahigh Mn concentration have been successfully grown on GaAs(001) substrates by molecular beam epitaxy. The morphology dependences on Mn concentration and growth temperature are investigated. High Mn concentration and high growth temperature are both necessary for the growth of nanowires. All the (In,Mn)As nanowires are self-aligned along [−110]GaAs, and therefore have the shape magnetic anisotropy with the easy axis along the alignment orientation of the nanowires.     

Nanowire manipulation on surfaces through electrostatic self‐assembly and magnetic interactions

A method for fabricating aligned nanowire arrays on surfaces is shown. Gold and segmented Au/Ni/Au nanowires of high aspect ratio have been prepared by template electrosynthesis, and functionalized with charged short alkanethiols that can be ionized in aqueous solutions. Different distributions of funtionalized nanowires could be obtained on large surfaces from nanowire aqueous suspensions, avoiding aggregation due to electrical repulsion. Due to the high magnetic anisotropy of segmented Au/Ni/Au nanowires chaining of aligned nanowires could be obtained by applying a low magnetic field. While electrostatics favours side wire interactions due to the high aspect ratio, concurrent electrostatics and applied magnetic field yields end‐to‐end interaction and linear alignment without bifunctionalization. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

水基磁性液体的场诱导光衍射特性的研究

设计了一套用于研究磁性液体的场诱导静态和动态光衍射特性的实验装置.通过在样品的后方放置一个适当孔径的光阑,实现了定量研究其衍射特性的目的.采用沿衍射方向移动光阑的方法定量地研究了衍射光的空间分布.设计了一个指数衰减函数用来拟合动态衍射实验的数据,获得动态衍射的特征时间.以两种不同浓度的表面包覆型水基氧化铁磁性液体样品为例,定量地实验研究了其场诱导光衍射特性与外磁场强度和样品浓度的关系.结果表明,磁性液体的场诱导结构对光的静态和动态衍射与外磁场的强度和磁性液体的浓度有关.其静态衍射与外磁场强度呈非单调的关系|其动态衍射的特征时间与外磁场的强度成反比,与磁性液体的浓度成正比.对实验中观察到的场诱导静态和动态衍射特性的物理机理进行了详细的分析.     

磁场对模板法制备的Co纳米线结构和磁性的影响

在用二次氧化法制备的高度有序的氧化铝模板上通过交流电化学方法制备了Co纳米线阵列．研究了外加磁场及电解液pH值对纳米线生长的影响．在pH值为6.0和6.5的电解液中分别在不加磁场和沿纳米线轴向施加0.3 T磁场情况下制备了hcp结构的Co纳米线阵列．实验数据表明,沉积时外加磁场和调节pH值能有效影响纳米线中hcp结构的Co晶粒的易磁化轴沿纳米线长轴方向生长．由于晶粒的磁晶各向异性和纳米线沿长度方向的宏观形状各向异性叠加,制备的Co纳米线阵列具有高垂直各向异性,高矫顽力和较高矩形比． 关键词： Co纳米线阵列 织构 磁性     

Thickness-dependent magnetic domain structures of Co ultra-thin film investigated by scanning transmission X-ray microscopy

Thickness-dependent magnetic domain structure of ultrathin Co wedge films (0.3 nm–1.0 nm) sandwiched by Pt layers was investigated by scanning transmission x-ray microscopy (STXM) employing X-ray magnetic circular dichroism (XMCD), utilizing elliptically polarized soft x-rays and electromagnetic fields, with a spatial resolution of 50 nm. The magnetic domain images measured at the Co L₃ edge showed the evolution of the magnetic domain structures from maze-like form to the bubble-like form as the perpendicular magnetic field was applied. The asymmetric domain expansion of a 500 nm-scale bubble domain was also measured when the in-plane and perpendicular external magnetic field were applied simultaneously.     

Electronic structure of paramagnetic In<Subscript>1-x</Subscript>Mn<Subscript>x</Subscript> As nanowires

The electronic structure, spin splitting energies, and g factors of paramagnetic In_1-xMn_xAs nanowires under magnetic and electric fields are investigated theoretically including the sp-d exchange interaction between the carriers and the magnetic ion. We find that the effective g factor changes dramatically with the magnetic field. The spin splitting due to the sp-d exchange interaction counteracts the Zeeman spin splitting. The effective g factor can be tuned to zero by the external magnetic field. There is also spin splitting under an electric field due to the Rashba spin-orbit coupling which is a relativistic effect. The spin-degenerated bands split at nonzero k_z (k_z is the wave vector in the wire direction), and the spin-splitting bands cross at k_z = 0, whose k_z-positive part and negative part are symmetrical. A proper magnetic field makes the k_z-positive part and negative part of the bands asymmetrical, and the bands cross at nonzero k_z. In the absence of magnetic field, the electron Rashba coefficient increases almost linearly with the electric field, while the hole Rashba coefficient increases at first and then decreases as the electric field increases. The hole Rashba coefficient can be tuned to zero by the electric field.     

Superconducting mechanism of YBaCuO superconductors

Electromagnetic properties of YBaCuO superconductor were measured on applying an external magnetic field to study non-volatile magnetic effect. The voltage increases with increase in applied magnetic flux, but it becomes constant at about 10⁻² T. The appearance of the voltage is ascribed to the trapping of magnetic flux. By changing the density of external magnetic flux, changes in inductance of a coil in which a superconducting bar inserted were also measured. The results showed that the filament model was valid to explain the mechanism of the occurrence of a voltage in superconducting sample. It was concluded that the electromagnetic properties arose from the interaction between the trapped magnetic flux and weak link of the filament formed in the superconducting bulk.     

Magnetic properties of cobalt nanowires: Study by polarized SANS

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic cobalt nanowires embedded into Al₂O₃ matrix have been studied using polarized small-angle neutron scattering. A comprehensive analysis of contributions to the scattering intensity was carried out, including nonmagnetic (nuclear) contribution, magnetic contribution depending on the magnetic field, and nuclear-magnetic interference indicating the correlation between the magnetic and nuclear structures. Experiments have revealed an anomalously low value of the magnetic contribution as compared to the nuclear one. This behavior is interpreted in terms of low coherence of the magnetic structure caused by the anisotropy of Co crystallites as compared with the large coherency of nuclear structure of nanowires.     

Studies on magnetic photonic band-gap material at microwave frequency

The microwave transmission characteristics of magnetic photonic band-gap (MPBG) materials, in which the periodic structures are composed of alternating layers of polycarbonate with and without ferromagnetic nanowires, are studied. We present a theoretical method to investigate the transmission spectra of the MPBG material. The band-gap effect varies with the periodic parameters of the MPBG structure. Our calculation can describe well the experimental measurement value. The influence of the applied static magnetic field on the MPBG and the ferromagnetic resonance phenomenon has been studied.     

Theoretical and experimental study of Fe/Cr nanometric quasiperiodic multilayers

An experimental and theoretical study of magnetization curves of Fe/Cr nanometric magnetic films grown with the structure of the quasiperiodic Fibonacci sequence is presented. Fe ferromagnetic films with interfilm exchange coupling provided by intervening Cr non-ferromagnetic layers were grown on MgO (100) by dc magnetron sputtering at 300 ^°C. The magnetization curves were investigated using the magneto-optical Kerr effect with the external field applied along the easy axis. The theoretical approach for this system is based on a realistic phenomenological model that includes the following contributions to free magnetic energy: Zeeman, cubic magneto-crystalline anisotropy, as well as bilinear and biquadratic exchange energies. Our numerical results are in very good agreement with the experimental data.     

水基磁性液体的场诱导光衍射特性的研究

设计了一套用于研究磁性液体的场诱导静态和动态光衍射特性的实验装置.通过在样品的后方放置一个适当孔径的光阑,实现了定量研究其衍射特性的目的.采用沿衍射方向移动光阑的方法定量地研究了衍射光的空间分布.设计了一个指数衰减函数用来拟合动态衍射实验的数据,获得动态衍射的特征时间.以两种不同浓度的表面包覆型水基氧化铁磁性液体样品为例,定量地实验研究了其场诱导光衍射特性与外磁场强度和样品浓度的关系.结果表明,磁性液体的场诱导结构对光的静态和动态衍射与外磁场的强度和磁性液体的浓度有关.其静态衍射与外磁场强度呈非单调的关系;其动态衍射的特征时间与外磁场的强度成反比,与磁性液体的浓度成正比.对实验中观察到的场诱导静态和动态衍射特性的物理机理进行了详细的分析.     

Anisotropic and large low-field magnetic entropy change in a La4/3Sr5/3Mn2O7 single crystal

We have studied the magnetocaloric effect (MCE) in a bilayered La_4/3Sr_5/3Mn₂O₇ single crystal with applied field along both ab-plane and c-direction. Due to the quasi-two-dimensional structure, the crystal exhibits a strong anisotropy in the MCE. The difference of magnetic entropy change between two crystallographic directions depends on external magnetic fields and has a maximum of 2 J/kg K. A large low-field magnetic entropy change, reaching 3.2 J/kg K for a magnetic field change of 15 kOe, is observed when the applied field is along ab-plane. This large low-field magnetic entropy change is attributed to the rapid change of magnetization in response to external magnetic fields in the easy magnetizing plane.