Magnetostatic interactions in dense nanowire arrays

We demonstrate the importance of magnetostatic interactions in dense arrays of ferromagnetic nanowires. Beginning from a simple micromagnetic model, we have calculated the interaction field for saturated magnetization in the plane of the array (perpendicular to the axes of the wires) and normal to the plane, using a hybrid (numerical and analytical) strategy. The slope of interaction field versus wire length changes dramatically at the transition between a dipolar regime (at very small lengths) and a monopolar regime (for longer nanowires). We present the interaction fields and the applied fields at saturation for large nanowire arrays. These results are compared with experiment for electrodeposited arrays, and very good agreement is obtained. This shows that the high field behavior of such arrays is dominated by magnetostatic effects and that a nanowire array behaves like a double-sided distribution of magnetic monopoles.     

Contribution of magnetostatic interaction to magnetization reversal of Fe3Pt nanowires arrays: A micromagnetic simulation

Using the micromagnetic simulations, we have investigated the magnetization reversal and magnetostatic interaction of Fe₃Pt nanowires arrays with wire diameters lower than 40 nm. By changing the number of interacting nanowires, N, interwire distance, a, and wire diameter, D, the effects of magnetostatic interaction on coercivity and remanence are investigated in detail. According to the simulated results, the contribution to the stray field induced by surface perpendicular magnetization at the end of wires is established.     

Study on magnetization reversal of cobalt nanowire arrays by magnetic force microscopy

The magnetic properties of self-assembly cobalt nanowire arrays formed in anodic porous alumina template were investigated by nanosize imaging method and macroscopic magnetic measurement. We have successfully made a wire-by-wire observation of magnetization reversal of a cobalt nanowire array using magnetic force microscopy with a home-made FePt tip. The nanowires in this medium have uniaxial anisotropy with easy axis along the wire due to the large aspect ratio of the wires (30 nm in diameter and 300 nm in length). Considering the nanowires as single-domain structures, we can obtain the average DC demagnetization curve from nanosize images by calculating the number of wires in each magnetized direction, and the results agreed well with the DC demagnetization curve measured by macroscopic measurement. The magnetostatic field between wires was evaluated by a new nanosize imaging method. Macroscopic measurement shows that reversible magnetization occurs in this medium. Nanosize images of the remanent and saturated states prove that the reversible magnetization processes mainly take place inside individual wires and reversed wires induced by magnetostatic field just give a little contribution to the reversible magnetization.     

Temperature dependence of the effective anisotropy in Ni nanowire arrays

Magnetic hysteresis in Ni nanowire arrays grown by electrodeposition inside the pores of anodic alumina templates is studied as a function of temperature in the range between 5 K and 300 K. Nanowires with different diameters, aspect ratios, inter-wire distance in the array and surface condition (smooth and rough) are synthesized. These microstructure parameters are linked to the different free magnetic energy contributions determining coercivity and the controlling magnetization reversal mechanisms. Coercivity increases with temperature in arrays of nanowires with rough surfaces and small diameters ─33 nm and 65 nm─ when measured without removing the alumina template and/or the Al substrate. For thicker wires ─200 nm in diameter and relatively smooth surfaces─ measured without the Al substrate, coercivity decreases as temperature rises. These temperature dependences of magnetic hysteresis are described in terms of an effective magnetic anisotropy K_a, resulting from the interplay of magnetocrystalline, magnetoelastic and shape anisotropies, together with the magnetostatic interaction energy density between nanowires in the array. The experimentally determined coercive fields are compared with results of micromagnetic calculations, performed considering the magnetization reversal mode acting in each studied array and microstructure parameters. A method is proposed to roughly estimate the value of K_a experimentally, from the hysteresis loops measured at different temperatures. These measured values are in agreement with theoretical calculations. The observed temperature dependence of coercivity does not arise from an intrinsic property of pure Ni but from the nanowires surface roughness and the way the array is measured, with or without the alumina template and/or the aluminum support.     

Creation of helical vortices during magnetization of aligned carbon nanotubes filled with Fe: theory and experiment

We report a novel magnetic phenomenon consisting of the formation of helical spin configurations during the magnetization of densely packed ferromagnetic nanowires encapsulated inside carbon nanotubes. We studied the hysteresis loops when the magnetic fields are applied parallel and perpendicular to the nanotubes axes. We also performed theoretical calculations on aligned nanowire arrays that clearly indicate the creation of helical spin vortices in the hysteresis loops. The latter are caused by the presence of strong dipolar interactions among neighboring wires.     

Template synthesis of single-crystal Cu nanowire arrays by electrodeposition

Cu nanowire arrays have been synthesized using potentiostatic electrodeposition within the confined nanochannels of a porous anodic alumina membrane. The Cu nanowire arrays and the individual nanowires have been characterized using SEM, TEM, SAED, HREM and XRD. Investigation results reveal that the Cu nanowire arrays having high wire packing densities are highly ordered over large areas. The individual Cu nanowires (diameter ∼60 nm) were single-crystal and found to be dense and continuous with uniform diameters throughout their entire length. An optimum ECD condition (at lower overpotentials) for the synthesis of single-crystal Cu nanowires was also discussed. Received: 19 April 2001 / Accepted: 28 April 2001 / Published online: 20 June 2001     

镍锰铁氧体纳米线阵列的制备与表征

采用真空负压灌注技术, 结合溶胶-凝胶法在多孔氧化铝模板的纳米孔洞中成功制备了平均直径为80 nm左右的Ni_{1- x}Mn_xFe₂O₄(x=0, 0.25, 0.5, 0.75) 纳米线阵列. XRD结果显示所制备的纳米线阵列为立方尖晶石结构, SEM和TEM的结果表明纳米线是由大量不同晶体取向的亚微晶粒联接组成. 磁测量结果显示, 随着Mn掺杂浓度的增加, 饱和磁化强度先增加而后减小, 这种变化与离子在尖晶石结构中的替代、占位变化有关. 相比于块体材料的NiFe₂O₄, 由于非线性磁结构比例的增加, 导致了线体NiFe₂O₄的饱和磁化强度降低.     

Magnetic and magnetization properties of electrodeposited fcc CoPt nanowire arrays

Magnetic and magnetization properties of fcc Co_1−xPt_x (x?0.3) alloy nanowires fabricated by electrodeposition into self-synthesized anodic alumina templates are investigated. Magnetization curves, measured for varying wire geometries, show a crossover of easy axis of magnetization from parallel to perpendicular to the nanowire axis as a function of the diameter and length. The measured values of coercivity (H_c) and remanent squareness (SQ) of CoPt nanowire arrays, as a function of angle (θ) between the field and wire axis, support the crossover of easy axis of magnetization. The curling mode of the magnetization reversal process is observed for CoPt nanowire arrays. At low temperatures, the easy axis for magnetization of the nanowires is observed to deviate from the room-temperature orientation.     

Fabrication and magnetic properties of Fe3O4 nanowire arrays in different diameters

Fe₃O₄ nanowire arrays with different diameters of D=50, 100, 150 and 200 nm were prepared in anodic aluminum oxide (AAO) templates by an electrodeposition method followed by heat-treating processes. A vibrating sample magnetometer (VSM) and a Quantum Design SQUID MPMS magnetometer were used to investigate the magnetic properties. At room temperature the nanowire arrays change from superparamagnetism to ferromagnetism as the diameter increases from 50 to 200 nm. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements show that the blocking temperature T_B increases with the diameter of nanowire. The ZFC curves of D=50 nm nanowire arrays under different applied fields (H) were measured and a power relationship between T_B and H were found. The temperature dependence of coercivity below T_B was also investigated. Mössbauer spectra and micromagnetic simulation were used to study the micro-magnetic structure of nanowire arrays and the static distribution of magnetic moments of D=200 nm nanowire arrays was investigated. The unique magnetic behaviors were interpreted by the competition of the demagnetization energy of quasi-one-dimensional nanostructures and the magnetocrystalline anisotropy energy of particles in nanowires.     

Magnetic characterization of FeCo nanowire arrays by first-order reversal curves

FeCo nanowire arrays have been obtained by current pulse electrodeposition into nanoporous alumina templates. First-order reversal curve (FORC) diagrams have been used to investigate magnetostatic interaction and average coercivity of individual FeCo nanowires embedded in porous alumina templates. The FeCo nanowires with a wires length up to 3 μm and wires diameter ranging from 25 to 50 nm showed interacting single-domain behavior. Using FORC diagrams, the spread of coercivity distribution was seen to be almost independent of the wires diameter, but with increase in diameter the inter-wire magnetostatic interaction was increased. It was found that for arrays with higher diameter, the coercivity of the arrays is lower than the average coercivity of the individual wires. It was detected that an increase in wire diameter results in a considerable increase in the spread of the distribution in the H_u direction of FORC distribution. Curve fitting on the experimental data proved a relatively linear relation between interaction field and square diameter of the nanowires.     

Magnetic properties of amorphous β-FeOOH nanowire arrays

β-FeOOH nanowire arrays with diameters of 50–200 nm have been fabricated by electrochemical deposition using two-step anodic porous alumina templates. The as-prepared nanowires are homogeneous and have large aspect ratios. The selected area electron diffraction photo performed on a single wire was used to confirm the amorphous crystal structure further. The magnetic properties of these nanowire arrays were firstly investigated by using a SQUID magnetometry. The ZFC and FC studies show that these nanowire arrays exhibit spin-freezing phenomena at low temperature. The temperature-dependent magnetization curves show that the Neel transition temperatures are much lower than that of bulk material. Moreover, hysteresis was found at 5 K and the coercivities up to about 1500 Oe. The size-dependent magnetic properties were also investigated. These abnormal magnetic behaviours can be interpreted in terms of the amorphous crystal structure and the low dimensionality of the nanowire arrays.     

Synthesis and morphology of silicon oxide nanowires from a free jet activated by electron-beam plasma

Silicon oxide nanowires were synthesized from monosilane–argon–hydrogen mixture by the gas-jet electron-beam plasma chemical deposition method with simultaneous oxygen injection into the vacuum chamber. The synthesis was performed on monocrystalline silicon substrates covered with micron and nanometer tin catalyst particles. The nanowires are formed the via vapor–liquid–solid mechanism in the “catalyst-on-bottom” mode, in which many nanowires grow from one catalyst particle. The process of synthesizing nanowires on a substrate with catalyst consists of three stages: heating to synthesis temperature, hydrogen plasma treatment, and nanowire growth. In the substrate region corresponding to the jet axis, different structures are formed depending on the catalyst particle size. For catalyst particles under 100 nm, there are formed structures of chaotically oriented and interlaced bundles of silica nanowires. For catalyst particles of 0.3–1 micron, there are formed oriented arrays of cylindrically shaped nanowire bundles (“microropes”). Cocoon-like structures are formed for catalyst particles of more than 1 micron.We propose a model of nanowire growth by this method, which is based on nonuniform heating of a catalyst particle by a directed plasma flow. It was found that for synthesis of oriented microrope arrays the initial tin film thickness should be less than 100 nm and the synthesis process should include a hydrogen plasma treatment stage.     

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.     

Growth and magnetic properties of single crystalline Ni nanowire arrays prepared by pulse DC electrodeposition

Highly textured Ni nanowire arrays were fabricated into anodic aluminum oxide(AAO) templates by pulse DC electrodeposition.The applied voltage and pH value of electrolytes were found strongly affecting the microstructure and magnetic properties of Ni nanowire arrays.Low applied potential and pH value both prefer to form polycrystalline fcc Ni nanowires.Increasing the applied potential or pH value favors the Ni [220] texture and even eventually forms the [220] oriented single crystal Ni wires,while exorbitan...     

Electrochemical deposition of Co nanowire arrays into self-organized titania nanotubes

Arrays of Co magnetic nanowires with hexagonal symmetry have been prepared by electrodeposition into self-aligned titania nanotubes membranes. The nanowires have been investigated by X-ray diffraction, SEM, and VSM magnetometry. Magnetization curves of arrays of Co wires indicate that the easy direction of magnetization is a direction perpendicular to the axis of the wire.     

Ni纳米线阵列的铁磁共振研究

通过改变氧化电压和酸性溶液制备了孔径、孔隙率不同的阳极氧化铝模板，用电沉积方法在模板中制备了Ni有序纳米线阵列，并用铁磁共振技术和振动样品磁强计对其进行了研究.研究表明Ni纳米线阵列存在着较强的偶极相互作用，偶极相互作用与纳米线的形状各向异性之间的竞争决定了纳米线的易磁化方向.随着纳米线阵列密度的增加，线间的偶极相互作用增加，使得纳米线易磁化方向从平行于纳米线方向渐趋向于垂直于纳米线的平面内. 关键词： 纳米线 铁磁共振 偶极相互作用 氧化铝模板     

Numerical study of guided modes in arrays of metallic nanowires

We numerically investigate the band structure and guided modes within arrays of metallic nanowires. We show that bandgaps appear for a range of array geometries and that these can be used to guide light in these structures. Values of attenuation as low as 1.7 dB/cm are predicted for arrays of silver wires at communications wavelengths. This is more than 100 times smaller than the attenuation of the surface plasmon polariton modes on a single silver nanowire.     

电化学沉积Fe单晶纳米线生长中的取向控制

利用电化学沉积方法，发现了一种能够动态地控制铁纳米线生长方向的沉积方法，利用该方法沉积了包括［110］取向，［200］取向及非晶态三种结构和取向的一维Fe纳米线阵列.对于三种纳米线阵列，测量了它们的磁特性，分析发现具有[200]择优取向纳米线阵列的方形度，各向异性特性和矫顽力都比[110]取向阵列有很大的改善. 关键词： 磁性纳米线 电化学沉积 取向控制     

Micromagnetic simulation on the dynamic susceptibility spectra of cobalt nanowires arrays: the effect of magnetostatic interaction

<正>Micromagnetic simulations have been performed to obtain the dynamic susceptibility spectra of 4×4 cobalt nanowire arrays with different spatial configurations and geometries.The susceptibility spectra of isolated wires have also been simulated for comparison purposes.It is found that the susceptibility spectrum of nanowire array bears a lot of similarities to that of an isolated wire,such as the occurrences of the edge mode and the bulk resonance mode. The simulation results also reveal that the susceptibility spectrum of nanowire array behaves like that of single isolated wire as the interwire distance grows to an extent,which is believed due to the decrease of magnetostatic interaction among nanowires,and can be further confirmed by the static magnetic hysteresis simulations.In comparison with single nanowire,magnetostatic interaction may increase or decrease the resonance frequencies of nanowire arrays assuming a certain interwire distance when the length of array increases.Our simulation results are also analysed by employing the Kittel equation and recent theoretical studies.     

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