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     

Magnetic behaviour of arrays of nickel nanowires with small diameter

Arrays of nickel nanowires have been fabricated within a template of porous alumina by electrochemical deposition. Measurements of magnetic hysteresis loops were performed at room temperature with a vibrating sample magnetometer. Coercivity and squareness of the arrays are closely related to l/D and diameter of the nanowires, and the angle θ between the normal line of the alumina surface and the applied magnetic field. For the same diameter of 10 nm, the coercivity and squareness increase remarkably with l/D when the l/D is less than 100. The diameter and angle θ dependences of coercivity do not follow the relationships of curling, fanning or coherent rotation mode of magnetization while thermal activation for magnetization reversal becomes remarkable for the arrays of Ni nanowires with the diameter less than 18 nm but the same l/D of 50. The coercivity of the arrays with the magnetic field perpendicular to the film surface is linear with D^−3/2 of Ni nanowires. From the fitting line, the critical diameter for superparamagnetism at room temperature and pure coercivity for such Ni nanowire arrays are found to be 6 nm and 1200 Oe respectively.     

Tailoring the magnetic properties of Fe asymmetric nanodots

Asymmetric dots as a function of their geometry have been investigated using three-dimensional (3D) object oriented micromagnetic framework (OOMMF) code. The effect of shape asymmetry of the disk on coercivity and remanence is studied. Angular dependence of the remanence and coercivity is also addressed. Asymmetric dots are found to reverse their magnetization by nucleation and propagation of a vortex, when the field is applied parallel to the direction of asymmetry. However, complex reversal modes appear when the angle at which the external field is applied is varied, leading to a non-monotonic behavior of the coercivity and remanence.     

The size and space arrangement roles on coercivity of electrodeposited Co1−xCux nanowires

Magnetization curves with various magnetic field orientations and nanowire diameters were measured at room temperature. The measured coercivity as a function of angle (θ) between the field and wire axis reveals that the coercivity decreases with increasing value of θ for various nanowires. Theoretically, based on Monte Carlo simulation we investigated the magnetization reversal modes of the Co_1−xCu_x nanowires and obtained also the θ dependence of the coercivity. Comparing the simulated with the experimental results, we find that the magnetocrystalline anisotropy plays an important role on the magnetic properties of Co_1−xCu_x nanowires, and the magnetization reversal process in the Co_1−xCu_x nanowires could not be understood by the classical uniform rotation mode in the chain-of-sphere model.     

Simulations of magnetic microstructure in thin film elements used for programmable motion of magnetic particles

The results of two-dimensional micromagnetic modeling of magnetization patterns in Permalloy ellipses under the influence of rotating constant-amplitude magnetic fields are discussed. Ellipses of two different lateral sizes have been studied, 0.5 μm×1.5 μm and 1 μm×3 μm. The amplitude of the rotating magnetic field was varied between simulations with the condition that it must be large enough to saturate or nearly saturate the ellipse with the field applied along the long axis of the ellipse. For the smaller ellipse size it is found that the magnetization pattern forms an S state and the direction of the net magnetization lags behind the direction of the applied field. At a critical angle of the rotating magnetic field the direction of the magnetization switches by a large angle to a new S state. Both the critical angle and the angle interval of the switch depend on field amplitude. For this new state, it is instead the applied field direction that lags behind the magnetization direction. The transient magnetization patterns correspond to multi-domain patterns including two vortices, but this state never exists for the equilibrated magnetization patterns. The behavior of the larger ellipse in rotating field is different. With the field applied along the long-axis of the ellipse, the magnetization of the ellipse is nearly saturated with a vortex close to each apex of the ellipse. As the field is rotated, this magnetization pattern remains and the net-magnetization direction lags behind the direction of the field until for a certain angle of the applied field an equilibrium multi-domain state is created. Comparisons are made with corresponding experimental results obtained by performing in-field magnetic force microscopy on Permalloy ellipses.     

Directed magnetic field induced assembly of high magnetic moment cobalt nanowires

A directed magnetic field induced assembly technique was employed to align two phase (h.c.p. + f.c.c.) cobalt nanoparticles in a mechanically robust long wire morphology. Co nanoparticles with an average size of 4.3 nm and saturation magnetization comparable to bulk cobalt were synthesized by borohydride reduction followed by size selection and magnetic field induced assembly. The coercivity of these nanowires was higher than their nanoparticle counterpart due to shape anisotropy. The experimental coercivity values of the nanowires were lower than the predictions of the coherent rotation, fanning and curling models of coercivity due to the preponderance of superparamagnetic particles with zero coercivity.     

Types of the jump phenomenon in the angular dependence of the noncollinear exchange bias

Based on the principle of minimal energy and the coherent rotation model, two types of the jump phenomena, complete and incomplete jump phenomenon, are proved to exist in the angular dependence of the exchange bias with noncollinear unidirectional and uniaxial anisotropies. It is found that the transition between complete and incomplete jump phenomena occurs on condition that the exchange-coupling constant exceeds a critical value. Additionally, two different modes of the magnetization rotation, the whole-plane rotation, and the half-plane rotation are present in the magnetization reversal process, and they are dependent on the direction of the external field. Furthermore, the equations of the critical angle, at which orientation the exchange bias field reaches a maximum value and the coercivity disappears, are also derived in this paper. The numerical calculations in this paper are consistent with the relevant experimental observations, indicating that our method to study the angular dependence of the exchange bias as well as the magnetization reversal behaviors is valid. Our discussion about the jump phenomenon, the critical angle, and the modes of the magnetization reversal can explain the observed differences in results between different experiments.     

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.     

Magnetization anisotropy of Ni dots with several tens of nanometer diameter

We have studied the magnetization of Ni dot with 50 to 70 nanometer diameter and 12 nanometer thickness using a magnetic force microscopy with an in-plane magnetic field. The Ni dots were prepared using self-assembled dot patterns with poly (styrene-b-methyl mathacrylate) diblock copolymers on Ni film and ion etching. It was found that the remanent magnetization direction of the dot was perpendicular to the plane as prepared. From the vibrating sample magnetometer measurement, a hysteresis loop was found in the perpendicular magnetization. When an in-plane external magnetic field was applied, the magnetization was rotated into a horizontal direction with low coercivity along the field direction.     

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.     

小尺寸正方形铁磁薄膜边界效应对磁化翻转过程的影响

用自旋动力学方法系统地研究磁偶极相互作用表现的边界效应对小尺寸正方形铁磁薄膜的磁化翻转过程的影响.在确定的磁偶极相互作用强度下,针对不同的单轴各向异性强度和不同的磁化角(外磁场与易轴间的夹角),具体给出矫顽场与磁化角及单轴各向异性强度之间的依赖关系和-些有代表性的磁滞回线,并给出磁化翻转过程中-些有代表性的微观磁结构.模拟结果表明:磁偶极相互作用表现的边界钉扎作用与单轴各向异性场之间的竞争决定磁滞回线的形状和矫顽场的大小,从而在不同磁化角情况下会导致不同的矫顽场机理.本文提出可有效地描述正方形铁磁性薄膜复杂微观磁畴结构的形成与演变的五磁畴模型.这种五磁畴模型既能直接揭示单轴各向异性正方形铁磁薄膜的几何特性和物理特性,也方便于磁化翻转过程的分析.     

Reversal modes in asymmetric Ni nanowires

We have investigated the evolution of the magnetization reversal mechanism in asymmetric Ni nanowires as a function of their geometry. Circular nanowires are found to reverse their magnetization by the propagation of a vortex domain wall, while in very asymmetric nanowires the reversal is driven by the propagation of a transverse domain wall. The effect of shape asymmetry of the wire on coercivity and remanence is also studied. Angular dependence of the remanence and coercivity is also addressed. Tailoring the magnetization reversal mechanism in asymmetric nanowires can be useful for magnetic logic and race-track memory, both of which are based on the displacement of magnetic domain walls. Finally, an alternative method to detect the presence of magnetic drops is proposed.     

纳米晶永磁Pr8Fe87B5反磁化机理研究

用熔体快淬法制备了Pr2Fe14B/α-Fe纳米复合永磁样品.测量了样品的起始磁化、反磁化过程、矫顽力和剩磁与外场的关系，以及样品的磁粘滞性.经分析认为材料的矫顽力主要由非均匀的钉扎机理决定，但由于交换硬化的软磁相的可逆转动使得这种反磁化机理不同于单相永磁材料的钉扎行为.磁粘滞性表明热激活主要源于硬磁相的不可逆磁化行为. 关键词： 纳米复合永磁 矫顽力 剩磁 磁粘滞     

Monte carlo simulation of magnetization and coercivity of free magnetic clusters

Based on Monte Carlo method, the oscillatory behaviour of the average magnetic moment as a function of the cluster sizes and the temperature dependences of magnetic moment with different sizes have been studied. It is found that the oscillations superimposed on the decreasing moment are associated with not only the geometrical structure effects but also the thermal fluctuation. The hystereses and thermal coercivities for free clusters with zero and finite uniaxial anisotropies have been calculated. The simulated thermal dependence of the coercivity is consistent with the experimental result, but does not fit the T^α law in the whole temperature range. It is evident that an easy magnetization direction and an anisotropy resulting from the spin configurations exist in the free clusters with the pure exchange interaction, which is also proved by the natural angle and energy distribution of clusters. A systematic theoretical analysis is also made to establish the relationship between natural angle and coercivity.     

Angular dependence of coercivity in Pr---Fe---B sintered magnets

Hysteresis loops were measured for varying angles of the applied field at different temperatures in oriented sintered magnets of compositions Fe₇₅Pr₁₇B₈ and Fe₅₃Pr₁₇B₃₀. While at elevated temperatures the coercive field increases monotonically with the angle of the applied field, a minimum in the angular dependence is found below room temperature. The temperature dependence of coercivity for the field applied parallel and with an angle of 45° to the alignment direction is well described by a nucleation model in the whole temperature range. The results are compared to Fe---Nd---B magnets, showing that the occurrence of a minimum in the angular dependence is related to the ratio of the experimental to the theoretical coercive fields.     

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.     

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     

FeCo二元合金纳米线阵列的磁化反转

利用交流电化学沉积方法在氧化铝模板中制备了一维结构的Fe_xCo_1-x(0 ≤ x ≤ 0.51)二元合金纳米线阵列.X射线衍射结果显示,单质Co纳米线为(100)择优取向的hcp结构,FeCo合金纳米线则呈现(110)择优取向的bcc结构,而且衍射峰随纳米线中Fe含量的增加向低角度偏移.室温磁性测量结果显示, FeCo合金纳米线具有较好的磁特性.与Co纳米线相比,Fe的引入改善了Co纳米线的磁性能,使其呈现出较大的矫顽力和较高的矩形比.采用一致转动模型和对称扇形机理的球链模型分别计算了FeCo合金纳米线的矫顽力, 发现其磁化反转机理与对称扇形机理的球链模型相符合.     

Stress effect on the magnetization process in Ni-Cu-Zn ferrite in a weak field

The effect of stress on the magnetization process for Ni-Cu-Zn ferrite was studied by the measurement of the nonsaturated hysteresis loop under external applied stress. A weak field was applied to estimate the effect of stress during the reversal magnetization process. The experimental results show that the nonreversible reversal magnetization process is affected considerably by the compressive stress in the applied field from 0.20 to 0.84 Oe. The compressive stress acts as positive field to impede the nonreversible process which takes place during the reversal magnetization process. The tensile stress had a counter effect but the effect was too small. Stress perpendicular to detecting direction seems to exhibit an effect opposite to that of parallel stress. The sample was toroidal to estimate the differencein the effect of stress between detecting direction and radial direction perpendicular to the detecting direction. The effect of stress in the detecting direction is larger than in the radial direction for the Ni-Cu-Zn ferrite with negative magnetostriction.     

Spin-wave excitation by direct current in obliquely magnetized nanostructures

The magnetization dynamics of magnetic nanostructures magnetized at an arbitrary out-of-plane angle is investigated with the spin-wave formalism. The magnetic excitations driven by a spin-polarized direct current are considered to be standing spin-wave modes appropriate for nanopillar structures. The spin waves grow exponentially above a certain critical value of the current density and their post-threshold nonlinear dynamics leads to magnetization oscillations in the microwave range. Due to demagnetizing fields, the current-driven excitation strongly depends on the direction of the applied external magnetic field. In order to calculate the microwave oscillation frequency we derive an equation of motion for the spin-wave amplitude as a function of the out-of-plane angle of the applied field. The results are compared with recent experimental data as well as with another theoretical approach.