NiFe2O4纳米线阵列的制备与磁性

在氧化铝模板的纳米孔洞中, 用电化学的方法沉积铁镍合金纳米线，经过550℃30h氧化处理 , 成功制备出 NiFe2O4纳米线阵列. 分别用扫描电子显微镜 (SEM) 、透射电 子显微镜 (TEM) 、x射线衍射仪 (XRD) 和振动样品磁场计 (VSM) 对样品的形貌、晶体结构 和磁学性质进行了表征测试. SEM和TEM观察结果显示氧化铝模板的孔洞分布均匀，孔心距约 为110nm; 纳米线的直径约为70nm. XRD显示纳米线阵列的物相结构为NiFe2O4; VSM测试结果表明，NiFe2O4纳米线阵列膜的易磁化方向垂直于膜面. 当垂直 磁化时磁滞回线的矩形比约为05，矫顽力为41×103A/m，比氧化处理前的铁镍合金 纳米线阵列都有显著提高. 关键词： 纳米线 Ni Fe2O4 矫顽力     

Magnetic properties of a two-dimensional spatially ordered array of nickel nanowires

The structural and magnetic characteristics of two-dimensional spatially ordered arrays of magnetic nickel nanowires embedded in the anodized alumina template have been investigated. It has been shown using small-angle polarized-neutron diffraction that, there exists the samples under investigation, in a highly ordered hexagonal structure of pores and magnetic nanowires separated by a characteristic distance d = 106 ± 2 nm. An analysis has been made of different contributions to neutron scattering, such as the nonmagnetic (nuclear) contribution, the magnetic contribution dependent on the magnetic field, and the interference contribution indicating a correlation between the magnetic and nuclear structures. The performed analysis of the results obtained has demonstrated that, when the magnetic field is applied perpendicular to the longitudinal axis of the nanowire in a completely magnetized sample, there arise demagnetizing fields around each nanowire that form a regular hexagonal lattice.     

Magnetic behaviour of arrays of Ni nanowires by electrodeposition into self-aligned titania nanotubes

Arrays of Ni nanowires electrodeposited into self-aligned and randomly disordered titania nanotube arrays grown by anodization process are investigated by X-ray diffraction, SEM, rf-GDOES and VSM magnetometry. The titania nanotube outer diameter is about 160 nm, wall thickness ranging from 60 to 70 nm and 300 nm in depth. The so-obtained Ni nanowires reach above 100 nm diameter and 240 nm length, giving rise to coercive fields of 98 and 200 Oe in the perpendicular or parallel to the nanowires axis hysteresis loops, respectively. The formation of magnetic vortex domain states is also discussed.     

Magnetic nanowire arrays in anodic alumina membranes: Rutherford backscattering characterization

Systematic study of magnetic nanowire arrays grown in anodic alumina membranes (AAM) has been done by means of Rutherford backscattering spectroscopy (RBS). The AAM used as templates were morphologically characterized by using high resolution scanning electron microscopy (HRSEM), fast Fourier transform (FFT) and atomic force microscopy (AFM). The highly ordered templates with a mean pore diameter size of 30 nanometers, a mean inter-pore spacing of 100 nm and lengths ranging from 4 to 180 microns were obtained through two-steps anodization process, and the Ni and Co nanowire arrays were grown by electrodeposition techniques. The main attention is addressed to Ni nanowire arrays. RBS results allowed us to determine the real depth profile of atomic composition of the obtained nanowire arrays. In addition, the RBS spectra fitting showed that the porosity increased from the top to the bottom of the samples. Two phenomenological models are proposed to understand the apparition of that secondary porosity and a linear relation between the total amount of electrodeposited Ni and the electrodeposition time was obtained. As an example, it is also reported the relation between RBS results and magnetic properties, such as coercive field and remanence/saturation magnetization ratio of the samples. Particularly, for Ni nanowires arrays obtained by using voltage pulses, it is demonstrated that the larger the nanowires, the higher the definition for easy axis parallel to the nanowire length is possible. PACS 82.80.Yc; 81.16-c; 75.75.+a     

Fabrication, morphology and structural characterization of ordered single-crystal Ag nanowires

Highly aligned Ag nanowires have been synthesized by dc electrodeposition within a hexagonal close-packed nanochannel anodic aluminum oxide template. The pore diameter varies from 20 nm to 50 nm depending on the anodization voltage and temperature for the two types of aqueous solutions, sulphuric and oxalic acids, respectively. The size and morphology of the Ag nanowire arrays were measured by scanning electron microscopy and transmission electron microscopy. The images indicate that the highly aligned Ag nanowires grow in the uniform nanochannels of the anodic alumina template and that the size of the nanowires depends on the size of the nanochannels. X-ray diffraction, selected area electron diffraction pattern and high-resolution transmission electron microscopy images show that the Ag nanowires are single-crystal. The temperature coefficient of resistivity (temperature range from 4.2 K to 300 K) of the Ag nanowire arrays decreases with decreasing diameter of the nanowires. Received: 5 November 2001 / Revised version: 12 March 2002 / Published online: 6 June 2002     

Electrodeposition and magnetic properties of Ni nanowire arrays on anodic aluminum oxide/Ti/Si substrate

Ni nanowire arrays with different diameters have now been extended to directly fabricate in porous anodic alumina oxide (AAO) templates on Ti/Si substrate by direct current (DC) electrodeposition. An aluminum film is firstly sputter-deposited on a silicon substrate coated with a 300 nm Ti film. AAO/Ti/Si substrate is synthesized by a two-step electrochemical anodization of the aluminum film on the Ti/Si substrate and then used as template to grow Ni nanowire arrays with different diameters. The coercivity and the squareness in parallel direction of Ni nanowires with about 10 nm diameters are 664 Oe and 0.90, respectively. The Ni nanowire arrays fabricated on AAO/Ti/Si substrates should lead to practical applications in ultrahigh-density magnetic storage devices because of the excellent properties.     

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.     

Surface structure of an ultrathin alumina film on Ni3Al(111): a dynamic scanning force microscopy study

The surface structure of an ultrathin alumina film on a Ni3Al(111) substrate has been studied by dynamic scanning force microscopy. The alumina film exhibits a hexagonal superstructure with a lattice parameter of 4.14 nm and a (1/sqrt[3] x 1/sqrt[3])R30 degrees substructure. Two domains rotated by 24 degrees are present. The film is terminated by a hexagonal lattice of oxygen ions with a lattice parameter of 0.293 nm, which is rotated by 30 degrees with respect to the substrate lattice. The nodes of the 4.14 nm superstructure and the 2.39 nm substructure are pinned on points of the substrate lattice, where the surface atomic lattice is almost commensurable. The oxygen lattice is perfectly hexagonal close to these nodes and disordered in the surrounding regions.     

Magnetic and structural properties of Fe–Co nanowires fabricated by matrix synthesis in the pores of track membranes

Fe_1-x Co _x nanowires are obtained by electrochemical deposition into the pores of track-etched membranes. The characteristics of the growth process that allow controlling the length and aspect ratio of the nanowires are established. The elemental composition and magnetic properties of the nanowires depend on the diameter of the track-etched pores, which varies from 30 to 200 nm, and the electrochemical potential U (650–850 mV), which determines the nanowire growth rate. According to the results of elemental analysis and the Mössbauer spectroscopy data, the Co content in Fe_1-x Co _x lies in the range of x=0.20?0.25. It is found that the orientation of the magnetic moment of Fe–Co nanoparticles in the wires depends both on the track pore size d and on the nanowire growth rate. Thus, the magnetic moments in nanowires grown in 50-nm-diameter pores are oriented within 0°–40° with respect to the nanowire axis. The magnetic properties of the nanowires are explained in the framework of a theoretical model describing the magnetic dynamics of nanocomposites, which was extended to include the relaxation of the magnetization vector and to take into account interaction between the particles. The key physical parameters important for the technological applications of the nanowires are determined, their dependence on the nanowire growth conditions is traced, and the possibility of controlling them is established.     

Ordered tellurium nanowire arrays and their optical properties

High-density tellurium (Te) nanowire arrays were prepared in the nanochannels of an anodic aluminum membrane (AAM) template using the electrochemical deposition method. The as-synthesized Te nanowires, typically 60 nm in diameter and up to 40 μm in length, possess a hexagonal single crystalline structure following [001] growth direction. The optical polarization properties of Te nanowire arrays embedded in AAM were investigated using an optic parameter oscillator in the wavelength range from 0.7 to 1 μm. The high optical polarization of the Te nanowire arrays embedded in the AAM assembly system was observed. PACS 81.05.Cy; 78.67.-n; 82.80.Fk.     

Coercive field behavior of permalloy antidot arrays based on self-assembled template fabrication

High-density magnetic antidot arrays have been fabricated by deposition of Fe₂₀Ni₈₀ thin films on self-assembled nanoporous alumina membranes (NAM) with high-order hexagonal symmetry. The magnetic properties induced by the size and the geometry configuration of the holes introduced in a Fe₂₀Ni₈₀ thin film are discussed based on hysteresis loops measured as a function of temperature. The precursor NAMs have pore diameters ranging between 35 and 95 nm (55 and 75 nm after the film deposition) and a lattice parameter of 105 nm. An enormous increase of coercitivity, as compared with the corresponding continuous films, was observed for temperatures between 2 and 300 K. This effect depends on the size and surface density of holes in the Fe₂₀Ni₈₀ antidot arrays. Rutherford backscattering spectrometry (RBS) measurements were performed in order to better clarify the magnetic material that was eventually deposited within the NAM pores.     

Long range ordering in self-assembled Ni arrays on patterned Si

We have succeeded in aligning self-assembled structures by using a lithographically defined stripe. The 140 nm wide by 100 nm high SiO₂ strip is shown to guide the assembly of 500 nm latex spheres so that spheres are aligned along the strip and are in registration on either side of the strip. This method can be used to increase long-range ordering in magnetic storage systems without compromising the density. Inverse sphere Ni arrays were made by electrodeposition through the latex template. We also show that the hexagonal symmetry of the resulting inverse sphere Ni arrays can be simulated using the approach presented below.     

A histogram-based segmentation method for characterization of self-assembled hexagonal lattices

Lattice characterization techniques are often used to quantify the effects of different anodization conditions on nano-porous anodized aluminum oxides. In this work, we develop a comprehensive hexagonal lattice characterization method to evaluate the amount of ordering of the lattice and localize the domains of the image and report their characteristics. A robust preprocessing is proposed to find pores’ centroids. Different domains of SEM images usually have different orientations. Pores orientation distribution is analyzed using angle-histogram. The valleys of angle-histogram are employed as thresholds to separate different dominant orientations. We show that using orientation as a distinguishing feature of different domains, significantly improves the robustness of the algorithm against tolerance parameters. Some new parameters are introduced to exactly characterize each of the domains and the whole lattice.     

Alumina template-assisted electrodeposition of Bi2Te2.7Se0.3 nanowire arrays

Bi₂Te_2.7Se_0.3 nanowire arrays have been fabricated by electrodeposition into the pores of an anodic aluminum oxide (AAO) template followed by annealing at 300 °C under Ar atmosphere. The as-prepared nanowires were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The nanowires are uniform single crystalline with diameter of ∼14 nm.     

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.     

Magnetic nanowires

We review recent developments in the research on magnetic nanowires electrodeposited into pores of membranes. Typical nanowires fabricated by this method have a diameter in the range 30–500 nm for a length of the order of 10 μm, and can be composed of a stack of layers of different metals with thicknesses in the nanometer range (multilayered nanowires). We describe the preparation methods and present typical examples of structural characterization. We review the magnetic properties with examples of results on both arrays of nanowires and isolated nanowires. We then describe the magnetoresistance properties of multilayered nanowires, and their interest for their understanding of the CPP–GMR and the determination of spin diffusion lengths. The last section is an overview on the perspectives of future research.     

Nanoporous alumina membrane prepared by nanoindentation and anodic oxidation

The fabrication of nanopatterned surfaces at large scale attracts the interest of research groups from a wide range of areas as biotechnology, nanoelectronics and nanomagnetism. An extended method to pattern the surface in the nanoscale is the fabrication of ordered arrays of nanoelements based on porous templates as Nanoporous Anodic Aluminium Oxide (NAAO). One of the challenges of the NAAO fabrication, based on self-organized methods, is the control of the symmetry and lattice parameter of the ordered nanoporous films. In this work, we present a combined method based on Atomic Force Microscopy (AFM) nanoimprint and anodic oxidation of Al surface. AFM nanoindentations substitute the first anodization process and even more important, allow us to control the symmetry and the lattice parameter of the ordered arrays. In addition, by using AFM nanoimprint method it is possible to select the region were the ordered alumina grows. We demonstrate that square nanoporous arrays of alumina with lattice parameter of 105 nm can be obtained by this method.     

Magnetic properties of ordered nanowires of the quasi-two-dimensional antiferromagnet SpFeMn(C<Subscript>2</Subscript>O<Subscript>4</Subscript>)<Subscript>3</Subscript>

Ordered arrays of nanowires of the photochromic antiferromagnet SpFeMn(C₂O₄)₃ (where Sp is 1-{(1′,3′,3′-trimethyl-6-nitro-5′-chlorospiro[2H-1-benzopyran-2,2′-indolin]-8-yl)methyl}pyridinium) have been fabricated in anodized aluminum oxide pores with diameters of 20 and 200 nm. It has been revealed that the growth of the spin-glass phase with noncollinear ordering of spins in nanowires is suppressed in favor of the uniaxial antiferromagnetic phase. A decrease in the nanowire diameter leads to an increase in the anisotropy of the magnetic resonance spectra. This is associated with the magnetocrystalline anisotropy that considerably exceeds the anisotropy of the nanowire shape.     

Ordered magnetic nanohole and antidot arrays prepared through replication from anodic alumina templates

Highly ordered arrays of Ni nanoholes and Fe₂₀Ni₈₀ antidots have been prepared, respectively, by replica/antireplica processing and sputtering techniques using nanoporous alumina membranes as templates. Geometrical characteristics as nanohole/antidot diameter, interpore distance and the overall hexagonal symmetry of arrays are controlled through the original templates. Experimental data on their hysteresis and magnetic domain structure have been taken by vibrating sample magnetometry and magnetic force microscopy, respectively. An analysis of the magnetization process, resulting magnetic anisotropy and magnetic domain structure is summarized considering the influence of those geometry aspects. In particular, the hexagonal symmetry and the density of nanohole/antidots determine the overall magnetic behavior, which is of interest in future high-density magnetic storage systems.