Obvious Magnetic Anisotropy Research Articles

High-frequency electromagnetic properties of compositionally graded FeCoB-SiO2 granular films deposited on flexible substrates

In order to break through the conductive percolation threshold limitation of the conventional granular films to increase resistivity (ρ), while keeping large complex permeability (μ̃), a series of compositionally graded FeCoB-SiO2 granular films with different multilayer structures were deposited on flexible substrates. The high-frequency electromagnetic properties were investigated. The experimental results showed that the compositionally graded granular films had obvious in-plane uniaxial magnetic anisotropy and the coercivity in the easy axis (Hce) was as low as 3.97 Oe. Both the ρ (∼15.4 mΩ·cm) and μ̃ (μi'=276.3 and μ″max=441.4) of the compositionally graded FeCoB-SiO2 granular films were simultaneously higher than those of the conventional FeCoB-SiO2 granular films. Two resonance peaks of complex permeability spectrum were also observed.

Magnetic Nanotubes: Synthesis, Properties, and Applications

This article presents a comprehensive review of recent progress of research dedicated to magnetic nanotubes (MNTs). The review mainly covers the recent achievements in the syntheses, properties, and applications of MNTs. After introducing the significance of MNTs and the magnetic characteristics of elements in the periodic table, the article starts with a brief overview of the existing fabrication pathways for MNTs followed by a detailed insight in selected methods. Properties and promising applications are discussed to guide future developments of MNTs. Special emphasis is put on the magnetic properties that are different from those of the corresponding bulk materials, such as obvious magnetic anisotropy, angle dependent magnetization and distinctive magnetization reversal. Finally, the paper concludes with perspectives and outlook for MNTs.

Enhanced high-frequency electromagnetic properties of FeCoB–SiO2/SiO2 multilayered granular films

A series of FeCoB–SiO2/SiO2 multilayered granular films with various thickness were prepared by alternating co-sputtering FeCoB and SiO2 targets and single-sputtering SiO2 target. As-deposited films were annealed in a magnetic field at different temperature. The high-frequency magnetic and electrical transport properties were investigated. The films annealed above 250°C show good soft magnetic properties and obvious uniaxial magnetic anisotropy. The resistivity (ρ), coercivity (Hce) decrease and complex permeability (μ˜), ferromagnetic resonance frequency (fr) increase with increasing annealing temperature and thickness. The desirable high ρ (∼5.92mΩcm), considerable frequency linewidth (Δf∼4.02GHz) and high μ˜ (μ′=120 at low frequency, μmax″=80) were obtained for the 252nm films annealed at 250°C. The amorphous or nanograin phase and strong exchange coupling effect are responsible for the good high-frequency magnetic performance. The electrical transport properties show the films with high ρ and large μ˜ are near the conductive percolation threshold. Moreover, the Δf, damp coefficient (α) and Gilbert damping parameter (G) were discussed based on phenomenological Landau–Lifshitz–Gilbert equation.

Synthesis and magnetic properties of ordered barium ferrite nanowire arrays in AAO template

BaFe 12O 19 nanowire arrays having single magnetic domain size (≤460 nm) in anodic aluminum oxide (AAO) templates were prepared by sol–gel and self-propagating high-temperature synthesis techniques. The diameter of the nanowire arrays is approximately 70 nm and the length is about 2–4 μm. The specimens were characterized using X-ray diffraction, vibrating sample magnetometer, field emission scan electron microscope, atomic force microscopy and microwave vector network analyzer. The magnetic properties of BaFe 12O 19 nanowire arrays embedded in AAO templates were measured by VSM with a field up to 1274 KA/m at room temperature. The results indicate that the nanowire arrays exhibit large saturation magnetization and high coercivity in the range of 6000 Oe and an obvious magnetic anisotropy with the easy magnetizing axis along the length of the nanowire arrays, probably due to the shape anisotropy and magneto-crystalline anisotropy. Finally the microwave absorption properties of the nanowires were discussed.

Perpendicular magnetic anisotropy in 70 nm CoFe 2O 4 thin films fabricated on SiO 2/Si(1 0 0) by the sol–gel method

Cobalt ferrite CoFe 2O 4 films were fabricated on SiO 2/Si(1 0 0) by the sol–gel method. Films crystallized at/above 600 °C are stoichiometric as expected. With increase of the annealing temperature from 600 °C to 750 °C, the columnar grain size of CoFe 2O 4 film increases from 13 nm to 50 nm, resulting in surface roughness increasing from 0.46 nm to 2.55 nm. Magnetic hysteresis loops in both in-plane and out-of-plane directions, at different annealing temperatures, indicate that the films annealed at 750 °C exhibit obvious perpendicular magnetic anisotropy. Simultaneously, with the annealing temperature increasing from 600 °C to 750 °C, the out of plane coercivity increases from 1 kOe to 2.4 kOe and the corresponding saturation magnetization increases from 200 emu/cm 3 to 283 emu/cm 3. In addition, all crystallized films exhibit cluster-like structured magnetic domains.

In situ synthesis of plate-like Fe2O3 nanoparticles in porous cellulose films with obvious magnetic anisotropy

Nanocomposite cellulose films with obvious magnetic anisotropy have been prepared by in situ synthesis of plate-like Fe2O3 nanoparticles in the cellulose matrix. The influence of the concentrations of FeCl2 and FeCl3 solutions on the morphology and particle size of the synthesized Fe2O3 nanoparticles as well as on the properties of the composite films has been investigated. The Fe2O3 nanoparticles synthesized in the cellulose matrix was γ-Fe2O3, and its morphology was plate-like with size about 48 nm and thickness about 9 nm, which was totally different from those reported works. The concentration of FeCl2 and FeCl3 solution has little influence on the particle size and morphology of the Fe2O3 nanoparticles, while the content of Fe2O3 nanoparticles increased with the increase of the concentration of the precursor solution, indicating that porous structured cellulose matrix could modulate the growth of inorganic nanoparticles. The unique morphology of the Fe2O3 nanoparticles endowed the composite films with obvious magnetic anisotropy, which would expand the applications of the cellulose based nanomaterials.

Fabrication and magnetic properties of Ni0.5Zn0.5Fe2O4 nanofibres by electrospinning

NiZn ferrite/polyvinylpyrrolidone composite fibres were prepared by sol–gel assisted electrospinning. Ni0.5Zn0.5Fe2O4 nanofibres with a pure cubic spinel structure were obtained subsequently by calcination of the composite fibres at high temperatures. This paper investigates the thermal decomposition process, structures and morphologies of the electrospun composite fibres and the calcined Ni0.5Zn0.5Fe2O4 nanofibres at different temperatures by thermo-gravimetric and differential thermal analysis, x-ray diffraction, Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The magnetic behaviour of the resultant nanofibres was studied by a vibrating sample magnetometer. It is found that the grain sizes of the nanofibres increase significantly and the nanofibre morphology gradually transforms from a porous structure to a necklace-like nanostructure with the increase of calcination temperature. The Ni0.5Zn0.5Fe2O4 nanofibres obtained at 1000 °C for 2 h are characterized by a necklace-like morphology and diameters of 100–200 nm. The saturation magnetization of the random Ni0.5Zn0.5Fe2O4 nanofibres increases from 46.5 to 90.2 emu/g when the calcination temperature increases from 450 to 1000 °C. The coercivity reaches a maximum value of 11.0 kA/m at a calcination temperature of 600 °C. Due to the shape anisotropy, the aligned Ni0.5Zn0.5Fe2O4 nanofibres exhibit an obvious magnetic anisotropy and the ease magnetizing direction is parallel to the nanofibre axis.

Template electrodeposition to cobalt-based alloys nanotube arrays

Parallel uniform arrays of amorphous ferromagnetic Co 81Ni 19 and Co 37Fe 63 alloy nanotubes with outer diameter around 325–365 nm, wall thickness of 30–60 nm and length of over 40 μm were prepared by a direct current electrodeposition with mercury cathode using porous anodic aluminum oxide membrane as template. The morphology, structure, composition and magnetic property were studied. The results showed that mercury cathode is the key factor to form amorphous alloy nanotubes, and the as-prepared nanotube arrays exhibit obvious uniaxial magnetic anisotropy and the easy magnetization direction is perpendicular to the nanotubes axis. The mechanism of formation of Co based alloy nanotubes was also discussed.

Fabrication and Magnetic Properties of Amorphous Co1-xPx Alloy Nanowire Arrays

Amorphous Co1-xPx alloy nanowire arrays with different sizes and 6.8 wt% x 9.9 wt% were fabricated in an anodic aluminum oxide film by electrodeposition. Transmission electron microscopy, selected area electron diffraction, X-ray fluorescence spectrometer, inductively coupled plasma-atomic emission spectrometer, X-ray diffraction spectrometer and vibrating sample magnetometer are employed to study the morphology, chemical composition, structure and magnetic properties of the nanowire arrays, respectively. The results show that the Co1-xPx alloy nanowire arrays with x 6.8 wt% are amorphous. Due to the shape anisotropy effect, the coercivity of nanowire arrays is large for the applied field along the nanowires axis, and nanowire arrays have obvious magnetic anisotropy. The amorphous Co-P alloy nanowire arrays are good candidates for ultra-high-density magnetic recording media.

A multistep ac electrodeposition method to prepare Co nanowires with high coercivity

It is known that the ac electrodeposition method with low current density can grow compact metal nanowires, but the length of those nanowires is very short. In contrast, the ac electrodeposition method with high current density can grow long metal nanowires. However these long nanowires are not compact and contain lots of defects. In this paper, we describe a multistep ac electrodeposition method to fabricate long metal nanowires with compact structure uniformly filled upon a porous anodic aluminum oxide template. Using this method, Co nanowires with high coercivity (Hc∥) and remnant ratio (Mr/Ms) have been prepared under relatively low deposition current density. The Co nanowires exhibited obvious magnetic anisotropy with the easy axis along the axial direction of nanowires. The maximal Hc∥ (2900 Oe) and Mr/Ms (0.95) were optimal for the perpendicular magnetic recording materials. The magnetic microstructure of Co nanowires is also discussed in this paper.

Fabrication and magnetic behavior of chemical deposited Ni–P nanowire and nanotube arrays

Ni–P nanowire and nanotube arrays were fabricated by chemical deposition using porous anodic aluminum oxide (AAO) as templates at room temperature. The morphologies of Ni–P nanowires and nanotubes are obtained with field emission scanning electron microscope and transmission electron microscope. X-ray diffraction and selected area electron diffraction are used to study the phase structure and chemical composition. The influence of pretreatment for AAO template on the deposition of nanowires and nanotubes is studied by comparing the growth mechanisms between nanowires and nanotubes. M– H hysteresis loops determined by vibrating sample magnetometer indicate that the nanowire and nanotube arrays obtained possess obvious magnetic anisotropy.

Structural and magnetic properties of electrodeposited Ni70Fe30 nanowire array

Ordered Ni70Fe30 nanowire array was fabricated in a porous alumina template by alternating current electrodeposition. The structural and magnetic properties of the as-obtained nanowire array were investigated by SEM, TEM, XRD, EDS and VSM. The results indicate that the as-obtained Ni70Fe30 nanowires exhibit a diameter of about 69.9 nm and aspect ratio of more than 60. Meanwhile, a preferred orientation [110] of bcc lattice was observed. The as-obtained nanowire array has an obvious magnetic anisotropy, of which the easy direction is perpendicular to the surface of the array. Moreover, after annealed, the Ni70Fe30 nanowire array exhibits an enhanced magnetic anisotropy.

Structural and magnetic properties of Co and Co71Ni29 nanowire arrays prepared by template electrodeposition

A comparative study on the structural and magnetic properties of Co and Co71Ni29 nanowire arrays prepared by AC electrodeposition in alumina templates has been presented. The Co and Co71Ni29 nanowires observed by SEM and TEM have a 45 nm diameter and exhibit high aspect ratio. Also, the nanowires of both Co and Co71Ni29, determined by XRD, have an identical crystallographic structure. The Co71Ni29 nanowires exist in a cobalt solid solution. Both the as-obtained Co and Co71Ni29 nanowire arrays measured by VSM show obvious magnetic anisotropy, dominated by shape anisotropy. Compared to the Co nanowire arrays, Co71Ni29 nanowire array shows an enhanced coercivity Hc (⊥) and approximate square ratio Mr/Ms(⊥).

Pulsed electrodeposition of monocrystalline Ni nanowire array and its magnetic properties

By means of a porous template without removing the aluminium substrate, a technique of pulsed electrodeposition with an intermittent symmetric square pulse has successfully been applied to fabricate Ni nanowire array. The as-obtained nanowires have a diameter of about 60 nm and exhibit high aspect ratio of more than 50. The electron diffraction pattern investigation demonstrates that the nanowires are single crystal. Moreover, a highly preferential orientation [2 2 0] of the as-obtained Ni nanowires with high purity decided by XRD has been obtained, and the preferred orientation is weakened remarkably by an annealing process. Furthermore, the investigation of magnetic properties by VSM indicates that the as-obtained Ni nanowire array has an obvious magnetic anisotropy and exhibits a good thermal stability.

MAGNETIC PROPERTIES OF NANOCRYSTALLINE CO-FERRITE FILMS DEPOSITED ON SINGLE-CRYSTAL SiO2 SUBSTRATES USING PULSED LASER DEPOSITION

Co-ferrite films were prepared using pulsed laser deposition with both post-annealing and in situ heating processes. Magnetic properties of these films were studied in the function of temperature, film thickness, and substrate. The films using post-annealing processes exhibited isotropic microstructure, and the coercivity showed no obvious magnetic anisotropy and no strong dependence on film thickness. Co-ferrite films using in situ heating exhibited (111) highly textured structure and possessed perpendicular anisotropy as well as large coercivity. The preferential texture and magnetic anisotropy were closely associated with substrate temperature and thickness. Perpendicular Hc over 12.5 kOe was obtained in the 33 nm Co-ferrite film deposited on single crystal quartz substrate at 550°C. The high coercivity and perpendicular coercivity may be attributed to the nanocrystalline grain, textured orientation, and large residual strain in these films since large residual strain may induce strong stress anisotropy.

Magnetic and Distribution of Magnetic Moments in Amorphous Fe89.7 P10.3 Alloy Nanowire Arrays

Binary amorphous Fe89.7P10.3 alloy nanowire arrays in diameter of about 40nm and length of about 3 μm have been fabricated in an anodic aluminium oxide template by electrodeposition. Magnetic properties of the samples are investigated by mean of vibrating sample magnetometer, transmission Mössbauer spectroscopy and conversion electron Mössbauer spectroscopy at room temperature. It is found that the nanowire arrays have obvious perpendicular magnetic anisotropy and are ferromagnetic at room temperature, with its Mössbauer spectra consisting of six broad lines. The average angles between the Fe magnetic moment and the wire axis are about 14° inside and 28° at the end of the amorphous Fe89.7P10.3 alloy nanowire arrays, respectively. The magnetic behaviour is decided by the shape anisotropy and the dipolar interaction between wires. In addition, the magnetic moments distribution is theoretically demonstrated by using the symmetric fanning mechanism of the spheres chain model.

Structural and Magnetic Properties of Co–Ni Nanowire Array Prepared by Alternating Current Electrodeposition

Abstract Ordered Co43Ni57 nanowire array has been fabricated in a porous alumina template by alternating current electrodeposition. The as-obtained nanowires exhibit a diameter of about 45 nm and aspect ratio of more than 30. X-ray diffraction indicates that the Co43Ni57 nanowires crystallize in Co (fcc) solid solution with a highly preferential orientation (111). The Co43Ni57 nanowire array has an obvious magnetic anisotropy. However, the magnetic properties of such nanowire arrays exhibited a bad thermal stability at the medium temperature of 200 °C.

Studies on high-moment soft magnetic FeCo/Co thin films

The dependences of soft magnetic properties and microstructures of the sputtered FeCo (=Fe65Co35) films on Co underlayer thickness tCo, FeCo thickness tFeCo, substrate temperature Ts and target-substrate spacing dT−S are studied. FeCo single layer generally shows a high coercivity with no obvious magnetic anisotropy. Excellent soft magnetic properties with saturation magnetization μ0Ms of 2.35 T and hard axis coercivity Hch of 0.25 kA/m in FeCo films can be achieved by introducing a Co underlayer. It is shown that sandwiching a Co underlayer causes a change in orientation and reduction in grain size from 70 nm to about 10 nm in the FeCo layer. The magnetic softness can be explained by the Hoffmann's ripple theory due to the effect of grain size. The magnetic anisotropy can be controlled by changing dT−S and a maximum of 14.3 kA/m for anisotropic field Hk is obtained with dT−S=18.0 cm.

NixPb1-x Nanowire Arrays: Effects of Annealing

Ni(x)Pb(1-x) nanowire arrays were successfully fabricated by AC electrodeposition within the nanopores of ordered porous alumina films prepared by a two-step anodization. Transmission electron microscopy analyses showed that the Ni-Pb nanoarrays are polycrystalline with dimension uniformity around 20 nm in diameter and lengths up to several micrometers. X-ray diffraction results revealed that the face-centered-cubic (fcc) Ni and fcc Pb peaks are detected when the Ni component (x) is below 0.71, indicating that the Ni(x)Pb(1-x) nanoarrays do not form metastable phase alloy. Hysteresis loops determined by vibrating sample magnetometer indicated that the Ni(x)Pb(1-x) nanoarrays obtained possess obvious magnetic anisotropy, and the perpendicular coercivity was lower than that of pure Ni nanowries before and after annealing. Annealing under magnetic field was carried out to examine the effect of a magnetic field on magnetic properties using an electromagnet field up to 0.3 T.

Structural and magnetic properties of electrochemically assembled Ni–Pb/Al 2O 3 nanostructures

Ni–Pb/Al 2O 3 nanostructures having composition of Ni 84 at.%, Pb 16 at.% were successfully fabricated by AC electrochemically assembled within the nanopores of ordered porous alumina films prepared by a two-step anodization. SEM images revealed that the pore arrays are regularly arranged throughout the alumina film and parallel each other. TEM analyses showed that the Ni–Pb nanoarrays are polycrystalline with dimension uniformity around 20 nm in diameter and lengths up to several μm. X-Ray diffraction results revealed that the Ni–Pb nanoarrays do not form metastable alloy phase. Hysteresis loops determined by VSM indicated that the Ni–Pb/Al 2O 3 nanostructures obtained possess obvious magnetic anisotropy, and the perpendicular coercivity is lower than that of Ni nanowires before and after annealing.