Cobalt antidot arrays on membranes: Fabrication and investigation with transmission X-ray microscopy

We have developed a method to fabricate ferromagnetic antidot arrays on silicon nitride membrane substrates for electron or soft X-ray microscopy with antidot periods ranging from 2 μm down to 200 nm. Observations of cobalt antidot arrays with magnetic soft X-ray microscopy show that for large periods, flux closure states occur between the antidots in the as-grown state and on application of a magnetic field, domain chains are created which show a spin configuration at the chain ends comprising four 90° walls. Pinning of the domain chain ends plays an important role in the magnetization reversal, determining the length of the chains and resulting in preservation of the domain chain configuration on reducing of the applied magnetic field to zero.     

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.     

Controlled Cu content of electrodeposited CoCu nanowires through pulse features and investigations of microstructures and magnetic properties

CoCu alloy nanowire arrays embedded in anodic alumina template were fabricated by ac pulse electrodeposition. Different off-times between pulses in an electrolyte with constant concentration of Co⁺² and Cu⁺² and acidity of 4 were employed. The effect of deposition parameters on the alloy contents, microstructures and magnetic properties of Co_xCu_1−x nanowires were studied. It is shown that Co content decreased by increasing the off-time between pulses in a wide range (x = 0.53-0.07). These results are in consistence with saturation magnetization, which was reduced with increase in the off-time between pulses. It was also found that by optimizing the off-times, it is possible to fabricate CoCu nanowires with mixed phase of hcp Co, fcc Cu and fcc CoCu crystal phase.     

Depth dependence of Néel wall pinning on amorphous CoxSi1−x films with diluted arrays of elliptical antidots

Diluted arrays of elliptical antidots have been fabricated by optical lithography, electron beam lithography and plasma etching on amorphous Co₇₄Si₂₆ magnetic films with a well-defined uniaxial anisotropy. The magnetic behavior of two identical antidot arrays but with different hole depth in comparison with film thickness has been studied by transverse magneto-optical Kerr effect. Significant differences appear in the coercivity depending on whether the magnetic film is completely perforated or not, indicating a much more effective domain wall pinning process when the depth of the holes is smaller than the magnetic film thickness.     

Magnetic properties of hexagonal ferrite dot arrays

Patterned magnetic media have been considered as one of the promising candidates for future ultra-high-density magnetic recording. In this paper, a new kind of patterned medium based on hexagonal ferrite have been studied. We have successfully fabricated strontium ferrite dot arrays by electron beam lithography. Their magnetic properties are evaluated by magnetic force microscopy (MFM) and superconducting quantum interference device (SQUID). The results show the dot arrays have perpendicular anisotropy. Dots with the lateral size larger than 500 nm show multidomain magnetization configuration in the initial magnetization state. However, with dot size decreased to 500 nm, all the dots have single-domain configuration both in the initial magnetization state and remanent magnetization state.     

Phase composition and magnetic characteristics of Fe-filled multi-walled carbon nanotubes

The work addresses the correlation between the phase composition and the magnetic characteristics of aligned Fe-filled multi-walled carbon nanotubes (Fe-MWCNTs) grown by pyrolysis of ferrocene on oxidized Si substrates. In a combinatorial approach we exploited the extremely high gradients of the technological parameters temperature and ferrocene flow across the surface of a substrate positioned close to the reactor wall to obtain a large variation in the structural and magnetic properties of the Fe-MWCNTs. In this way, we established several clear correlations between the Fe-filling phase composition and the overall magnetic characteristics of the aligned Fe-MWCNTs. The α-Fe rich samples, which possess a more ordered graphitic sheet structure, a higher degree of preferred crystalline orientation of the metal filling and much larger metal crystallites in comparison with the carbide-rich samples, show a much stronger magnetic anisotropy with easy axis perpendicular to the substrate and unusually high values of the coercive field H_c and the saturation field H_s. The changes in the measured saturation magnetisation M_s and the H_c values correlate well with the variation of the α-Fe content and the filling crystallinity. A special annealing treatment of the samples causes a distinct increase of the α-Fe quantity and an increase of the measured average grain size. The respective magnetic characteristics show a significant increase of the overall magnetic moment and decrease of the coercive field. The correlation between the structural and the magnetic characteristics of the annealed samples matches quite well the respective correlations in the case of as-deposited samples.     

Determination of anisotropy constants of protein encapsulated iron oxide nanoparticles by electron magnetic resonance

Angle-dependent electron magnetic resonance was performed on 4.9, 8.0, and 19 nm iron oxide nanoparticles encapsulated within protein capsids and suspended in water. Measurements were taken at liquid nitrogen temperature after cooling in a 1 T field to partially align the particles. The angle dependence of the shifts in the resonance field for the iron oxide nanoparticles (synthesized within Listeria-Dps, horse spleen ferritin, and cowpea chlorotic mottle virus) all show evidence of a uniaxial anisotropy. Using a Boltzmann distribution for the particles’ easy-axis direction, we are able to use the resonance field shifts to extract a value for the anisotropy energy, showing that the anisotropy energy density increases with decreasing particle size. This suggests that surface anisotropy plays a significant role in magnetic nanoparticles of this size.     

Autocatalytic redox fabrication and magnetic studies of Co-Ni-P alloy nanowire arrays

Uniform and large-scale Co-Ni-P alloy nanowire arrays have been fabricated by autocatalytic redox reaction in an anodic alumina membrane (AAM). The images of Co-Ni-P alloy nanowire arrays and single nanowires are obtained by scanning electron microscope (SEM) and transmission electron microscope (TEM), respectively. Selected area electron diffraction (SAED), X-ray diffraction (XRD) and energy dispersive spectra (EDS) are employed to study the morphology and chemical composition of the nanowires. The results indicate that the Co-Ni-P nanowire arrays are amorphous in structure. The magnetic property of Co-Ni-P nanowire arrays is characterized using a vibrating sample magnetometer (VSM). The hysteresis loops show that the easily magnetized direction of Co-Ni-P nanowire arrays is parallel to the nanowire arrays and that it has obvious magnetic anisotropy as a result of the shape anisotropy.     

Magnetic anisotropy engineering in square magnetic elements

Square magnetic elements with side in the 100–500 nm range have been fabricated using the focused ion beam (FIB) milling technique from a 10 nm thick, single-crystal Fe film, epitaxially grown on MgO(0 0 1). Thanks to the good crystal quality of the film, magnetic elements with well-defined magnetocrystalline anisotropy have been prepared, while the fine control of the size and shape of the magnets allows for the effective engineering of the anisotropic behavior of the magnetostatic energy that determines the so-called configurational anisotropy. Micromagnetic calculations and experiments show that the angular dependence of the transverse susceptibility has a strong dependence on the material parameters as well as on the static applied field. This allows the effective engineering of the total anisotropy of the magnets.     

Synthesis and magnetic properties of spinel CoFe2O4 nanowire arrays

Spinel CoFe₂O₄ nanowire arrays were synthesized in nanopores of anodic aluminum oxide (AAO) template using aqueous solution of cobalt and iron nitrates as precursor. The precursor was filled into the nanopores by vacuum impregnation. After heat treatment, it transformed to spinel CoFe₂O₄ nanowires. The structure, morphology and magnetic properties of the sample were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The results indicate that the nanowire arrays are compact. And the individual nanowires have a high aspect ratio, which are about 80 nm in diameter and 10 μm in length. The nanowires are polycrystalline spinel phase. Magnetic measurements indicate that the nanowire arrays are nearly magnetic isotropic. The reason is briefly discussed. Moreover, the temperature dependence of the coercive force of the nanowire arrays was studied.     

Fabrication and magnetic properties of amorphous Co0.71Pt0.29 nanowire arrays

Highly ordered Co_0.71Pt_0.29 alloy nanowire arrays have been fabricated successfully by direct current electro-deposition into the pores of a porous anodic aluminum oxide (AAO) template. SEM and TEM images reveal that the nanowires of array are uniform, well isolated, and parallel to one another. The aspect ratio of nanowires is over 200. XRD and EDS pattern indicates that amorphous Co_0.71Pt_0.29 structure was formed during electro-deposition. In amorphous sample, magnetocrystal anisotropy is very small, therefore, shape anisotropy plays a dominant role which leads to strong perpendicular anisotropy. High coercivity (Hc=1.7 kOe) and squareness (Mr/Ms) around 0.7 were obtained in the samples when the field was applied parallel to the axis of the nanowires. However, when it changed to polycrystalline structure after annealing, due to the competition of magnetocrystal anisotropy and shape anisotropy, the sample did not display perpendicular anisotropy.     

Electromagnetic wave absorption properties of Fe3C/carbon nanocomposites prepared by a CVD method

Dendrite-shaped iron nanowires with 50-200 nm in diameter and 10-20 μm in length were prepared by the CVD method from Fe(CO)₅ as a starting source. Ethanol was cracked on the surface of the resultant iron nanowires to form the Fe₃C/carbon nanocomposites, in which nanosized carbon beads covered the surface of Fe₃C. Resin compact of the resultant Fe₃C/carbon nanocomposites had excellent electromagnetic wave absorption ability in the range of 0.9-9.0 GHz, and such available absorption range more enhanced compared to that observed on the resin compact prepared from the original iron nanowires by the hybridization of magnetic (Fe₃C) and dielectric (carbon) materials.     

Configurational spin reorientation phase transition in magnetic nanowire arrays

Classical microscopic spin reorientation phase transitions (RPT) are the result of competing magnetocrystalline anisotropies. RPTs can also be observed in discrete macroscopic systems induced by competing shape anisotropies and magnetostatic coupling. Such a configurational RPT was recently observed in series of self-organized hexagonal arrays of 2.5 μm long, 25-60 nm diameter circular permalloy nanowires grown in anodic alumina matrix. This RPT is a crossover transition from a one-dimensional easy axis “wire” behavior of weakly interacting uniaxial nanowires to a two-dimensional behavior of strongly coupled “wire film” having an easy plane anisotropy. It is shown that RPT takes place due to the competition between the intrinsic dipolar forces in individual wires and the external dipolar field of interacting nanowires in the array. The crossover occurs at a volume ratio of 0.38 for 65 nm periodicity. The experimental results are in agreement with the semi-analytical calculations of the dipolar interaction fields for these arrays of circular ferromagnetic nanowires, and are interpreted in terms of the Landau phase transition theory. The conditions for the crossover and the order of the phase transition are established. Based on the contribution to the magnetic energy from the flower state at the ends of the wires, it is concluded that the observed transition is of the first order.     

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.     

Thermally activated magnetization reversal process of self-assembled Fe55Co45 nanowire arrays

We have investigated the temperature dependence of the magnetic properties and the magnetic relaxation of the Fe₅₅Co₄₅ nanowire arrays electrodeposited into self-assembled porous alumina templates with the diameter about 10 nm. X-ray diffraction (XRD) pattern indicates that the nanowire arrays are BCC structure with [1 1 0] orientation along the nanowire axes. Owing to the strong shape anisotropy, the nanowire arrays exhibit uniaxial magnetic anisotropy with the easy magnetization direction along the nanowire axes. The coercivity at 5 K can be explained by the sphere chains of the symmetric fanning mechanism. The temperature dependence of coercivity can be interpreted by thermally activated reversal mechanism as being the localized nucleation reversal mechanism with the activation volume much smaller than the wire volume. Strong field and temperature-dependent magnetic viscosity effects were also observed.     

Microstructures and coercivities of SmCox and Sm(Co,Cu)5 films prepared by magnetron sputtering

We have investigated the influence of composition and annealing conditions on the magnetic properties and microstructural features of SmCo_x films that were prepared by sputtering and subsequent annealing. A huge in-plane coercivity of 5.6 T was obtained from an optimally annealed Sm–Co film, which was attributed to the nanometer sized polycrystalline microstructure of the highly anisotropic SmCo₅ phase. Although a high density of planar defects were observed in the films that were annealed at high temperatures, they did not act as strong pinning sites for domain wall motion. The effect of Cu on [SmCo_4.5(9 nm)/Cu(xnm)]₁₀ multilayer thin films was also studied. An appropriate Cu content increased the coercivity.     

Magnetization reversal of single domain permalloy nanowires

Magnetization reversal process and magnetoresistance (MR) hysteresis of single domain permalloy nanowires are numerically investigated by using OOMMF. It is shown that the abrupt jumps in the magnetoresistance are due to the domain formation and domain wall propagation so that a magnetic domain suddenly switches from one state into another. A nonmonotonic angular dependence of the jump (switching) field is found. Coherent rotation mode is responsible for the smooth variation of MR curves. The nucleation pattern of newly born domains depends on the tilted angle of external field.     

Magnetization of cobalt nanodiscs

We have studied the magnetization and AC susceptibility of a dilute colloidal dispersion of ε$\epsilon$-Co nanodiscs. The temperature dependence of the magnetization implies that the nanodisc moments become largely static below a blocking temperature which exceeds room temperature. The related anisotropy energy is significantly larger than the magnetocrystalline anisotropy for bulk ε$\epsilon$-Co, which may result either from surface modifications to the crystalline anisotropy, or perhaps from shape anisotropy. The saturation moment is found to be temperature independent, over a wide range of temperatures and also close to the blocking temperature, although the field at which saturation occurs varies considerably with temperature. A coercive field of 1700 Oe is found at low temperature, indicating that magnetization reversal involves the coordinated rotation of the moments in individual nanodiscs. Aging effects are observed in the AC susceptibility, implying that the spatial coherence of the room temperature dispersions increases over time, leading to static short-range order.     

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.