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.     

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.     

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 ordered Fe60Pb40 nanowire arrays electrodeposited in AAO templates

Ordered ferromagnetic-nonmagnetic heterogeneous Fe₆₀Pb₄₀ nanowire arrays were successfully fabricated by alternating current (AC) electrodeposition into nanoporous alumina templates. Transmission electron microscopy (TEM) image and selected-area diffraction (SAED) pattern analysis showed that the Fe₆₀Pb₄₀ nanowires are polycrystalline with an average diameter of 22 nm and lengths up to several micrometers. X-ray diffraction (XRD) observations indicated that α-Fe and fcc Pb phase coexist and do not form metastable alloy phase. The as-deposited samples were annealed at 200, 300, 400 and 500 ^°C, respectively. Magnetic measurements showed that nanowires have high magnetic anisotropy with their easy axis parallel to the nanowire arrays, and the coercivity of the samples increased with the annealing temperature up to 400 ^°C and reached a maximum (2650 Oe). The change of magnetic properties associated with the microstructure was discussed.     

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.     

Anomalous magnetization processes and non-symmetrical domain wall displacements in L10 FePt particulate films

Anomalous magnetization processes and non-symmetrical domain wall displacements in the minor loop of L1₀ FePt particulate films were investigated by magnetization measurements and in situ magnetic force microscopy. Magnetization (M) decreases dramatically on increasing the magnetic field to ∼3 kOe after which M becomes small and constant in the range of 5–20 kOe as observed in the successive measurement of minor loops. The domain wall displacement is non-symmetrical with respect to the field direction. The anomalous magnetization behavior was attributed to the non-symmetrical domain wall displacement and large magnetic field required for domain wall nucleation. Energy calculations from modeling suggest that non-symmetrical domain wall displacement is caused by the existence of metastable domains in which the domain edges are stuck to the particle boundaries.     

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.     

Mössbauer study of Fe0.92−xCoxP0.08 nanowire arrays

Arrays of Fe_0.92−xCo_xP_0.08 (0.22≤x≤0.78) ternary alloy nanowires were fabricated in anodic aluminium oxide templates by electrochemical deposition. The broadened peaks in transmission Mössbauer spectra and the halo in selected area electron diffraction patterns indicate that the structure of Fe_0.92−xCo_xP_0.08 nanowires is amorphous. However, the short-range order of Fe_0.92−xCo_xP_0.08 nanowires has a bcc structure with a [110]-preferred orientation that is parallel to the nanowires. The magnetic texture results in the magnetic moment direction of the Fe atoms being along the nanowires. The short-range order around the Fe atoms reaches a minimum at x=0.45. With increasing Co content, the average hyperfine field decreases, while the isomer shift and quadrupole splitting remain almost constant, which result from the variation of 3d and 4s electron volume density at the Fe sites.     

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 ordered 20 nm Co-Pb nanowire arrays

Ordered Co-Pb nanowire arrays embedded in anodic alumina template were successfully fabricated by electrodeposition. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) observations revealed that the Co-Pb nanowires were polycrystalline with uniform diameters around 20 nm and lengths up to several micrometers. Magnetic measurements showed that the coercivity and remanence of the as-deposited Co-Pb nanowires decreased with the increase of the Pb content. After annealing the Co-Pb nanowires present higher coercivities (2.4-2.5 kOe) than that of pure Co nanowires (2.1 kOe) and the dependence of coercivity and remanence on the Pb content is inconspicuous. A phase separation of Co and Pb occurred after annealing. The familiar pinning model was employed to explain the above experimental results.     

Magnetic layer thickness dependence of magnetization reversal in electrodeposited CoNi/Cu multilayer nanowires

The magnetization reversal of electrodeposited CoNi/Cu multilayer nanowires patterned in an array using a hole template has been investigated. The reversal mode is found to depend on the CoNi layer thickness t(CoNi); with increasing t(CoNi) a transition occurs from coherent rotation to a combination of coherent and incoherent rotation at around t(CoNi)=51 nm. The reversal mode has been identified using the magnetic hysteresis loops measured at room temperature for CoNi/Cu nanowires placed at various angles between the directions of the nanowire axis and external fields using a vibrating sample magnetometer. The nanowire samples have a diameter of ∼250 nm and constant Cu layer thickness of 4.2 nm with various t(CoNi) ranging from 6.8 nm to 7.5 μm. With increasing t(CoNi), the magnetic easy axis moves from the direction perpendicular to nanowires to that parallel to the nanowires at around t(CoNi)=51 nm, indicating a change in the magnetization reversal mode. The reversal mode for the nanowires with thin disk-shaped CoNi layers (t(CoNi)=6.8, 12 and 17 nm) is of a coherent rotation type, while that for long rod-shaped CoNi layers (t(CoNi)=150 nm, 1.0, 2.5 and 7.5 μm) can be consistently explained by a combination of coherent rotation and a curling mode. The effects of dipole–dipole interactions between nanowires and between adjacent magnetic layers in each nanowire on the reversal process have been discussed.     

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.     

Structure and magnetic properties of CoxPb1−x nanowire arrays

Ordered Co_xPb_1−x nanowire arrays embedded in the porous anodic aluminum oxide (AAO) template have been fabricated by electrodeposition. XRD experiments prove that neither hexagonal-close-packed (hcp) nor face-centered-cubic (fcc) Co peaks are detected when the Co component (x) is below 0.91. The coercivity (H_c) and squareness (M_r/M_s) are found to increase with ferromagnetic Co component and the maximum value is at the position x=1 (pure Co nanowires). Annealing effects cause H_c and M_r/M_s increase, which surpasses the pure Co nanowires in the 0.2<x<0.6 at the annealing temperature of 700 °C. Microstructure change during annealing process is proposed to explain the magnetic properties change of samples.     

The effect of aging time and calcination temperature on the magnetic properties of α-Fe/Fe3O4 composite

Composites of α-Fe/Fe₃O₄ having dimensions in the range of 100–150 nm have been prepared by disproportion method. The structure and morphology are investigated by XRD and TEM. XRD shows that the metal has got the BCC structure. TEM shows balls of metallic iron about 100-nm-wide stuck to magnetite grains. Magnetic measurement shows that the sample aged for 3 h and calcined at 200 °C has the maximal saturation magnetization corresponding to the highest concentration of α-Fe in the final sample.     

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.     

Permeability-frequency spectra of (Fe1−xcox)73.5Cu1Nb3Si13.5B9 nanocrystalline alloys under different magnetic fields

Under various amplitude of AC magnetic fields domain wall motion is the main mechanism in the magnetization process. This includes domain wall bulging and domain wall displacing. In this paper complex permeability-frequency spectra of (Fe_1−xCo_x)_73.5Cu₁Nb₃Si_13.5B₉ (x=0,0.5$x=0,0.5$) nanocrystalline alloys were measured as a function of the AC magnetic field, ranging from 0.001 to 0.04 Oe. Obvious changes have been found in complex permeability spectra for alloy x=0$x=0$ with the change of the amplitude of AC magnetic field, but variation of AC magnetic field has little effect on complex permeability spectra for alloy x=0.5$x=0.5$. This is attributed to the increased pinning field after substitution of Fe with Co in Fe_73.5Cu₁Nb₃Si_13.5B₉ nanaocrystalline alloy.     

Soft magnetic properties and giant magneto-impedance effect of Fe73.5−xCrxSi13.5B9Nb3Au1 (x=1–5) alloys

In this paper, the effect of microstructural and surface morphological developments on the soft magnetic properties and giant magneto-impedance (GMI) effect of Fe_73.5−xCr_xSi_13.5B₉Nb₃Au₁ (x=1, 2, 3, 4, 5) alloys was investigated. It was found that the Cr addition causes slight decrease in the mean grain size of α-Fe(Si) grains. AFM results indicated a large variation of surface morphology of density and size of protrusions along the ribbon plane due to structural changes caused by thermal treatments with increasing Cr content. Ultrasoft magnetic properties such as the increase of magnetic permeability and the decrease of coercivity were observed in the samples annealed at 540 °C for 30 min. Accordingly, the GMI effect was also observed in the annealed samples.     

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.     

Studies of the relaxed amorphous phase in the Fe80Nb6B14 alloy

The paper presents measurements of magnetic permeability, magnetic after-effects, magnetostriction, DSC and XPS for the Fe₈₀Nb₆B₁₄ amorphous alloys preliminary annealed for 1 h at temperatures ranging from 300 to 770 K. It was shown that annealing out of free volume and internal stresses causes a decrease of magnetostriction coefficient and leads to the formation of the energetically stable relaxed amorphous state. The XPS spectra show local fluctuation of boron density. This effect was attributed to the formation of small iron clusters—the characteristic feature for the relaxed amorphous phase.     

Spectral analysis of the precessional switching of the magnetization in an isotropic thin film

We study how a magnetic field step triggers the precessional switching of the magnetization in an isotropic thin film. Using a variational approach, we make an analytical estimate of the switching frequency. We compare it to more general analytical models, and to the results obtained numerically by direct integration of the equations of motion. We show that the periodic motions of the three magnetization components can be described satisfactorily with truncated Fourier expansions, indicating a relatively high spectral purity of the magnetic response. Our analytical expressions are simple enough to be physically transparent at first sight, in contrast to the results of the more elaborate models that treat also anisotropy.