The synthesis and the magnetic properties of Gd-doped FexCo1−x/CoyFe3−yO4 micro-octahedrons composites

Gd³⁺-substituted micro-octahedron composites (Fe_xCo_1−x/Co_yGd_zFe_3−y−zO₄) in which the Fe-Co alloy has either a bcc or fcc structure and the oxide is a spinel phase were fabricated by the hydrothermal method. The X-ray diffraction (XRD) patterns indicate that the as-synthesized Gd³⁺-substituted micro-octahedron composites are well crystallized. Scanning electron microscopy (SEM) images show that the final product consists of larger numbers of micro-octahedrons with the size ranging from 1.3 to 5 μm, and the size of products are increased with increasing the concentration of KOH. The effect of the Co²⁺/Fe²⁺ ratio (0?Co²⁺/Fe²⁺?1) and substitution Fe³⁺ ions by Gd³⁺ ions on structure, magnetic properties of the micro-octahedrons composites were investigated, and a possible growth mechanism is suggested to explain the formation of micro-octahedrons composites. The magnetic properties of the structure show the maximal saturation magnetization (107 emu/g) and the maximal coercivity (1192 Oe) detected by a vibrating sample magnetometer.     

Microstructure and soft magnetic properties of CoFeZrO thin films

Nanogranular CoFeZrO thin films were successfully prepared by radio frequency reactive magnetron sputtering in O₂/Ar atmosphere. The magnetic properties and microstructure were investigated. It is found that the Co_17.08Fe_49.76Zr_16.24O_16.91 films show the best soft magnetic properties: magnetic coercivity of 0.3 Oe; anisotropy field of 44.9 Oe; saturation magnetization of 16.8 kG; electrical resistivity of 462.8 μΩ cm. The effective permeability of the films reaches 800 and flattens up to 2 GHz.     

Magnetic, structural and micro-structural properties of mechanically alloyed nano-structured Fe48Co48V4 powder containing inter-metallic Co3V

The Fe₄₈Co₄₈V₄ alloy was synthesized in a planetary high-energy ball-mill under an argon atmosphere. The structure, microstructure and magnetic properties of the mechanically alloyed powders were investigated by X-ray diffraction, Scanning Electron Microscopy and a Vibration Sample Magnetometer, respectively. During the mechanical alloying of Fe₄₈Co₄₈V₄, inter-metallic Co₃V appears. The lattice parameter decreases up to 55 h of milling time with an oscillation and then increases from 55 to 125 h of milling time. The coercivity increases during the milling treatment from 49 to 58 Oe. The saturation magnetization has some fluctuations during the milling treatment and finally reaches ∼190 emu/g at 125 h.     

Magnetic and structural studies of mechanically alloyed nanostructured Fe49Co49V2 powders

Nanostructured Fe₄₉Co₄₉V₂ powders were produced by high energy milling at different milling times and then examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The saturation magnetization and coercivity of samples were measured at room temperature by a vibration sample magnetometer (VSM). Structural studies show that as the milling time increases from 0 to 125 h, the average grain size reduces from 130 to about 8-10 nm, while the microstrain increases up to 1.7%. The lattice parameter decreases from 0 to 36 h and then increases up to 125 h. According to the XRD patterns, the formation of intermetallic compound of (Fe, Co)V after about 16 h affects the magnetic properties. The coercivity totally increases up to 61 Oe due to the introduction of microstrain during the milling process. Magnetic measurements reveal that the saturation magnetization has some fluctuations during the milling treatment and finally at 125 h reaches about 180 emu/g     

Effect of Co underlayer on soft magnetic properties and microstructure of FeCo thin films

High saturation magnetization soft magnetic FeCo (=Fe₆₅Co₃₅) films were prepared using a thin Co underlayer. The FeCo/Co films exhibited a well-defined in-plane uniaxial anisotropy with easy axis coercivity (H_ce) of 10 Oe and hard axis coercivity (H_ch) of 3 Oe, and a half reduction of H_c with H_ce=4.8 Oe and H_ch=1.0 Oe was obtained when the composition was adjusted to 25 at% Co. The effective permeability of the films remains flat around 250 to 800 MHz. The saturation magnetostriction was 5.2×10⁻⁵ and the intrinsic stress was 0.8 GPa in FeCo single layer, both were slightly reduced by Co underlayer. The Co underlayer changed the preferred orientation of the FeCo films from (2 0 0) to (1 1 0) but more significantly, reduced the average grain size from ∼74 to ∼8.2 nm. It also reduced the surface roughness from 2.351 to 0.751 nm. The initial stage and interface diffusion properties were examined by TEM and XPS.     

Preference orientation and magnetic properties of CoxFe3−xO4 nanocrystalline thin films on quartz substrate

Nanocrystalline spinel ferrite thin films of Co_xFe_3−xO₄ (x=0.3$x=0.3$, 0.5, 0.8, and 1.0) have been prepared by RF sputtering on quartz substrate without a buffer layer at room temperature and annealed at the temperature range from 200 to 600 °C in air. The as-sputtered films exhibit the preferred orientation and the high magnetization and coercivity. After annealing, the preferred orientations become poor, but the magnetization and coercivity increase. The sample with a magnetization of 455 emu/cm³, a coercivity of 2.8 kOe, a remanence ratio of 0.72, and a maximum energy product of 2.4 MGOe has been obtained. The influence of Co ions and annealing temperature on the magnetic properties has been discussed.     

Enhanced antiferromagnetic coupling in dual-synthetic antiferromagnet with Co2FeAl electrodes

We study dual-synthetic antiferromagnets (DSyAFs) using Co₂FeAl (CFA) Heusler electrodes with a stack structure of Ta/CFA/Ru/CFA/Ru/CFA/Ta. When the thicknesses of the two Ru layers are 0.45 nm, 0.65 nm or 0.45 nm, 1.00 nm, the CFA-based DSyAF has a strong antiferromagnetic coupling between adjacent CFA layers at room temperature with a saturation magnetic field of ∼11,000 Oe, a saturation magnetization of ∼710 emu/cm³ and a coercivity of ∼2.0 Oe. Moreover, the DSyAF has a good thermal stability up to 400 °C, at which CFA films show B2-ordered structure. Therefore, the CFA-based DSyAFs are favorable for applications in future spintronic devices.     

High-frequency characteristics of CoFeVAlONb thin films

High-frequency characteristics of CoFeVAlONb thin films were studied. A thin film of Co_43.47Fe_35.30V_1.54Al_5.55O_9.93Nb_4.21 is observed to exhibit excellent magnetic properties; magnetic coercivity of 1.24 Oe, uniaxial in-plane anisotropy field of 66.99 Oe, and saturation magnetization of 19.8 kG. The effective permeability of the film is as high as 1089 and is stable up to 1.8 GHz, and with ferromagnetic resonance over 3 GHz. This film also has very high electrical resistivity of about 628 μΩ cm. These superior properties make it ideal for high-frequency magnetic applications.     

Effect of Fe/Sr mole ratios on the formation and magnetic properties of SrFe12O19 microtubules prepared by sol-gel method

The sol was obtained by sol-gel method. Then, the sol was dripped onto the absorbent cotton template. The gel was obtained after the evaporation of water. Strontium ferrite microtubules were prepared after carrying out calcination process at different temperatures. The phase, morphology and particle diameter and the magnetic properties of samples were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM), respectively. The effects of Fe³⁺/Sr²⁺ mole ratio and calcination temperature on the crystal structure, morphology and magnetic properties of ferrite microtubules were studied. The external diameters of obtained SrFe₁₂O₁₉ microtubules were found to range between 8 and 13 μm; the wall thicknesses ranged between 1 and 2 μm. When the Fe³⁺/Sr²⁺ mole ratio and the calcination temperature were 11.5 and 850 °C, respectively, the coercivity, saturation magnetization and remanent magnetization for the samples were 7115.1 Oe, 70.1 and 42.4 emu/g, respectively. The mechanism of the formation and variation in magnetic properties of the microtubules were explained.     

Influence of milling time on the structural, microstructural and magnetic properties of mechanically alloyed Ni58Fe12Zr10Hf10B10 nanostructured/amorphous powders

This paper investigates structural, microstructural and magnetic properties of amorphous/nanocrystalline Ni₅₈Fe₁₂Zr₁₀Hf₁₀B₁₀ powders prepared by high energy milling. Ball milling of Ni, Fe, Zr, Hf and B leads to alloying of the element powders at 120 h. The results show that at 190 h the amorphous content is at the highest level and the grain size is about 2 nm. The magnetic measurements reveal that the coercivity and the saturation magnetization reach about 20 Oe and 30 emu/g at 190 h and become approximately 5 Oe and 40 emu/g after a suitable heat treatment, respectively.     

Improvement of the magnetic properties of barium hexaferrite nanopowders using modified co-precipitation method

Barium hexaferrite BaFe₁₂O₁₉ powders have been synthesized using the modified co-precipitation method. Modification was performed via the ultrasonication of the precipitated precursors at room temperature for 1 h and the additions of the 2% KNO₃, surface active agents and oxalic acid. The results revealed that single phase magnetic barium hexaferrite was formed at a low annealing temperature of 800 °C for 2 h with the Fe³⁺/Ba²⁺ molar ratio 8. The microstructure of the powders appeared as a homogeneous hexagonal platelet-like structure using 2% KNO₃ as the crystal modifier. A saturation magnetization (60.4 emu/g) was achieved for the BaFe₁₂O₁₉ phase formed at 1000 °C for 2 h with Fe³⁺/Ba²⁺ molar ratio 8 using 5 M NaOH solution at pH 10 in the presence of 2% KNO₃. Moreover, the saturation magnetization was 52.2 emu/g for the precipitated precursor at Fe³⁺/Ba²⁺ molar ratio 12 in was achieved for the precipitated precursor ultrasonicated for 1 h and then annealed at 1200 °C for 2 h. Coercivities from 956.9 to 4558 Oe were obtained at different synthesis conditions.     

Planar Hall effect and magnetoresistance in Ni81Fe19 and Co square shaped thin films

Ni₈₁Fe₁₉ and Co thin films have been fabricated and their transport properties have been investigated for potential applications in ultra sensitive magnetic field sensors. The Ni₈₁Fe₁₉ films exhibit an anisotropic magnetoresistance (AMR) of 2.5% with a coercivity 2.5 Oe and the Co films exhibit an AMR of 0.7% with coercivity 11 Oe. Large planar Hall effect magnetoresistance values at room temperature are reported for both cases. An unbalanced Wheatstone bridge model is proposed to describe quantitatively the observed experimental Planar Hall Effect data.     

Effect of Cr on the magnetic properties of evaporated CoxCr1−x/Si(1 0 0) and CoxCr1−x/glass thin films

Series of Co_xCr_1−x thin films have been evaporated under vacuum onto Si(1 0 0) and glass substrates. Thickness ranges from 17 to 220 nm, and x from 0.80 to 0.88. Alternating gradient field magnetometer (AGFM) measurements provided saturation magnetization values ranging from 220 to 1200 emu/cm³. Values of squareness exceeding 0.8 have been measured. Coercive field may reach values up to 700 Oe, depending on the percentage of chromium, as well as the substrate nature and the direction of the applied magnetic field. The saturation magnetization value decreases as the Cr content increases. In order to study their dynamical magnetic properties, Brillouin Light Scattering (BLS) measurements have been performed on these samples. Stiffness constant value and anisotropy magnetic field were adjusted to fit the experimental BLS spectra. These results are analyzed and correlated.     

The multiferroic properties of Bi(Fe0.95Co0.05)O3 films

Bi(Fe_0.95Co_0.05)O₃ films were prepared on conductive indium tin oxide (ITO)/glass substrates by chemical solution deposition. Well saturated polarization hysteresis loop has been observed with a remnant polarization value of about 22 μC/cm² at room temperature. Weak ferromagnetism with saturation magnetization of about 3 emu/cm³ was observed at room temperature. The clear observation of both room temperature ferroelectric and ferromagnetic properties suggests the potential multiferroic applications of Bi(Fe_0.95Co_0.05)O₃.     

Structural and magnetic properties of self-assembled Sm-Co spherical aggregates

Self-assembled Sm-Co nanoparticles in the form of spherical aggregates (referred as nanospheres) with diameter ranging from 50 to 180 nm were achieved by means of polyol technique. The size distribution of the Sm-Co nanospheres can be regulated close to ∼100 nm by controlling the molar ratio of Sm:Co precursor. The spherical aggregates exhibited Sm₂Co₇ phase as a major constituent; while the aggregates obtained at higher Co concentration showed co-existence of Co-phase with Sm₂Co₇ phase. Upon annealing, the biphasic nature of nanospheres (Sm₂Co₇/Co) transformed into Sm₂Co₁₇ structure. By varying the Sm:Co precursor ratio from 1:5 to 1:9, the coercivity (H_c) and magnetization (M_s) values of the as-synthesized nanospheres can be tuned from 336 to 140 Oe and from 63.7 to 108 emu/g, respectively, and these values significantly improved after annealing. Maximum values of H_c (1050 Oe) at the Sm:Co molar ratio of 1:5 and M_s of 184.6 emu/g at the Sm:Co molar ratio of 1:9 were achieved in the annealed samples.     

Synthesis of α-Fe2O3 nanobelts and nanoflakes by thermal oxidation and study to their magnetic properties

α-Fe₂O₃ nanobelts and nanoflakes have been successfully synthesized by oxidation of iron-coated ITO glass in air. The X-ray diffraction, Raman spectrum and scanning electron microscopy are carried out to characterize the nanobelts and nanoflakes. The formation mechanism has been presented. Significantly, the magnetic investigations show that the magnetic properties are strongly shape-dependent. The magnetization measurements of belt-like and flake-like α-Fe₂O₃ in perpendicular exhibit ferromagnetic feature with the coercivity (H_c) and saturation magnetization (M_s) of 334.5 Oe and 1.35 emu/g, 239.5 Oe and 0.12 emu/g, respectively. For the parallel, belt-like and flake-like α-Fe₂O₃ also exhibit ferromagnetic feature with the H_c and M_s of 205.5 Oe and 1.44 emu/g, 159.6 Oe and 0.15 emu/g, respectively.     

Magnetic properties of Ni-substituted BiFeO3

BiFe_1−xNi_xO₃ ceramic powders with x up to 0.10 have been prepared by the sol-gel technique. The band gap of BiFeO₃ is 2.23 eV, and decreases to 2.09 eV for BiFe_0.95Ni_0.05O₃ and BiFe_0.90Ni_0.10O₃. The Mössbauer spectra show sextet at room temperature, indicating the magnetic ordering and the presence of only Fe³⁺ ions. Superparamagnetism with blocking temperature of 31 K for BiFe_0.95Ni_0.05O₃ and 100 K for BiFe_0.90Ni_0.10O₃ was observed. Enhanced magnetization at room temperature have been observed (1.0 emu/g for BiFe_0.95Ni_0.05O₃ and 2.9 emu/g for BiFe_0.90Ni_0.10O₃ under magnetic field of 10,000 Oe), which is one order larger than that of BiFeO₃ (0.1 emu/g under magnetic field of 10,000 Oe). The enhanced magnetization was attributed to the suppression of the cycloidal spin structure by Ni³⁺ substitution and the ferrimagnetic interaction between Fe³⁺ and Ni³⁺ ions.     

Synthesis of iron oxide nanoparticles under oxidizing environment and their stabilization in aqueous and non-aqueous media

Synthesis of magnetite (Fe₃O₄) nanoparticles under oxidizing environment by precipitation from aqueous media is not straightforward because Fe²⁺ gets oxidized to Fe³⁺ and thus the ratio of Fe³⁺:Fe²⁺=2:1 is not maintained during the precipitation. A molar ratio of Fe³⁺:Fe²⁺ smaller than 2:1 has been used by many to compensate for the oxidation of Fe²⁺ during the preparation. In this work, we have prepared iron oxide nanoparticles in air environment by the precipitation technique using initial molar ratios Fe³⁺:Fe²⁺?2:1. The phases of the resulting powders have been determined by several techniques. It is found that the particles consist mainly of maghemite with little or no magnetite phase. The particles have been suspended in non-aqueous and aqueous media by coating the particles with a single layer and a bilayer of oleic acid, respectively. The particle sizes, morphology and the magnetic properties of the particles and the ferrofulids prepared from these particles are reported. The average particle sizes obtained from the TEM micrographs are 14, 10 and 9 nm for the water, kerosene and dodecane-based ferrofluids, respectively, indicating a better dispersion in the non-aqueous media. The specific saturation magnetization (σ_s) value of the oleic-acid-coated particles (∼53 emu/g) is found to be lower than that for the uncoated particles (∼63 emu/g). Magnetization σ_s of the dodecane-based ferrofluid is found to be 10.1 emu/g for a volume fraction of particles ?=0.019. Zero coercivity and zero remanance on the magnetization curves indicate that the particles are superparamagnetic (SPM) in nature.     

Simultaneous variations of microstructure and magnetic properties of Ni58Fe12Zr20B10 nanostructure/amorphous alloy prepared by mechanical alloying

This study aims to evaluate magnetic and micro-structural properties of amorphous/nanocrystalline mechanically alloyed Ni₅₈Fe₁₂Zr₂₀B₁₀ powders with ball-milling time up to 190 h. Structural, micro-structural and thermal evaluations of the milled powders were carried out by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and differential scanning calorimetry (DSC) methods. Magnetic properties were also measured by a vibrating sample magnetometer (VSM) instrument. Results showed that the amorphous phase reached maximum value of 95% and the crystallite size was about 3 nm at the end of the milling. Magnetization saturation (M_s) decreased slightly and coercivity (H_c) reached to the highest value at 72 h of the milling time. At the 190 h of milling, the coercivity and saturation magnetization reached 18 Oe and 20 emu/g, respectively. While, after an appropriate amount of heat treatment, these two variables became approximately 2 Oe and 32 emu/g.     

Magnetic and structural properties of laminated Co65Fe35 films

We report on coercivity mechanisms in Co₆₅Fe₃₅ films. Using a unique High Target Utilisation Sputtering System (HiTUS) sputtering system we have produced a range of samples with controlled grain size in the range 6-30 nm. This has enabled an evaluation of the effects of different seed layers (Ru, Cu, Ta and Mg) and the effects of lamination of the films. We found that in 100 Å single-layer films the coercivity rises abruptly from 29 to 90 Oe with grain size. We found that samples with Cu or Ru seed layers show the largest reduction in coercivity. The effects of lamination are more complex since it leads to a reduction in grain size whereas coercivity increases. We believe this is due to RKKY coupling through the metallic interlayers.