Magnetic properties of high-Bs Fe–Cu–Si–B nanocrystalline soft magnetic alloys

In the present study, the magnetic properties and microstructures of newly developed Fe–Cu–Si–B alloys prepared by annealing the melt-spun ribbon have been studied. The average size and number density of nanocrystalline grains were about 20 nm and 10²³–10²⁴ m⁻³, respectively. The saturation magnetic flux density B_s for the present alloy is more than 1.8 T, that is about 10% larger than that of Fe-based amorphous alloys. Moreover, core loss P of the present alloy is about half of that of Si-steel up to B=1.7 T.     

Low-Temperature Magnetic Properties of Co Antidot Array

Cobalt antidot arrays with different thicknesses are fabricated by rf magnetron sputtering onto porous alumina substrates. Scanning electron microscopy and grazing incidence x-ray diffraction are employed to characterize the morphology and crystal structure of the antidot array, respectively. The temperature dependence of magnetic properties shows that in the temperature range 5K--300K, coercivity and squareness increase firstly, reach their maximum values, then decrease. The anomalous temperature dependences of coercivity and squareness are discussed by considering the pinning effect of the antidot and the magnetocrystalline anisotropy.     

Coercivity and squareness enhancement in ball-milled FeCo–MnO nanocomposites

FeCo–MnO nanocomposites were prepared by ball milling mixtures of Fe, Co and Mn, where MnO was obtained from the oxidation of Mn after or during the milling process. The coercivity and squareness of FeCo–MnO nanocomposites increase with increasing milling time. After annealing treatment, the coercivity and squareness increase and exhibit a maximum at 120 h milling time. The temperature dependence of the magnetic properties of FeCo–MnO was also studied and there is a distinct boundary at 120 K. The exchange-bias field and the coercivity decrease quickly with increasing temperature from 30 to 120 K. However, there are no shifted hysteresis loops and the coercivity decreases slightly with increasing temperature from 120 K to room temperature. The enhancement of the coercivity and squareness is mainly attributed to the exchange-bias effects and the reduced magnetic interactions between the FeCo particles by the efficient isolation in MnO matrix.     

Composition-Controlled Low Field Magnetostriction of TbFe Amorphous Films

Amorphous TbFe films are fabricated by dc magnetron sputtering, and their magnetostrictions at low field are examined over a wide range of terbium content （from 32at.% to 70at.%）. It is found that the terbium content plays an important role in the magnetic and magnetostrictive properties of TbFe films. TbFe film soft magnetic properties and low field magnetostriction can be efficiently improved by controlling the terbium at an optimum content. The magnetostriction at lower magnetic field is increased with the increase of terbium content up to 48.2at.%. After reaching the maximum value, further increase of terbium content would result in a great decrease of the low field magnetostriction. By contrast, at higher magnetic field, the magnetostriction is decreased monotonically with the increase of the terbium content.     

The influence of composition and field annealing on magnetic properties of FeCo-based amorphous and nanocrystalline alloys

We report the influence of composition and very high transverse field annealing on the magnetic properties and structure of four FeCo-based amorphous and nanocrystalline alloys. The compositions (Fe₅₀Co₅₀)₈₉Zr₇B₄ and (Fe₆₅Co₃₅)₈₉Zr₇B₄ were investigated changing the Fe:Co ratio from 50:50 to 65:35. (Fe₅₀Co₅₀)₈₅Zr₂Nb₄B_8.5 was chosen to investigate Nb substitution for Zr in an FeCo-based alloy. This substitution is shown to increase the magnetostrictive constant, _λS${\lambda }_{\mathrm{S}}$, of the nanocrystalline alloy from 36×10⁻⁶ to 54×10⁻⁶. The composition (Fe₆₅Co₃₅)₈₄Cr₅Zr₇B₄ was studied to investigate the influence of Cr on intergranular coupling across the amorphous matrix. Samples of each composition were annealed in the amorphous state at 300 °C and in the nanocrystalline state at 600 °C. Field annealing was performed in 17 T transverse field in an inert atmosphere. Frequency-dependent magnetic properties were measured with an automatic recording hysteresisgraph. Static magnetic properties were measured with a vibrating sample magnetometer. The mass-specific power loss of the alloys decreased with field annealing in both the nanocrystalline and amorphous states for some frequency and induction combinations. Furthermore, the hysteresis loops are sheared after field annealing, indicating a transverse magnetic anisotropy. The nanocrystalline (Fe₅₀Co₅₀)₈₅Zr₂Nb₄B_8.5 composition has a lower relative permeability than the other compositions.     

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.     

Structural and magnetic features of heterogeneously nucleated Fe-oxide nanoparticles

Iron oxide nanoparticles of diameter 14 nm were synthesized by applying Pt seed-assisted heterogeneous thermal decomposition of Fe(CO)₅ in a two-stage procedure. The intense heating treatment resulted in a remarkable mean volume increment compared to previous studies. This method is able to control the nanoparticle mean diameter, keeping the demand for thermal energy at low levels. High-resolution electron microscopy images and the corresponding electron diffraction patterns revealed the appearance of a FePt₃ core in each nanoparticle, surrounded by highly crystallized inverse spinel Fe₃O₄ formed after atmospheric oxidation, as shown by a combination of X-ray diffraction and chemical analysis. Magnetic measurements indicated that the presence of Pt-rich core does not cause any visible modification to the values of saturation magnetization and anisotropy constant of nanoparticles, compared to homogeneously nucleated iron oxide particles of the same size.     

Spin dynamics at low microwave frequencies in crystalline Fe ultrathin film double layers using co-planar transmission lines

Magnetic damping has been studied in magnetic double layers using a network analyzer (NA) with a coplanar transmission line. The magnetic films consisted of ultrathin crystalline films of Fe separated by an Au spacer. The films were deposited on GaAs(0 0 1) substrates using the molecular beam epitaxy (MBE) technique. NA-ferromagnetic resonance (NA-FMR) measurements were carried out along the magnetic hard axis, allowing one to follow the frequency FMR linewidth down to the 1 GHz range of frequencies. It will be shown that the FMR linewidth in the NA-FMR measurements is not entirely described by Gilbert damping. The additional contribution in the frequency linewidth increases with decreasing frequency, and is most likely caused by dipolar fields associated with an inhomogeneous RF field around the coplanar transmission line.     

Thermal treatment effects in the self-assembly of FePt nanoparticle arrays

This work concentrates on the influence of synthetic mechanisms of FePt nanoparticles on their self-arrangement and some structural and magnetic properties as studied by means of different electron microscopy techniques and SQUID magnetometry. High-reflux points associated with long boiling durations seem adequate to increase the iron precursor's decomposition yield and facilitate the simultaneous cubic to tetragonal FePt transformation, in single-phase FePt nanoparticles. Nevertheless, such conditions also result in the loss of long-range arrangement and in the appearance of coalescence effects. A core–shell structure comprising of FePt and Fe₃O₄ is favored under mild thermal conditions during synthesis, which seems to confront the undesirable atomic diffusion. Additionally, particle isolation due to the surfactant coating leads in an hcp-symmetry self-assembly. Moreover, working at lower temperatures allows a homogeneous mixture between different phases producing binary composite arrays.     

Ni80Fe20 permalloy nanoparticles: Wet chemical preparation, size control and their dynamic permeability characteristics when composited with Fe micron particles

Ni₈₀Fe₂₀ permalloy nanoparticles (NPs) have been prepared by the polyol processing at 180 °C for 2 h and their particle sizes can be precisely controlled in the size range of 20-440 nm by proper addition of K₂PtCl₄ agent. X-ray diffraction results show that the Ni-Fe NPs are of FCC structure, and a homogeneous composition and a narrow size distribution of these NPs have been confirmed by scanning electron microscopy assisted with energy dispersion spectroscopy of X-ray (SEM-EDX). The saturation magnetization of ~440nm NPs is 80.8 emu/g that is comparable to that of bulk Ni₈₀Fe₂₀ alloys, but it decreases to 28.7 emu/g for ~20 nm NPs. The coercive force decreases from 90 to 3 Oe with decreasing NP size. The wide range of particle size is exploited to seek for high permeability composite particles. The planar type samples composed of the NiFe NPs exhibit low initial permeability due to the deteriorated magnetic softness and low packing density. However, when they are mixed with Fe micron particles, the initial permeability significantly increases depending on the mixing ratio and the NiFe NP size. A maximum initial permeability is achieved to be ~9.1 at 1 GHz for the Fe-10 vol%NiFe (~20 nmΦ), which is about three times that of pure Fe micron particles. The effects of Ni-Fe particle size, volume percentage and solvent on the static and dynamic permeability are discussed.     

Finite size effects in shell nanoparticles

The finite size effects on magnetic properties of shell particles in the shape of truncated octahedron are studied using classical O(3) Heisenberg model. Both considered cases with nearest neighbor ferromagnetic (FM) and antiferromagnetic (AFM) exchange interactions show the existence of the well-defined ground state. The FM particles show FM order, while AFM particles show freezing in the non-collinear structure with very low magnetization due to the presence of frustrated triangular facets. The variation of magnetization, susceptibility and specific heat as a function of size shows considerable differences from that in the three-dimensional (3D) particles. We also observe effects of shell geometry, i.e. the fraction of sites in facets, edges and vertexes, which affects the apparent ordering temperature.     

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.     

Order-disorder transformation in FePt nanoparticles studied by ferromagnetic resonance

We have investigated the ferromagnetic resonance (FMR) response of as-made and temperature annealed FePt magnetic nanoparticles. The as-made nanoparticles, which have been fabricated by a chemical route, crystallize in the low magnetic anisotropy fcc phase and have a diameter in the range of 2-4 nm. The annealing of the particles at high temperatures (T_A=550, 650 and C) in an inert Ar atmosphere produces a partial transformation to the high magnetocrystalline anisotropy L1₀ phase, with a significant increase in particle size and size distribution. FMR measurements at X-band (9.5 GHz) and Q-band (34 GHz) show a single relatively narrow line for the as-synthesized particles and a structure of two superimposed lines for the three annealed samples. The origin of this line shape has been attributed to the presence of the disordered fcc phase. Assuming that the system consists of a collection of identical particles with a random distribution of easy axes, we have been able to estimate a mean value for the magnetic anisotropy constant of the particles in the fcc phase, K∼2×10⁶ erg/cm³. The measured line shape in the annealed samples can be explained if we consider that the magnetic anisotropy of the particles has a gaussian distribution with a relatively broad width.     

Synthesis and application of iron-filled carbon nanotubes coated with FeCo alloy nanoparticles

FeCo alloy nanoparticles were coated onto the iron-filled carbon nanotubes by an electroless plating method. As-prepared samples were then annealed at different temperatures. The morphologies and structures of the samples were characterized, and the relationship between the soft magnetic properties and the Fe/Co ratios of the samples was established. The microwave absorbing characteristics of the samples were also evaluated. The results show that the soft magnetic characteristics of iron-filled carbon nanotubes can be improved after being coated with FeCo alloy nanoparticles and then heat treated, which results in more effective microwave absorption.     

High concentration of hematite nanoparticles in a silica matrix: Structural and magnetic properties

The α-Fe₂O₃/SiO₂ nanocomposite containing 45 wt% of hematite was prepared by the sol-gel method followed by heating in air at 200 °C. The so-obtained composite of iron(III) nanoparticles dissolved in glassy silica matrix was investigated by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometry. XRPD confirms the formation of a single-phase hematite sample, whereas TEM reveals spherical particles in a silica matrix with an average diameter of 10 nm. DC magnetization shows bifurcation of the zero-field-cooled (ZFC) and field-cooled (FC) branches up to the room temperature with a blocking temperature T_B=65 K. Isothermal M(H) dependence displays significant hysteretic behaviour below T_B, whereas the room temperature data were successfully fitted to a weighted Langevin function. The average particle size obtained from this fit is in agreement with the TEM findings. The small shift of the T_B value with the magnetic field strength, narrowing of the hysteresis loop at low applied field, and the frequency dependence of the AC susceptibility data point to the presence of inter-particle interactions. The analysis of the results suggests that the system consists of single-domain nanoparticles with intermediate strength interactions.     

Microstructural and magnetic studies of L10 FePt–SiO2 nano-composite thin film with columnar structure for perpendicular magnetic recording

(Fe₅₀Pt₅₀)_100−x-(SiO₂)_x films (x=0–30 vol%) were grown on a textured Pt(0 0 1)/CrRu(0 0 2) bilayer at 420 °C using glass substrates. FePt(0 0 1) preferred orientation was obtained in the films. Interconnected microstructure with an average grain size of about 30 nm is observed in the binary FePt film. As SiO₂ is incorporated, it precipitates as particles are dispersed at FePt grain boundaries. When the content of SiO₂ is increased to 13 vol%, columnar FePt with (0 0 1) texture separated by SiO₂ is attained. The FePt columns have a length/radius ratio of 2:1. Additionally, the mean grain size is reduced to about 13 nm. The development of this well-isolated columnar structure leads to an enhancement in coercivity by about 44% from 210 to 315 kA/m. As the SiO₂ content exceeds 20 vol%, a significant ordering reduction is found accompanied by a transformation of preferred orientation from (0 0 1) to (2 0 0) and the columnar structure disappears, resulting in a drastic degradation in magnetism. The results of our study suggest that isolated columnar, grain refined, (0 0 1)-textured FePt film can be achieved via the fine control of SiO₂ content. This may provide useful information for the design of FePt perpendicular recording media.     

THz Generation by Optical Rectification and Competition with Other Nonlinear Processes

We present a study of the competition between tera-hertz （THz） generation by optical rectification in （110） ZnTe crystals, two-photon absorption, second harmonic generation and free-carrier absorption. The two-photon nonlinear absorption coefficient, second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently. The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced, upon focusing, by free-carrier absorption. The reduction of the generated THz radiation upon tight focusing is explained, provided that one also takes into account diffraction effects from the sub-wavelength THz source.     

Magnetic properties of p-doped GaMnN diluted magnetic semiconductors containing clusters

Magnetic properties of p-doped GaMnN diluted magnetic semiconductors, having both randomly distributed Mn ions and Mn_xN_y clusters, are presented under the theory based on the hole-mediated ferromagnetism. The critical temperature of the second order phase transition between ferromagnetic and paramagnetic phases and the magnetization as a function of temperature are obtained from the free energy calculation. The Curie temperature of the p-doped GaMnN containing clusters depends not on the type of clusters but on the composition rate of clusters. The behavior of the spontaneous magnetization as a function of temperature is strongly affected by carrier concentration. The p-doped GaMnN diluted magnetic semiconductors containing clusters have room temperature ferromagnetism regardless of the magnetic type of clusters, as long as hole-mediated spin-spin interactions occur in them.     

A study on microstructure,magnetic properties and kinetics of the nanocrystallization of Fe40Ni38B18Mo4 metglass

This study has investigated the microstructure and magnetic properties of Fe₄₀Ni₃₈B₁₈Mo₄ at various degrees of crystallization from the amorphous state. TEM and XRD studies confirmed that phases forming after crystallization at temperatures around 414 and 522 °C were cubic (Fe, Ni, Mo)₂₃B₆ phase and FCC (Fe, Ni) solid solution. The growth behavior and morphology of the nanocrystalline phases have been studied as a function of time and temperature. Nanoparticles were lying in the size range of 10–20 nm and they were stable below 522 °C. Kissinger approach, Ozawa method and Yi Qun Gao method were employed to determine and compare the kinetic parameters of the crystallization processes. A growth mechanism of crystallizing phases was proposed on the basis of these results. Magnetic properties mainly coercivity and saturation magnetization of as-received and heat-treated samples were evaluated.     

Properties of laser gain of Li:CdZnO/ZnMgO quantum well

The many-body optical gain in Li:CdZnO/MgZnO quantum well (QW) structures with spontaneous polarization and piezoelectric polarization, and ferroelectric dipole moment is investigated by using the non-Markovian gain model with many-body effects. The CdZnO/MgZnO QW structure with high Cd composition is found to have smaller optical gain because the strain-induced piezoelectric polarization and the spontaneous polarization in the well increase with the inclusion of Cd. The internal field is reduced due to the additional polarization by Li in the CdZnO/MgZnO QW structure. These results show that Li:CdZnO-based QW lasers are promising candidates for optoelectric applications in visible and UV regions.