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, 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     

Structural and magnetic properties of Co50Ni50 powder mixtures

In the present work, morphological, structural, thermal and magnetic properties of nanocrystalline Co₅₀Ni₅₀ alloy prepared by high energy planetary ball milling have been studied by means of scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. The coercivity and the saturation magnetization of alloyed powders were measured at room temperature by a vibration sample magnetization. Morphological observations indicated a narrow distribution in the particle and homogeneous shape form with mean average particle size around 130 μm². The results show that an allotropic Co transformation hcp→fcc occurs within the three first hours of milling and contrary to what expected, the Rietveld refinement method reveals the formation of two fcc solid solutions (SS): fcc Co(Ni) and Ni(Co) beside a small amount of the undissolved Co hcp. Thermal measurement, as a function of milling time was carried out to confirm the existence of the hcp phase and to estimate its amount. Magnetic measurement indicated that the 48 h milled powders with a steady state particles size have the highest saturation (105.3 emu/g) and the lowest coercivity (34.5 Oe).     

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.     

Microwave-assisted synthesis of SrFe12O19 hexaferrites

Ultra-fine and homogeneous SrFe₁₂O₁₉ hexaferrites were synthesized by a microwave-assisted calcination route. The calcined precursors were prepared by a sol-gel auto-combustion method using Fe(NO₃)₃·9H₂O, Sr(NO₃)₂ and citric acid as starting materials. The structures, powder morphology and magnetic properties of the products were characterized by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer. The results showed that microwaves are helpful to reduce the calcination temperature and shorten the calcination time. The ferrites with saturation magnetization, remanence and intrinsic coercivity of 54.80 emu/g, 29.52 emu/g and 5261 Oe, respectively, were obtained in samples calcined at 800 °C for 80 min.     

Effect of sintering temperature and the particle size on the structural and magnetic properties of nanocrystalline Li0.5Fe2.5O4

Sintering temperature and particle size dependent structural and magnetic properties of lithium ferrite (Li_0.5Fe_2.5O₄) were synthesized and sintered at four different temperatures ranging from 875 to 1475 K in the step of 200 K. The sample sintered at 875 K was also treated for four different sintering times ranging from 4 to 16 h. Samples sintered at 1475 K have the cubic spinel structure with a small amount of α-Fe₂O₃ (hematite) and γ-Fe₂O₃ (maghemite). The samples sintered at≤1275 K do not show hematite and maghemite phases and the crystals form the single phase spinel structure with the cation ordering on octahedral sites. Particle size of lithium ferrite is in the range of 13-45 nm, and is depend on the sintering temperature and sintering time. The saturation magnetization increased from 45 to 76 emu/g and coercivity decreases from 151 to 139 Oe with an increase in particle size. Magnetization temperature curve recorded in ZFC and FC modes in an external magnetic field of 100 Oe. Typical blocking effects are observed below about 244 K. The dielectric constant increases with an increase in sintering temperature and particle size.     

Preparation and magnetic properties of anisotropic (Sm,Pr)Co5/Co composite particles

Anisotropic (Sm,Pr)Co₅/Co nanocomposite particles have been fabricated by chemical coating the 2 h ball milled (Sm,Pr)Co₅ flakes with Co nanoparticles. The Co nanoparticles were synthesized with mean particle sizes in the range of 20-50 nm. The nanocomposite particles present [0 0 1] out-of-plane texture and improved magnetic properties, e.g., an enhanced remanent magnetization of 72 emu/g for (Sm,Pr)Co₅/Co and 66 emu/g for (Sm,Pr)Co₅. In addition, by using the 8 h ball milled powders (much smaller than the 2 h ball milled powders) as the starting materials, Co nanoparticles can also be successfully coated on the surface of the flakes. A plausible mechanism for the formation of Co nanoparticles on the surface of (Sm,Pr)Co₅ flakes is discussed.     

Phase formation and change of magnetic properties in mechanical alloyed Ni0.5Co0.5Fe2O4 by annealing

The effects of milling time and annealing temperature on phase formation, microstructure and magnetic properties of nickel-cobalt ferrite synthesized from oxide precursors by mechanical alloying were studied. The study of milling time effects on phase formation of milled materials showed that if milling continues up to 55 h, single phase nano-sized nickel-cobalt ferrite is obtained. Also, magnetic properties of powders versus milling time and annealing at different temperatures extensively changed, so that annealing at 1200 °C increased the magnetization saturation of the as-milled powder from 15.1 to 53.6 emu/g. X-ray powder diffraction technique (XRD) with Cu-Ka radiation was employed for phase identification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also used to determine the morphology and size of the particles. The magnetic properties were measured by a vibration sample magnetometer (VSM).     

Synthesis of (Mg0.476Mn0.448Zn0.007)(Fe1.997Ti0.002)O4 powder and sintered ferrites by high energy ball-milling process

(Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder prepared by high energy ball-milling process were consolidated by microwave and conventional sintering processes. Phases, microstructure and magnetic properties of the ferrites prepared by different processes were investigated. The (Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder could be prepared by high energy ball-milling process of raw Fe₃O₄, MnO₂, ZnO, TiO₂ and MgO powders. Prefired and microwave sintered ferrites could achieve the maximum density (4.86 g/cm⁻³), the average grain size (15 μm) was larger than that (10 μm) prepared by prefired and conventionally sintered ferrites with pure ferrite phase, and the saturation magnetization (66.77 emu/g) was lower than that of prefired and conventionally sintered ferrites (88.25 emu/g), the remanent magnetization (0.7367 emu/g) was higher than that of prefired and conventionally sintered ferrites (0.0731 emu/g). Although the microwave sintering process could increase the density of ferrites, the saturation magnetization of ferrites was decreased and the remanent magnetization of ferrites was also increased.     

Characterization of nanocrystalline FeSiCr powders prepared by ball milling

FeSi₁₀Cr₁₀ powder was mechanically alloyed by high energy planetary ball milling, starting from elemental powders. The microstructural and magnetic properties of the milled powders were characterized by scanning electron microscopy, X-ray diffraction, ⁵⁷Fe Mössbauer spectrometry and a vibratory sample magnetometer.After 3 h of milling, the formation of two bcc solid solutions α-Fe₁ (Si, Cr) and α-Fe₂ (Si, Cr) is observed. Their grain sizes decrease with increase in milling time attaining, at 15 h of milling, 23 and 11 nm, respectively. Mössbauer spectra of the milled powder show the presence of two components. One is a ferromagnetic type with a broad sextuplet. Its distribution of hyperfine field is characterized by high and low hyperfine field’s peaks and a mean value of 26.5 T. The other is a single paramagnetic peak. Its low concentration increases to ∼4% at 15 h of milling. These results can be explained by different atomic environments affected by Si or/and Cr elements, as well as the increased disordered grain boundaries.Magnetic measurements of the milled FeSi₁₀Cr₁₀ alloy powder exhibit a soft ferromagnetic character with a decrease of both magnetization at saturation (Ms) and coercive force (Hc) with milling time attaining values of Ms=151 emu/g and Hc=2500 A/m at 30 h of milling time.     

Crystal structure and magnetic properties of MnxBi100−x (x=48, 50, 55 and 60) compounds

Mn_xBi_100−x (x=48, 50, 55 and 60) alloys were prepared by the induction melting technique, and subjected to melt spinning and subsequent ball milling. XRD shows that the as-milled powders were mainly composed of LTP MnBi. Increasing melt spinning speed and reducing annealing treatment time can restrain the segregation of Mn from MnBi liquid during the peritectic reaction, which increases the LTP MnBi content. High energy ball milling results in the coercivity increase of MnBi powders. With increasing milling time, the coercivity increases initially and then decreases gradually. After ball milling for 4 h, the coercivity of the Mn_xBi_100−x powders is 11.4 kOe for x=48 and 14.8 kOe for x=55. The optimal composition of Mn₄₈Bi₅₂ with more LTP has an M_2.2 T of 49.98 emu/g and an M_r of 33.57 emu/g.     

Influence of annealing temperature on morphological and magnetic properties of La0.9Sr0.1MnO3

The nanocrystalline samples of La_0.9Sr_0.1MnO₃ (LSMO) have been prepared by the combustion method. The thermo gravimetric analysis of precursor was carried out. The X-ray diffraction study confirms the rhombohedral crystal structure without any other impurity phases. The morphology and magnetic properties change with annealing temperature. The saturation magnetization increases linearly and coercivity of the nanoparticles varies significantly as annealing temperature increases. The maximum saturation magnetization and lower coercivity found for the sample heat treated at 1200 °C are 52.5 emu/g and 10.7 Oe respectively.     

Magnetocaloric effect in high-energy ball-milled Gd5Si2Ge2 and Gd5Si2Ge2/Fe nanopowders

Evolution of structure and magnetocaloric properties in ball-milled Gd₅Si₂Ge₂ and Gd₅Si₂Ge₂/0.1 wt% Fe nanostructured powders were investigated. The high-energy ball-milled powders were composed of very fine grains (70–80 nm). Magnetization decreased with milling time due to decrease in the grain size and randomization of the magnetic moments at the surface. The magnetic entropy change (ΔS_M) was calculated from the isothermal magnetization curves and a maximum value of 0.45 J/kg K was obtained for 32 h milled Gd₅Si₂Ge₂ alloy powder for a magnetic field change of 2 T while it was still low in Fe-contained alloy powders. The thermo-magnetic measurements revealed that the milled powders display distribution of magnetic transitions, which is desirable for practical magnetic refrigerant to cover a wide temperature span.     

Effect of surface-topology on magnetic properties in (La0.6Eu0.4)0.67Ca0.33MnO3 of anisotropic shape of nanoplates

(La_0.6Eu_0.4)_0.67Ca_0.33MnO₃ has been prepared in the shape of nanoplates of single crystallites (an orthorhombic structure) through polymer templates. HRTEM images reveal 18, 25, and 30 nm thicknesses of plates after heating a precursor powder at 873, 1073, and 1273 K in air for 2 h. These values present average crystallite size determined from broadening of the X-ray diffraction peaks. A spin-glass-like surface (GS) overlayer (3-5 nm thickness) in such plates facilitates a ferromagnetic→ferrimagnetic reordering with markedly suppressed Curie point T_C, i.e., as small as 90 K in a 873 K heated sample, from the parent value 268 K. The T_C point increases to 103 K (or 120 K) when heating at higher temperature 1073 (or 1273 K), during which the core grows at the expense of the overlayer. The GS tailors as high coercivity H_c as 617 Oe in the zero field cooled (ZFC) sample that is decreased to 500 Oe in the field cooled (FC) sample in the surface spin-freezing along the field direction. The H_c-value (ZFC) that steps down successively to 252 Oe on the overlayer is thinned down by heating at 1273 K. Samples heated at 873, 1073, or 1273 K have regularly increased saturation magnetization 35.3, 63.9, or 69.6 emu/g in ZFC, while 43.7, 70.2, or 75.5 emu/g in FC measured at 10 K. The ferrimagnetic reordering are described based on the scenario of an antiferromagnetic exchange coupling between the Eu³⁺ and Mn³⁺ (or Mn⁴⁺) sublattices.     

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.     

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.     

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.     

Preparation of Co3O4 nanoparticles by nonhydrolytic thermolysis of [Co(Pht)(H2O)]n polymers

Benzenedicarboxylate complexes, especially phthalate ones, can be significant precursors for the preparation of nano-sized metal and metal oxides. The injection of organic surfactants such as oleic acid (OA) and triphenylphosphine into molecular precursors has yielded samples with size control, narrow size distributions and crystallinity of individual nanocrystals. Fourier transform infrared and X-ray photoelectron spectroscopy revealed that the OA molecules were adsorbed on the ferromagnetic nanoparticles by chemisorption. The temperature-dependent magnetization curve in zero-field-cooled and field-cooled exhibit weak ferromagnetism of the Co₃O₄ nanoparticles. At 300 K the remanent magnetization is 0.02 emu/g, the coercive field is 441 Oe and the magnetization at saturation is 1.05 emu/g.     

Effect of thermal treatment on the microstructure and magnetic properties of a bulk amorphous Fe72Al5P10Ga2C6B4Si1 alloy

The structure and chemical composition of the phases that form in the controlled crystallization of a bulk amorphous Fe₇₂Al₅P₁₀Ga₂C₆B₄Si₁ alloy are studied by differential scanning calorimetry, transmission electron microscopy, and x-ray diffraction. It was established that, when the alloy is annealed at a temperature above the glass transition point, a nanocrystalline structure consisting of three phases arises. The magnetic properties of amorphous and nanocrystalline samples were studied with a vibrating-sample magnetometer. The coercive force and saturation magnetization of amorphous samples were found to be 1 Oe and 130 emu/g, respectively. The structure and chemical composition of the forming phases and their correlation with the magnetic properties of the samples were determined. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 12, 2004, pp. 2158–2163. Original Russian Text Copyright ? 2004 by Abrosimova, Aronin, Kabanov, Matveev, Molokanov, Rybchenko.