• Journal of Magnetics
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• v.10 no.4
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• pp.145-148
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• 2005

Effect of vacuum annealing on the microstructures and magnetic properties of $Fe_{84}Si_6B_{10}$ films has been investigated as a function of annealing temperature. X-ray diffraction and transmission electron microscopy were employed to analyze crystallization behavior of the films. Permeability of the films was measured at various frequencies by one-turn coil method. When the films were annealed below 673 K, the coercivity of the films did not change a lot (${\~}$1500 A/m) although the grain size of a crystalline phase in the partially crystallized films increased gradually up to about 16 nm. It then increased rapidly as the films became almost fully crystalline mostly with $\alpha$-(Fe,Si) phase at and above 723 K. On the other hand, the electrical resistivity of the films decreased monotonically with the increase of annealing temperature. The permeabilities of the films annealed at 473${\~}$673 K were all over 1000, showing the optimum value of 3500 at 523 K, and almost constant up to 300 MHz. However, those of the as-deposited and fully crystallized films were lower than 1000 and unstable at the same frequency range.

• Journal of Powder Materials
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• v.9 no.6
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• pp.381-393
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• 2002

Recent developments in nanocrystalline and nanocomposite rare earth-transition metal magnets are reviewed and emphasis is placed on research work at IFW Dresden. Principal synthesis methods include high energy ball milling, melt spinning, mold casting and hydrogen assisted methods such as reactive milling and hydrogenation-disproportionation-desorption-recombination. These techniques are applied to NdFeB-, PrFeB- and SmCo-type systems with the aim to produce high remanence magnets with high coercivity. Concepts of maximizing the energy density in nanostructured magnets by either inducing a texture via anisotropic HDDR or hot deformation or enhancing the remanence via magnetic exchange coupling are evaluated. With respect to high temperature applications melt spun $Sm(Co_{0.74}Fe_{0.1}Cu_{0.12}Zr_{0.04})_{7.5}$ ribbons were prepared, which showed coercivities of up to 0.53 T at 50$0^{\circ}C$. Partially amorphous $Nd_{60}Fe_xCo_{30-x}Al_{10}(0{\leq}x{\leq}30)$ alloys were prepared by copper mold casting. The effect of transition metal content on the glass-forming ability and the magnetic properties was investigated. The $Nd_{60}Co_{30}Al_{10}$ alloy exhibits an amorphous structure shown by the corresponding diffraction pattern. A small substitution of Co by 2.5 at.% Fe results In the formation of Fe-rich crystallites embedded in the Nd-rich amorphous matrix. The Fe-rich crystallites show hard magnetic behaviour at room temperature with a coercivity value of about 0.4 T, relatively low saturation magnetization and a Curie temperature of 500 K.

• Journal of Powder Materials
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• v.10 no.1
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• pp.46-50
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• 2003

Lee et al. reported that a mixture of Cu and Ta, the combination of which is characterized by a positive heat of mixing, $\{Delta}H_{mix}$ of +2 kJ/㏖, can be amorphized by mechanical alloying(MA). It is our aim to investigate to what extent the MA is capable of producing a non-equilibrium phase with increasing the heat of mixing. The system chosen is the binary $Cu_{30}Mo_{70}$ with $\{Delta}H_{mix}$=+19 kJ/㏖. The mechanical alloying was carried out using a Fritsch P-5 planetary mill under Ar gas atmosphere. The vial and balls are made of Cu containing 1.8-2.0 wt.%Be to avoid contaminations arising mainly from Fe when steel balls and vial are used. The MA powders were characterized by the X-ray diffraction, EXAFS and thermal analysis. We conclude that two phase mixture of nanocrystalline fcc-Cu and bcc-Mo with grain size of 10 nm is formed by the ball-milling for a 3:7 mixture of pure Cu and Mo, the evidence for which has been deduced from the thermodynamic and structural analysis based on the DSC, X-ray diffraction and EXAFS spectra.

• Journal of the Korean Ceramic Society
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• v.45 no.5
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• pp.297-302
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• 2008

To direct the evolution of nanostructure and immobilize ${\gamma}-Al_2O_3$ catalyst, nanocrystalline La-doped-$Al_2O_3$ powder were prepared by the sol-gel process with addition of an amphiphilic block copolymer template (pluronic P123: $(poly(ethyleneoxide)_{20}-poly(propyleneoxide)_{70}-poly(ethyleneoxide)_{20})$. The dried gel is amorphous, whereas heating at temperature above $700^{\circ}C$ leads to the formation of nanocrystalline ${\gamma}$ and ${\delta}-Al_2O_3$ and these two phases is kept until $1100^{\circ}C$. ${\alpha}-A1_2O_3 $starts to form at $1200^{\circ}C$ with $LaAl_{11}O_{18}$. The surface morphology and crystal structure has been observed by field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD). Solid state $^{27}Al$ MAS NMR indicates two types of local environment, i.e. octahedral and tetrahedral sites. The surface area and pore size was compared among these powders using the BET nitrogen adsorption measurements.

• Journal of Magnetics
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• v.16 no.3
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• pp.300-303
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• 2011

The effect of the hot-compaction temperature on the microstructure and magnetic properties of anisotropic nanocrystalline magnets was investigated. The hot-compaction temperature was found to impact both the magnetic properties and the microstructure of die-upset magnets. The remanence of the isotropic precursor increases slightly with the improved hot-compaction temperature, and the grains start to grow on the flake boundary at higher hot-compaction temperatures. After hot deformation, it was found that the change in the magnetic properties was the inverse of that observed with the hot-compaction temperature. Microstructural investigation showed that die-upset magnets inherit the microstructural characteristics of their precursor. For the die-upset magnets, hot pressed at low temperature, scarcely any abnormal grain growth on the flake boundary can be seen. For those hot pressed at higher temperatures, however, layers with large equiaxed grains could be observed, which accounted for the poor alignment during the hot deformation, and thus the poor magnetic properties.

• Journal of the Korean Magnetics Society
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• v.1 no.2
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• pp.37-41
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• 1991

Amorphous $Fe_{68.5}Co_5M_3Cu_1Si_{13.5}B_9(M=Nb, Mo, Mn, Cr)$ alloys were prepared by using rapidly quenching techinque and were annealed above their crystallization temperatures. Coercive force, initial permeability and AC power loss of the annealed $Fe_{68.5}Co_5M_3Cu_1Si_{13.5}B_9(M=Nb, Mo, Mn, Cr)$ alloys have been studied systematically. Nanocrystallines are formed in the annealed alloys which include Mo and Nb. Remarkably improved soft magnetic properties are obtained in the alloys whose average grain size is around 10 nm. However, soft magnetic properties of the alloys are degraded when grain size is less than IOnm or larger than 15nm. It is considered that the degradation of soft magnetic properties in the alloys whose average grain size is less than 10 nm is due to the Fe-rich amorphous phase retained at grain boundary during the initial crystallization process.

• Journal of the Korean Magnetics Society
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• v.5 no.5
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• pp.864-873
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• 1995

Amorphous $Fe_{73.5}Cu_{1}Nb_{3}Si_{16.5}B_{6}$ ribbons were annealed for different time at $500^{\circ}C$ and $552^{\circ}C$, just before and after the exothermic reaction in DSC curve. The development of nanocrystalline phase was investigated by means of $M\"{o}ssbsuer$ spectroscopy. The crystalline phase consists mainly of $DO_{3}Fe-Si$. Though slight in amount (5%), another ferromagnetic phase which could be presumed $t-Fe_{3}B$ was detected Si content of $DO_{3}Fe-Si$, Si/(Fe+Si), was 0.218 under the heat treatment at $500^{\circ}C$ for 60 min and 0.222 at $552^{\circ}C$ for 10 min. Since then both of those values decreased with time until 120 min and finally these two values remained constant at 0.210. The variation in Si content with annealing time results in the variation in the hyperfine field and the isomer shift. The increase in the mean hyperfine fields and the decrease in the mean isomer shifts of Fe-Si are caused by the increase in Si content. The volume fractions of residual amorphous phase rapidly decrease during the early stage of annealing and come nearer to saturation after 120 min both at $500^{\circ}C$ and $552^{\circ}C$. The decrease in the mean hyperfine field of residual amorphous. in spite of slight changes in the volume fractions of Fe-Si and of residual amorphous after 120 min. is caused by the increase in the content of Nb and B in residual amorphous phase. The saturated volume fraction of the crystalline phase was 81% for $500^{\circ}C$ (180 min) and 77% for $552^{\circ}C$ (960 min), different from expectation.

• Journal of the Korean Magnetics Society
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• v.5 no.1
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• pp.1-7
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• 1995

${(Fe_{0.85}Co_{0.15})}_{75}Al_{7}B_{18-x}Nb_{x}(x=2,\;4\;and\;6\;at%)\;and\;{(Fe_{0.85}Co_{0.15})}_{75}Al_{y}B_{21-y}Nb_{4}(y=3,\;5,\;7,\;9\;at%)$ alloys were prepared by a single-roll quenching method. Microstructure and magnetic properties of the alloys such as saturation magnetization, initial permeability, coercive force and power loss have been investigated as functions of composition and armea1ing temperature. Nanocrystallines are obtained by armealing of as-prepared amorphous alloys in all compositions except the alloy of 9 at% AI. Saturation magnetization increases after armea1ing and, decreases with Nb content. However, AI and B affects the saturation magnetization insignificantly. Initial perrreability of nanocrystallized alloy at 50 kHz is improved roore than twice compared to that of the as-prepared alloy. Coercive force and core loss reach less than half after armea1ing.

• Journal of Magnetics
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• v.11 no.1
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• pp.30-35
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• 2006

A thorough study about the influences of Mn substitution for Fe on the microstructure and magnetic characteristics of $Fe_{73.5-x}Mn-{x}Si_{13.5}B_{9}Nb_{3}Cu_1$ (x = 1, 3, 5) alloys prepared by the melt-spinning technique has been performed. Nanocomposites composed of nanoscale $(Fe,Mn)_{3}Si$ magnetic phase embedded in an amorphous matrix were obtained by annealing their amorphous alloys at $535^{\circ}C$ for 1 hour. The addition of Mn causes a slight increase in the mean grain size. The Curie temperatures of the initial amorphous phase and of the nanocrystals phase decreased, while the Curie temperature of the remaining amorphous phase remained nearly constant with increasing Mn content. Soft magnetic properties of the crystallized samples have been significantly improved by a proper thermal treatment. Accordingly, the giant magnetoimpedance effect is observed and ascribed to the increase of the magnetic permeability, and the decrease of the coercivity of the samples. The increased magnetic permeability is resulted from a decrease in the magnetocrystalline anisotropy and saturation magnetostriction.

• Journal of Magnetics
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• v.21 no.2
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• pp.192-196
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• 2016

The Fe-Si-B-Nb-Cu alloys containing Ca and Al were rapidly solidified to thin ribbons by melt-spinning. The ribbons were ball-milled to make powders, and then mixed with 1 wt.% water glass and 1.5 wt.% lubricant. The mixed powders were burn-off, and then compacted to form toroidal-shaped cores, which were heat treated to crystallize the nano-grain structure and to remove residual stress of material. The characteristics of the powder cores were analyzed using a differential scanning calorimetry (DSC) and a B-H meter. The microstructures were observed using transmission electron microscope (TEM). The optimized soft magnetic properties (${\mu}_i$ and $P_{cv}$) of the powder cores were obtained from the Ca and Al containing alloys after annealing at $530^{\circ}C$ for 1 h. The core loss of Fe-Si-B-Nb-Cu-based powder cores was reduced by the addition of Ca element, and the initial permeability increased due to the addition of Al element.