D. P. Yang

High-temperature giant magnetoimpedance in Fe-based nanocrystalline alloy

Giant magnetoimpedance (GMI) was investigated from room temperature up to 823 K in an Fe-based nanocrystalline Fe73.0Cu1.0Nb2.5V1.0Si13.5B9.0 ribbon. With an increment of the measuring temperature (T), GMI shows notable enhancement followed by a declining dependence, yielding a maximum value around 603 K where the relative GMI is nearly four times that at room temperature. The field at the peak of the GMI vs Hdc curve decreases monotonically with T, but around T=603 K there superimposes a trough-shaped variation. The thermal evolution of the soft magnetic property and magnetic anisotropy is suggested to be responsible for the high-temperature GMI features. Discussion on the intergrain exchange magnetic coupling through the amorphous boundaries in the two-phase Fe-based nanocrystalline alloy is also given.

Longitudinally driven giant magnetoimpedance effect in stress-annealed Fe-based nanocrystalline ribbons

A high-frequency longitudinally driven giant magnetoimpedance (GMI) effect has been measured in stress-annealed Fe73Cu1Nb1.5V2Si13.5B9 nanocrystalline ribbons. Based on how the impedance phase varies with the external magnetic field, it becomes clear that the imaginary part of the complex permeability, μ″, which is related to magnetic losses, plays an important role in the high-frequency longitudinally driven GMI effect. The transverse anisotropy field Hk can be readily determined by a sharp minimum in the curve of the impedance phase as a function of the external magnetic field. This provides a new method for measuring the magnetic anisotropy field in such systems.

Mössbauer spectroscopic and x-ray diffraction studies of Fe/SiO2 nanocomposite soft magnetic materials

Nanocomposite high resistive soft magnetic materials Fe/SiO2 (Fe volume fraction 50%) have been synthesized using a wet chemical reaction method. A series of metal-ceramic Fe/SiO2 nanocomposite powder samples were obtained by annealing the precursor at temperatures from 400 to 900 °C in the presence of a reducing agent. The compositions of the precursor and the successive heat-treated samples have been investigated by 57Fe Mossbauer spectroscopy and x-ray diffraction, which revealed that the annealing process reduces nanosize granular ferrihydrite to α-Fe, and indicated that 800 °C is the optimum annealing temperature.

Study of the nitrogen diffusion mechanism in R2Fe17

Previously, nuclear magnetic resonance experiments and diffusion calculations have indicated that the distribution of nitrogen atoms in a Y2Fe17Nx particle with intermediate N content is characterized by a nitrided region and an unnitrided region. In order to directly detect this two‐region configuration, x‐ray diffraction experiments have been carried out on systematically ground nitrogenated samples. Furthermore, x‐ray diffraction and 89Y nuclear magnetic resonance on vacuum‐annealed samples show that the two‐region configuration is stable, and that the nitrogen atoms do not diffuse further into the particle. Thermal conductivity detection measurements indicate that only 5% of the inserted N atoms can be released by vacuum annealing at the nitrogenation temperature.

Nitrogen diffusion mechanism in the R2Fe17 lattice

In this report, a diffusion analysis has been extended to a lattice containing two interstitial sites, and the results obtained are used to understand: (1) the formation of a nitrided/unnitrided N configuration for intermediate N content and (2) the abnormally small (apparent) diffusion frequency factor, both of which characterize the newly developed R2Fe17 nitrides. It turns out that the diffusion mechanism for N atoms in the R2Fe17 lattice is a chemical reaction diffusion rather than a free‐diffusion process.

X-ray diffraction, magnetization and nuclear magnetic resonance study of Y2Fe17-xGax

A combined x-ray diffraction, magnetization and nuclear magnetic resonance ( and ) study of the system is presented. It is found that the substitution of larger Ga atoms for Fe causes a transformation from the hexagonal to rhombohedral structure and an overall average lattice expansion of per formula unit per Ga atom. Measurements on magnetically aligned powders yielded values for the anisotropy field and the average moment per Fe atom. Consistent with a change from planar to uniaxial anisotropy, the anisotropy field decreases with increasing Ga content and extrapolates to zero for . The average moment per Fe atom as well as the hyperfine field also decrease with Ga content. Using a calculated value of for the Y moment, values for the hyperfine coupling constants of and were obtained for the on-site and transferred contributions, respectively. The experimental results are discussed in terms of a model for the substitution of Ga for Fe in this system, and its relationship to the enhanced Curie temperature.

Mössbauer spectroscopy study of the rhombohedral phase Y2Fe17Nx with intermediate nitrogen content (0⩽x⩽2.8)

Mossbauer spectra were obtained at 15 K from samples of rhombohedral Y2Fe17 and its nitrides Y2Fe17Nx, with nominal nitrogen contents of x=0.6, 1.2, 1.8, 2.4, and 2.8. The spectra have been analyzed using a model which is consistent with the crystallographic structure and with magnetization that lies along a general direction in the hexagonal basal plane. The hyperfine field values for the four crystallographically inequivalent Fe sites change differently upon nitrogenation. For each of the Y2Fe17Nx samples with intermediate N content, the spectrum can be described as a superposition of two components, one from the nitrided shell of the sample particle and one from the outer portion of its unnitrided core. These results, along with earlier 57Fe nuclear magnetic resonance measurements, yield a consistent picture for the two-phase configuration of Y2Fe17Nx and its hyperfine field behavior.

QUANTITATIVE ANALYSIS OF THE NITROGENATION PROCESS IN Y2FE17NX BASED ON A TWO-REGION CONFIGURATION

A quantitative analysis has been carried out for the nitrogenation process using two groups of Y2Fe17Nx samples, one obtained by successively allowing longer nitrogenation times and the other by systematically varying the particle size. A detailed calculation using the two-region model results in an analytical solution for the nitrogen distribution as a function of nitrogenation time and particle size, which agrees with the observations in both groups of samples. This model describes the entire nitrogenation process (usually longer than 10 h), and predicts a diffusion parameter D0 with the correct order of magnitude (∼10−6 m2/s).

A Mössbauer Effect Study on the Structural Components in Potassium-Promoted Iron Oxide Catalysts for Dehydrogenation of Ethylbenzene

Potassium-promoted iron oxide catalysts for dehydrogenation of ethylbenzene to styrene belong to one kind of complex oxide catalysts with a spinel structure and they exhibit good catalytic properties. In this work, two series of potassium-promoted catalysts were prepared with different potassium-to-iron ratios and using different calcining temperatures. Mossbauer spectroscopy has been used to determine the optimal potassium amount and the lowest calcining temperature for spinel structure formation, and to detect other structural components in the catalysts. Information was obtained for K2CO3-promoted iron oxides which may prove useful for industrial applications of this type of catalysts.

Correlation between MI Effect and Transverse Anisotropy in Stress-Annealed Nanocrystalline Alloys: A Mössbauer Effect Study

In order to understand the correlation between the large and sensitive magneto-impedance (MI) effect in the tensile stress-annealed Fe-based nanocrystalline alloy Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and the transverse magnetic anisotropy on a microscopic scale, a Mossbauer effect study has been carried out. A series of ribbon samples were subject to thermal annealing with tensile stress a up to 60 MPa. MI measurements showed that the effective field of transverse magnetic anisotropy H k increases with increasing σ. Analysis of Mossbauer spectra provided direct evidence that there is an increasing transverse magnetic structure with increasing σ in these tensile stress-annealed samples. This work confirms, on the microscopic scale, that the transverse magnetic anisotropy arises from stress-annealing and is essential for the giant magneto-impedance effect.