Y. D. Zhang

Effect of magnetic field on the superparamagnetic relaxation in granular Co-Ag samples

In order to study the effect of applied magnetic field on the superparamagnetic relaxation behavior of small Co particles, magnetization measurements were carried out on as-prepared and annealed granular samples of Co20Ag80 and Co25Ag75. Values of the superparamagnetic blocking temperature TB− were obtained from the characteristic peak in the zero-field-cooled magnetization. Consistent with existing models, it was found that the initial decrease of TB− with applied magnetic field is quadratic. An estimate of the magnetic anisotropy “energy density” Ku yielded a value which is two orders of magnitude greater than the value for bulk cobalt. The results reported here underscore the importance of considering the effect of superparamagnetic relaxation on the performance of nanostructured magnetic materials.

Microstructure and magnetic behavior of carbon-coated Co nanoparticles studied by nuclear magnetic resonance

Spin-echo nuclear magnetic resonance (NMR) experiments have been carried out at 4.2 K and room temperature on carbon-coated nanoscale (average diameter ≈20 nm) face-centered-cubic (fcc) Co particles prepared by the Kratschmer carbon arc process for 0⩽H⩽9.0 kOe. Information concerning the magnetic structure and paramagnetic relaxation behavior of the nanoscale particle system has been obtained.

Three dimensional alignment of magnetic particles

In this article, novel magnetic particle alignment methods, which we refer to as dynamic alignment, are presented. In the process of a dynamic alignment, the magnetic field direction is periodically changed while the particle orientation is captured in epoxy resin. Depending on the manner of the variation of the magnetic field, particles can be aligned in various patterns. By employing the dynamic alignment methods described in this article, the alignment of magnetic particles can be achieved such that particles are aligned along all three crystalline axes.

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.

Nuclear magnetic resonance study of polycrystalline Sr1-xCaxRuO3 (0le xle1.0)

A zero-field spin-echo nuclear magnetic resonance (NMR) study of 99,101 Ru nuclei in polycrystalline powder samples of magnetically ordered Sr1 - x Cax RuO3 (0 x 1.0) is reported. The NMR spectrum for SrRuO3 at 1.3 K consists of two peaks at 64.4 MHz and 72.2 MHz corresponding to the 99 Ru and 101 Ru isotopes, respectively, and a hyperfine field of 329 kG. With the replacement of Sr by Ca, the peaks broaden somewhat, although there is essentially no change in the peak frequencies. However, the two resonance peaks show a significant reduction in height with intensity being shifted into the wings. Although electric quadrupole effects are present, the results suggest that the reduction of peak intensity when Ca is added is due mainly to a progressive loss of Ru moments participating in the magnetic ordering. The loss is a consequence of the dilution of the ferromagnetic exchange coupling between the moments. A measure of the spontaneous magnetization temperature dependence for SrRuO3 ( 21 K) has been obtained from the shift of the peak frequencies. It was found that the spontaneous magnetization decreases with a Bloch law T 3/2 temperature dependence characteristic of spin-wave excitations. From the coefficient of the temperature dependence, a value of 2.4 meV was obtained for the exchange integral which is consistent with the observed ordering temperature.

Effect of Ga substitution on the Sm sublattice anisotropy in Sm2Fe17-xGax (0 <= x <= 8)

In order to study the unique behavior of the Sm sublattice anisotropy, magnetization curves along both the easy and hard directions were obtained at 10 K for Sm2Fe17−xGax (0⩽x⩽8) powders which were aligned using either static- or dynamic-field alignment methods. The Fe sublattice contribution was isolated by making similar measurements on Y2Fe17−xGax (0⩽x⩽8), as the presence of Y eliminates the rare-earth sublattice contribution. The results show that the Sm sublattice anisotropy is dominant for x⩾1. It increases with increasing Ga content, reaching a (broad) maximum of 1.8×107 erg/cm3 for x≈2–3, and then decreases and becomes negative for x≈5.5. This work shows that the unique behavior of the magneto-crystalline anisotropy for Sm2Fe17−xGax arises from the variation of the Sm-sublattice contribution with Ga substitution.

Nitrogen location in R2Fe17 compounds: an NMR study

Abstract 14 N, 89 Y and 1 H spin-echo NMR experiments have been carried out in order to study the nitrogen location in the R 2 Fe 17 lattice with R = Gd, Lu and Y. The observed 14 N spectrum for the Gd-compound indicates that essentially all of the N atoms occupy the octahedral interstitial sites. However, the ‘two-peak’ 14 N spectra for the Y-compounds and, to a lesser degree the Lu-compound, indicate that some of the N atoms enter the tetrahedral interstitial sites in addition to the octahedral sites. A consideration of both 14 N and 89 Y spectra indicates that the probability for a N atom to enter a tetrahedral site in the Y 2 Fe 17 lattice is much smaller than that for octahedral site occupation. This study reveals that the local structure of Y 2 Fe 17 N x may change with the nitrogenation conditions.

Nuclear magnetic resonance study of R2Fe17 (R=Y, Sm, and Gd) hydrides

In order to study the location of hydrogen atoms and the effects of their insertion into R2Fe17, spin–echo NMR experiments have been carried out on the hexagonal Y2Fe17H x (x=0, 3.0, 4.7), rhombohedral Sm2Fe17H x (x=0, 1.7, 5.7), and mixed‐phase Gd2Fe17H x (x=0, 2.3, 5.8) compounds. 1H and 89Y spectra obtained from Y2Fe17H x clearly demonstrate that the hydrogenation process is reversible upon vacuum annealing. For both Y2Fe17H x and Sm2Fe17H x , the 1H spectra show two broad peaks; the peaks are tentatively assigned to H atoms in the tetrahedral and octahedral interstitial sites, and a hydrogen filling scheme is proposed. For Gd2Fe17H x , a single broad 1H peak near 70 MHz is observed; application of an external magnetic field indicates that the hyperfine field has the same direction as the net magnetization.

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.