Youhui Gao

Electron holography of Nd-Fe-B nanocomposite magnets

Abstract Magnetization distribution in Nd–Fe–B nanocomposite magnets was investigated by electron holography, using a new pole piece apparatus dedicated to observations of nanocrystalline ferromagnetic materials. The exchange coupling between the magnetically soft and hard grains of 20–30 nm was experimentally verified by this microscopic study with improved resolution.

Superstructures of self-assembled cobalt nanocrystals

Uniform three-dimensional superstructures of spherical cobalt nanocrystals are produced by the interplay between dipolar interaction and applied magnetic field. An anomalous low-temperature magnetic behavior is observed, indicating that uncompensated surface spins become ordered below 10 K, as evidenced by the presence of two magnetic phases that superimpose in hysteresis loops as compared to measurements at 20 K. The approach discussed here provides a framework for applications such as high-performance mesomagnets, microelectronic and magnetic devices fabrication, and can be extended to other nanocomposite materials fabrication if cobalt particles can act as carriers for other nanoparticles.

Electron holography of Fe-based nanocrystalline magnetic materials (invited)

Magnetic domain structures of nanocrystalline magnetic materials were extensively investigated by electron holography with a change in temperature or magnetic field applied. In both soft and hard magnetic materials, the distribution of lines of magnetic flux clarified in situ by electron holography was found to correspond well to their magnetic properties. An attempt to produce a strong magnetic field using a sharp needle made of a permanent magnet, whose movement is controlled by piezo drives has been presented. This article demonstrates that the attempt is promising to investigate the magnetization process of hard magnetic materials by electron holography.

Nonuniform magnetic structure in Nd2Fe14B/Fe3B nanocomposite materials

The magnetic structure of a Nd4Fe77Co3B16 two-phase nanocomposite is studied by Lorentz microscopy and electron holography in this article. The big domains consisting several grains show strong intergranular exchange interaction. Some bright spots inside domains reflect a nonuniform magnetic structure that is characterized as a magnetic vortex using electron holography. It results from the strong intergranular exchange interaction and the randomly oriented Nd2Fe14B particles. This nonuniform magnetic structure is extensive in two-phase nanocomposites, and has an important influence on the magnetic properties. The internal stray field is dramatically weakened because of the zero divergence of magnetization in the magnetic vortex, and small Neff is observed. With the numerical simulation, it is found that the formation of the magnetic vortex induces a concave demagnetization curve and decay of the coercivity. This is the reason why the coercivity of practical magnets deviates greatly from that of theoretica...

Electron holography of core-shell Co/CoO spherical nanocrystals

Phase profiles of core-shell Co∕CoO nanocrystals are imaged using off-axis electron holography. Contributions of mean inner potential and magnetic induction to the phase shift are extracted, respectively, to quantitatively characterize the nanocrystal geometry and magnetic spin arrangement in a self-assembled nanocrystal chain. It is also found that the spin of 11nm diameter Co core encapsulated with 5-nm-thick CoO shell remains stable even at an elevated temperature of 200°C, indicating that the magnetically thermal stability of cobalt nanocrystals is remarkably improved due to the exchange coupling between the ferromagnetic core and antiferromagnetic shell.

Direct observation of magnetization reversal in thin Nd2Fe14B film

By utilizing a sharp magnetic needle made of a sintered Nd2Fe14B permanent magnet, we were able to produce a strong magnetic field of up to approximately 560kA∕m in a transmission electron microscope and succeeded in observing the magnetization reversal in a thin film of sintered Nd2Fe14B by in situ Lorentz microscopy. The increase in the magnetic field induced by the magnetic needle led to a pair of straight magnetic domain walls forming abruptly from the grain boundary. The successive nucleation in the neighboring grain started at the grain boundary adjacent to the magnetic domain wall formed first. Eventually, the magnetic domain walls in the neighboring grains connected and moved continuously and finally disappeared.

Magnetic microstructure of Fe84Nb7B9 alloys observed by electron holography

The microstructure, magnetic properties and magnetic structure of crystallized Fe84Nb7B9 samples are studied in this article. It is found that the size of the body-centered-cubic Fe crystallites is independent of the annealing temperature. The volume fraction of the amorphous phase decreases with an increase in annealing temperature and iron boride precipitates form at temperatures over 973 K. Excellent soft magnetic properties are obtained in the sample annealed at 923 K which has large uniform domains with smooth and straight walls, whereas precipitates of Fe boride in the sample annealed at 973 K degrade the magnetic softness. Their magnetization processes are pure magnetization rotation dominated by uniaxial anisotropy. The thick amorphous phase matrix and strong internal stray field in the sample annealed at 773 K lead to a magnetic ripple structure. The domain wall movement is strongly pinned by anisotropy fluctuations. The longitudinal wavelength of the magnetic ripples increases with a decrease in...

Domain structures of nanocrystalline Fe90Zr7B3 alloy studied by Lorentz microscopy

Abstract As-quenched Fe90Zr7B3 alloy has been crystallized at 773, 923 and 973 K, it is found that the specimens annealed at 773 and 923 K have grains with the same size, but the one annealed at 773 K has a thick amorphous matrix and a broad grain size distribution. Big domains and smooth domain walls are observed in the specimen annealed at the optimum condition (923 K), and excellent magnetic softness is obtained. On the other hand, the one annealed at 973 K has very large grains (40 nm). Very small domains with irregular walls are observed, indicating a weak intergranular exchange coupling. Through in situ Lorentz microscopy, a relaxation of internal stress in the specimen annealed at 773 K is observed at anelevated temperature (333 K). The internal stress observed is considered to be one of the important factors that degrade the soft magnetic properties. Based on the results of differential thermal analysis, a two-step annealing, where the as-quenched specimen is pre-heated at 723 K and subsequently annealed at 773 K, is utilized. The domain structure of the specimen treated by the two-step annealing is analyzed. It is found that the size of the domain is larger than that of the specimen annealed at 773 K, suggesting the possibility of control of the soft magnetic properties.

Microstructure and magnetic properties of cobalt nanocrystals

Cobalt nanocrystals are synthesized by rapid pyrolysis of cobalt carbonyl in an inert atmosphere. The size distribution and particle shape of the crystals are controlled by varying the surfactant, its concentration and the reaction temperature. The particles are observed to contain abundant planar defects and their self-assemblies are found to be strongly dependent on their magnetic states. Magnetic order in nanocrystal chains are studied by electron holography.

Correlation Exchange Length in Nanocrystalline Soft Magnetic Materials Characterized by Electron Holography

Magnetic ripple structure of Fe84Nb7B9 is studied by electron holography. The correlation exchange length, effective exchange and anisotropy constants are estimated from a field dependence of ripple wavelength. The function of ripple theory is adjusted with a decay constant, which reflects influence of stray field on the ripple structure. A magnetic hardness at elevated temperature is also observed and analyzed.