Weihua Mao

MAGNETIC PROPERTIES AND MAGNETIC DOMAIN STRUCTURES OF NDFE10.5MO1.5 AND NDFE10.5MO1.5NX

We succeed in preparing anisotropic magnetic powders with high performance based on the NdFe10.5Mo1.5Nx nitrides. The properties of these materials are favorable for permanent magnet application. The domain structures of the NdFe10.5Mo1.5 and NdFe10.5Mo1.5Nx were studied by using magnetic force microscopy. Upon nitrogenation, a domain structure transition from complex maze to simple stripe was found. This transition is due to the strongly uniaxial magnetocrystalline anisotropy induced by interstitial nitrogen atoms. Together with magnetic measurements, we have calculated the domain wall energy γ, exchange constant A, domain wall thickness δ, and critical single-domain particle size Dc of NdFe10.5Mo1.5 and NdFe10.5Mo1.5Nx.

Variation of magnetic properties as a function of Mo content in the RFe12−xMox series and their nitrides (R=Y, Nd, Gd)

The structure and magnetic properties of RFe12−xMox series and their nitrides with R=Y,Gd,Nd and x=0.8,1.0,1.5,2.0,2.5 have been studied by x‐ray‐diffraction and magnetic measurements. It is found that with the increment of Mo content x, the lattice parameters increase, whereas the saturation magnetization, Curie temperature, and anisotropy field decreases in the RFe12−xMox series. Upon nitrogenation lattice parameters, saturation magnetization, and Curie temperature increase. Correspondingly, the easy c‐axis magnetocrystalline anisotropy of the Fe sublattice decreases in YFe12−xMoxNy and GdFe12−xMoxNy but increases in NdFe12−xMoxNy although its variation with the Mo content is the same as in their original counterparts. NdFe12−xMoxNy compound in the low Mo content shows great potential for permanent magnet applications.

Structural and magnetic properties of H and N modified Y(Fe, M)12 compounds (M Ti, V, Mo, Cr, W)

Abstract A series of Y(Fe, M) 12 compounds and their hydrides and nitrides, where M  Ti, V, Mo, Cr, W, were prepared single phase. The effects of interstitial H and N atoms on the structural and magnetic properties have been studied by using the X-ray diffraction and magnetic measurements. The hydrides and nitrides still retain the tetragonal structure, but the unit cell volumes are expanded compared to their original counterparts. The saturation magnetization and Curie temperature are increased by both hydrogenation and nitrogenation, whereas the easy c -axis anisotropy related to the Fe-sublattice is enhanced in the hydrides but decreased in the nitrides.

The effects of hydrogen disproportionation, desorption, and recombination on the structure and magnetic properties of Sm2Fe17Nx and NdFe10Mo2Nx compounds

Hydrogen disproportionation, desorption, and recombination (HDDR) has been used as a pretreatment to prepare high performance Sm2Fe17Nx and NdFe10Mo2Nx compounds. Isotropic Sm2Fe17Nx and NdFe10Mo2Nx compounds with intrinsic coercivity larger than 12 and 4 kOe have been obtained by nitriding the HDDR treated powders, respectively. It is found that the magnetic properties are sensitively dependent on the time and temperature of the HDDR process, which determine the grain size, nitrogen content, and the amount of α‐Fe in the nitrides.

Micromagnetic study of Nd2Fe14B/α-Fe and Nd2Fe14B/Fe3B nanocomposite magnets

Abstract The remanence enhancement, phase fractions, and coercivity mechanism of Nd2Fe14B/α-Fe and Nd2Fe14B/Fe3B nanocomposite magnets were studied using the micromagnetic theory. It was found that magnetocrystalline anisotropy energy density K, together with saturation magnetization Ms of the soft magnetic phase, plays a key role in the actual value of coercive fields for both Nd2Fe14B/α-Fe and Nd2Fe14B/Fe3B nanocomposite magnets. The differences between magnetic properties of Nd2Fe14B/α-Fe and Nd2Fe14B/Fe3B were successfully explained. Theoretical results support the experimental values.

A study on the effect of hydrogen in the compounds with -type structure

The structural and magnetic properties of hydrides with -type structure have been studied by means of magnetic measurements, the neutron powder diffraction technique and self-consistent spin-polarized band calculations (LMTO-ASA). We found that the hydrides retain the -type structure, but with an increase of unit-cell volume. The neutron diffraction results indicate that hydrogen atoms occupy the interstitial 2b sites. Magnetic measurements carried out on , and their hydrides show that both the Curie temperature and the saturation magnetization can be enhanced by introducing interstitial hydrogen atoms. Band-structure calculations and spin-fluctuation theory give a fair description of the enhancement of the magnetization and Curie temperature.

Nitrogenation of the magnetic compound R(Fe,M)12

The diffusion mechanism of nitrogen atoms in ThMn12-type crystal structure compounds was investigated by using x-ray diffraction, Mossbauer spectroscopy measurements, thermopiezic analysis, and differential thermal analysis. The studies indicate that the distribution of nitrogen atoms in a R(Fe,M)12 particles with intermediate N content is characterized by a nitrided region and an unnitrided region. The results confirmed that this two-phase region is stable, and the nitrogen atoms do not diffuse further into the particle. Based on the characteristics of the nitrogenation, a trapping diffusion model is applied to the R(Fe,M)12 system. On the basis of this study high performance magnetic powders of NdFe10.5Mo1.5Nx were successfully prepared.

EFFECT OF INTERSTITIAL NITROGEN ON THE STRUCTURAL AND MAGNETIC PROPERTIES OF NDFE10.5V1.5NX

The NdFe10.5V1.5Nx nitrides crystallize in the ThMn12-type structure. The nitrogen atoms occupy interstitial sites, and their most important effects are on the crystal fields around the rare earth ion sites. The variation of anisotropy fields of NdFe10.5V1.5Nx as a function of the nitrogen content x is presented. The crystal field interaction parameters are determined by using single-ion model. In the light of this study, high performance magnetic powders based on NdFe10.5V1.5Nx were successfully prepared.

SYNTHESIS AND CHARACTERIZATION OF HARD MAGNETIC MATERIALS: PRFE10.5V1.5NX

The PrFe10.5V1.5 intermetallics and their nitrides were successfully synthesized. In terms of magnetocrystalline anisotropy, PrFe10.5V1.5Nx are characteristics of an easy axis from 0 K to Curie temperature, with an anisotropy field up to 152.9 kOe at 1.5 K and 108.4 kOe at room temperature. In combination with a high Curie temperature of 820 K and a large saturation magnetization of 157.46 emu/g at 1.5 K and 142.77 emu/g at room temperature, these nitrides are favorable for permanent magnet applications. As a preliminary attempt, magnetic powders based on PrFe10.5V1.5Nx were obtained with a maximum energy product of 16.0 and 28.8 MGOe at room temperature and 1.5 K, respectively.

Microstructure and domain structures of Sm0.56GdxDy0.44−x(Co0.68Fe0.22Cu0.08Zr0.02)7.22 magnets with low temperature coefficients

The microstructure and domain structures of high performance temperature-compensated sintered magnets with composition of Sm0.56GdxDy0.44−x(TM)7.22 (TM=Co0.68Fe0.22Cu0.08Zr0.02)7.22 (0.0⩽x⩽0.4) have been investigated by using electron microscopy and magnetic force microscopy. Both microstructure and magnetic properties of these magnets show a significant dependence on the content of heavy rare earth Gd and Dy. The intrinsic coercive force increases significantly with increasing Gd content. It is found that there are two different types of grains in the magnets, which show different domain structures. The larger magnetocrystalline anisotropy, larger cell size, and coarser grain boundary region give rise to a higher coercivity in the Gd-rich magnets.