Benpei Cheng

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.

High coercivity in mechanically milled ThMn12-type Nd–Fe–Mo nitrides

Starting from carefully homogenized Nd10Fe90-yMoy (y=12, 10, 7) alloys and by appropriate mechanical milling, the as-milled microstructure consisting of a nanoscale mixture of severely distorted 1:12 phase and substitutional α-Fe-based solid solution was obtained. This kind of as-milled microstructure was thought to have a critical effect on the formation of iron-free nanocrystalline 1:12 phase during subsequent annealing. Upon nitrogenation, the sample of Nd10Fe78Mo12Nx exhibited a record-high coercivity of 13.1 kOe at 293 K. Measurements of initial magnetization curve and a family of demagnetization curves engendered under different maximum applied fields Hm were carried out, and the results revealed the domain-wall pinning at grain boundaries as the coercivity mechanism. A low Mo-content sample of Nd10Fe83Mo7Nx with iHc∼5.8 kOe, Br∼6.8 kG, and (BH)max∼7.0 MG Oe was made by optimizing the preparation conditions.

High-performance Pr2Fe14C-type magnets prepared by melt spinning

Without annealing, Pr13Fe80-xCoxC5B2 (x = 0-20) ribbons, with a coercive force. of more than 1.0 T have been prepared by melt spinning. Due to Co addition, rectangularity of demagnetization curves is significantly improved. The remanence and maximum energy product of the samples are obviously increased with the increase of x (x less than or equal to 15). A 3 maximum energy product of 145.7 kJ/m(3) which is the highest value among all that previously reported in the R2Fe14C-type (R = Nd or Pr) ribbons, has been achieved in the ribbons (x = 15) prepared directly by melt spinning. The Curie temperatures of 2:14:1 carbides of the ribbons in the as-quenched state increase linearly with X at an average rate 12K/at%Co. Similar to B and Cu, Co addition is found to accelerate the formation of 2:14:1 carbides

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.

MAGNETIC PROPERTIES OF ANISOTROPIC ND(FE,MO)12NX POWDERS

Anisotropic magnets based on Nd(Fe,Mo)12Nx nitrides were prepared by a milling process. Particle size dependence of the coercivity has been investigated. The optimizing size which accompanies the highest coercive force was examined by transmission electron microscopy. A coercive force iHc of up to 6.2 kOe, and a maximum energy product (BH)max of up to 12.0 MG Oe were obtained. The mechanism of the coercivities is discussed with a comparison of the powders prepared by mechanical alloying. The variation of coercive force as a function of temperature is reported.

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.

Neutron diffraction studies on YFe10.5Mo1.5Cx prepared by arc-melting

Abstract Neutron power diffraction studies are performed with different reactors on YFe10.5Mo1.5Cx (x = 0.3 and 0.6) prepared by arc-melting. The refinements of the neutron diffraction data indicate that the carbides retain the ThMn12-type structure with an increase of unit cell volume and most of carbon atoms occupy the interstitial 2b sites. The effects of carbon introduced by arc-melting are similar to that of interstitial nitrogen introduced by the gas-solid phase reaction, which is of significance in the technical applications due to a better stability of these carbides.

Study of the coercivity of Pr(Mo, Fe)12Nx

A study of the permanent magnetic properties of Pr(Mo, Fe)12Nx powders has been carried out by using a mechanical alloying technique. The variations of coercive force and remanence as a function of composition and crystallizing temperature have been investigated. A coercive force of up to 6.0 kOe was obtained. The temperature dependence of the coercive force is presented in comparison with that of Nd2Fe14B‐type magnets.

Thermal expansion anomalies of R(Fe, M)12 (R=Y, Nd; M=Mo and Si)

Structural and thermal-expansion anomaly studies on R(Fe,M)12 (R=Nd and and Y, M=Mo and Si) compounds were performed by x-ray diffraction. Mo atoms occupy the 8i site. While Si atoms occupy the 8f and 8j sites but not the 8i site. Thermal-expansion anomaly shows only in ab plane in the Mo compounds, while becomes very weak and along with only the c axis in the Si compounds. The anomaly was attributed to the contribution of the interactions of short Fe–Fe distances similar to the previous explanation on other R–Fe intermetallics and that of other strongly positive interactions such as 8j-8j.