Zhenxi Wang

Structure and magnetic properties of TbMn6−xAlxSn6 compounds

In the present work, the influence of the substitution of Al for Mn on the structure and magnetic properties of TbMn6−xAlxSn6 compounds was investigated. With increasing Al concentration the lattice constants a, c, and the unit-cell volume V decrease monotonically. The TbMn6−xAlxSn6 compounds exhibit ferrimagnetic ordering. The magnetic ordering temperature first decreases with increasing Al content up to x=1.5, then increases with further increasing Al content. As for the unsubstituted material, it was found that this series of compounds exhibits a second magnetic transition at lower temperature that is attributed to spin reorientation. The spin-reorientation temperature and the magnetic anisotropy field first decrease with increasing Al content, go through a minimum at x=0.5, then increase with further increasing Al content. The saturation magnetization decreases monotonically with increasing Al content. Two metamagnetic transitions are found in TbMn6Sn6 compounds, one at 0.44 T, and the other at about ...

Achievement of high coercivity in sintered R-Fe-B magnets based on misch-metal by dual alloy method

The R-Fe-B (R, rare earth) sintered magnets prepared with different ratio of alloys of MM-Fe-B (MM, misch-metal) and Nd-Fe-B by dual alloy method were investigated. As expected, the high ratio of MM-Fe-B alloy degrades the hard magnetic properties heavily with intrinsic coercivity lower than 5 kOe. When the atomic ratio MM/R ≤ 21.5% the magnetic properties can reach a practical level of Br ≥ 12.1 kGs, Hcj ≥ 10.7 kOe, and (BH)max ≥ 34.0 MGOe. And the effect of Hcj enhancement by the grain boundary diffusion process is obvious when MM/R ≤ 21.5%. It is revealed that the decrement of intrinsic magnetic properties of R2Fe14B matrix phase is not the main reason of the degradation of the magnets with high MM ratio. The change of deteriorated microstructure together with phase component plays fundamental roles in low Hcj. In high MM ratio magnets, (a) after annealing, Ce atoms inside main phase are inclined to be segregated in the outer layer of the main phase grains; (b) there is no thin layer of Ce-rich phase a...

Catalyst-seeded synthesis and field emission properties of flowerlike Si-doped AlN nanoneedle array

Flowerlike Si-doped AlN nanoneedle array is grown from cobalt particles seeded on Si substrate by evaporating AlCl(4) and SiCl(4) in NH(3) atmosphere. The nanoneedles are several microns in length, and their base and tip diameters are in the range of 50-150 and 5-30 nm, respectively. Field emission (FE) measurements show that the Si-doped AlN nanoneedle array has a low turn-on field (similar to 1.8 V/mu m) and a high field enhancement factor (similar to 3271), indicating that it is a promising candidate for FE applications. In addition, this catalyst-seeded and Si-doped method may facilitate the development of efficient AlN nanostructure FE devices. (c) 2006 American Institute of Physics.

Giant magnetoimpedance and colossal magnetoresistance in La0.75Sr0.25MnO3 at room temperature

The impedance as well as the resistance and reactance of the La0.75Sr0.25MnO3 manganite increase with increasing ac frequency due to the skin effect of the metal state below Curie temperature. The manganite La0.75Sr0.25MnO3 shows not only the colossal magnetoresistance effect but also the giant magnetoimpedance effect. For the giant magnetoimpedance, the magnetic field influences the impedance by affecting the penetration depth via the transverse permeability, whereas for the colossal magnetoresistance the magnetic field tends to align the local spins, leading to the decrease of the dc resistance via the interplay between a strong electron–phonon coupling and the double exchange effect of spin alignment on electron kinetic energy.

Giant magnetoimpedance and permeability in CoFeNiSiB amorphous ribbon

In the present work, the effect of longitudinal and transverse magnetoimpedances connected with the transverse permeability and longitudinal permeability for amorphous ribbons of CoFeNiSiB has been investigated in detail. Our results strongly show that there exists a correlation between the magnetic field dependences of the magnetoimpedance and the magnetic permeability.

Anisotropy of grain boundary diffusion in sintered Nd-Fe-B magnet

The grain boundary diffusion (GBD) was applied to magnetically aligned Nd-Fe-B sintered magnets on their different surfaces. The demagnetization curves show better squareness for Dy diffusing from the pole surfaces than that from the side surfaces. Kerr magnetic domain patterns on magnet surfaces in aligned and unaligned magnets were observed together with in situ optical microscope images. It indicates that the anisotropy of the GBD effect is attributed to the anisotropic texture of the grain boundary Nd-rich phase in the magnet.

Enhancement of room temperature magnetoresistance in La0.67Sr0.33Mn1−xTixO3 manganites

Abstract The enhancement of room temperature magnetoresistance can be induced by the Ti substitution for Mn in La 0.67 Sr 0.33 Mn 1− x Ti x O 3 , which may be connected with the shift of the Curie temperature from a high temperature to the near room temperature by the Ti substitution. This experimental observation may provide a route to improve CMR behavior of the perovskite manganites at room temperature.

MAGNETIC-PROPERTIES OF ND-FE-COB PERMANENT MAGNETIC-ALLOYS

Permanent magnets in the Nd16.5(Fe1−xCox)76.5B7 system were prepared by powder metallurgical methods. Magnetic hysteresis loops, with the magnetic field parallel and perpendicular to the aligning direction, were measured in the temperature range from 4.2 to 450 K. The magnetic anisotropy constants K1 and K2 of Nd2(Fe1−xCox)14B with 0≤x≤0.4 were determined by using aligned sintered magnets Nd16.5(Fe1‐xCox)76.5B7. The intrinsic coercive force iHc, magnetic anisotropy constants K1 and K2, and spin reorientation temperature decrease monotonically with increasing Co concentration.

Giant magnetoimpedance effect in Fe79.5P12C6Mo0.5Cu0.5Si1.5 nanocrystalline ribbons

Abstract Nanocrystalline structure was observed in Fe 79.5 P 12 C 6 Mo 0.5 Cu 0.5 Si 1.5 ribbons after annealed at temperatures from 350 to 500°C. After annealed at 350°C, the ribbon Fe 79.5 P 12 C 6 Mo 0.5 Cu 0.5 Si 1.5 consists of nanoparticles bcc α-Fe(Si, C) and amorphous matrix. Nanoparticles α-Fe(Si, C) and Fe 3 P appear in Fe 79.5 P 12 C 6 Mo 0.5 Cu 0.5 Si 1.5 ribbon after annealed at 500°C. The giant magnetoimpedance effect was found in nanocrystalline Fe 79.5 P 12 C 6 Mo 0.5 Cu 0.5 Si 1.5 ribbons. In the frequency range of 0.1–10 MHz, there is a maximum of magnetoimpedance ratio at 6.9, 1.9, and 1.5 MHz for samples with annealing temperatures of 350, 400, and 450°C, respectively. The value of (Z(0)-Z(65Oe))/Z(0) for the nanocrystalline ribbon Fe 79.5 P 12 C 6 Mo 0.5 Cu 0.5 Si 1.5 with annealing temperature 450°C can reach the value of 13% at 1.5 MHz. The GMI effect in Fe 79.5 P 12 C 6 Mo 0.5 Cu 0.5 Si 1.5 ribbon annealed at 500°C is very small (less than 2%).

Study of sintered Nd-Fe-B magnet with high performance of Hcj (kOe) + (BH)max (MGOe) > 75

We have developed a Nd-Fe-B sintered magnet of extremely high performance. The intrinsic coercivity Hcj is as high as 35.2 kOe (2803kA/m) together with the maximum energy product (BH)max of 40.4 MGOe (321.6kJ/m3). These values result in Hcj (kOe) + (BH)max (MGOe) > 75. Between 293 K (20oC) and 473 K (200oC), the temperature coefficients of remanence and intrinsic coercivity are αBr = −0.122 %/°C and αHcj = −0.403%/°C, respectively. A maximum operating temperature of 503 K (230oC) is obtained when permeance coefficient Pc = −B/H = 2. Grain boundary diffusion (GBD) technique on magnet surface has been developed to increase Hcj by 3.6 kOe without significantly decrease of Br and (BH)max. The intrinsic coercivity of the GBD treated magnet Hcj(C) has a linear relationship with that of the untreated magnet Hcj(B) between 200 K and 473 K (in unit of kOe): Hcj(C) = 1.03Hcj(B) + 2.38. The enhancement of Hcj by GBD treatment has contributions not only from the improvement of microstructure but also from the increas...