Coercivity Of Nd Research Articles

Magnetic Properties of Nd–Fe–B–Cr Nanocrystalline Composite Magnets

Magnetic properties and phase composition of Nd 4.5 Fe 77 B 18.5 exchange-coupled nanocrystalline composite magnet have been investigated as a function of heat treatment by means of differential thermal analysis, magnetization, X-ray diffraction, and 57 Fe Mossbauer measurements. Cr content dependence of magnetic properties and phase composition of Cr-added Nd 4.5 (Fe 1-x Cr x ) 77 B 18.5 has also been investigated. By annealing up to 853 K, Nd 4.5 Fe 77 B 18.5 amorphous ribbon crystallizes in both t-Fe 3 B and Nd 2 Fe 23 B 3 . These two compounds are soft magnets and the specimen does not have coercivity. Hard magnetic Nd 2 Fe 14 B is precipitated by annealing up to 943 K, and the specimen becomes hard magnetic. Nd 4.5 Fe 77 B 18.5 nanocrystalline composite magnet annealed up to 943 K consists of 10 mole% Nd 2 Fe 14 B and 90 mol% t-Fe 3 B, and the small amount of hard magnetic phase results in large coercivity of this material. With increasing Cr content, coercivity of Nd 4.5 (Fe 1-x Cr x ) 77 B 18.5 optimally heat-treated ribbon increases, but remanence decreases. In all the specimens, hard magnet Nd 2 Fe 14 B is formed and the amount is found to decrease with increasing Cr content. This is one of the reasons for the reduction of remanence. Soft magnetic phases which crystallize in the specimen changes from t-Fe 3 B to Fe 2 B and α-Fe depending on Cr content. Hyperfine magnetic field of Fe-B compound decreases as a function of Cr content, while those of Nd 2 Fe 14 B and α-Fe are independent of Cr content. Therefore, Cr atoms mainly exist in Fe-B compound, not in Nd 2 Fe 14 B and α-Fe, and reduce the magnetization and Curie temperature of soft magnetic phases. In addition, the total amount of soft magnetic compounds decreases by the Cr-substitution for Fe. These influences of Cr atoms result in the decrease of the remanence of this material.

Coercivity of Nd(Dy) - Fe - B bonded magnets made from the inert-gas-atomized powders

The coercivity behaviour of bonded magnets made from sieved inert-gas-atomized Nd(Dy) - Fe - B powders has been investigated. For magnets with particle sizes , the coercivity is controlled by a nucleation mechanism as in the sintered magnet. For the magnet with particle sizes of , the hardening mechanism is mainly controlled by nucleation as in the sintered magnet but the hardening mechanism of domain wall pinning or the nucleation of a single domain, which usually appears in melt-spun materials, may also be involved. The coercivity behaviour of bonded magnets made from gas-atomized powders seems to be between those of the sintered and melt-spun magnets depending on the particle size. Meanwhile it has been found that the demagnetizing field of bonded magnets made from the inert-gas-atomized powders is very small.

High pressure compacting of high speed steel powders: J. Asami, T. Aizawa (University of Tokyo, Tokyo, Japan)

Nd–Fe–B oxide powders with various pH were prepared using chloride and nitrate precursors including NdCl3·6H2O, FeCl3·6H2O, H3BO3, Nd2O3, Fe(NO3)3·9H2O, HNO3, citric acid (CA), ethylene glycol (EG) by Pechini type sol–gel method. The pH of chloride and nitrate base sols were 0 and 2.2, respectively. Mixed oxide powders were obtained by calcination and annealing the gels. These oxides by using a reduction-diffusion process under high vacuum and employing CaH2 as reducing agent at 800 °C were hated to prepare Nd2Fe14B nanoparticles. The role of pH on phase, morphologies, microstructure, and magnetic properties of the powders were investigated. The results show that with a decrease in pH, the average particle size and coercivity of Nd–Fe–B oxide powders were decreased and increased, respectively. Nd2Fe14B nanoparticles were formed successfully after reduction process. The average particle size of reduction treated products were 30 and 65 nm for powders which made of chloride and nitrate base metal salts, respectively. Final powders which made of chloride and nitrate base metal salts had a saturation magnetization of 127.7 emu/g and 122.8 emu/g while the coercivity of samples were 3.32 kOe and 1.82 kOe, respectively. The experimental results in the angular dependence of coercivity indicated that the normalized coercivity of the permanent magnets Hc(θ)/Hc(0) obeys the 1/cosθ law and intermediate between the 1/cosθ law and Stoner-Wohlfarth formula for different Nd2Fe14B magnets which made of nitrate and chloride base metal salts, respectively. Also, the results show that different Nd2Fe14B magnets which made of nitrate and chloride base metal salts had the maximum energy product of 5 and 16 MGOe, respectively. The Henkel plot showed that magnetic phases in synthesized NdFeB magnets which made of chloride and nitrate base metal salts were coupled by exchange and dipolar interactions, respectively. Different average particle size, morphology and microstructure were the reasons for variation of magnetic properties.

Relation of remanence and coercivity of Nd,(Dy)-Fe,(Co)-B sintered permanent magnets to crystallite orientation

For sintered permanent magnets based on Nd2Fe14B having magnetically oriented crystallites, a simple method of measuring the degree of alignment by x-ray diffraction is described, using the inverse pole figure technique. The results are related to a distribution function for the easy axis which is a direct measure of remanence. The fraction of the ideal [001] texture component in both Nd-Fe-B and Nd,Dy-Fe,Co-B magnets is enhanced by doping with oxygen, resulting in a measurable increase in remanence and improved loop squareness. The observed increase of the intrinsic coercivity iHc with better crystallite orientation is also discussed.

The effect of the cooling rate on the intrinsic coercivity of some NdFeB based permanent magnets

The effects of furnace cooling, air cooling and quenching in liquid argon from 1100°C on the coercivity of Nd 16.4Fe 75.7B 7.9 (A), Nd 17Fe 78.5B 5Cu 1.5 (B) and Nd 17Fe 79B 4 (C) sintered magnets have been investigated. In magnet A, a considerable fall in H cl, occurred on quenching whilst air or furnace cooling caused successively less deterioration in H cl . The coercivity of quenched magnet A was partially restored during annealing at 600 or 680 °C. In magnet B, no significant fall in H cl was observed on cooling at different cooling rates, and a considerable improvement in H cl occurred on subsequent annealing at 600°C. Magnet C showed a remarkable increase in H cl on quenching whilst air cooling or furnace cooling resulted in successively smaller improvements in the H cl values. The coercivity of magnet C dropped drastically after quenching and then annealing at 600°C. In magnets A and B the coercivity behaviour on cooling was related mainly to the melting temperature and to the wettability characteristics of the intergranular phase(s) in these magnets, and to the formation of metastable phases on quenching, whereas in magnet C, redistribution and dissolution of soft magnetic phase(s) during the high temperature (1100°C) treatment could be responsible for the changes in H cl .

Magnetic Properties and Corrosion Characteristics of Nd-(Fe,Co,Ni)-B Pseudo-Ternary System

Melt-spun ribbons and sintered materials with the composition Nd 15 (Fe 1-x-y Co x Ni y ). 77 B 8 were investigated with respect to the saturation magnetization, coercivity, Curie temperature and corrosion characteristics. It was found that simultaneous substitution of Co and Ni atoms for Fe atoms enhances the intrinsic coercivity of sintered magnets in the composition range 0.2?x?0.4 and 0.0?y?0.2, and also improves the corrosion resistance. A further addition of Ti atoms to the system increases the coercivity and makes the demagnetization curve rectangular. For example, the (BH) max and intrinsic coercivity in Nd 15 (Fe 0.585 Co 0.30 Ni 0.10 Ti 0.015 ) 77 B 8 are as high as 33.8 MGOe and 10.4 kOe respectively.

Coercivity and Grain Boundary Morphology in Nd–Fe–B Sintered Magnets

The microstructure of Nd15Fe77B8 sintered permanent magnets has been investigated under a 400 kV high-resolution transmission electron microscope. With particular concern about the microstructural changes in the Nd–Nd2Fe14B interfaces, the effects of isothermal heat treatment at 870 K have been studied for a more comprehensive understanding of the coercivity-microstructure relationship in the Nd–Fe–B sintered magnets. The atomic-scale steps along the grain boundary can be observed in as-sintered samples. The steps disappear after heat treatment, and very flat interfaces are formed. This atomic-scale alignment at the grain boundary, therefore, is one of the factors affecting the coercivity of Nd–Fe–B sintered magnets.

The influence of Ga-substitution on the coercivity of Nd-(Fe,Co)-B sintered permanent magnets

In Co-substituted Nd-Fe-B sintered magnets, several additional phases such as Nd(Fe,Co)/sub y/B/sub ?/ with y=2, 3, and 4 have been found besides the hard magnetic phase and the Nd-rich intergranular phase. The low coercivity of the magnet is attributed to the occurrence of these soft ferromagnetic phases. In Ga-doped Nd-(Fe,Co)-B sintered magnets, new intergranular phases such as Nd(Ga,Fe,Co) and Nd(Ga,Fe,Co)/sub 2/ have been found instead of the soft ferromagnetic Co-rich phases. From X-ray microanalysis data it is concluded that Ga substitutes the transition-metal atoms in the 2:14:1-phase, and Ga is not detected in the Nd-rich phase at all. The increase of the coercivity of Ga-doped magnets is correlated to a higher volume fraction of intergranular phases and an increased wetting during sintering due to these phases.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Principles and techniques of electron microscopy

Nd–Fe–B oxide powders with various pH were prepared using chloride and nitrate precursors including NdCl3·6H2O, FeCl3·6H2O, H3BO3, Nd2O3, Fe(NO3)3·9H2O, HNO3, citric acid (CA), ethylene glycol (EG) by Pechini type sol–gel method. The pH of chloride and nitrate base sols were 0 and 2.2, respectively. Mixed oxide powders were obtained by calcination and annealing the gels. These oxides by using a reduction-diffusion process under high vacuum and employing CaH2 as reducing agent at 800 °C were hated to prepare Nd2Fe14B nanoparticles. The role of pH on phase, morphologies, microstructure, and magnetic properties of the powders were investigated. The results show that with a decrease in pH, the average particle size and coercivity of Nd–Fe–B oxide powders were decreased and increased, respectively. Nd2Fe14B nanoparticles were formed successfully after reduction process. The average particle size of reduction treated products were 30 and 65 nm for powders which made of chloride and nitrate base metal salts, respectively. Final powders which made of chloride and nitrate base metal salts had a saturation magnetization of 127.7 emu/g and 122.8 emu/g while the coercivity of samples were 3.32 kOe and 1.82 kOe, respectively. The experimental results in the angular dependence of coercivity indicated that the normalized coercivity of the permanent magnets Hc(θ)/Hc(0) obeys the 1/cosθ law and intermediate between the 1/cosθ law and Stoner-Wohlfarth formula for different Nd2Fe14B magnets which made of nitrate and chloride base metal salts, respectively. Also, the results show that different Nd2Fe14B magnets which made of nitrate and chloride base metal salts had the maximum energy product of 5 and 16 MGOe, respectively. The Henkel plot showed that magnetic phases in synthesized NdFeB magnets which made of chloride and nitrate base metal salts were coupled by exchange and dipolar interactions, respectively. Different average particle size, morphology and microstructure were the reasons for variation of magnetic properties.

Preparation and magnetic properties of R-Fe-B permanent magnet materials containing Fe 3B as the main phase

The coercivity of Nd or Pr containing Fe 3B based permanent magnet materials can be improved to approximately μ 0 H c = 0.45 T by partial substitutions of the rare earth atoms by Tb or Dy. Full substitution leads to low coercivities due to the formation of the magnetically soft phase R 3Fe 62B 14, which replaces the magnetically hard R 2Fe 14B phase.

Ga added Nd-Fe-B sintered and die-upset magnets

The addition of Ga to Nd-Fe-B sintered and die-upset magnets in order to improve thermal stability was examined. The Ga addition increases the coercivity of Nd(Fe/sub 0.72/Co/sub 0.2/B/sub 0.08/)/sub 5.6/ sintered magnets markedly. Ga addition does not enhance coercivity of sintered magnets with lower Co substitution. Ga addition increases coercivity for Nd-Fe-B die-upset magnets. No decrease in coercivity due to Co substitution of up to 20 at.% is observed for Nd/sub 14/Fe/sub 79.25/B/sub 6/Ga/sub 0.75/ die-upset magnets. This is an advantage of the die-upset magnets compared with the sintered magnets. Ga has a tendency to occur in the Nd-rich phase at higher concentrations than 2 wt.%, and Ga content in the 2/14/1 matrix is around 0.2 wt.% for Nd/sub 14/Fe/sub 79.25/BGa/sub 0.75/ die-upset magnets. For sintered magnets, Ga is distributed between the matrix and the Nd-rich phase. H/sub A/ of the 2/14/1 matrix is increased by Ga addition up to 2.5 at.%. The correlation between H/sub A/ and iH/sub c/ is not good. The Ga-containing Nd-rich phase is observed in the sintered magnets, and the role of Ga in this phase needs to be evaluated to understand coercivity enhancement by Ga addition. >

The comparative performance of Nd(Fe, Co)B magnets formed by rapid quenching and by sintering

The structures and magnetic properties of crystalline Nd(Fe, Co)B alloys have been studied by X-ray diffraction, differential thermal analysis, transmission electron microscopy and vibrating sample magnetometry. It was observed that the dependences of the magnetization and the Curie temperature of crystallization on the concentration of cobalt are similar to those of other crystal alloys. On transforming the amorphous alloy to the crystalline alloy, a b.c.c. phase, similar to α-Fe, is formed first and then tetragonal Nd 2(Fe, Co) 14B is formed at about 600°C. The mechanism of coercivity of Nd(Fe, Co)B magnets formed by quenching and sintering is also discussed.