Y. Kanai

Effect of strength of intergrain exchange interaction on magnetic properties of nanocomposite magnets

Micromagnetic calculation using a discrete model was applied to computer simulation of the magnetic properties of nanocomposite magnets, and the effect of the strength of the intergrain exchange interaction on magnetic properties was studied. A reduction of the strength of the intergrain exchange interaction suppressed the averaging effect of anisotropy in hard grains and increased coercivity. An excessive decrease in the strength, however, enabled magnetization in soft grains to reverse and again decreased coercivity. The maximum energy product did not increase by reducing the strength of the intergrain exchange interaction when the grain size was relatively large. On the other hand, the maximum energy product of a magnet with a relatively small grain size increased by reducing the strength of the intergrain exchange interaction. This suggests that we can obtain a large maximum energy product in a magnet with a small grain size and a suitably weak strength of the intergrain exchange interaction. Nanostructure suitable to a nanocomposite magnet was discussed based on the calculation.

Maximum energy product of isotropic Nd-Fe-B-based nanocomposite magnets

The maximum energy products, (BH)max, of isotropic Nd-Fe-B-based nanocomposite magnets were calculated by a three dimensional model (10×10×10) that incorporated non-uniform magnetization reversals by subdivisions of the grains. Based on the results of our simulation, the largest obtainable (BH)max for a sample with 60% soft and 40% hard grains is about 290 kJ/m3. This value is lower than the previous crude estimate of 400 kJ/m3 but is almost twice as large as the experimental measured maximum of 160 kJ/m3 suggesting that further improvement in (BH)max is possible if the nanostructure of an actual magnet can made close to the theoretically ideal structure.

Computer simulation of magnetic properties of anisotropic nanocomposite magnets

Magnetic properties of anisotropic nanocomposite magnets, composed of Nd/sub 2/Fe/sub 14/B and /spl alpha/-Fe grains, were analyzed by computer simulation using the micromagnetic theory. All of the remanence, the coercivity, and the maximum energy product increased with increasing the alignment of Nd/sub 2/Fe/sub 14/B grains. In particular, the increase in coercivity was enhanced by the loss of the averaging effect of magnetic anisotropy. The maximum energy product for a fully aligned magnet versus amount of /spl alpha/-Fe showed a peak at 50% /spl alpha/-Fe, and the maximum energy product of approximately 690 kJ/m/sup 3/ was obtained. This value is much larger than the theoretical limitation for the single-phase Nd/sub 2/Fe/sub 14/B magnet but smaller than that calculated for a nanocomposite magnet with the Sm/sub 2/Fe/sub 17/N/sub x///spl alpha/-Fe layer structure.

Flux loss in nanocomposite magnets

The irreversible flux loss due to exposure to an elevated temperature was evaluated systematically for nanocomposite magnets with various coercivity values. It was found that the short-term and long-term flux losses in some nanocomposite magnets are smaller than in a conventional isotropic Nd-Fe-B magnet despite their small coercivity values. The origins of the observed small values for flux loss were studied by using the proposed model. It was clarified that the observed small values for the short-term flux loss in the nanocomposite magnets can be attributed to the squareness of a demagnetization curve of the nanocomposite magnets not deteriorating at an elevated temperature. The small values for the irreversible susceptibility of the nanocomposite magnets reasonably explained their small values for long-term flux loss.

Improvement in hard magnetic properties of rapidly quenched SmFeN flakes by Zn-coating

Abstract Hard magnetic properties of rapidly quenched SmFeN flakes with thin Zn-coating layers, which were prepared by vapor deposition and successive annealing, were studied. The coating reduced the amount of α-Fe precipitated on the surfaces of the flakes and improved the coercivity by 2–3 kOe, although it did not affect the remanence.

Flux stability in nanocomposite magnets

Abstract The stability of flux in Nd–Fe–B based nanocomposite magnets was studied and it was found that nanocomposite magnets have a high stability of flux in spite of their small coercivity. The observed high stability can be attributed to a square demagnetization curve at an elevated temperature and a small irreversible susceptibility in the nanocomposite magnets.

Flux stability of a NdFeB based magnet consolidated from a mixture of isotropic and anisotropic powders

Abstract Flux stability was studied for NdFeB magnets consolidated by direct joule heating from a mixture of anisotropic and isotropic powders. Addition of anisotropic powder beyond 70% degraded the short-term stability as well as the long-term stability significantly. The magnetic after effect constant did not depend on the fraction of anisotropic powder. Thus, the long-term stability can be estimated by measuring the irreversible susceptibility.