We have been improving magnetic properties of permanent magnets through microstructure control. The main researches were (1) development of high-performance Fe-Cr-Co magnets by recrystallization texture and high coercivity Mn-Sn-Co-N-based alloys. We have been also improving magnetic properties in (2) rare earth magnets. Namely, a) Dy reduction in Nd-Fe-B sintered magnets, b) development of high performance Sm-Fe-N magnets and c) highly anisotropic Nd-Fe-B HDDR powders. In addition, we have studied about (3) the usage of hard magnetic materials for electromagnetic wave absorbers. Recently, the use of microwaves in the GHz range has increased because of the demand for large data transmission. However, the problem of electromagnetic interference (EMI) has become serious, and much attention has been paid to microwave absorption materials. We have investigated magnetic loss of permanent magnet materials at natural resonance frequency, and have succeeded in the development of new microwave absorbers using M-type (or W-type ferrite) and RE-Fe-B (RE: rare earth) compounds. In this article, researches described above, especially researches about microwave absorbers, are summarized.

During the last decade, coercivity enhancement of permanent magnets has been one of the major issues due to the strong demands of high-performance magnets for the traction motors of electric/hybrid vehicles. For this purpose, the study on the coercivity mechanism of permanent magnets is essentially important. However, this has been uncleared regardless of the past long-discussed studies. Recently, we have tacked this issue using the multi-scale coercivity analysis. As macroscopic coercivity analyses, two different approaches were employed. One was the first-order reversal curve (FORC) analysis, which revealed the dominant magnetization reversal process, i.e., single- or multi-domain magnetization reversal. The other was the energy barrier analysis based on the magnetic viscosity measurements. On the other hand, as a microscopic coercivity analysis, an elemental-magnetization reversal measurement was attempted. In this paper, these multi-scale coercivity analyses have been reviewed.

Single phase samples of a cobaltite BaCo₆O₁₁ and its Fe-substituted derivatives BaFe_xCo_6-xO₁₁, which have R-type hexagonal ferrite structures, were successfully synthesized by a high-pressure and high-temperature method. BaCo₆O₁₁ was found to be a ferromagnetic metal. Mixed valent Co^3.4+ cations at the octahedral B(1) and B(2) sites contribute to the metallic conduction, whereas Co³⁺ cations at the trigonal-bipyramidal B(3) site give local magnetic moments, which order ferromagnetically below the Curie temperature of 17 K. The Fe substitution largely increases the magnetic transition temperature and changes the ground state to a ferrimagnetic semiconductor owing to the introduction of localized Fe³⁺ cations at the B(1) and B(2) sites.

The magnetic properties and microstructure of the W and/or La added Sm-Fe-N magnetic powders were investigated. The W and La+W added Sm-Fe-N powders exhibited higher coercivity of 2.1 and 2.0 MAm^-1 than that of the powder without La and W addition (1.5 MAm^-1), respectively, while the La added powder had lower coercivity of 1.0 MAm^-1. Initial recoil curves of each powders indicated that volume fraction of single domain particles was higher in the W and La+W added powders than in the La added powder, and the coercivity of these powders strongly depended on particles size. The lattice constant slightly changed with La and W addition. Microstructural observation revealed that W tended to condense in the small particle, and the W was detected from Sm₂Fe₁₇N₃(01̄1) plane.