Kurima Kobayashi

A new demagnetization correction method is explained in this paper. Since a magnetization curve in an open magnetic circuit is considerably affected by the demagnetizing field which depends on the shape of the magnetic material, the demagnetization correction is significantly important to evaluate the magnetic properties of magnetic materials. In the new demagnetization correction method, a proper closed magnetic circuit curve is obtained in a way that a curve defined as a mathematical function is repeatedly modified in a computer feedback loop. In the method, the distribution of the demagnetizing field inside the magnetic material is taken into account by using finite element method in the computer calculation. Some calculation results are shown about Nd-Fe-B sintered magnets, and it is revealed that the new demagnetization correction method is able to reproduce the measured curves in the closed magnetic circuit with much higher accuracy than that of the classical demagnetization correction method.

We investigated (Sm,Zr)(Fe,Co)11.0-11.5Ti1.0-0.5 compounds as permanent magnet materials. Good magnetic properties were observed in (Sm0.8Zr0.2)(Fe0.75Co0.25)11.5Ti0.5 powder containing a limited amount of the α-(Fe, Co) phase, including saturation polarization (Js) of 1.63 T, an anisotropic field (Ha) of 5.90 MA/m at room temperature, and a Curie temperature (Tc) of about 880 K. Notably, Js and Ha remained above 1.5 T and 3.70 MA/m, respectively, even at 473 K. The high-temperature magnetic properties of (Sm0.8Zr0.2)(Fe0.75Co0.25)11.5Ti0.5 were superior to those of Nd2Fe14B.

The magnetization and demagnetization mechanisms in the mechanically polished surface layers (SL) of the c-plane and a-b plane of Nd-Fe-B sintered magnets were investigated. The magnetic interaction between the SL and the interior of the magnet was clarified by using vibrating sample magnetometer measurements of a whole sample and magneto-force microscopy observations of the domain structure of the SL layer. The polishing eliminated the Nd-rich grain boundary phases, which was only about 2 nm thick, from the SL crystal grains in the sintered magnets. The a-b plane polishing caused the independent magnetic reversal of the a-b plane SL, which was about 5.5 μm thick, as in the platy samples. The coercivities (μ₀H_c) of the SL were less than 0.3 T. In contrast, the c-plane polishing did not produce independent magnetic reversal of the SL, although the coercivity of bulk samples was clearly decreased by the formation of the c-plane SL. The grains in the SL should form clusters that alter the demagnetizing factors, depending on the shape of cluster as a magnetic unit. The magnetic interaction among the SL crystal grains is expected to play an important role in the coercivity mechanism of Nd-Fe-B sintered magnets.

We studied NdFe₁₁TiN_x compounds as permanent magnet materials. The (Nd_0.7,Zr_0.3)(Fe_0.75Co_0.25)_11.5Ti_0.5N_0.52 powder that contained a limited amount of the α-(Fe, Co) phase shows fairly good magnetic properties, such as a saturation polarization (J_s) of 1.68 T and an anisotropic field (H_a) of 2.88 (Law of approach to saturation) – 4.0 MA/m (Intersection of magnetization curves). Both properties are comparable to those of the Nd₂Fe₁₄B phase.

The coercivities (H_c), saturation magnetizations (J_s), and anisotropy constants K₁ of (Nd_1−x, Dy_x)₂Fe₁₄B sintered magnets (x = 0 to 1.0) are determined using high magnetic field measurements up to about 8 MA/m. The H_a, J_s, and K₁ values of samples with x < 0.5 are similar to those obtained in previous studies using single crystals; the H_c values for the sintered magnets are reported here for the first time. The magnetization into the single-domain states (SDS) in Dy-substituted sintered magnets is completed in lower fields than with Dy-free magnets (for example, 0.64 MA/m in the x = 1.0 sample and 1.6 MA/m in the x = 0.25 sample). The volume fraction of magnetized SDS grains when applying fields less than the demagnetizing fields of the samples also increases with increasing Dy content, i.e., 22 vol. % for a Dy-free sample and 48 vol. % for a Nd-free sample. A characteristic magnetization behavior of large pinning-like magnetization curves appeared in samples that were initially magnetized in a direction perpendicular to the c-axis and successively magnetized in the parallel direction. The apparent pinning field increased with increasing Dy concentration and reached 2.8 MA/m for the Nd-free samples. The origin of this behavior is the tilting of the c-axis in the crystal grains of sintered magnets and the clear preference for crystal grains in Dy-substituted samples to form SDS grains.