Effects of Anisotropy Field and easy axis dispersions on square-ness ratio for HDDR-processed NdFeB powders

• Main Authors: Akagi, F., Shirai, T., Kariya, R.
• Format: Conference Proceeding
• Language: English
• Subjects: Conferences; Magnetics
• ISSN: 2150-4601
• DOI: 10.1109/INTMAG.2018.8508047

NdFeB magnetic powders that are produced by a hydrogenation decomposition desorption recombination (HDDR) process consist of small grains with highly anisotropic energy [1]-[3]. Therefore, HDDR-processed NdFeB magnet powders are expected to have a high squareness ratio and high coercivity (H_{c}) to obtain the high maximum energy product (BH _{max}) that is required in highly efficient motors of hybrid or electric vehicles. The squareness ratio is defined as the value of the magnetic field at 90% of the remanent magnetization divided by H_{c}. However, the squareness ratio is much lower than the expected value of 1.0, and the H_{c} is lower than a third of the average anisotropy fields. A previous study using a micromagnetic simulator has shown that when an anisotropy field (H_{k}) dispersion of grains was assumed to be a Gaussian distribution with a coefficient variation (\sigma H_{k} {/\lt \mathrm {H}} _{k} \gt ) of 30%, the squareness ratio corresponded with an experimental value [4]. However, Nishio et al. showed that the H_{k} of a single crystal was 7600 kA/m [5]. Therefore, when the \sigma H_{k} {/\lt \mathrm {H}} _{k} \gt is 30%, the \lt H_{k} \gt + 3 \sigma H_{k} is unrealistically higher than the H_{k} of a single crystal. In this study, the H_{k} dispersion of the grains was assumed to be a horizontally flipped lognormal distribution, and the effects of the H_{k} dispersion of the grains on the squareness ratio were investigated by using a micromagnetic simulator. Moreover, Nishio et al. showed that an easy axis (c-axis) inclination angle was distributed within ± 20° [6], so we also investigated the effects of the c-axis dispersion on the squareness ratio.