Hysteresis Modeling of NdFeB Magnets with High Nd

The Stoner-Wohlfarth hysteresis model can describe very well the hysteresis curves since the assumptions of the model are compatible with the studied samples. These conditions are (i) single domain particle size and (ii) inexistence of interactions between the particles. These conditions are obtained in Nd2Fe14B magnets with nanocrystalline grain size below 200 nm and with high Nd, which promotes a paramagnetic phase at the grain boundaries. In the present study, both isotropic and anisotropic NdFeB magnets are modeled.

Effect of Mechanical Milling on the Magnetic Properties of SrFe12O19

In this work, a single phase strontium ferrite sample was mechanically milled for 60 h. It has been found that the crystallite size reduced by 85%. A dramatic drop (from 4500 to 1600 Oe) in the coercive filed was also observed. The observed reduction in the coercivity was attributed to the reduction of the single domain particle size.

Application of Micromagnetic Models for Barium Ferrite Magnets

The applicability of micromagnetics for phases with high magnetocrystalline anisotropy as barium ferrite Ba2Fe12O19and Nd2Fe14B is discussed. The Stoner-Wohlfarth model is very suitable for such phases, and also for PtFe and PtCo. It was discussed how to take into account the total energy of the system for grain size above the single domain particle size. For this situation of large grain size, the net magnetostatic energy of the system cannot be neglected. From energy considerations, it follows that the coercive force should decrease with the inverse of the square root of the grain size.

Squareness of NdFeB Stoner-Wohlfarth Hysteresis

Hysteresis curves were calculated according the Stoner-Wohlfarth (SW) model using values of K2/K1from zero up to 0.25 (where K2is the 2ndorder anisotropy constant and where K1is the 1storder anisotropy constant). The SW calculations assume Nd2Fe14B nanocrystalline magnets with single domain particle size (grain diameter less than 150 nm). The effect of K2on several index of merit of magnets as BHmaxand squareness are discussed. As a general result, increasing K2, the intrinsic coercivity increases. However, the increase of K2almost does not improve the BHmaxof isotropic magnets. It is found that the effect of the K2/K1ratio is coupled with the effect of texture, i.e., the effect of K2is more significant for pronounced texture.

Nucleation as Coercivity Mechanism in NdFeB Magnets

In the first part of this article, the SW-CLC (Stoner-Wohlfarth with CLC modification) model is discussed for nanocrystalline magnets, as melt-spun exchange coupled NdFeB. In the second part, the effect of grain size of coercivity for large grain size, above the single domain particle size, is addressed. The Kondorsky law observed for large grain size only can be due to domain wall displacement phenomena, where there is nucleus expansion. There are two main situations, one for nanocrystalline grain size, where the SW-CLC model is obeyed, and other for large grain size, where the coercive field decreases proportionally to the square root of the grain size.