Influence of defect thickness on the angular dependence of coercivity in rare-earth permanent magnets

Abstract

International audienceThe coercive field and angular dependence of the coercive field of single-grain Nd$_{2}$Fe$_{14}$B permanent magnets are computed using finite element micromagnetics. It is shown that the thickness of surface defects plays a critical role in determining the reversal process. For small defect thicknesses reversal is heavily driven by nucleation, whereas with increasing defect thickness domain wall de-pinning becomes more important. This change results in an observable shift between two well-known behavioral models. A similar trend is observed in experimental measurements of bulk samples, where a Nd-Cu infiltration process has been used to enhance coercivity by modifying the grain boundaries. When account is taken of the imperfect grain alignment of real magnets, the single-grain computed results appears to closely match experimental behaviour

Coercivity global model and magnetization reversal in fine hexaferrites

“…. As it was stated in previous coercivity analysis using this model , Eq. implies that the magnetic properties in the activation volume are close to the main phase properties, since γ is the main phase domain wall energy.…”

“…Here k is the Boltzmann constant, T is the absolute temperature, M s is the saturation magnetization, and β is an effective demagnetization coefficient. Then, a simple and essential proposal is pointed up in the model : the moment configuration within the nucleus must have necessarily similarities with a magnetic domain wall since a domain wall constitutes the non‐uniform moment configuration of the lowest energy. In that case, E o is assumed to be E o = γ ′ S where γ ′ is the surface energy of the domain formed and S is the surface of the activation volume.…”

Magnetic SANS study of a sintered Nd–Fe–B magnet: Estimation of defect size

“…In particular, the micromagnetic simulations reported in Ref. [13] show that defects smaller than 10 nm might be responsible for the reduced coercive field of Nd-Fe-B magnets, although experimental data concerning this matter are still limited in the literature. To obtain further understanding of the impact of lattice imperfections (here: the Nd-rich interphases) on the magnetic properties of Nd-Fe-B-based sintered magnets, an experimental technique which allows one to resolve the nanometer-scale spin microstructure in the bulk of the magnet is required.…”

Computer simulation of coercivity improvement due to microstructural refinement

“…2,3,17 A collective effect of large non-magnetic phases located at the corners of Nd 2 Fe 14 B grains has also been reported for a sintered magnet by computer simulations based on micromagnetic theory. 18 However, these micromagnetic simulations have not clearly determined the effects of the size of the non-magnetic phases.…”