Coercivity Mechanism of (Nd0.8Ce0.2)2.4Fe12Co2B Ribbons with Ferromagnetic Grain Boundary Phase

Abstract

Understanding the coercivity mechanism has had a substantial impact on developing economically more attractive RE-based (RE = rare earth) permanent materials because of price volatility of key RE metals (i.e., Nd and Dy) in recent years. In this work, we investigated the microstructure and magnetic properties of melt-spun (Nd 0.8 Ce 0.2 ) 2.4 Fe 12 Co 2 B ribbons and annealed samples at 773 K for 15 min with 1 Tesla (T) magnetic field to better understand the coercivity mechanism. We found hard magnetic grains were surrounded by thin and continuous layers along the grain boundaries (GBs) with a high concentration of ferromagnetic elements (Fe + Co >74 at%). The obvious positive peak in the δM plot and the interaction domain structure observed by Lorentz magnetic microscopy indicate that there is strong exchange coupling interaction through the ferromagnetic GB phase between hard magnetic grains. The annealing in an applied magnetic field of 1 T increases the remanence by enhancing the exchange coupling interaction, leading to a maximum product energy ((BH) max ) which is 16% higher than that of melt-spun ribbons. We also studied the temperature dependence of the coercivity in a temperature range of 300-500 K, and proposed that the coercivity of melt-spun (Nd 0.8 Ce 0.2 ) 2.4 Fe 12 Co 2 B ribbons with ferromagnetic GB phase at room temperature was from the combination of strong domain-wall pinning and nucleation. The same mechanism works in the annealed ribbons.

An evolving switching surface model for ferromagnetic hysteresis

“…Depending on whether κ = 1 or 0, (24) reduces to an inverse sigmoid function of the type x/(1 − |x|) or tanh −1 (x), respectively. The last relation along with (19) and (20) are used to compute the evolution of h r for a given variation of the applied magnetic field h (see Appendix for details).…”

“…Experiments and micromagnetic theories suggest that the coercivity in RE magnets arise due to a combined effect of domain wall pinning and nucleation 60 . It is observed through advanced imaging techniques that reversed domains nucleate right from the beginning of magnetization of a virgin NdFeB specimen 24 . Thus, a combination of nucleation and pinning causes considerable amount of initial magnetization as compared to a solely pinning-type magnet.…”

“…Here,K 1 = K 1 (1−χ (r) )/µ 0 and f (h r ) are defined in (24). Consequently, the switching surface (19) now becomes…”

“…For example, in sintered RE magnets like NdFeB, nucleation is found to be dominant [18][19][20][21][22] , whereas, in most of the steels like Si steel, MnMg steel, the coercivity arises due to domain wall pinning 23 . Recent experiments by Li et al 24 have shown that both nucleation and pinning mechanisms play equally important roles in the magnetization/demagnetization of melt-spun NdFeB ribbons and powders.…”

The effects of Co on the enhancement of magnetic properties by modifying the intergranular phase in Nd-Fe-B alloys

“…For non-magnetic intergranular phase of hot-deformed Nd-Fe-B magnets, substitution of Co for Fe led to the formation of a Co-enriched layer at the edges of 2:14:1 grains resulting in a reduction of the magnetocrystalline anisotropy of 2:14:1 phase 9 , whereas the segregation of Co was found at the interface between the intergranular phase and 2:14:1 phase acting as nucleation sites for reversed magnetic domains 10 . On the other hand, the enrichment of Co was observed at the magnetic intergranular phase and further promoted by magnetic field annealing, which resulted in an improvement of remanence by strengthening the exchange coupling interaction between hard magnetic grains of Nd-Ce-Fe-Co-B alloys 11 . Hence, the effect of Co addition on the coercivity needs to be further investigated.…”