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 (Nd0.8Ce0.2)2.4Fe12Co2B 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 (Nd0.8Ce0.2)2.4Fe12Co2B 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.
Highlights
Nd2 Fe14 B based permanent magnets (PMs) have attracted considerable research interest since their discovery in 1984, and are widely used in various fields including electronic, hybrid electric vehicles, and wind turbines because of their excellent magnetic properties [1,2,3]
We investigated the microstructure and magnetic properties of melt-spun (Nd0.8 Ce0.2 )2.4 Fe12 Co2 B ribbons as well as samples annealed in a 1 T magnetic field to obtain a deeper insight into the coercivity mechanism
We proposed the coercivity in melt-spun (Nd0.8 Ce0.2 )2.4 Fe12 Co2 B ribbons with ferromagnetic grain boundaries (GBs) phase at room temperature was from the combination of strong domain-wall pinning and nucleation
Summary
Nd2 Fe14 B based permanent magnets (PMs) have attracted considerable research interest since their discovery in 1984, and are widely used in various fields including electronic, hybrid electric vehicles, and wind turbines because of their excellent magnetic properties [1,2,3]. The powders pulverized from melt-spun Nd-Fe-B ribbons are currently used as raw materials for the production of bonded magnets and hot-pressed magnets [4]. There has been great effort in developing economically more attractive RE-based PMs (RE = rare earth) because of the limited natural resources and high cost of key RE metals (i.e., Nd and Dy) [5,6]. The element cerium (Ce), would be one of the suitable elements to form alloys by partially substituting Nd because of the earth abundance and fairly low cost [7,8,9]. In 2015, Pathak et al, reported an unexpected increase of the coercivity (1409 A·m−1 ) and (BH)max (100 kJ·m−3 ) for melt-spun (Nd0.8 Ce0.2 )2.4 Fe12 Co2 B ribbons by simultaneous substitution of Nd by Ce, and Fe by Co [10]. The segregation of heavy elements was Materials 2017, 10, 1062; doi:10.3390/ma10091062 www.mdpi.com/journal/materials
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