Coercivity generation of surface Nd2Fe14B grains and mechanism of fcc-phase formation at the Nd/Nd2Fe14B interface in Nd-sputtered Nd–Fe–B sintered magnets

Abstract

The coercivity of surface grains of a Nd-Fe-B sintered magnet can be measured unequivocally if the magnet is a thin slab with the orientation of its c-axis parallel to the largest surface. Due to the loss of part of the neodymium-rich grain boundary phase, the surface Nd 2 Fe 14 B grains have low coercivity after machining. When a Nd layer is sputter deposited on the surface and annealed in the temperature range from 500 to 675°C, the surface coercivity recovers to its full value, which is identical to the coercivity of the bulk magnet from which the slab was cut. Transmission electron microscopy has indicated the formation of a fcc interfacial phase having a lattice constant a = 0.548 nm between the surface grains and the Nd layer in the specimen whose surface coercivity was recovered. In addition to a double hexagonal closed packed phase ͑metallic Nd͒, A-type Nd 2 O 3 has been observed in the inner region of the Nd film away from the interface. A-type Nd 2 O 3 having a hexagonal structure with a = 0.382 nm and c = 0.598 nm is the most stable phase of the neodymium oxides with 2:3 stoichiometry. Since the fcc phase is derived from metastable C-type Nd 2 O 3 having a cubic structure with a = 1.108 nm, the fcc phase is also a metastable phase. Therefore it is considered that the formation of the fcc phase results from the interfacial energy with the Nd 2 Fe 14 B grains.

A study on the Nd-rich phase evolution in the Nd–Fe–B sintered magnet and its mechanism during post-sintering annealing

“…The addition of Cu promotes this microstructural evolution of Nd-rich GBP and forms a thin Cu-enriched layer along the Nd-rich GBP//Nd 2 Fe 14 B interface because of the eutectic decomposition of Nd-Cu at 520°C [6,7]. Crystal structure changes of the Nd-rich phase, induced by PSA, have been also reported using a simple model system [8][9][10]. The most stable hNd 2 O 3 phase is formed away from the Nd 2 Fe 14 B//Nd-rich phase interface, but the meta-stable fcc-NdO 2 phase (a = 0.54 nm), which is related to the C-Nd 2 O 3 (Ia 3, a = 1.108 nm) phase, is formed at the interface in the sample whose coercivity was recovered during 550°C annealing [8][9][10].…”

“…Crystal structure changes of the Nd-rich phase, induced by PSA, have been also reported using a simple model system [8][9][10]. The most stable hNd 2 O 3 phase is formed away from the Nd 2 Fe 14 B//Nd-rich phase interface, but the meta-stable fcc-NdO 2 phase (a = 0.54 nm), which is related to the C-Nd 2 O 3 (Ia 3, a = 1.108 nm) phase, is formed at the interface in the sample whose coercivity was recovered during 550°C annealing [8][9][10]. This suggests that the meta-stable fccNdO 2 (or C-Nd 2 O 3 ) could be stabilized by the energy of the interface with the Nd 2 Fe 14 B phase [8][9][10].…”

“…Since the magnetic properties of Nd-Fe-B sintered magnets are highly influenced by the microstructure of Nd-rich grain boundary phases (GBP), the effects of PSA on the microstructural changes of the Nd-rich GBP have been studied [4][5][6][7][8][9][10][11][12]. It is reported that the roughness of the Nd-rich GBP is enhanced during PSA.…”

Effect of Cu Addition on the Phase Equilibria in Nd-Fe-B Sintered Magnets

“…If the liquid phase after the high temperature annealing exists at the center of triple junctions, as described above, it is considered that the precipitated dhcp-Nd and hcp-Nd 2 O 3 from liquid phase do not contact with Nd 2 Fe 14 B main phases. Fukagawa and Hirosawa 18) have reported that dchp-Nd does not contribute to the recover of coercivity even after annealing. Therefore, it can be said that the roles of high temperature annealing are to precipitate the fcc-NdO from the liquid phase along the Nd 2 Fe 14 B grains and to move the residual liquid phase at the center of triple junctions.…”

Effect of cyclic sintering process for NdFeB magnet on microstructure and magnetic properties

“…The Nd-rich phases near the main phase are reported to be metastable oxide phases [9,10]. Many researchers have shown that the Nd-rich phase was essential to improve coercivity and thermal stability for sintered Nd-Fe-B magnets.…”