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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

2008; American Institute of Physics; Volume: 104; Issue: 1 Linguagem: Inglês

10.1063/1.2952556

1520-8850

Tomoki Fukagawa, S. Hirosawa,

Magnetic properties of thin films

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 Nd2Fe14B 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 Nd), A-type Nd2O3 has been observed in the inner region of the Nd film away from the interface. A-type Nd2O3 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 Nd2O3 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 Nd2Fe14B grains.