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.
We have investigated the magnetism of the grain boundary (GB) phase in a Nd14.0Fe79.7Cu0.1B6.2 sintered magnet using soft x-ray magnetic circular dichroism (XMCD) at the Fe L2,3-edges. Soft XMCD spectra were measured from the fractured surface that was confirmed to be covered with a thin GB phase by Auger electron spectroscopy. The magnetic moment of Fe in the GB phase was estimated to be mGB=1.4 μB at 30 °C using the sum rule analysis for XMCD spectra, which is 60% of that of Fe in the Nd2Fe14B compound. The temperature dependence of mGB evaluated with reference to Fe in the Nd2Fe14B phase indicated that the Curie temperature of the GB phase is more than 50 °C lower compared to that of Nd2Fe14B.
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