Spin-reorientation transition in nano-, micro- and single-crystalline Nd2Fe14B

Spin reorientation of Nd2Fe14B with different degree of grain sizes was detected by measuring the temperature dependence of the ac susceptibility. The studied materials include: (1) a spherical Nd2Fe14B single crystal that shows no coercivity. (2) Microcrystalline Nd2Fe14B obtained by induction-melting or by sintering: Due to the presence of random-oriented multi-domain grains (≥5000 nm), the bulk ingot shows no coercivity, but sintered materials do. (3) Microcrystalline Nd2Fe14B produced by the hydrogenation-disproportionation-desorption-recombination (HDDR): The HDDR treated materials show a coercivity due to the presence of single-domain grains of a size of about 200 endash 300 nm. (4) Nanocrystalline Nd2Fe14B prepared by mechanical alloying and by rapid quenching: Due to the presence of ultrafine Nd2Fe14B grains (<50nm), these materials show a high coercivity and an enhanced remanence. For the first time, a decrease of the spin-reorientation temperature, an intrinsic magnetic property, with decreasing the Nd2Fe14B grain size is found. The decrease of the spin-reorientation temperature in nanocrystalline Nd2Fe14B can be ascribed to the strong inter-grain exchange change coupling among nanocrystalline Nd2Fe14B. copyright 1997 American Institute of Physics