SPIN-REORIENTATION TRANSITION IN NANO-CRYSTALLINE, MICRO-CRYSTALLINE AND SINGLE-CRYSTALLINE ND2FE14B

Spin reorientation of Nd2Fe14B with different degree of grain sizes wa s detected by measuring the temperature dependence of the ac susceptib ility. The studied materials include: (1) a spherical Nd2Fe14B single crystal that shows no coercivity. (2) Microcrystalline Nd2Fe14B obtain ed by induction-melting or by sintering: Due to the presence of random -oriented multi-domain grains (greater than or equal to 5000 nm), the bulk ingot shows no coercivity, but sintered materials do. (3) Microcr ystalline Nd2Fe14B produced by the nation-disproportionation-desorptio n-recombination (HDDR): The HDDR treated materials show a coercivity d ue to the presence of single-domain grains of a size of about 200-300 nm. (4) Nanocrystalline Nd2Fe14B prepared by mechanical alloying and b y rapid quenching: Due to the presence of ultrafine Nd2Fe14B grains (< 50 nm), these materials show a high coercivity and an enhanced remane nce. For the first time, a decrease of the spin-reorientation temperat ure, an intrinsic magnetic property, with decreasing the Nd2Fe14B grai n size is found. The decrease of the spin-reorientation temperature in nanocrystalline Nd2Fe14B can be ascribed to the strong inter-grain ex change change coupling among nanocrystalline Nd2Fe14B. (C) 1997 Americ an Institute of Physics.