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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 200–300 nm. (4) Nanocrystalline Nd2Fe14B prepared by mechanical alloying and by rapid quenching: Due to the presence of ultrafine Nd2Fe14B grains (<50 nm), 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.

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The magnetic properties of nanocrystalline hard magnetic and soft magnetic are summarized. When the grain size becomes of the order f the magnetic exchange length exchange coupling occurs. The different concepts of exchange coupling in these materials are discussed. Exchange coupling leads in isotropic hard magnetic materials to a remanence enhancement. Soft magnetic materials exhibit due to exchange coupling a lower coercivity, lower losses and consequently also improved properties.

Nanocomposite ribbons consisting of hard-magnetic R2Fe14B (R=rare-earth) and soft-magnetic Fe3B phases were produced by melt spinning and subsequent heat treatment. Alloys of composition Nd4−xSmxFe77.5B18.5, with x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5 all showing strong remanence enhancement, were selected to investigate the effect of the magnetocrystalline anisotropy of the R2Fe14B-type phase on the exchange coupling with the soft-magnetic phase in the nanocomposite magnets. An enhancement of exchange coupling due to anisotropy reduction was found, as expected on the basis of the model of Kneller and Hawig. A maximum of the energy product (BH)max was achieved for x=0.2 after annealing at 700 °C. The thermomagnetic behavior also changes upon substitution of Sm for Nd.