COMPUTER-SIMULATION OF SINGLE-PHASE NANOCRYSTALLINE PERMANENT-MAGNETS

Demagnetizing curves have been calculated numerically for three-dimens ional micromagnetic model assemblies of randomly oriented, magneticall y hard, exchange coupled, uniaxial nanocrystals as typified by rapidly quenched Nd2Fe14B. The curves were obtained as a sequence of static e quilibrium states in an incrementally changing applied field. The magn etization distribution in each state was obtained by minimizing the su m of the exchange, anisotropy and Zeeman energies of the assembly, usi ng a modified LaBonte method, with computational elements as small as 1.11 nm (roughly 1/4 the domain wall thickness in Nd2Fe14B]. For compu tational economy, internal dipolar interactions were ignored in the en ergy minimization. For a material with the magnetic constants of stoic hiometric Nd2Fe14B. tests showed that these interactions contribute le ss than 3% to the energy. On increasing the model grain size from 4.4 to 36 nm, the reduced remanence fell from 76 to 54% and the reduced in trinsic coercivity mu(0i)H(C)M(S)/K-U increased from 0.16 to 0.46 (jus t under half the Stoner-Wohlfarth value); both sets of results are in reasonable agreement with experimental values. The energy product, eva luated for Nd2Fe14B, ranged from similar to 224 kJ/m(3) for 10 nm grai ns to similar to 128 kJ/m(3) for 36 nm grains. For grain sizes greater than or equal to 20 run, spatial magnetization variation was confined to domain walls centred on the grain boundaries, For grain sizes decr easing below about twice the domain wall thickness, spatial magnetizat ion variation extended to the interior of the grains and exhibited inc reasingly long-range correlations. (C) 1998 Elsevier Science B.V. All rights reserved.