MODELING OF EXCHANGE-SPRING PERMANENT-MAGNETS

Micromagnetic modelling using the finite-element method reveals the co rrelation between the local arrangement of the magnetic moments on a l ength scale of several nanometers and the magnetic properties of nanoc omposite permanent magnets. Theoretical limits for remanence and coerc ivity are derived for Nd2Fe14B/Fe3B/alpha-Fe nanocrystalline permanent magnets. The coercive field shows a maximum at an average grain size of D = 20 nm. Intergrain exchange interactions override the magneto-cr ystalline anisotropy of the Nd2Fe14B grains for smaller grains, wherea s exchange hardening of the soft phases becomes less effective for lar ger grains. The calculations show a linear trade off of remanence and coercivity as a function of the alpha-Fe to Fe3B ratio. The coercive f ield H-c and the remanence J(r) cover the range (H-c, J(r)) = (340 kA/ m, 1.4 T) to (610 kA/m, 1.1 T) for a composite magnet containing 40% N d2Fe14B, (60 - x)% Fe3B, and x% alpha-Fe. The numerical integration of the Gilbert equation yields the transient magnetic states during irre versible switching and thus reveal how reversed domains nucleate and e xpand. (C) 1998 Elsevier Science B.V. All rights reserved.