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- 저자 : Helmut Kronmüller (검색결과 1 건)
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MICROMAGNETISM OF HARD AND SOFT MAGNETIC MATERIALS
Helmut Kronmüller 한국자기학회 1995 韓國磁氣學會誌 Vol.5 No.5
High performance magnetic materials are characterized by the combination of outstanding magnetic properties and optimized microstructures, e.g., nanocrystalline composites of multilayers and small particle systems. The characteristic parameters of the hysteresis loops of these materials vary over more than a factor of 10^6 with optimum values for the coercive field of several Tesla and permeabilities of 10^6. Within the framework of the computational micromagnetism (nanomagnetism) using the finite element method the upper and lower bounds of the coercive field of different types of grain ensembles and multilayers have been determined. For the case of nanocrystalline composites the role of grain size, exchange and dipolar coupling between grains and the degree of grain alignment will be discusses in detail. It is shown that the largest coercivities are obtained for exchange decoupled grains, whereas remanence enhancing requires exchange coupled grains below 20 ㎚. For composite permanent magnets based on Nd₂Fe₁₄B with an amount of ~ 50% soft a-Fe-phase coercivities of μ_oHc = 0.75 T, a remanence of 1.5 T and an energy product of 400 kJ/㎥ is expected.<br/> In nanocrystalline systems the temperature dependence of the coercivity is well described by the relation μ_oHc = (2 K₁/Ms) α-Neffμ_oMs, where the microstructural parameters α and Neff take care of the short-range perturbations of the anisotropy and Neff is related to the long-range dipolar interactions. Neff is found to follow a logarithmic grain size dependence μ_oHc = (2 K₁/Ms) α-Neff(βInD)μ_oMs. Several trends how to achieve the ideal situation α → 1 and Neff → 1 → 0 will be discussed.
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