Micromagnetism and microstructure of hard magnetic materials

Abstract

High-performance magnetic materials are based on outstanding intrinsic magnetic properties and optimized microstructures and alloy compositions. The interactions between these three parameters in general are rather complex and cannot be treated explicitly by the theory of micromagnetism. Instead numerical methods have to be applied in order to determine the characteristic properties of hysteresis loops. Within the framework of computational micromagnetism (nanomagnetism) using the finite-element method the coercive fields of different types of grain ensembles have been determined. In the case of nanocrystalline composites the roles of grain size, exchange and dipolar coupling between grains will be discussed in detail. It is shown that, in sintered magnets, large coercivities require magnetic de-coupling between the grains, whereas regions with exchange coupling reduce the coercive field drastically, but, however, increase the remanence. Nanocrystalline composite materials with remanence enhancement and high coercivities are shown to require soft grains with diameters of twice the wall width of the hard magnetic phase. For an amount of 50% -Fe coercivities of , a remanence of 1.5 T and an energy product of are expected. A quantitative analysis of the numerical results for and the remanence leads to logarithmic dependences on grain size.

Explosive shock processing of Pr2Fe14B/α–Fe exchange-coupled nanocomposite bulk magnets

“…It is well known that for optimum exchange coupling, the size of the soft magnetic ␣-Fe grains should be approximately twice that of the effective exchange range L ex . 27,28 A larger grain size above 2L ex results in hysteresis loops of contracted shape, and corresponding deterioration of magnetic properties. The optimum grain size 2L ex has been found to be around 15 nm in Nd 2 Fe 14 B/ ␣-Fe nanocomposites, which is very similar to the grain size of ␣-Fe phase observed via HRTEM analysis of the shock-compacted samples.…”

Structure of macrodomain walls in polytwinned magnets

“…The role of these defects was studied in a number of papers using the finite-element version of the micromagnetic method. 3,4 There is another group of intermetallic permanent magnets with high uniaxial anisotropy in the tetragonal L1 0 phase including CoPt, FePt, and FePd. Unlike other permanent magnets, their microstructure is dominated not by grain boundaries, but by twin boundaries and antiphase boundaries.…”

The effect of CoPt crystallinity and grain texturing on properties of exchange-coupled Fe/CoPt systems

“…This suggests that the coupling behavior between the hard and the soft magnet needs to be improved. In order to achieve better coupling, the effects of various factors such as interface conditions, 15,16 soft-hard volume fraction, 17 microstructures, [18][19][20][21][22][23] etc., have been investigated. There have been some theoretical and empirical works carried out to elucidate how these factors can alter the coupling mechanism.…”