The Coercivity - Remanence Tradeoff in Nanocrystalline Permanent Magnets
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The Coercivity – Remanence Tradeoff in Nanocrystalline Permanent Magnets Laura H. Lewis and David C. Crew Materials and Chemical Sciences Division, Energy Sciences and Technology Dept., Brookhaven National Laboratory, Upton, New York 11973-5000 USA ABSTRACT The energy product (BH)max is a figure of merit quantifying the maximum amount of useful work that can be performed by the magnet. The energy product is determined by the magnetic remanence and the coercivity which, as extrinsic properties, are determined by the magnets’ microstructure. Thus, in principle, magnetic material microstructures may be tailored to obtain defined parameters to produce optimal permanent magnets. However, as asserted by the eponymous Murphy, “Nature favors the hidden flaw”. While there is still much undeveloped potential in nanomagnetic materials, with relevant length scales on the order of 100 Å, accumulating evidence strongly suggests that maximum remanence and maximum coercivity are mutually exclusive in nanocrystalline magnetic materials. Diverse experimental and computational results obtained from nanocrystalline Nd2 Fe14 B-based magnets produced by meltspinning techniques and subjected to various degrees of thermomechanical deformation confirm this conclusion. Recent results obtained from temperature-dependent magnetic measurement, magnetic force microscopy and simple micromagnetic modeling will be reviewed and summarized. The results, while somewhat discouraging, do hint at possible materials design routes to sidestep the inherent performance limitations of the magnetic nanostructures.
INTRODUCTION The permanent magnet material Nd2 Fe14 B in its nanocrystalline form exhibits very favorable properties conferred by the rapid solidification process: simplified processing, good corrosion resistance and high magnetic hardness provided by a nanoscale grain size on the order of 200 nm. The metallurgical properties of the Nd2 Fe14 B compound allow it to be thermomechanically deformed, or “die-upset”, a process that significantly improves the remanence Br of the magnet [1]. This increase in remanence results stems from two effects: (a) the exchange enhancement of the magnetization in the intergranular regions [2] and (b) an increased degree of crystallographic texture of the magnet. Both effects enhance the energy product (BH)max, which is a figure of merit that quantifies the maximum amount of useful work that may be performed by the magnet. The energy product is determined by the magnetic remanence and the coercivity which, as extrinsic properties, are largely determined by the magnets’ microstructure. Thus, in principle, it should be possible to tailor magnetic material microstructures to obtain target properties that yield optimal permanent magnets. Unfortunately, the increase in the remanence upon die-upsetting is always accompanied by a decrease in the coercivity Hci of the magnet. The reasons underlying this phenomenon are unclear. This coercivity decrease dilutes the gains in the energy product produced by the increased remanence a
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