The Coercivity - Remanence Tradeoff in Nanocrystalline Permanent Magnets

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 Nd₂Fe₁₄B-based magnets produced by melt-spinning 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.