Giant coercivity of dense nanostructured spark plasma sintered barium hexaferrite

TL;DR

This paper demonstrates that spark plasma sintering can produce dense barium hexaferrite magnets with extremely fine grains, achieving coercivity close to the theoretical limit and significantly improving their magnetic energy product.

Contribution

It introduces a method to produce dense, nanostructured ferrite magnets with unprecedented coercivity and energy product, surpassing previous limitations.

Findings

Achieved coercive field of 0.5 T in dense nanostructured ferrite magnets.

Produced approximately 80% dense material with crystallites smaller than 100 nm.

Reached the highest energy product of 8.8 kJ/m^3 in isotropic ferrite magnets.

Abstract

Due to the limited rare-earth elements resources, ferrite magnets need to be improved drastically. Ideally, for a true hard magnet, the coercive field should be larger than the saturation magnetization, which is not yet realized for ferrites. Thus, an alternative can be found in making very fine grain ferrite magnets, but it is usually impossible to get small grains and dense material together. In this paper, it is shown that the spark plasma sintering method is able to produce approximately 80% of dense material with crystallites smaller than 100 nm. The as-prepared bulk sintered anisotropic magnets exhibits coercive field of 0.5 T which is approximately 60% of the theoretical limit and only a few percentage below that of loose nanopowders. As a result, the magnets behave nearly ideal (-1.18 slope in the BH plane second quadrant) and the energy product reaches 8.8 kJ m-3, the highest value achieved in the isotropic ferrite magnet to our knowledge.