Towards Rare-Earth-Free Permanent Magnets: Fe-(CuMn) Nanocomposites

Abstract

Ultrastrong permanent magnets are materials used for conversion of mechanical to electric
energy (and vice-versa) in alternative energy, consumer electronics, and military applications
because of their enormous magnetic energy storage. Rare-earth elements (REEs) are principle
components of strong permanent magnets, contributing a magnetocrystalline anisotropy that
makes rare-earth magnets very resistant to demagnetization. The price of many of these critical
elements has recently skyrocketed, causing significant economic pressure. It is therefore
necessary to explore routes to the development of REE-free permanent magnet materials. One
route is to create a Fe-(CuMn) nanocomposite, due to potential for dramatically enhanced
demagnetization resistance in nanostructured systems. This investigation focuses on
understanding the magnetic and structural properties of Cu30Mn70.
Many Mn-rich binary alloys, like Cu30Mn70, are antiferromagnetic (AF) but are weakly
ferromagnetic (FM) at lower Mn concentrations. We anticipate that when an Mn-rich alloy is
embedded in a strongly FM matrix like Fe, exchange interactions between the AF Mn-rich
component and the FM matrix cause a massive increase in energy product via “exchange
anisotropy” (HE).
Nanostructured Cu30Mn70 ribbons were synthesized by rapid solidification using the meltspinning
technique. Magnetic measurements on the as-spun ribbons after field-cooling to 10 K
surprisingly reveal a giant HE of ~10 kOe, unprecedented in CuMn alloys. Structural
characterization shows existence of two face-centered-cubic phases with slightly different latticeparameters.
We hypothesize that the larger lattice-parameter phase is Mn-rich (AF), the other
Mn-poor (FM). Exchange interactions between these nanoscaled regions would explain the giant
HE.
This research was conducted under ONR grant #N000141010533.