Ensuring the future affordability of wind turbines, computers and electric cars

June 1, 2016

Technologies from wind turbines to electric vehicles rely on critical materials called rare-earth elements. These elements, though often abundant, can be difficult and increasingly costly to come by. Now, scientists looking for alternatives have reported in ACS' journal Chemistry of Materials a new way to make nanoparticles that could replace some rare-earth materials and help ensure the continued supply of products people have come to depend on.

Rare-earth elements have unique characteristics that make them very useful. For example, the world's strongest magnets are made with neodymium. A little too powerful for your refrigerator, these magnets are incorporated into computer disk drives, power windows and . But are challenging to mine and process, and prices can rise quickly in a short period of time. Given the increasing demand for rare earths, Alberto López-Ortega, Claudio Sangregorio and colleagues set out to find substitutes for use in strong magnets.

The researchers used a mixed iron-cobalt oleate complex in a one-step synthetic approach to produce magnetic core-shell nanoparticles. The resulting materials showed strong magnetic properties and energy-storing capabilities. Their approach could signal an efficient new strategy toward replacing rare earths in and keeping costs stable, the researchers say.

More information: Strongly exchange coupled core|shell nanoparticles with high magnetic anisotropy: a strategy towards Rare Earth -free permanent magnets, DOI: 10.1021/acs.chemmater.6b00623

Abstract
Antiferromagnetic(AFM)|ferrimagnetic(FiM) core|shell (CS) nanoparticles (NPs) of formula Co0.3Fe0.7O|Co0.6Fe2.4O4 with mean diameter from 6 to 18 nm have been synthesized through a one-pot thermal decomposition process. The CS structure has been generated by topotaxial oxidation of the core region, leading to the formation of a highly monodisperse single inverted AFM|FiM CS system with variable AFM-core diameter and constant FiM-shell thickness (~2 nm). The sharp interface, the high structural matching between both phases and the good crystallinity of the AFM material have been structurally demonstrated and are corroborated by the robust exchange-coupling between AFM and FiM phases, which gives rise to one among the largest exchange bias (HE) values ever reported for CS NPs (8.6 kOe) and to a strongly enhanced coercive field (HC). In addition, the investigation of the magnetic properties as a function of the AFM-core size (dAFM), revealed a non-monotonous trend of both HC and HE, which display a maximum value for dAFM = 5 nm (19.3 and 8.6 kOe, respectively). These properties induce a huge improvement of the capability of storing energy of the material, a result which suggests that the combination of highly anisotropic AFM|FiM materials can be an efficient strategy towards the realization of novel Rare Earth-free permanent magnets.

Provided by: American Chemical Society

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