Magnetic nanostructuring and overcoming Brown's paradox to realize extraordinary high-temperature energy products.

Balamurugan Balasubramanian,David J Sellmyer,Pinaki Mukherjee,Bhaskar Das,Priyanka Manchanda,Ralph Skomski

doi:10.1038/srep06265

Balamurugan Balasubramanian, David J Sellmyer + Show 4 more

Open Access

https://doi.org/10.1038/srep06265

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Journal: Scientific Reports	Publication Date: Sep 2, 2014
Citations: 56	License type: CC BY 4.0

Affiliation: University of Nebraska–Lincoln

Abstract

Nanoscience has been one of the outstanding driving forces in technology recently, arguably more so in magnetism than in any other branch of science and technology. Due to nanoscale bit size, a single computer hard disk is now able to store the text of 3,000,000 average-size books, and today's high-performance permanent magnets—found in hybrid cars, wind turbines, and disk drives—are nanostructured to a large degree. The nanostructures ideally are designed from Co- and Fe-rich building blocks without critical rare-earth elements, and often are required to exhibit high coercivity and magnetization at elevated temperatures of typically up to 180 °C for many important permanent-magnet applications. Here we achieve this goal in exchange-coupled hard-soft composite films by effective nanostructuring of high-anisotropy HfCo7 nanoparticles with a high-magnetization Fe65Co35 phase. An analysis based on a model structure shows that the soft-phase addition improves the performance of the hard-magnetic material by mitigating Brown's paradox in magnetism, a substantial reduction of coercivity from the anisotropy field. The nanostructures exhibit a high room-temperature energy product of about 20.3 MGOe (161.5 kJ/m3), which is a record for a rare earth- or Pt-free magnetic material and retain values as high as 17.1 MGOe (136.1 kJ/m3) at 180°C.

Highlights

Nebraska Center for Materials and Nanoscience and Department of Physics and Astronomy, University of Nebraska, Lincoln, NE68588 (USA)
The fabrication of the nanostructures requires the realization of high magnetocrystalline anisotropy, easy-axis alignment, and a suitable nanostructure with high Hc and high Ms The magnetic anisotropy is often reduced in nanoparticles due to disorder and surface effects, and this leads to thermally activated magnetization reversal at high temperatures, which subsequently destroys the permanent-magnet properties[18,19,20]
Note that the room-temperature initial magnetization curves were measured along the easy-axis direction for Hf-Co and Hf-Co:Fe-Co nanocomposite films and the results reveal a nucleation-type coercivity mechanism (Fig. S7 in Supplementary Information)

Summary

Introduction

The nanostructures ideally are designed from Co- and Fe-rich building blocks without critical rare-earth elements, and often are required to exhibit high coercivity and magnetization at elevated temperatures of typically up to 180 6C for many important permanent-magnet applications We achieve this goal in exchange-coupled hard-soft composite films by effective nanostructuring of high-anisotropy HfCo7 nanoparticles with a high-magnetization Fe65Co35 phase. Top-end permanent-magnet materials such as Nd2Fe14B and SmCo5 contain critical rare-earth elements with raw-materials supply bottlenecks, and many alternative materials exhibit somewhat reduced anisotropy constants of the order of 10 Mergs/cm[3] or contain expensive Pt14–17 Subject to these materials constraints, our approach focuses on Co alloys with heavy transition metals and offers the benefit of effective nanostructuring on a coarser scale of 2dB < 20 nm. Thermally activated reversal of individual nanoparticles is unimportant in the present context, because the magnetic nanoparticles are exchange-coupled in both bulk magnets and thin films

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