Magnets with a low-temperature coefficient of coercivity (TCC) have various applications in a varying temperature environment. Rare-earth magnets like NdFeB are widely used, but they usually have a large negative TCC. Here, it is first experimentally demonstrated that α″-Fe16N2 foils, as a candidate for rare-earth-free magnets, have an ultralow positive TCC (0.4 Oe/K) from 300 K to 425 K. It is two orders of magnitude smaller than that of the commercial NdFeB magnets in this temperature range. The α″-Fe16N2 foils are made from as-rolled iron foils (25 µm) by a low-temperature nitridation process (< 473 K). The microstructure of these iron foils is tuned before the nitridation by a combined oxidation and reduction process to induce pores and defects that could significantly enhance the diffusivity of nitrogen atoms in the foils. The fabricated α″-Fe16N2 foils exhibit the specific saturation magnetization up to 222 emu/g at 300 K (reduced iron foils ~ 205 emu/g) and the coercivity 1.1 kOe. The synthesized α″-Fe16N2 foils have an ultralow TCC and a high saturation magnetization besides its usages of low-cost and environment-friendly raw materials. These combined unique features make α″-Fe16N2 a promising rare-earth-free alternative for many applications required less temperature dependence of magnetic properties.
Bibliographical noteFunding Information:
Part of this work was carried out in the Characterization Facility, the University of Minnesota, which receives partial support from NSF through the MRSEC program. Authors also thank the partial support from the Institute for Rock Magnetism, Department of Earth Science, the University of Minnesota for the use of instruments. We are grateful to Dr. John Larson, Dr. Frank Johnson, Mr. Rich Greger, and Mr. Chad from Niron Magnetics Inc. For their help in foil preparation.
© 2019 Acta Materialia Inc.
- Low-temperature nitridation
- Permanent magnet
- Porous structure
- Rare-earth-free magnets
- α″-FeN foils