Synthesis of α-Fe16N2 foils with an ultralow temperature coefficient of coercivity for rare-earth-free magnets

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 alpha ''-Fe16N2 foils, as a candidate for rareearth-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 alpha ''-Fe16N2 foils are made from as-rolled iron foils (25 mu m) 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 alpha ''-Fe16N2 foils exhibit the specific saturation magnetization up to 222 emu/g at 300 K (reduced iron foils similar to 205 emu/g) and the coercivity 1.1 kOe. The synthesized alpha ''-Fe16N2 foils have an ultralow TCC and a high saturation magnetization besides its usages of low-cost and environmentfriendly raw materials. These combined unique features make alpha ''-Fe16N2 a promising rare-earth-free alternative for many applications required less temperature dependence of magnetic properties. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.