J.M.D. Coey1 P. Stamenov1 S.B. Porter1 M. Venkatesan1 T. Iriyama2

1, Trinity College Dublin, Dublin, , Ireland
2, Daido Steel Co. Ltd, Nagoya, , Japan

Sm2Fe17N3 was discovered about seven years after Nd2Fe14B [1,2], and it seemed to offer intrinsic magnetic properties that were superior (TC, K1) or comparable (Ms) to those of its famous predecessor. However, the promise of the new material to challenge Nd2Fe14B did not materialize. The 2:17 nitride powder, prepared by a low-temperature gas-phase interstitial modification process was difficult to orient and was not stable at the temperatures needed to process dense sintered magnets [3]. An early result was zinc-bonded Sm2Fe17N3 [4] with an energy product of 84 kJm3 but a rather low coercivity of 480 kAm-1, less than 5 % of the anisotropy field (2K1/Ms ≈ 11 MAm-1). Improvements were later obtained in polymer-bonded magnets produced by injection moulding [5]. Work continued both in Japan and China to develop a coercive powder suited for isotropic bonded magnets. Attempts to make fully-dense magnets by explosive compaction [6] or spark sintering [7] yielded interesting results, but these methods are not suited to large-scale production.

Here we report on the magnetic properties of melt-spun Sm-Fe-N powder, which has superior corrosion resistance and thermal stability compared to melt-spun Nd-Fe-B. The powder, with a crystallite size of approximately 15 nm deduced from Scherrer broadening of the X-ray reflections was in the form of flakes 10 µm thick and up to 100 µm in diameter. Composition was checked by EDX microprobe analysis. The Nitroquench powder exhibits a room-temperature coercivity of 690 kAm-1after saturation in 14 T, with an isotropic remanence of 700 kAm-1 in zero applied field and an extrapolated saturation magnetization of 1230 kAm-1. The remanence ratio Mr/Ms = 57 % (63% if the remanence is measured in zero internal field), is reflected in a preferred orientation seen in 57Fe Mössbauer spectra of magnetized powder; spectra obtained after saturating an immobilized powder absorber either in-plane or perpendicular to the sample plane exhibit distinctly different relative intensities of the ΔM = 0 absorption lines. The remanence enhancement is attributed to fact that the nanocrystallite size is not very much greater than the exchange length. The maximum energy product for the powder, assuming full density, is 162 kJm-3. The Nitroquench powder may be used to produce isotropic polymer-bonded magnets with an energy product > 100 kJm-3.

Keywords: Permanent magnets; nanocomposites; interstitial intermetallics.

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[7] T. Saito, J. Magn. Magn. Mater 369 184 (2014)