Sm-Fe-N revisited; remanence enhancement in melt-spun Nitroquench material

Following its discovery in the aftermath of Nd2Fe14B, Sm2Fe17N3 seemed to offer intrinsic magnetic properties that were superior or comparable to those of its famous predecessor. But the promise of the new material to challenge Nd2Fe14B was not realized, mainly because the 2:17 nitride powder, prepared by a low-temperature gas-phase interstitial modification process, was unstable at the temperatures needed to process dense sintered magnets. Here we discuss the magnetic properties of Nitroquench, a melt-spun Sm-Fe-N material, which offers superior corrosion resistance and thermal stability compared to melt-spun Nd-Fe-B. The powder, with a crystallite size of approximately 30 nm is in the form of flakes 15-18 mu m thick and about 100 mu m in diameter. Room-temperature coercivity is 690 kAm(-1) after saturation in 14 T, with a remanence of 92 Am(2)kg(-1) and an extrapolated saturation magnetization of 160 Am(2)kg(-1). The remanence enhancement is reflected in a preferred orientation seen in Fe-57 Mossbauer spectra of magnetized isotropic powder, which exhibits different relative intensities of the Delta M = 0 absorption lines according to the direction of the field used to saturate the magnetization. When measured in zero internal field, the remanence ratio M-r/M-s is 64%. The remanence enhancement is attributed to a nanocrystallite size that is not very much greater than the exchange length. The maximum energy product for the powder, assuming full density, is 162 kJm(-3). Nitroquench powder may be used to produce bonded magnets with an energy product > 100 kJm(-3).