Occupancy of 4d-states in T (T: Mo, Nb) from NEXAFS L 3,2 Spectra of Nd-Fe-B-T Alloys

Abstract

8) alloys were studied by X-ray absorption spectroscopy. The NEXAFS spectra of NdFeB ribbons produced by the melt spinning technique (as quenched, AQ) and subsequently annealed (TT) were taken at Mo and Nb L 3,2 -edge. We calculated the variations in occupancy of the 4d-states (in both additives), fi nding an opposite trend in their fi lling. The most signifi cant changes were observed in the T 2 Nd 7 system, for which the electron occupancy in the 4d levels was modifi ed to an excess of 1 electron/atom (after thermal treatment). The correlation between these results and magnetic features of the alloys are evidenced.Keywords: Nd-Fe-B based magnets, additive, electron occupancy.Introduction. Neodymium-iron-boron nanocomposite permanent magnets were developed in the early 1990s [1,2]. Their low neodymium content, less than the stoichiometric amount of 12 at.%, made them cheaper and more corrosion resistant. Moreover, their higher boron content improves the amorphization ability. These systems, called exchange-spring magnets, exhibit interesting magnetic properties due to the exchange coupling between the hard and soft magnetic phases at nanoscale. The coexistence of both types of magnetic phases results in rema nence and magnetic energy product ((BH) max ) enhancement. However, the presence of the soft phases reduces the coercivity. The use of alloying components such as Mn, Cu, Nb, Ti, and Mo, among others, helps to control the grain growth during the melt-spinning production of these kind of magnets, improving in this way the magnetic response of the material [3][4][5]. In particular, the addition of Mo and Nb was reported to be very effective for grain refi nement of nanocomposites, leading to the improvement of the coercivity after annealing [6]. However, the mechanisms that cause the suppression of α-Fe formation are not fully understood, as well as the role of Mo or Nb as an additive component that helps to stabilize the Nd 2 Fe 14 B phase (ϕ-phase). Urse et al. explained the coercivity enhancement by the pinning of domain walls at grain boundaries where the additive precipitates, in the case of Modoped Nd-Fe-B thin fi lms [7]. For Nb-doped ribbons, it was found that a higher concentration of this additive is required than in the case of Mo in order to obtain higher coercivities and (BH) max [8]. Thus, a study of the electronic state of these elements becomes appropriate to elucidate the role they play as additives in Nd-Fe-B magnets. Synchrotron radiation is a powerful tool that helps in this study. The information on the electronic structure of occupied/unoccupied 4d-electron states in these alloys was derived from near-edge X-ray absorption fine structure (NEXAFS) measurements.The purpose of the present work is to quantify, by X-ray absorption spectroscopy, variations in occupancy of the 4d-states of molybdenum and niobium added to NdFeB alloys produced by the melt spinning technique, both as quenched (AQ) and subsequently annealed (TT), and to identify correlations between these results and magnetic feat...