MAGNETIC PROPERTIES OF RAPIDLY QUENCHED (Nd, Pr)2 (Fe,Co)14B-TYPE ALLOYS

The coercivity Hc; of rapidly quenched Nd-Fe-B is linked to the multiphase structure of the alloys: high coercivities require the suppression of the &Nd solid solution and the exictence of the intergranular Nd-rich phase. In quarternary Pr-Fe-Co-B alloys H,i and the remanence J, depend in the same way on Co content as in the Nd-FeCo-B system. However, compared with rapidly quenched Nd-Co-B samples, as-quenched Pr-Co-B alloys show huge coercivities as a result of the favourable intrinsic magnetic properties. Again, magnetic hardness is not only coupled to the grain size but also to the existence of secondary phases. In Nd-Fe-B type magnets the principal phase which is responsible for the hard magnetic properties is the tetragonal NdzFe14B (4 -) phase. However, optimum magnetic properties are only achieved if an appropriate off-stoichiometric composition is chosen. This applies equally to powder-metallurgically produced magnets where a smooth Nd-rich grain boundary phase is supposed to reduce possible nucleation sites for reverse domains [I] as to rapidly quenched material where a similar Nd-rich phase might act as a pinning site for domain walls [2]. By alloying Co to rapidly solidified Nd-Fe-B it is possible to optimize the magnetic properties further and to prepare isotropic magnets with an outstanding temperature characteristic of both the remanence, J,, and the coercivity, Hci , respectively 131. So far, the effect of a substitution of Nd by Pr in the quarternary Nd-Fe-CUB system and especially the coercivities of Co-rich alloys remained to be investigated. Besides this latter topic we will first of all concentrate on a systematic study of the influence of composition on the magnetic properties of ternary Nd-Fe-B. Details of the sample preparation are given elsewhere [4]. Figure 1 presents the variation of the coercivity Hci and the remanence J, along the tie-line which connects the a-Fe solid solution with the NdzFel4Bphase in the ternary Nd-Fe-B phase diagram [5]. It is obvious that Hc; remains at relatively low values of less than 4 kA/cm a t the Fe-rich compositions while high coercivities are only observed if the alloy composition lies in the three phase field built up by 6, the B-rich phase, Ndl-,FerB4, and the Nd solid solution. We believe that this dependence is due to the occurrence of metastable minority phases of a) a supersaturated &Nd solid solution which forms at high Fe-concentrations as a result of the peritectic formation of 4, and of b) a Nd-rich intergranular phase which represents the Nd-enriched melt which solidifies as the last component during melt spinning. Both phases were clearly identified by field ion microscopy in combination with atom probe techniques [6] and partly by MoPbauer spectroscopy [7]. With increasing Nd-content the remanence decreases continuously due to the increasing amount of the nonmagnetic Ndrich phase. The same dependence as in figure 1 is observed for concentrations along the line at which the B-content remains constant while the Fe/Nd ratio is Fig. 1. Coercivity H,; and remanence J, of rapidly quenched Nd2,Felo0-3~B~ vs. B-concentration x. Fig. 2. Coercivity Hci and remanence J, of rapidly quenched NdxFeg5-,Bs vs. Nd-concentration x. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888279 C8 612 JOURNAL DE PHYSIQUE changed (Fig. 2). In comparison, samples with concentrations within the three phase region mentioned above show only a minor variation of the coercivity with increasing Nd-content, the remanence staying almost constant (Fig. 3). These relationships were also found for mechanically alloyed Nd-Fe-B [8] which underlines a close similarity with rapidly quenched material. We have already shown that substitutions of Fe by Co up to 30 at % result in increased H,; values while the remanence, J,, almost remains constant [3]. Beyond 30 at % Co, H,; and J, decrease and finally drop below the values of the unalloyed sample. In the comparable Pr-Fe-Co-B system we observe the same dependence of H,; and J, on the amount of Co substitution (Fig. 4). However, on the Co-rich side, H,; increases again up to a maximum value of 15.2 kA/cm for the ternary Pr15C077B8 alloy. By comparison, the value for the kA/cm