The Role of Cluster Formation on the Magnetic Properties of Nd-Fe-Al-based Magnetic Alloys
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The Role of Cluster Formation on the Magnetic Properties of Nd-Fe-Al-based Magnetic Alloys I. Betancourt and R. Valenzuela Institute for Materials Research, UNAM. P.O. Box 70-360, Mexico DF 04510, Mexico ABSTRACT We present a detailed investigation of the role of the disorder degree on magnetic properties for melt spun and die cast intermetallic Nd-Fe-Al-based alloys. For melt spun alloys, the roll speed was varied between 30 m/s and 5 m/s, which resulted in a clear dependence of coercivity on increasing roll speed, showing a steep decrease (from 250 kA/m to 10 kA/m). The results are interpreted in terms of a gradual transition from fully disordered microstructure (30 m/s sample) to short range ordered one (i.e., a clustered microstructure at 5 m/s), which is a consequence of a diminished cooling rate. On the other hand, die-cast alloy affords cluster formation due to this very low cooling rate compared with the melt spinning process and in consequence, coercivity values of 284 kA/m were observed. Further alloying addition (B, Cu, Dy) on the original Nd-FeAl composition, exhibited coercivity enhancement (up to 388 kA/m) due to the presence of small Nd-based crystallites, which act as pinning centers.
INTRODUCTION Disordered intermetallic magnetic alloys have usually been studied as excellent soft magnetic materials due to their very high permeability and vanishing coercivity values, both a consequence of a negligible anisotropy and a lack of defects, which enhances the domain wall displacement under an applied magnetic field [1]. Due to these microstructural features, no permanent magnetic properties were expected in amorphous alloys, since the coercivity mechanism requires in general, high magnetocrystalline anisotropy and/or small precipitates acting as pinning centers. Recently, a new disordered alloy, based on Nd-Fe-Al, was announced as permanent magnet [2], because of its coercivity values (within the range 250- 300 kA/m). A rather simple die-casting processing route was used to prepare this alloy [2]. The origin of coercivity Hc in these amorphous alloys has been ascribed to short-range order structures, in the form of highly interacting magnetic Nd, Nd-Fe-Al clusters comprising a few hundreds atoms embedded in an amorphous matrix [3]. Such particular microstructure is afforded by low cooling rate during the die-casting process. In addition, pinning of domain walls is also believed to play a significant role in attaining considerable Hc values, since nanometric hcp Nd crystallites have been observed in NdFeAl alloys, depending on composition and processing route [4,5]. In contrast, NdFeAl alloys prepared by melt spinning techniques show fairly lower coercivity values at room temperature when compared with their bulk counterparts [1]. This behaviour has been ascribed to a fully disordered structure afforded by very high cooling rates (tpically 105-6 K/seg) during the melt spinning process, which enables highly unrelaxed structure as a consequence of the internal stresses induced during the rapid soli
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