Nanocrystalline Nd₂Fe₁₇synthesized by high-energy ball milling: crystal structure, microstructure and magnetic properties

Abstract

Nanocrystalline Nd(2)Fe(17) powders have been obtained by means of high-energy ball milling from nearly single-phase bulk alloys produced by arc melting and high temperature homogenization annealing. The rhombohedral Th(2)Zn(17)-type crystal structure of the bulk alloy remains unaltered after the milling process, with almost unchanged values for the cell parameters. However, the severe mechanical processing induces drastic microstructural changes. A decrease of the mean crystalline size down to around 10 nm is observed, giving rise to a considerable augmentation of the disordered inter-grain boundaries. This modification of the microstructure affects the magnetic behaviour of the milled powders, although the magnetic structure remains collinear ferromagnetic. While a unique ferro-to-paramagnetic transition temperature, T(C) = 339 ± 2 K, is observed in the bulk alloy, the nanocrystalline samples exhibit a more likely distribution of T(C) values. The latter seems to be responsible for the significant broadening of the temperature range in which magneto-caloric effect is observed, and the lowering of the maximum value of the magnetic entropy change.

Magneto-caloric effect in the pseudo-binary intermetallic YPrFe17 compound

“…[13,20,30]. The ratio c/a ~ 1.54 is in good agreement with those reported for the rhombohedral crystal structure in these 2:17-type alloys [13,29,30].…”

“…No traces of the hexagonal Th 2 Ni 17 -type crystal structure have been found, as confirmed by neutron diffraction from which information of the whole sample is attained, in contrast with XRD. Whereas in other 2:17 pseudo-binary alloys of this family a disordered rhombohedral structure has been proposed in order to explain the strong intensity reduction observed for many reflexions [27,28], in the present case there is no such a reduction, even though when compared with the diffraction patterns of the binary Y 2 Fe 17 , Pr 2 Fe 17 or Nd 2 Fe 17 alloys [13,29]. From the fit of the diffraction patterns it is evident that the 6c site corresponding to the R atoms is equally shared between Y and Pr with the same atomic coordinates.…”

On the broadening of the magnetic entropy change due to Curie temperature distribution

“…[5][6][7] In fact, the nanocrystallization of ferromagnetic R 2 Fe 17 compounds (R ¼ Pr and Nd) by means of ball milling has been proved as an illustrative example of this method, which leads to the enhancement of the refrigerant temperature range and the refrigerant capacity. 8,9 Moreover, the Curie temperature (T C ) is strongly affected by the modification of the Fe-Fe interatomic distances in a number of Fe-based alloys exhibiting magneto-volume anomalies. 10 Changes in the material microstructure can produce local inhomogeneities around the Fe atoms, and a distribution of Fe-Fe interatomic distances affecting the exchange interactions.…”

“…13 SEM images reveal that the Pr 2 Fe 17 and Nd 2 Fe 17 milled powders are agglomerations of micronic grains (0.5-10 lm) with rounded borders and narrower size distribution as the milling-time is increased. 8,9 Higher magnification images obtained by TEM show grains formed by crystallites of nanometric size; the histograms describing the crystallite size distribution for the BM-10h and BM-20h compounds (powders ball-milled for 10 and 20 h, respectively) follow log-normal functions with average size of the crystallites below 25 nm (see Table I). For the BM-40h samples, not enough crystallites In Fig.…”

“…Hence, we could picture the nanostructured powders having a distribution of Fe-Fe interatomic distances around the value for the Bulk alloys giving rise to a distribution of Curie temperature values. 8,9 Then, we describe the temperature dependence of the magnetization M(T) by a superposition of individual power-law functions, according to 17 …”