Magnetic properties of melt-quenched Ni-rich amorphous and bcc Zr–Ni alloys (abstract)

Abstract

It has been demonstrated recently that the structure of melt-quenched Zr–Ni ribbons in the vicinity of 90 at. % Ni was strongly dependent on the quenching rate: at the highest quenching rate, the alloys Zr10Ni90 and Zr9Ni91 could be prepared with an amorphous structure whereas at lower quenching rates the Zr9Ni91 alloy was prepared as a solid solution of Zr in Ni with a body-centered cubic (bcc) structure. In a preliminary measurement we have established a Curie point (Tc) of 66 K for the a-Zr9Ni91 ribbon which was much smaller than the value Tc=235 K reported for an a-Zr10Ni90 ribbon. This discrepancy has motivated us to perform the present detailed magnetization study in the temperature range 5 to 300 K. It was also of interest to compare the Curie point of the amorphous and bcc phase with the same chemical composition. The real component χ′ of the AC susceptibility as well as the magnetization isotherms up to H=18.5 kOe have been investigated. The measurements of χ′ yielded T=41, 66, and 70 K for a-Zr10Ni90, a-Zr9Ni91, and bcc-Zr9Ni91 ribbons, respectively. The para-ferromagnetic transitions were found to be fairly sharp although in some of these samples several transitions differing by a few degrees only could be observed. In some of the samples, however, very faint magnetic transitions indicated by a small broad peak could also be detected around 250 K. This feature can be attributed to some residual phases of either an unknown Zr–Ni intermetallic compound or of Ni(Zr) solid solution precipitates. The close coincidence of this peak with the Curie temperature reported by Kaul suggests that his value (Tc=235 K) cannot be representative of the amorphous Zr–Ni matrix but it rather may originate from ferromagnetic precipitates. The magnetization isotherms for the amorphous and bcc-Zr9Ni91 alloy indicated the development of a significant magnetization around the Tc values deduced from χ′. The saturation magnetization at the lowest temperature amounted to about 7 emu/g in both structural modifications. This value is reasonable in view of the reduction of Tc with respect to pure fcc-Ni. The present Tc data indicate that the critical concentration for the onset of magnetic ordering in the Zr–Ni system should be slightly below 90 at. % Ni. It can be concluded from this study that the magnetic behavior of Zr–Ni alloys at about 90 at. % Ni content is characteristic for very weak itinerant ferromagnets and the results will be analyzed in this framework with the help of Arrott plots.