Tunneling states in metallic glasses: Model structures for Ni80P20 and Cu33Zr67

Abstract

A computer model based on the analogy between local environments in the crystalline and amorphous states is presented and used to generate and relax structures which simulate two typical metallic glasses, namely ${\mathrm{Ni}}_{80}$ ${\mathrm{P}}_{20}$ and ${\mathrm{Cu}}_{33}$ ${\mathrm{Zr}}_{67}$. It is found that this model gives a physically realistic picture of the systems examined; in particular, the short-range order is properly described, yielding good agreement between calculated and measured coordination numbers. The structures are then analyzed in terms of single-atom tunneling taking place between two metastable minimum-energy positions. The probability of occurrence of these "two-level systems" is shown to be strongly correlated with the degree of relaxation and thus with the density. Moreover, they are seen to be associated with voids in the structure, which disappear upon relaxation. It is therefore proposed that the relaxation procedure is analogous to the physical process of annealing metallic glasses below their glass transition temperatures. Hence, it is concluded that a reduction of the various low-temperature anomalies in these materials should result from the annealing process.