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Nanosecond-width pulse voltages were applied to a pure noble metal, i.e., face-centered cubic (fcc) structure iridium (Ir), of nanometer-sized contact (NC) forms. The dynamics of structure and the variation in the conductance of the NCs in the nanosecond electrical energization were investigated by in situ high-resolution transmission electron microscopy. We demonstrated that even in the pure noble metal NCs, crystalline phases transferred to amorphous phases by the nanosecond electrical energization. The inverse phase transformation, i.e., crystallization, was also observed by subsequent energization. The results showed that the phase control can be conducted even in the pure noble metal NCs of a fcc structure. The conductance of the NCs per the unit area of cross section varied in response to their phase variation; the average conductance ratio between the crystalline and amorphous phases was 0.86. The present study revealed that the pure noble metal NCs are applicable to nanometer-sized phase-change memory devices. • Amorphization of pure noble metal nanocontacts. • The phase control of iridium nanocontacts by nanosecond electrical energization. • The realization of high-temperature fatigueless phase-change memory devices. [ABSTRACT FROM AUTHOR]

The structural dynamics of pure niobium (Nb) contacts with a width of several nanometers (nanocontacts (NCs)) during the energization using nanosecond pulse-width voltages was observed in situ by transmission electron microscopy. The conductance of Nb NCs was simultaneously measured. As pulse waves with 4 ns width and 0.88 V in average height were impressed to crystal-state Nb NCs, phase transformation into amorphous states was observed. Subsequently, average pulse-width voltages lower than that of the amorphization voltage, i.e., 0.32 V, were impressed to amorphous state NCs, resulting in the inverse phase transformation, i.e., crystallization. In particular, the impression of the lower pulse-width voltages caused stepwise crystallization in the amorphous state NCs, corresponding to multiple stepwise conductance variations. The step height in the stepwise conductance variation depended on the external shape of NCs, the injected energy, and the stage of the stepwise crystallization. This stepwise conductance variation leads to the applications of Nb NCs to optimized ultimate fine multi-level nonvolatile memory devices. • Amorphization of niobium nanocontacts. • Stepwise crystallization of niobium nanocontacts by nanosecond energization. • Conductance variations by stepwise crystallization. • Niobium nanocontacts suitable for advanced functional nanodevices. [ABSTRACT FROM AUTHOR]