Structural stability of Ni quantum point contacts under electrical stresses

We have investigated the structural stability of quantum point contacts (QPCs) of ferromagnetic metals under electrical stresses, using a spectroscopic method called ``electromigration (EM) spectroscopy[1]''; i. e., we have applied a feedback-controlled electrical break junction method to Ni QPC samples and obtained a histogram of the critical junction voltages, $V_{c}$, at which there occurred one-by-one atom removal due to EM. The obtained histogram shows that $V_{c}$ is distributed over a range of from 0.2 $\sim $ 0.4 V, which are consistent with the surface diffusion potential, $E_{D}$, of Ni. It was found that, although the local current density through the Ni QPCs is higher than 10$^{10}$ A/cm$^{2}$, the Ni QPC is stable, as long as the junction voltage is less than a certain threshold voltage determined by $E_{D}$ of Ni. The present result indicates that the junction voltage, rather than the current density, plays a critical role in EM in metallic QPCs and that the EM spectroscopy is a powerful tool for determining the structural stability of electrically-biased atomic-scale systems. [1] A. Umeno and K. Hirakawa, Appl. Phys. Lett. 94, 162103 (2009)