Mesoscopic Stoner Instability in Metallic Nanoparticles Revealed by Shot Noise

We study sequential tunneling through a metallic nanoparticle close to the Stoner instability coupled to parallel magnetized electrodes. Increasing the bias voltage successively opens transport channels associated with excitations of the nanoparticle’s total spin. For the current this leads just to a steplike increase. The Fano factor, in contrast, shows oscillations between large super-Poissonian and sub-Poissonian values as a function of bias voltage. We explain the enhanced Fano factor in terms of generalized random-telegraph noise and propose the shot noise as a convenient tool to probe the mesoscopic Stoner instability.

Figure 1

Nanoparticle states and possible transitions. Solid (dotted) lines indicate tunneling of majority (minority) spin electrons.

Figure 2

Current as a function of bias voltage. Arrows mark onset of transport through excited states. $N_G=n+\frac{1}{2}+\frac{2n+3}{8}\frac{\Delta }{E_C}-0.0001$, $n\in \mathbb{N}$, $E_C=10\Delta$, $J=0.97\Delta$, ${\Gamma }_L={\Gamma }_R$, $p=0.3$, $T=0.001\Delta$. Higher steps become increasingly difficult to resolve.

Figure 3

Fano factor as a function of bias voltage for different asymmetries of the tunnel couplings. Other parameters as in Fig. 2.