Using Single Quantum States as Spin Filters to Study Spin Polarization in Ferromagnets

By measuring electron tunneling between a ferromagnet and individual energy levels in an aluminum quantum dot, we show how spin-resolved quantum states can be used as filters to determine spin-dependent tunneling rates. We also observe magnetic-field-dependent shifts in the magnet’s electrochemical potential relative to the dot’s energy levels. The shifts vary between samples and are generally smaller than expected from the magnet’s spin-polarized density of states. We suggest that they are affected by field-dependent charge redistribution at the magnetic interface.

Figure 1

(inset) Cross-sectional device schematic. (a) Differential conductance vs $V$ for device Ni#1 with one Ni electrode, and (b) for device Co#1 with one Co electrode. Magnetic fields are applied to cause Zeeman splitting of the spin-up and spin-down resonances.

Figure 2

$dI/dV$ vs voltage and magnetic field for (a,b) device Ni#1 and (c) Co#1. The scales extend from 0 to (a,b) $0.2\mu \mathrm{S}$ and (c) $2\mu \mathrm{S}$. White indicates $dI/dV$ values beyond the scale maximum, and in (c) black indicates negative values.

Figure 3

(a) Current vs voltage curve for device Co# 1 for $B=1\mathrm{T}$, showing the range of $V$ where tunneling occurs via one pair of Zeeman-split energy levels. (b) Energy-level diagrams at each current step. Black horizontal arrows show the threshold tunneling transition. Gray arrows depict other transitions which contribute to the current. (c) The tunneling rates ${\gamma }_{\uparrow }$, ${\gamma }_{\downarrow }$, and ${\gamma }_N$ were determined as described in the text. (d) Tunneling polarization for device Co#1.