Energy dispersion of tunnel spin polarization extracted from thermal and electrical spin currents

Abstract

The energy dispersion of the tunnel spin polarization of ferromagnetic tunnel contacts $(\mathrm{C}{\mathrm{o}}_{70}\mathrm{F}{\mathrm{e}}_{30}/\mathrm{MgO})$ to semiconductors (silicon and germanium) has been extracted from thermal and electrical spin currents. It is demonstrated that the variation of thermal and electrical spin currents with bias voltage can be described simultaneously using the same energy dispersion of the tunnel spin polarization, whereby the electrical spin current is determined by the value of the tunnel spin polarization, whereas the energy derivative of the polarization determines the thermal spin current. Variations in the behavior of the spin currents for different devices can be correlated to the degree of energy dispersion of the tunnel spin polarization. It is also shown that the application of a modest voltage can switch the thermal spin current to zero and can even reverse its sign. The characterization of the energy dispersion of the tunnel spin polarization provides a tool for the engineering of the spin-dependent thermoelectric properties of ferromagnetic tunnel contacts.