  The electronic and magnetic bulk properties of half-metallic Heusler alloys
such as Co$_{2}$FeSi, Co$_{2}$FeAl, Co$_{2}$MnSi and Co$_{2}$MnAl are
investigated by means of {\em ab initio} calculations in combination with Monte
Carlo simulations. The electronic structure is analyzed using the plane wave
code Quantum Espresso and magnetic exchange interactions are determined using
the KKR method. From the magnetic exchange interactions the Curie temperature
is obtained via Monte Carlo simulations. In addition, electronic transport
properties of the trilayer systems consisting of two semi-infinite platinum
leads and a Heusler layer in between are obtained from the fully relativistic
KKR method by employing the Kubo-Greenwood formalism. The focus is on
thermoelectric properties, namely the Seebeck effect and its spin dependence.
It turns out that already thin Heusler layers provide highly polarized currents
within the systems. This is attributed to the recovery of half-metallicity with
increasing thickness. The absence of electronic states of the spin down
electrons around the Fermi level suppresses the contribution of this spin
channel to the total conductivity. This strongly influences the thermoelectric
properties of such systems and results in polarized thermoelectric currents.
