  Thermal spin-transfer torque describes the manipulation of the magnetization
by the application of a heat flow. The effect has been calculated theoretically
by Jia et al. in 2011. It is found to require large temperature gradients in
the order of Kelvins across an ultra thin MgO barrier. In this paper, we
present results on the fabrication and the characterization of magnetic tunnel
junctions with 3 monolayer thin MgO barriers. The quality of the interfaces at
different growth conditions is studied quantitatively via high-resolution
transmission electron microscopy imaging. We demonstrate tunneling magneto
resistance ratios of up to 55% to 64% for 3 to 4 monolayer barrier thickness.
Magnetic tunnel junctions with perpendicular magnetization anisotropy show
spin-transfer torque switching with a critical current of 0.2 MA/cm$^2$. The
thermally generated torque is calculated ab initio using the
Korringa-Kohn-Rostoker and non-equilibrium Green's function method. Temperature
gradients generated from femtosecond laser pulses were simulated using COMSOL,
revealing gradients of 20 K enabling thermal spin-transfer-torque switching.
