In the field of spin caloritronics recent theoretical models suggested a significant influence of the interfaces of the magnetic tunnel junction (MTJ) on the thermal transport. In this work magnetothermopower measurements are carried out on CoFeB/MgO/CoFeB nanopillars and an unexpected increase of the thermovoltage with the diameter of the nanopillars is observed. To understand this behavior the thermal profiles are computed by finite element simulations. The observed behavior with the pillar diameter could only be reproduced in simulations by considering a far lower effective thermal conductivity of the MgO than the intrinsic thin-film value. In agreement with theoretical predictions, a finite thermal conductivity of the MgO/CoFeB interface can explain this observation. This is experimental evidence of the influence of the thermal resistance of the MgO/CoFeB interfaces on magnetothermovoltage measurements and is in agreement with recent theoretical predictions. The measured magnetothermovoltage is around 4.5 μV and the simulated temperature difference is about 2 K across the tunnel barrier, which resulted in a magnetic contribution of the thermopower of $\mathrm{\Delta }{S}_{\mathrm{MTJ}}\approx -2.25\mu \mathrm{V}{\mathrm{K}}^{-1}$. This value was about 20 times smaller than the result obtained by the typically used thermal conductivity of MgO thin films.

• Received 16 January 2017

DOI:https://doi.org/10.1103/PhysRevB.95.104441