  Spin-dependent charge transport in magnetic tunnel junctions (MTJs) can be
manipulated by a temperature gradient, which can be utilized for spintronic and
spin caloritronic applications. Evaluation of the thermally induced phenomena
requires knowledge of the temperature differences across the oxide tunnel
barrier adjacent to the ferromagnetic (FM) leads. However, it is challenging to
accurately measure thermal properties of an oxide tunnel barrier consisting of
only a few atomic layers. In this work, we experimentally interrogate the
temperature evolutions in Ru/oxide/FM/seed/MgO (oxide=MgO, MgAl2O4; FM=Co,
CoFeB; seed=Pt, Ta) structures having perpendicular magnetic anisotropy using
ultrafast thermometry. The Ru layer is optically thick and heated by ultrafast
laser pulses; the subsequent temperature changes are monitored using
thermoreflectance of Ru and magneto-optic Kerr effect (MOKE) of the FM layers.
We independently measure the response times of Co and CoFeB magnetism using
quadratic MOKE and obtain {\tau}em=0.2 ps for Co and 2 ps for CoFeB. These time
scales are much shorter than the time scale of heat transport through the oxide
tunnel barrier, which occurs at 10-3000 ps. We determine effective thermal
conductivities of MgO and MgAl2O4 tunnel barriers in the range of 0.4-0.6 W m-1
K-1, comparable to an estimate of the series conductance of the Ru/oxide and
oxide/FM interfaces and an order of magnitude smaller than the thermal
conductivity of MgO thin films. We find that the electron-phonon thermal
conductance near the tunnel barrier is only a factor of 5-12 larger than the
thermal conductance of the oxide tunnel barrier. Therefore, the drop in the
electronic temperature is approximately 20-30% larger than the drop in the
phonon temperature across the tunnel barrier.
