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

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Thermal Conductivity of Oxide Tunnel Barriers in Magnetic Tunnel  Junctions Measured by Ultrafast Thermoreflectance and Magneto-optic Kerr  Effect Thermometry