Thermal Conductivity of Oxide Tunnel Barriers in Magnetic Tunnel Junctions Measured by Ultrafast Thermoreflectance and Magneto-Optic Kerr Effect Thermometry
Year: 2020
Type: article
Abstract: 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 $\mathrm{Ru}/\mathrm{oxide}/\mathrm{FM}/\mathrm{seed}/\mathrm{Mg}\mathrm{O}$ (oxide = $\mathrm{Mg}\mathrm{O}$, ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$; FM = $\mathrm{Co}$, ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$; seed = $\mathrm{Pt}$, $\mathrm{Ta}$) structures having perpendicular magnetic anisotropy using ultrafast thermometry. The $\mathrm{Ru}$ layer is optically thick and heated by ultrafast laser pulses; the subsequent temperature changes are monitored using the thermoreflectance of $\mathrm{Ru}$ and magneto-optic Kerr effect (MOKE) of the FM layers. We independently measure the response times of $\mathrm{Co}$ and $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$ magnetism using quadratic MOKE and obtain ${\ensuremath{\tau}}_{e\ensuremath{-}m}=0.2$ ps for $\mathrm{Co}$ and 2 ps for $\mathrm{Co}\text{\ensuremath{-}}\mathrm{Fe}\text{\ensuremath{-}}\mathrm{B}$. 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 $\mathrm{Mg}\mathrm{O}$ and ${\mathrm{Mg}\mathrm{Al}}_{2}{\mathrm{O}}_{4}$ tunnel barriers in the range of 0.4--0.6 W ${\mathrm{m}}^{\ensuremath{-}1}$ ${\mathrm{K}}^{\ensuremath{-}1}$, comparable with an estimate of the series thermal conductance of the $\mathrm{Ru}/\mathrm{oxide}$ and oxide/FM interfaces and an order of magnitude smaller than the thermal conductivity of $\mathrm{Mg}\mathrm{O}$ 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.
Source: Physical Review Applied
Authors: Hyejin Jang, Luca Marnitz, Torsten Huebner, Johannes Kimling, Timo Kuschel, David G. Cahill
Institutions: University of Illinois Urbana-Champaign, Bielefeld University
Language: English
Subfield: Condensed Matter Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W2994019679