AA-05 Tunnel magneto-Seebeck effect and thermal conductivity of tunnel barriers
Creators: Timo Kuschel
Publisher: name: IEEE
Year: 2025

A magnetic tunnel junction (MTJ) that consists of an insulating tunnel barrier sandwiched by two magnetic electrodes is not only the main building block of magnetoresistive random-access memory (MRAM) devices, it is also the basis of potential spin caloritronic applications based on the tunnel magneto-Seebeck (TMS) effect. Here, a temperature gradient induces Seebeck voltages across the tunnel junction depending on the magnetization orientation of the two magnetic electrodes [1,2]. For example, the combination of TMS and the anomalous Nernst effect (ANE) in MTJs opens the possibility of detecting thermal gradients in three dimensions on the nano scale [3]. The thickness of the tunnel barrier and the choice of electrode material determines the TMS strength [4,5]. In addition, the thermal conductivity of the tunnel barrier also depends on the barrier’s thickness and is a crucial quantity when Seebeck coefficients of MTJs have to be identified [3,6]. So far, estimates of thermal conductivity have been used such as values which are one order of magnitude below the thermal conductivity of bulk MgO or MgAl2O4 (MAO). However, only experimentally determined values for the thermal conductivity of tunnel barriers will result in reliable quantitative numbers for the TMS coefficients. Recently, several techniques have been utilized to determine the thermal conductivity of tunnel barriers. TMS measurements before and after the dielectric breakdown to get rid of parasitic AHE contributions have been conducted in combination with finite element modelling [7]. For example, here we extracted 0.7 W/(K m) for the thermal conductivity of ultra-thin MAO which is almost two orders of magnitude below the bulk value. In addition, ultrafast thermoreflectance and magneto-optic Kerr effect thermometry have been used to identify the thermal conductivity of ultrathin MgO and MAO [8]. The results of 0.4-0.6 W/(K m) now provide the basis for more accurate estimations of TMS coefficients. In my contribution, the TMS will be introduced and the determination of the thermal conductivity of ultrathin MgO and MAO will be presented. Finally, the impact of the barrier's thermal conductivity on the TMS is discussed.References: [1] M. Walter et al., Nat. Mater. 10, 742 (2011); N. Liebing et al., Phys. Rev. Lett. 107, 177201 (2011). [2] T. Kuschel et al., J. Phys. D.: Appl. Phys. 52, 133001 (2018). [3] U. Martens, TK et al., Commun. Phys. 1, 65 (2018). [4] T. Huebner, TK et al., Phys. Rev. B 96, 214435 (2017). [5] A. Boehnke, TK et a., Nat. Commun. 8, 1626 (2017). [6] J. Zhang et al., Phys. Rev. Lett. 115, 037203 (2015). [7] T. Huebner, TK et al., J. Phys. D.: Appl. Phys. 51, 224006 (2018). [8] H. Jang, TK et al., Phys. Rev. Applied 13, 024007 (2020).
A magnetic tunnel junction (MTJ) that consists of an insulating tunnel barrier sandwiched by two magnetic electrodes is not only the main building block of magnetoresistive random-access memory (MRAM) devices, it is also the basis of potential spin caloritronic applications based on the tunnel magneto-Seebeck (TMS) effect. Here, a temperature gradient induces Seebeck voltages across the tunnel junction depending on the magnetization orientation of the two magnetic electrodes [1,2]. For example, the combination of TMS and the anomalous Nernst effect (ANE) in MTJs opens the possibility of detecting thermal gradients in three dimensions on the nano scale [3]. The thickness of the tunnel barrier and the choice of electrode material determines the TMS strength [4,5]. In addition, the thermal conductivity of the tunnel barrier also depends on the barrier’s thickness and is a crucial quantity when Seebeck coefficients of MTJs have to be identified [3,6]. So far, estimates of thermal conductivity have been used such as values which are one order of magnitude below the thermal conductivity of bulk MgO or MgAl2O4 (MAO). However, only experimentally determined values for the thermal conductivity of tunnel barriers will result in reliable quantitative numbers for the TMS coefficients. Recently, several techniques have been utilized to determine the thermal conductivity of tunnel barriers. TMS measurements before and after the dielectric breakdown to get rid of parasitic AHE contributions have been conducted in combination with finite element modelling [7]. For example, here we extracted 0.7 W/(K m) for the thermal conductivity of ultra-thin MAO which is almost two orders of magnitude below the bulk value. In addition, ultrafast thermoreflectance and magneto-optic Kerr effect thermometry have been used to identify the thermal conductivity of ultrathin MgO and MAO [8]. The results of 0.4-0.6 W/(K m) now provide the basis for more accurate estimations of TMS coefficients. In my contribution, the TMS will be introduced and the determination of the thermal conductivity of ultrathin MgO and MAO will be presented. Finally, the impact of the barrier's thermal conductivity on the TMS is discussed.References: [1] M. Walter et al., Nat. Mater. 10, 742 (2011); N. Liebing et al., Phys. Rev. Lett. 107, 177201 (2011). [2] T. Kuschel et al., J. Phys. D.: Appl. Phys. 52, 133001 (2018). [3] U. Martens, TK et al., Commun. Phys. 1, 65 (2018). [4] T. Huebner, TK et al., Phys. Rev. B 96, 214435 (2017). [5] A. Boehnke, TK et a., Nat. Commun. 8, 1626 (2017). [6] J. Zhang et al., Phys. Rev. Lett. 115, 037203 (2015). [7] T. Huebner, TK et al., J. Phys. D.: Appl. Phys. 51, 224006 (2018). [8] H. Jang, TK et al., Phys. Rev. Applied 13, 024007 (2020).