Tunneling Magnetothermopower in Magnetic Tunnel Junction Nanopillars

We study tunneling magnetothermopower (TMTP) in $\mathrm{CoFeB}/\mathrm{MgO}/\mathrm{CoFeB}$ magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by an electric heater line. Thermopower voltages up to a few tens of $\mu \mathrm{V}$ between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence the thermal gradient. The thermopower signal varies by up to $10\mu \mathrm{V}$ upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresponds to a large spin-dependent Seebeck coefficient of the order of $100\mu \mathrm{V}/\mathrm{K}$ and a large TMTP change of the tunnel junction of up to 90%.

Figure 1

(a) Sketch of TMTP setup. A current $I_{\mathrm{heat}}$ through the HL creates a temperature gradient across the MTJ and the thermovoltage $V_{\mathrm{TP}}$ is measured in in-plane magnetic fields $H_S$. (b) Electron micrograph of typical device with HL, BC, and top contacts (TC1-TC3) to three nanopillars. The position of the MTJ nanopillar under TC2 is marked. (c) Cross section sketch of contact and layer structure along the dashed line in (b). (d) Simulated temperature for $P_{\mathrm{heat}}=60\mathrm{mW}$ in the MTJ as function of thickness $t$. $t=0$ corresponds to the bottom of the lowest CoFeB layer.

Figure 2

Magnetic field dependence of TMR and TMTP of two typical devices MTJ-3 (a),(b) and MTJ-2 (c),(d). (a) Easy axis TMR loop without (black) and with (red) an applied heater current of $I_{\mathrm{heat}}=38\mathrm{mA}$. The TMR switching fields $H_C^{-}$, $H_C^{+}$ are shifted due to the HL field $H_{\mathrm{heat}}$ by $\Delta H_C^{-}$, $\Delta H_C^{+}$. (b) Easy axis TMTP loops under applied dc heating powers of $P_{\mathrm{heat}}=21.5\mathrm{mW}$ (blue), 37.8 mW (red), and 58.05 mW (black). $H_{\mathrm{heat}}$ is compensated. Black dashed line is $V_{\mathrm{TP},\mathrm{short}}$ of shorted MTJ. (c) Angular dependent TMR loop for 360° in-plane rotation of static field ${\mu }_0{H}_S=30\mathrm{mT}$. (d) $V_{\mathrm{TP}}$ under same field rotation of (c). $V_{\mathrm{TP}}$ is derived under ac excitation with $I_{\mathrm{heat}}=60\mathrm{mA}$, $P_{\mathrm{heat}}=35.15\mathrm{mW}$, and $\pm H_{\mathrm{heat}}\approx \pm 6\mathrm{mT}$.

Figure 3

(a) $V_{\mathrm{TP}}$ as function of $P_{\mathrm{heat}}$ for parallel ($P$, red circles) and antiparallel (AP, black squares) orientation of MTJ-1. dc (symbols) and ac (lines) data agree. (b) TMTP ratio vs $P_{\mathrm{heat}}$ for ac (full squares) and dc measurements (open squares). Inset: TMTP vs TMR of the devices of Table . (c) HL resistance $R_{\mathrm{HL}}$ (left scale, circles) and increase of HL temperature $\Delta T_{\mathrm{HL}}$ (right scale, triangles) as a function of $P_{\mathrm{heat}}$.