Transverse Spin Seebeck Effect versus Anomalous and Planar Nernst Effects in Permalloy Thin Films

Transverse magnetothermoelectric effects are studied in Permalloy thin films grown on MgO and GaAs substrates and compared to those grown on suspended ${\mathrm{SiN}}_x$ membranes. The transverse voltage along platinum strips patterned on top of the Permalloy films is measured versus the external magnetic field as a function of the angle and temperature gradients. After the identification of the contribution of the planar and anomalous Nernst effects, we find an upper limit for the transverse spin Seebeck effect, which is several orders of magnitude smaller than previously reported.

Figure 1

(a) Setup 1: A Py film of $4\times 8{\mathrm{mm}}^2$ size on a $4\times 10{\mathrm{mm}}^2$ MgO substrate is subjected to a temperature gradient ${\overrightarrow{\nabla }}_{x}T$ along the $x$ axis between two Peltier elements with 7 mm spacing. The transverse voltage $V_y$ is probed with bond wires on both ends of the Pt detector strip. The magnetic field $\overrightarrow{H}$ is applied in the film plane at an angle of $\Theta$ to the $x$ axis. (b) Setup 2: A similar Py film patterned on a $500\times 500\mu {\mathrm{m}}^2$ ${\mathrm{SiN}}_x$ membrane. Besides an electrically isolated heater and thermometers two Pt detector strips labeled A and B are used.

Figure 2

Transverse voltage $V_y$ versus the magnetic field and angle $\Theta$. (a) Setup 1: Py on the MgO substrate; $-4.5\le {\mu }_{0}H\le 4.5\mathrm{mT}$, ${\overrightarrow{\nabla }}_{x}T\approx 3.6\mathrm{K}/\mathrm{mm}$. The noise level in each sweep is 10 nV. (b) Setup 2: Py on the ${\mathrm{SiN}}_x$ membrane; $-4\le {\mu }_{0}H\le 4\mathrm{mT}$, ${\overrightarrow{\nabla }}_{x}T\approx 240\mathrm{K}/\mathrm{mm}$ is applied over a distance of $200\mu \mathrm{m}$. Data are obtained at strip A in Fig. 1b. The noise level in each sweep is 50 nV.

Figure 3

Average A (a) and difference $\Delta$ (b) of the saturation values from Fig. 2a as a function of angle $\Theta$. The red lines show the $2A_0\sin (\Theta )\cos (\Theta )+c$ fit for (a) and the ${\Delta }_0\cos (\Theta )$ fit for (b).

Figure 4

${\Delta }_0$ versus the temperature at the Pt strip position for different ${\overrightarrow{\nabla }}_{x}T$. The inset shows the related average values $A_0$. The measurements are taken on a single MgO substrate. The green line indicates the results of the finite element simulation. The dotted lines are a guide to the eye.