Comparison between thermal and current driven spin-transfer torque in nanopillar metallic spin valves

We investigate the relation between thermal spin-transfer torque (TSTT) and the spin-dependent Seebeck effect (SDSE), which produces a spin current when a temperature gradient is applied across a metallic ferromagnet, in nanopillar metallic spin valves. Comparing its angular dependence (aSDSE) with the angle-dependent magnetoresistance measurements on the same device, we are able to verify that a small spin heat accumulation builds up in our devices. From the SDSE measurement and the observed current driven STT switching current of 0.8 mA in our spin valve devices, it was estimated that a temperature difference of 230 K is needed to produce an equal amount of TSTT. Experiments specifically focused on investigating TSTT show a response that is dominated by overall heating of the magnetic layer. Comparing it to the current driven STT experiments we estimate that only $\sim 10\%$ of the response is due to TSTT. This leads us to conclude that switching dominated by TSTT requires a direct coupling to a perfect heat sink to minimize the effect of overall heating. Nevertheless the combined effect of heating, STT, and TSTT could prove useful for inducing magnetization switching when further investigated and optimized.

Figure 1

(a) Schematic of the device structure used where an $F\left|N\right|F$ stack is sandwiched between a platinum (Pt) bottom contact and a Au top contact. Two Pt joule heaters are used to produce a thermal gradient across the stack and are insulated from the Pt bottom contact by a thin ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer. (b) In the angle-dependent measurements stack type A is used, consisting of a circular $F_1$ layer and rectangular $N$ and $F_2$ layers. The circular shape of $F_1$ ensures that there is no preferential in-plane direction for the magnetization such that it easily aligns with a small magnetic field. Rotating this small field will not influence the magnetization direction of $F_2$ giving an angle $\theta$ between $M_1$ and $M_2$. (c) For the thermal STT measurements stack type B is used, consisting of an in situ grown rectangular $F\left|N\right|F$ stack. Because of shape anisotropy two stable magnetic states are present, namely, parallel (P) and antiparallel (AP) magnetization alignments.

Figure 2

(a) The normalized aMR [18] (squares), aSDSE (triangles), and the difference between the two (circles) are plotted as a function of the angle between $M_1$ and $M_2$. (b) The MR measurement gives the resistance across the stack as a function of applied magnetic field $B$. (c) The spin-dependent Seebeck effect gives the Seebeck voltage as a function of applied magnetic field $B$.

Figure 3

(a) The magnetic minor switching loop of the $F_2$ layer for a type-B stack, where $B_1$ and $B_2$ represent the low and high switching fields, respectively. (b) Resistance of the stack as a function of direct current ($I_{\mathrm{dc}}$) sent through it. The switching from the antiparallel resistance state to the parallel state and vice versa is cause by the spin-transfer torque induced by $I_{\mathrm{dc}}$. A constant $B$ of 40 mT is applied to ensure that we are in the middle of the minor loop where both the parallel and the antiparallel magnetization alignments constitute a stable state.

Figure 4

The evolution of the minor loop switching fields $B_1$ and $B_2$ for: (a) Spin-transfer torque, induced by sending a dc current ($I_{\mathrm{dc}}$) through the stack. (b) Thermal STT, induced by a thermal gradient across the stack by sending an $I_{\mathrm{dc}}$ through the Pt joule heaters. (c) Overall temperature change, induced by a controllable heater. (c) The temperature of the $F_2$ layer extracted from 3D finite element modeling as a function of $I_{\mathrm{dc}}$ sent through the Pt joule heaters.

Figure 5

Measurements on a device with an $F\left|N\right|F$ stack with negligible dipole field coupling. (a) Resistance of the stack as a function of applied magnetic field $B$, spin valve measurement. (b) Minor loop switching measurement for the $F_2$ layer clearly showing no coupling as the loop is well centered around $B=0$.