Thermal spin torques in magnetic insulators

The damping of spin waves transmitted through a two-port magnonic device implemented on a yttrium iron garnet thin film is shown to be proportional to the temperature gradient imposed on the device. The sign of the damping depends on the relative orientation of the magnetic field, the wave vector, and the temperature gradient. The observations are accounted for qualitatively and quantitatively by using an extension of the variational principle that leads to the Landau-Lifshitz equation. All parameters of the model can be obtained by independent measurements.

Figure 1

Spin-wave propagation under a thermal gradient. 4.8-mm-long YIG strip fabricated on GGG substrate, width $\mathit{\text{w}}=100\mu \mathrm{m}$, thickness $\mathit{\text{t}}=20$ nm, 10-nm-thick Cu contact connected to Au electrodes, microprobes for both excitation and detection, Peltier elements $A$ and $B$ heat sunk by copper blocks (not shown).

Figure 2

Linewidth of the ferromagnetic resonance spectra at 4.2 GHz, as a function of temperature gradient. The slope changes sign upon flipping the field (top) or flipping the direction of propagation at fixed field orientation (bottom). $A\to B$ data are translated by 0.03 mT.

Figure 3

Linewidth as a function of frequency at a set temperature gradient, using microprobe (top), or metal contacts (bottom) for excitation. Wave vector based on HFSS calculation. The applied temperature gradients are indicated in the figure. Top: black line yields $\alpha =3.15\times {10}^{-4}$; red and blue lines using Eq. (5). Bottom: black line yields $\alpha =6.30\times {10}^{-4}$; red line using Eq. (5). The error bars indicate the noise level.