Temperature dependence of magnetization drift velocity and current polarization in ${\text{Ni}}_{80}{\text{Fe}}_{20}$ by spin-wave Doppler measurements

A spin-wave Doppler technique is used to measure the temperature dependence of both the magnetization drift velocity, which represents the magnitude of adiabatic spin-transfer torque, and the current polarization in current-carrying ${\text{Ni}}_{80}{\text{Fe}}_{20}$ wires. For current densities of ${10}^{11}\text{A}/{\text{m}}^2$, we obtain magnetization drift velocities decreasing from $4.8\pm 0.3$ to $4.1\pm 0.1\text{m}/\text{s}$ over a temperature range from 80 to 340 K. Interpretation of velocity values yields current polarization dropping from $0.75\pm 0.05$ to $0.58\pm 0.02$ over the same temperature range. Analysis indicates different temperature dependences for spin-up and spin-down conductivities, suggesting a strong impurity scattering of spin-down electrons.

Figure 1

(Color online) SEM image of a device with an $8\mu \text{m}$ wide Py strip. $S_{12}$ $\left(S_{21}\right)$ refers to the spin-wave transmission from rf port 2(1) to port 1(2) and electrons flow in the negative $x$ direction for positive current. In positive fields, $S_{12}$ is much stronger than $S_{21}$. The inset shows the zoomed-in image of the microwave antennas. The periodicity of the rf current is 750 nm and the center-to-center distance of the pair of antennas is $7\mu \text{m}$.

Figure 2

(Color online) (a) Real (red solid line) and imaginary (blue dashed line) part of $S_{12}$ transmission impedance in a $+40\text{mT}$ field at room temperature with no dc current. (b) The amplitude of $S_{12}$ and $S_{21}$ in the same $+40\text{mT}$ field. (c) $S_{12}$ and $S_{21}$ transmission impedance response when a current density of $\pm 1.2\times {10}^{11}\text{A}/{\text{m}}^2$ is applied in the Py wire. The frequency shift $\Delta f$ between the $S_{12}$ and $S_{21}$ curves changes sign when the current is reversed.

Figure 3

(Color online) (a) Magnetization drift velocity as a function of current density $\left(T=220\text{K}\right)$. Red line is the linear fit. (b) Magnetization drift velocity from 80 to 340 K for a current density of ${10}^{11}\text{A}/{\text{m}}^2$. (c) Spin polarization as a function of temperature. Some earlier measurements of spin polarization in Py are also shown for comparison. Inset shows $M_{\text{s}}t$ vs $T$ data.

Figure 4

(Color online) (a) Spin-up and spin-down conductivities as a function of temperature. (b) Simulated temperature dependence based on Eqs. (4, 5) and the resistivities plotted in the inset.