Spin-wave propagation and spin-polarized electron transport in single-crystal iron films

The techniques of propagating spin-wave spectroscopy and current-induced spin-wave Doppler shift are applied to a 20-nm-thick Fe/MgO(001) film. The magnetic parameters extracted from the position of the spin-wave resonance peaks are very close to those tabulated for bulk iron. From the zero-current propagating wave forms, a group velocity of 4 km/s and an attenuation length of about 6 $\mu \mathrm{m}$ are extracted for 1.6-$\mu \mathrm{m}$-wavelength spin wave at 18 GHz. From the measured current-induced spin-wave Doppler shift, we extract a surprisingly high degree of spin polarization of the current of $83\%$, which constitutes the main finding of this work. This set of results makes single-crystalline iron a promising candidate for building devices utilizing high-frequency spin waves and spin-polarized currents.

Figure 1

(a) Sketch of the Fe/MgO(001) stack and of the measurement geometry. (b) Optical microscope image of a PSWS device. (c) Scanning electron microscope image of the spin-wave antennas.

Figure 2

(a) Self-inductance spectrum measured at ${\mu }_{0}H=36$ mT for the device of Fig. 1. The peaks are identified as follows (in order of increasing frequency): the secondary MSSW peak corresponding to a wave vector $k_{\text{S}}=1.5\mathrm{rad}/\mu \text{m}$, the main MSSW peak corresponding to a wave vector $k_{\text{M}}=3.8\mathrm{rad}/\mu \text{m}$, and the PSSW1 peak corresponding to a standing spin-wave mode across the film thickness. (b) Dependence on the external magnetic field of the frequency of the main MSSW peak (diamonds) and of the PSSW1 peak (circles) for the same device. The squares show the frequency of the main MSSW peak for a different device, where the Fe strip is oriented along a [110] hard axis of Fe (see the insets for the device geometries). The lines are the calculated mode frequencies for each mode.

Figure 3

(a) Measured mutual-inductance spectra for the device with an edge-to-edge distance $D=1\mu \text{m}$ at a field ${\mu }_{0}H=58$ mT. (b) Dependence of the propagation time $\tau$ on the distance $D$. (c) Dependence of the logarithm of the normalized amplitude $A_{21}$ on the distance. (d) Dependence of the logarithm of $A_{21}$ on $\tau$.

Figure 4

Dependence of the group velocity (a), effective damping (b), and attenuation length (c) on the magnetic field. In each panel, the symbols are the measured data deduced using the method illustrated in Fig. 3 and the solid lines are the theoretical expectations. The dashed lines in panels (a) and (b) are the polynomial fit serving as a guide for the eyes. The dashed line in panel (c) is deduced from these two fits using the relation $L_{\text{att}}=v_{\text{g}}/\mathrm{\Gamma }$. The inset in panel (b) shows the frequency dependence of the FMR linewidth (full width at half maximum) measured on a similar film by broadband ferromagnetic resonance.

Figure 5

(a) Mutual-inductance spectra measured on the $D=1$ $\mu \mathrm{m}$ device under an applied field ${\mu }_{0}H=120$ mT and a dc electrical current $I=\pm 10$ mA. The imaginary parts of both $\mathrm{\Delta }{L}_{21}$ ($k>0$) and $\mathrm{\Delta }{L}_{12}$ ($k<0$) are shown for the two current polarities. The inset is a zoom of the wave forms allowing one to visualize the current-induced frequency shifts. (b) Current dependence of the effective spin velocity deduced from the part of the current-induced frequency shift which is odd in $k$. The red solid diamonds are for the main SW peak ($k_{\text{M}}=3.8$ rad/$\mu \mathrm{m}$) and the line is the corresponding linear fit. The black squares are for the secondary SW peak ($k_{\text{S}}=1.5$ rad/$\mu \mathrm{m}$). The red open diamonds are for the main SW peak under an applied field ${\mu }_{0}H=-120$ mT.