Transport and spin conversion of multicarriers in semimetal bismuth

In this paper, we report on the investigation of (i) the transport properties of multicarriers in semimetal Bi and (ii) the spin conversion physics in this semimetal system on a ferrimagnetic insulator, yttrium-iron-garnet. Hall measurements reveal that electrons and holes coexist in the Bi, with electrons being the dominant carrier. The results of a spin conversion experiment corroborate the results of the Hall measurement; in addition, the inverse spin Hall effect governs the spin conversion in the semimetal/insulator system. This study provides further insights into spin conversion physics in semimetal systems.

Figure 1

(a) X-ray diffraction patterns of Bi (20 nm)/YIG and of YIG. The crystalline peaks are identified as shown in the figure. (b) An atomic force microscopic view of the Bi. (c) Schematic image of the spin conversion device. ${\theta }_{\mathrm{H}}$ is the angle of an external magnetic field, $H$, to the YIG plane. (d) Illustration of different spin conversion principles in the ISHE and the IREE.

Figure 2

(a) Thickness dependence of the sheet resistance $R_{xx}$ of Bi/YIG at 300 K. The Hall resistance $R_{xy}$ is shown in the inset. The thickness dependencies of the sheet Hall coefficient $R_{\mathrm{H}}=R_{xy}/{\mu }_{0}H$ and the inverse Hall mobility ${{\mu }_{\mathrm{H}}}^{-1}=(R_{\mathrm{H}}/R_{xx}{)}^{-1}$ are also presented. (b) Magnetoresistance $[R_{xx}\left(H\right)-R_{xx}\left(0\right)]/R_{xx}\left(H\right)$ of Bi/YIG at 300 K. The fitting results using the two-carrier model are also displayed as a solid line. (c) Contour plot of Hall factor $\alpha =R_{\mathrm{H}}/R_0$ determined from the two-carrier model, and the value of α determined from the magnetoresistance. The results indicate a dominance of electron conductions. See the text for details.

Figure 3

(a) FMR spectra of YIG when the external magnetic field is set to be 0 (red solid line) and 180 (blue solid line) degrees, respectively. The microwave power was 1 mW. (b) Electromotive forces from Bi (20 nm)/YIG under FMR of the YIG. Red and blue open circles are experimental data at a magnetic field of 0 and 180 degrees, respectively. To subtract the thermal effect, the data are averaged (shown as a black solid line). (c, d) Excitation power dependence of the electromotive forces from Bi/YIG. The data are well fitted to a linear function.

Figure 4

Thickness dependence of the generated charge current in Bi/YIG. A monotonic increase in the charged current is clearly observed, indicating the lack of the IREE. The dashed lines labeled “IREE,” “ ${\lambda }_{\mathrm{s}}=8\mathrm{nm}$,” and “20 nm,” are the theoretical lines when we assume that the generation of the charge current is due to the IREE, the ISHE (where the spin diffusion length of the Bi is 8 nm), and the ISHE (where its spin diffusion length is 20 nm), respectively. The line “IREE” is a line assuming a certain IREE length scale.