Significant effect of interfacial spin moments in ferromagnet-semiconductor heterojunctions on spin transport in a semiconductor

Using controlled ferromagnet (FM) -semiconductor (SC) interfaces in SC-based lateral spin-valve (LSV) devices, we experimentally study the effect of interfacial spin moments in FM–SC heterojunctions on spin transport in SC. First-principles calculations predict that the spin moment of FM–SC junctions can be artificially reduced by inserting $3d$ transition metal V, Cr, or Cu atomic layers between FM and SC. When all-epitaxial FM–SC Schottky-tunnel contacts with a $0.4–0.5$-nm-thick V, Cr, or Cu interfacial layer are formed, we find that the spin signals in FM–SC LSV devices are significantly decreased at 8 K. When we increase the interfacial spin moment by inserting an $\sim 0.3$-nm-thick Co layer between FM and SC, the spin signals at 8 K are significantly enhanced again. From these experiments, we conclude that the interfacial spin moments at FM–SC interfaces are one of the important factors to achieve large spin signals even in SC-based spintronic devices.

Figure 1

Calculated spin moments of each atom along the Ge[111] direction and a schematic of a part of the stacked supercell, Fe (3 atoms)/TM (3 atoms)/${\mathrm{Co}}_2\mathrm{FeSi}$.

Figure 2

(a) $in situ$ RHEED patterns of the CFAS surface on the V, Cr, and Cu insertion layers on Fe-terminated Ge(111). (b) $M-H$ curves at 300 K for the 7–8-nm-thick CFAS films on the TM(V, Cr, or Cu)/Fe/Ge(111)/Si(111).

Figure 3

(a) Schematic of a fabricated LSV device and terminal configurations of four-terminal nonlocal and two-terminal local magnetoresistance measurements. A cross section of the CFAS/TM/Fe/Ge contact is enlarged on the left. (b) $\left|J\right|\text{−}{V}_{\mathrm{int}}$ characteristics for the Schottky-tunnel contacts (contact area $\sim$ 2.0 $\mu {\mathrm{m}}^2$) measured in the three-terminal configuration (inset) at 8 K for devices with V/Fe, Cr/Fe, and Cu/Fe insertion layers. (c) Nonlocal and (d) local magnetoresistance curves for each device at 8 K.

Figure 4

HAADF STEM images and EDX maps near the CFAS/TM/Fe/Ge interfaces in LSV devices for (a) V/Fe, (b) Cr/Fe, and (c) Cu/Fe. (d) Schematics of the interfacial structure expected from the structural analyses for each device.

Figure 5

(a) Nonlocal and (b) local magnetoresistance curves for an LSV device with $\mathrm{CFAS}(\sim 8$ nm)/$\mathrm{Co}(\sim 0.3$ nm)/$\mathrm{Fe}(\sim 0.3$ nm)/Ge contacts, measured at $I_{\mathrm{bias}}=-0.7$ mA and at 8 K.