Opposite Spin Asymmetry of Elastic and Inelastic Scattering of Nonequilibrium Holes Injected into a Ferromagnet

The spin asymmetry of elastic and inelastic scattering of nonequilibrium holes injected into Co thin films is examined using a $p$-type magnetic tunnel transistor. Spin-dependent transmission yields a positive or negative magnetocurrent depending on Co thickness and hole energy. Up to a critical thickness of about 3 nm, (quasi)elastic scattering dominates with a short attenuation length ($<1\mathrm{nm}$) and preferential attenuation of holes in the majority spin bands, consistent with spin-wave emission. At a larger Co thickness, inelastic scattering dominates with a larger attenuation length ($\sim 4\mathrm{nm}$) and opposite spin asymmetry.

Figure 1

Left: Schematic energy diagram of a $p$-type MTT of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{Co}/\mathrm{Au}/p$-type Si. The curve in the ${\mathrm{Al}}_2{\mathrm{O}}_3$ tunnel barrier represents the energy distribution of tunneling electrons (holes). Right: Collector hole current ($I_C$) versus emitter bias ($V_E$) in a $p$-type MTT with a 3.6 nm Co base.

Figure 2

Magnetic field dependence of the collector current in a $p$-type MTT with a 3.6 nm thick Co base at different emitter bias, as indicated in each panel. Labels P and AP denote parallel and antiparallel alignment, respectively, of the emitter and base magnetizations. $T=82\mathrm{K}$.

Figure 3

MC versus $V_E$ in $p$-type MTTs with different thickness of Co base ranging from 1.5 to 12 nm. $T=82\mathrm{K}$.

Figure 4

MC versus $V_E$ in $p$-type MTTs with an extra Au layer inserted into the base. Top panel: Co(8 nm) compared to $\mathrm{Co}(3.6\mathrm{nm})/\mathrm{Au}(2\mathrm{nm})/\mathrm{Co}(4.4\mathrm{nm})$. Bottom panel: ${\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{Co}(3.6\mathrm{nm})$ compared to ${\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{Au}(2\mathrm{nm})/\mathrm{Co}(3.6\mathrm{nm})$. $T=82\mathrm{K}$. The solid lines are a guide to the eyes.

Figure 5

Transmitted hole current $I_C/I_E$ for parallel state (top panel) and MC (bottom panel) as a function of Co thickness for an emitter bias of 0.6 (circles), 0.8 (open squares), and 1.4 V (triangles).