Current-Perpendicular-to-Plane Giant Magnetoresistance in Pseudo Spin Valves With $\hbox{Co}_{2}\hbox{Fe}(\hbox{Ge}_{0.5}\hbox{Ga}_{0.5})$ Heusler Alloy Ferromagnetic Layers and Cu/Ag Spacer

We studied the transport properties and micro-structure of current-perpendicular-to-plane pseudo spin valves (PSVs) with ferromagnetic ${\rm Co}_{2}{\rm Fe}({\rm Ge}_{0.5}{\rm Ga}_{0.5})$ (CFGG) Heusler alloy layers and a Cu or Ag spacer. The CFGG/Cu/CFGG PSVs showed an optimum annealing temperature at 300 $^{\circ}{\rm C}$, which is substantially lower than 500$^{\circ}{\rm C}$ for CFGG/Ag/CFGG PSVs. The Valet-Fert analysis yielded comparable interfacial spin asymmetries in CFGG/Cu (or Ag)/CFGG PSVs, which indicates that the Cu spacer has as good a band matching as that of the Ag spacer with the CFGG Heusler alloy. However, transmission electron microscopy observation revealed that the CFGG/Cu/CFGG tri-layered structure is thermally unstable compared to the CFGG/Ag/CFGG tri-layered structure, thus placing a limit on the annealing temperature for producing higher magnetoresistance output by using the Cu spacer.