Spin transport and spin-flip scattering in magnetic multilayer structures

The existence of spin-flip scattering at the interface between ferromagnetic (F) and nonmagnetic (N) layers of magnetoresistive F/N/F structures can significantly reduce the size of the magnetoresistance, limiting the sensitivity and increasing the power consumption of F/N/F devices such as GMR magnetic field sensors, magnetic read heads, and MRAM's~[1]. Detecting and measuring the degree of spin flip scattering in F/N/F structures can allow further optimization in such devices as well as increase the understanding of interfacial spin transport. Our nonlocal spin injection and detection experiments on mesoscopic Co-Al$_2$O$_3$-Cu-Al$_2$O$_3 $-Co spin valves provide evidence for the existence of interfacial spin-flip scattering in magnetoresistive devices~[2]. By extending the conventional picture of spin-dependent interfacial resistances (R$_{\uparrow}$, R$_{\downarrow}$) to include two additional spin-flip scattering channels (R$_{\uparrow\downarrow}$,R$_{\downarrow\uparrow}$)~[3] we have shown that the nonlocal resistance contains information about both the degree of spin polarization and the degree of spin-flip scattering at the F/N interface. The magnitudes of R$_{\uparrow\downarrow}$ and R$_{\downarrow\uparrow}$ depend on the relative orientation of the detector magnetization and the nonequilibrium magnetization in the normal metal. We have observed that the difference in spin-flip scattering between up and down channels vanishes at low temperatures, but for T$>$100K it increases nonlinearly with temperature. Further evidence for the presence of interfacial spin-flip scattering can be obtained from noise measurements, which are extremely sensitive to the microscopic transport details. \noindent [1] \textit{Spin Dependent Transport in Magnetic Nanostructures}, edited by S. Maekawa and T. Shinjo (Taylor \& Francis, New York, 2002). \noindent [2] S. Garzon, I. \v{Z}uti\'{c}, and R. A. Webb, Phys. Rev. Lett. \textbf{94}, 176601 (2005). \noindent [3] E. I. Rashba, Eur. Phys. J. B \textbf{29}, 513 (2002).