Oscillating tunneling magnetoresistance in magnetic double-tunnel-junction structures

Based on an extended Slonczewski model with double $\delta$-type potential barriers, we study spin-dependent resonant tunneling conductances in a double-tunnel-junction structure, in which two ferromagnetic electrodes are separated from a middle nonmagnetic layer of thickness a by two thin insulating layers, respectively. It is shown that as the thickness a is increased, the tunneling magnetoresistance, along with the tunneling conductance for parallel and antiparallel magnetization configurations, exhibits amplitude-varying oscillating behavior with a period of $a=\pi {/k}_F\hspace{0.5em}{(k}_F$ being the Fermi wave vector). This abnormal phenomenon is found to stem from the spin-dependent resonant transmission of electrons passing through the double-tunnel-junction structure.