Feature of Current-Induced Microwave Oscillation in Nano-Contacts Magneto-Resistive Devices After High-Temperature Annealing

We measured microwave spectra of nano-contacts magneto-resistive (NCMR) devices with a current-perpendicular-to-plane (CPP) spin-valve (SV) structure under dc current. This CPP-SV structure has a FeCo/FeCo-AlO $_{x}$ nano-oxide layer (NOL)/FeCo structure as reference/spacer/free layer, respectively. NCMR is a new MR effect whose origin is thought to be spin-dependent scattering due to confined domain walls at nano-contacts (NCs) inside NOL with a potentially high MR ratio. We have observed high-level oscillation with narrow line-width which has been reported in NCMR-SV devices with the MR ratio of 3%–5%. In this study, we investigated current-induced microwave oscillation in NCMR devices with high MR ratio of 7%–11% after high-temperature annealing in order to obtain larger integrated oscillation power. High-temperature annealing not only increases the MR ratio, but also causes instability in the reference layer magnetization. It also leads to an increase in the oscillation power but broadening of the line-width, compared with low MR ratio samples. It is suggested that large integrated oscillation power in NCMR device is caused by the distorted alignment between free and reference layer magnetization at ferromagnetic NCs.