Magnetization Reversal of Strongly Exchange-Coupled Double Nanolayers for Spintronic Devices

Contemporary spintronics devices, notably giant and tunnel magnetoresistance sensors with high linear ranges, stand to benefit from large bias fields in excess of 1 T that have been observed in exchange-coupled double layers using rare earth–transition metal ferrimagnets as pinning layers. However, before the strong coupling can be exploited, the reversal process must be understood at the scales of the smallest devices, where it would be effective. Little is known about these processes to date because of the technical difficulty of imaging magnetic structures smaller than 20 nm in fields of several Tesla. Here we image the nanoscale magnetic structures of a double layer comprising a polycrystalline Co/Pt multilayer, which is strongly exchange-coupled to an amorphous and magnetically hard ferrimagnetic TbFe alloy layer. High-resolution magnetic force microscopy provides snapshots of the magnetic structures along various applied fields between 0 and 7 T as the magnetization reversal takes place. In contrast to a magnetization reversal process governed by nucleation, followed by domain wall motion observed in weakly exchange-coupled systems, a complex nucleation-dominated magnetization process with three distinct reversal stages is found. Micromagnetic modeling reveals that the grain-to-grain variation of the Co/Pt anisotropy and the strength of the interfacial exchange coupling affect this reversal process. We show that we can control the coupling and thus the reversal process by using spacers at the ferrimagnet–ferromagnet interface, a useful option when designing spintronics devices.