Magnetic field distribution in ferromagnetic metal/normal metal multilayers using NMR

The origin of the interlayer exchange coupling in ferromagnetic-normal metal (FM-NM) multilayer structures has been the focus of intensive research efforts for more than a decade. NMR is an important method for investigating these systems. We have studied room temperature molecular-beam-epitaxy grown Co/Al and Fe/Cu multilayer structures using Co-59, Al-27, and Cu-63 nuclei as probes. The Co-59 FM-NMR results for Co/Al films indicate about 3 ML of Co is intermixed with Al at the interface. For both Co/Al and Fe/Cu samples substantial NM line broadening is observed with no indication of line structure or splitting. The line broadening increases with decrease of the NM layer thickness. The Ruderman-Kittel-Kasuya-Yosida (RKKY)-like interaction across the interface was investigated using a Fe/Cu sample with an insulating AlOx layer separating each Fe layer from adjacent Cu layers. Our Cu-63 results for this sample are very similar to results obtained without the insulating oxide barrier. This shows that the line broadening is not due to the RKKY-like interaction but interfacial roughness. Simulation of the local internal magnetic field using a simple rough interface model provides support for this suggestion.