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

Abstract

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 $\mathrm{Co}∕\mathrm{Al}$ and $\mathrm{Fe}∕\mathrm{Cu}$ multilayer structures using $^{59}\mathrm{Co}$, $^{27}\mathrm{Al}$, and $^{63}\mathrm{Cu}$ nuclei as probes. The $^{59}\mathrm{Co}$ FM-NMR results for $\mathrm{Co}∕\mathrm{Al}$ films indicate about 3 ML of Co is intermixed with Al at the interface. For both $\mathrm{Co}∕\mathrm{Al}$ and $\mathrm{Fe}∕\mathrm{Cu}$ 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 $\mathrm{Fe}∕\mathrm{Cu}$ sample with an insulating ${\mathrm{AlO}}_{\mathrm{x}}$ layer separating each Fe layer from adjacent Cu layers. Our $^{63}\mathrm{Cu}$ 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.