Domain coupling and resistance in perpendicularly magnetized metal-oxide bilayers

Two of the interesting problems in magnetic heterostructures are the exchange coupling in multilayers and the stabilization of domains with magnetization perpendicular to the film plane. In this work, these two phenomena are combined and the stabilization of a perpendicular domain structure in an ultra thin (<10nm) metallic layer, grown on a magnetic oxide (Yttrium Iron Garnet, YIG) layer with perpendicular anisotropy, by interlayer exchange coupling was studied. Further, this bilayer was used to study the physics of interlayer coupling and the associated magnetoresistance including the contribution of domain walls to spin-dependent scattering.;YIG thin films, deposited on a gadolinium gallium garnet (GGG) substrate using magnetron sputtering, were post-annealed to crystallize the film. The crystallization of the YIG film was optimized by X-ray diffraction, its perpendicular magnetization confirmed by two different magnetometry measurements, and the characteristic stripe domains imaged by magnetic force microscopy (MFM). Next, Fe films of various thicknesses were deposited on the YIG film. Under optimal growth conditions, and up to a critical thickness, the domain structure of Fe was found to be exchange coupled to that of YIG and showed the characteristic stripe domains structure. However, MFM imaging of such FM/YIG bilayers is not conclusive because of the possible effects of the stray magnetic fields from the substrate (YIG). To independently resolve the domain structure of the FM and YIG layers, clement specific X-ray photoemission electron microscopy (XPEEM) with magnetic circular dichroism (MCD) contrast, was performed at the Advanced Light Source. The XPEEM images revealed that the Co (FM) layer was coupled perpendicularly to the YIG layer up to a Co thickness of 5nm, but as the Co layer thickness was increased, domains of both the Co layer and the surface of the YIG film started to show an in-plane domain structure. Preliminary micromagnetic simulations showed that the volume exchange stiffness term at the interface plays a dominant role, when compared with the bilinear and biquadratic coupling contributions, in determining the interlayer coupling. Finally, the magnetoresistance (MR) of such FM/YIG bilayers, by lithographically patterning the FM layer on a YIG underlayer, was measured to both study the changes in the domain structure as a function of applied field and the contribution of the domain wall to the resistance. Since the YIG layer is insulating, such transport measurement would be dominated by the FM (Co) layer. MR measurement in longitudinal (field parallel to current direction) and transverse (field perpendicular to current direction) geometries revealed anisotropic MR (AMR) characteristics. However, the anomaly in the data when measured in the perpendicular mode (field perpendicular to the sample surface) was shown to be due to the Hall effect and domain coupling between FM and YIG layer. Detailed analysis of the perpendicular measurement showed sensitivity to the initial domain structure of the Co/YIG bilayer and this was interpreted in terms of the domain wall resistance contribution to the total MR.