Influence of interfacial magnetic ordering and field-cooling effect on perpendicular exchange bias and magnetoresistance in nanoporous IrMn/[Co/Pd] films

Abstract

We have studied systematically the effect of field cooling on the magnetic properties of continuous and porous IrMn/[Co/Pd] films. It is found that the coexistence of two ferromagnetic (FM) phases in the porous film, namely, hard-magnetic and soft-magnetic ones, with significantly different magnetic properties relates to the role of pore edges and modifies its magnetic and magnetoresistive properties. It is shown that annealing of the films with their subsequent cooling in an external magnetic field applied for aligning the magnetic moments in the antiferromagnetic (AFM) IrMn layer improves effectively the uniaxial perpendicular anisotropy of the [Co/Pd] layer and induces unidirectional anisotropy in its hard-magnetic regions, blocking simultaneously the soft-magnetic parts by pinning their magnetic moments along the film plane. Magnetoresistance of both continuous and porous films is found to be determined mainly by electron–magnon scattering, whereas the complex morphology of the porous film providing different orientations of exchange coupling at the AFM/FM interface in different film regions modifies significantly the spin-dependent electron transport. The revealed asymmetry of the field dependences of magnetoresistance is attributed both to unidirectional magnetic anisotropy of the FM layer and its splitting into magnetically nonequivalent regions in the porous films. The origin of the observed phenomenon is associated with a local influence on the orientation of AFM magnetic moments by an adjacent ferromagnet.