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Magnetic domain transformations in polycrystalline hard-soft giant-magnetoresistive-effect spin valves with Co-Cu-based artificial antiferromagnetic subsystems

The magnetic and transport properties of hard-soft giant-magnetoresistive-effect spin valves, with a Co/Cu/Co antiferromagnetically coupled sandwich as the hard layer, are investigated. The irreversibilities in the antiferromagnetically coupled Co/Cu/Co sandwiches prepared by DC-magnetron (DC = direct current) sputtering are shown to strongly depend on the magnetic history of the samples. The irreversibilities, evidenced from analysis of the giant-magnetoresistance and magnetization responses along minor loops, are attributed to domain-phase transformations. First, the effect of the buffer stack on the structural properties and on the quality of the antiferromagnetic coupling is investigated. Coupling strengths larger than 0.4 erg cm-2 have been achieved for deposition on buffers containing a Cr/Fe bilayer. A method is given for estimating the amount of remanence in the Co/Cu/Co sandwich, based on the analysis of the giant-magnetoresistive response upon switching the magnetization direction of the soft magnetic layer located in the buffer. The perfect antiferromagnetic alignment at zero field is attributed to the formation of a magnetic state with relatively large domains. The small amount of remanence detected in some samples is ascribed to the persistence at zero field of a significant density of domain walls. Small fluctuations in the thickness of the layers causing a lateral distribution of the coupling strength are expected to influence the remanence magnetic state and induce differences between samples.