In-plane and perpendicular exchange bias in [Pt/Co]/NiO multilayers

Exchange bias in [Pt/Co]/NiO multilayers were studied as a function of film thickness and [Pt/Co] layer repetition. A strong temperature dependence of the coercivity, H-c,and exchange bias field, H-ex, was observed for the thick and thinnest [Pt/Co]/NiO multilayers. While the thinnest [Pt(3 nm)/Co(1.25 nm)](4)/NiO multilayers exhibits no in-plane exchange bias field, a perpendicular H-ex perpendicular to similar to -150 Oe at 80 K was measured. By contrast, the thickest [Pt(12 nm)/CO(10 nm)](1)/NiO multilayers exhibited an in-plane H-ex/(sic) -600 Oe (with H-ex/(sic) -1300 Oe at 5 K) with no measurable perpendicular exchange bias field. The estimated interfacial exchange coupling energy implies the effective Co layer thickness contributing to the exchange bias is effective only in Co layer in contact with NiO bottom layer. AC susceptibility and the temperature dependence of H-ex show that the a 1.25 nm thick Co component enables perpendicular exchange bias with a reduced blocking temperature T-B similar to 200 K, compared to that (T-B similar to 250 K) for the thick [Pt/Co]/NiO multilayers. This is attributed to disordered CoPt phases that formed due to intermixing between Co and Pt during deposition.