We report on the magnetic reversal characteristics of exchange coupled ferrimagnetic (FI) $\mathrm{T}{\mathrm{b}}_{19}\mathrm{F}{\mathrm{e}}_{81}/\mathrm{T}{\mathrm{b}}_{36}\mathrm{F}{\mathrm{e}}_{64}$ heterostructures. Both layers are amorphous and exhibit strong perpendicular magnetic anisotropy. The investigated heterostructures consist of a Tb-dominated and a Fe-dominated FI layer. Thus, in the magnetic ground state the net moments of the individual layers are oppositely aligned due to antiferromagnetic coupling of Fe and Tb moments. By cooling the system below 160 K, a large positive and negative exchange bias (EB) effect appears for the Tb- and Fe-dominated layers, respectively. The biasing depends only on the initial magnetization state and is neither affected by a cooling field nor by loop cycling. The phenomenon can be explained by the presence of a hard magnetic Fe-dominated interfacial layer, which forms during the sputter deposition process due to interface mixing and resputtering effects. This interfacial layer acts as a pinning layer below a certain temperature, where its coercivity increases to values larger than the accessible magnetic field range. This assumption is further supported by introducing a 0.9-nm-thick Ru spacer layer, which causes the EB effect to vanish. The EB effect was further investigated for a sample series, where the thickness ratio of the two Tb-Fe layers was varied, while keeping the total thickness of the bilayers constant. Only samples where the individual layers are sufficiently thick reveal double shifted loops, indicating the high sensitivity of the observed bias effect with respect to the magnetic properties of the individual layers and their interfacial area.

• Received 7 November 2016

DOI:https://doi.org/10.1103/PhysRevB.95.104410