Thickness effects of ultrathin Pt spacer on perpendicular exchange coupling properties of TbFeCo bilayers with opposite phases

Manipulating magnetic exchange coupling properties is of great importance for device applications. Controlling exchange coupling of perpendicularly magnetized multilayers, however, is challenging. In this work, we fabricate ferrimagnetic TbFeCo/Pt/TbFeCo structures consisting of two Tb-rich and FeCo-rich layers, separated by ultrathin Pt layers with thickness ranged from 0.4 to 3 nm. The adjustability of perpendicular exchange coupling between two ferrimagnetic layers by means of thickness of Pt spacer is demonstrated. For the structure with 0.8 nm Pt spacer or thinner a sharp single switching is observed, indicating a strong interlayer coupling. A two-step reversal with a negative exchange bias is demonstrated for the structure with Pt spacer thickness in the range of 1.6–3 nm, showing macroscopic antiferromagnetic coupling. For the structure with 1.6 nm Pt a large interfacial perpendicular coupling energy density of ∼0.4 erg/cm² is obtained by minor loop measurements. The exchange coupling strength is shown to rapidly decrease with increasing thickness of Pt spacer. By varying temperature a transition from ferromagnetic coupling to antiferromagnetic coupling type is witnessed near the compensation temperature of FeCo-rich layer in the structures. A shift of the minor loop of several hundred Oersteds is observed. A large interfacial perpendicular coupling energy density of ∼1 erg/cm² is obtained in the structure with 2 nm Pt spacer at 150 K. Our results show TbFeCo/Pt/TbFeCo structure with controllable perpendicular exchange bias may be of great importance for spintronic applications.