Strong temperature dependence of antiferromagnetic coupling in CoFeB/Ru/CoFeB

The temperature dependence of saturation and spin-flop fields for artificial ferrimagnets (AFi) based on antiparallel coupled CoFeB/Ru/CoFeB trilayers has been investigated in a temperature range between 80K and 600K. The results presented in this paper are relevant for magnetic devices using this system, e.g. magnetic-random access memory based on spin-flop switching. In good accordance to the theory, the saturation field Hsat behaves like Hsat ~ H_0 (T/T_0)/sinh(T/T_0) with a characteristic temperature of T_0 = 150K. Within this model, the Fermi velocity for the Ru layer is of the order of 10^5m/s, therefore, explaining the strong variation of the coupling strength with the temperature in Ru based AFi. Furthermore, a strong uniaxial anisotropy of K_u = 2x10^3 J/m^3 with a small angular distribution of the anisotropy axes is observed for the AFi trilayers based on amorphous CoFeB alloys.