Spin-transfer-torque induced spatially nonuniform switching in ferrimagnets

Ferrimagnet (FiM), (FeCo)(1-)Gd-x(x), attracts research attention due to its ultrafast magnetic dynamics and finite net magnetization. Incorporating FiM into the magnetic tunnel junction will be beneficial to further improve the writing speed of magnetic random access memory. It is commonly assumed that the FeCo and Gd atoms are switched together due to the strong exchange coupling, which remains valid even if one performs the two-sublattice macrospin simulation. Interestingly, using the atomistic model developed by our group, it is clearly seen that different atoms are not switched together. In addition, our study reveals that the nature of switching is spatially nonuniform even in the small sample with the dimension of 20 x 20 nm(2). Furthermore, the characteristics of nonuniformity are completely different for samples with different Gd compositions (x). When x is close to the magnetization compensation point, successful switching cannot be obtained, but is accompanied by the stable oscillation. The atom type that dominates the oscillation is different from that predicted by the two-sublattice macrospin model. In addition, the size of a singular region is a non-monotonic function of current density. All these results can only be understood by considering the spatial nonuniform magnetization dynamics.