  Ferrimagnet (FiM), (FeCo)1-xGdx, 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 (MRAM). 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 nm-20 nm. Furthermore, the
characteristics of nonuniformity are completely different for samples with
different Gd composition (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 singular region is a non-monotonic function of current density. All
these results can only be understood by considering the spatial nonuniform
magnetization dynamics.
