  The interlayer antiferromagnetic coupling rare-earth/transition-metal bilayer
ferrimagnet systems have attracted much attention because they present
variously unusual temperature-and field-dependent nontrivial magnetic states
and dynamics. These properties and the implementation of their applications in
spintronics highly depend on the significant temperature dependence of the
magnetic exchange stiffness constant A. Here, we quantitatively determine the
temperature dependence of magnetic exchange stiffness A_{Py-Gd} and A_{Gd} in
the artificially layered ferrimagnet consisting of a Py/Gd bilayer, using a
measurement of anisotropic magnetoresistance (AMR) of the bilayer thin film at
different temperatures and magnetic fields. The obtained temperature dependence
of A_{Py-Gd} and A_{Gd} exhibit a scaling power law with the magnetization of
Gd. The critical field of spin-flop transition and its temperature dependence
can also be directly obtained by this method. Additionally, the experimental
results are well reproduced by micromagnetic simulations with the obtained
parameters A_{Py-Gd} and A_{Gd}, which further confirms the reliability of this
easily accessible technique.
