  We demonstrate that electromagnons can be used to directly probe the nature
of a phase transition between magnetically ordered phases in an improper
ferroelectric. The antiferromagnetic/paraelectric to
antiferromagnetic/ferroelectric phase transition in Cu$_{1-x}$Zn$_{x}$O ($x=0,
0.05$) alloys was tracked via the electromagnon response using terahertz
time-domain spectroscopy, on heating and cooling through the phase transition.
The transition was found to exhibit thermal hysteresis, confirming its
first-order nature, and to broaden under the influence of spin-disorder upon Zn
substitution. The energy of the electromagnon increases upon alloying, as a
result of the non-magnetic ions modifying the magnetic interactions that give
rise to the multiferroic phase and electromagnons. We describe our findings in
the context of recent theoretical work that examined improper ferroelectricity
and electromagnons in CuO from phenomenological and first-principles
approaches.
