  The intrinsic noncollinear spin patterns in rare-earth pyrochlore are
physically interesting, hosting many emergent properties, e.g. spin ice and
monopole-type excitation. Recently, the magnetic monopole excitation of spin
ice systems was predicted to be magnetoelectric active, while rare experimental
works have directly confirmed this scenario. In this work, we performed
systematic experimental investigation on the magnetoelectricity of
Dy$_2$Ti$_2$O$_7$ by probing the ferroelectricity, spin dynamics, and
dielectric behaviors. Two ferroelectric transitions at $T_{c1}$=25 K and
$T_{c2}$=13 K have been observed. Remarkable magnetoelectric coupling is
identified below the lower transition temperature, with a significant
suppression of the electric polarization upon applied magnetic field. It is
surprised that the lower ferroelectric transition temperature just coincides
with the Ising-spin paramagnetic transition point, below which the
quasi-particle-like monopoles are populated, indicating implicit correlation
between electric dipoles and spin moments. The possible magnetoelectric
mechanisms have also been discussed although a decent theory remains
unavailable up to date. Our results will stimulate more investigations to
explore multiferroicity in these spin ice systems and other frustrated magnets.
