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Experimental observation of magnetoelectricity in spin ice Dy$_2$Ti$_2$O$_7$

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.