  Magnetoelectric materials are attractive for several applications, including
actuators, switches, and magnetic field sensors. Typical mechanisms for
achieving a strong magnetoelectric coupling are rooted in transition metal
magnetism. In sharp contrast, here we identify CsEr(MoO4)2 as a magnetoelectric
material without magnetic transition metal ions, thus ensuring that the Er ions
play a key role in achieving this interesting property. Our detailed study
includes measurements of the structural, magnetic, and electric properties of
this material. Bulk characterization and neutron powder diffraction show no
evidence for structural phase transitions down to 0.3 K and therefore
CsEr(MoO4)2 maintains the room temperature P2/c space group over a wide
temperature range without external magnetic field. These same measurements also
identify collinear antiferromagnetic ordering of the Er3+ moments below TN =
0.87 K. Complementary dielectric constant and pyroelectric current measurements
reveal that a ferroelectric phase (P ~ 0.5 nC/cm2) emerges when applying a
modest external magnetic field, which indicates that this material has a strong
magnetoelectric coupling. We argue that the magnetoelectric coupling in this
system arises from a pseudo Jahn-Teller distortion induced by the magnetic
field.
