  The role of rare-earth (R) ions on the magnetodielectric (MD) coupling is
always intriguing and markedly different for different systems. Although many
reports are available concerning this aspect in frustrated 3d-transition metal
oxides, no such reports exist on higher d (4d/5d)-orbital based systems due to
the rare availability of highly insulating 4d/5d-systems. Here, we
systematically investigated the magnetic, dielectric, ferroelectric and
magnetodielectric behavior of the 6H-perovskites Ba3RRu2O9 for different
R-ions, namely, R= Sm, Tb and Ho, which magnetically order at 12, 9.5 and 10.2
K respectively. For R=Tb and Ho, the temperature and magnetic-field dependent
complex dielectric constant traces the magnetic features, which manifests MD
coupling in this system. A weak magnetic-field (H) induced transition is
observed for ~30 kOe, which is clearly captured in H-dependent dielectric
measurements. No MD coupling is observed for Ba3SmRu2O9. The MD coupling is
enhanced by a factor of 3 and 20 times for R=Tb and Ho, respectively, when
compared to that of the Nd-counterpart. These results evidence the gradual
enhancement of MD coupling with the introduction of heavier R-ions in this
series, which is attributed to their larger moment values. A weak improper
ferroelectricity is documented for Ho-member, which is absent for Nd. Our
investigation establishes dominating 4d(Ru)-4f(R) magnetic correlation in this
series for the heavier R-members.
