  (Ba/Sr)3NbFe3Si2O14 is a magnetoelectric multiferroic with an incommensurate
antiferromagnetic spiral magnetic structure which induces electric polarization
at 26 K. The structure, as revealed by x-rays and neutrons, as well as static
and dynamic magnetic and dielectric properties of these compounds have been
studied down to 6 K under different conditions. Both the compounds have similar
crystal structure but with different lattice constants down to 6 K. The Ba-and
Sr-compounds exhibit a transition at 26 K and 25 K respectively, as indicated
by the specific heat capacity and dc specific magnetization, into an
antiferromagnetic state. Although Ba and Sr are isovalent, they exhibit very
different static and dynamic magnetic behavior. The Ba-compound exhibits both
thermal and magnetic field hysteresis with the thermal hysteresis decreasing
with increasing magnetic field, a behavior typical of glasses. The glassy
behavior is also clearly seen in the ac susceptibility studies which show a
dispersive peak in the range 40 K to 90 K in the frequency range 10^1 Hz to
10^4 Hz. The dispersive behavior follows a cluster glass critical slowing
dynamics with a freezing temperature of 35 K and a critical exponent of 3.9, a
value close to the 3-D Ising model. The Sr-compound however does not exhibit
any dispersive behavior except for the invariant transition at 25 K in ac
susceptibility with no magnetic field hysteresis at all temperatures. The
dielectric constant studied in the frequency range 10^1 Hz to 10^6 Hz also
reflects the magnetic behavior of the two compounds. The Ba-compound has two
distinct dispersive peaks near TN and in the range 40 K to 125 K while the
Sr-compound has a single dispersive peak in the range 40 K to 80 K. The
activation energy of the high temperature dispersive peak in both compounds
however is found to be similar, 71 meV and 65 meV respectively for Ba- and
Sr-compounds.
