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Effect of divalent Ba cation substitution with Sr on coupled multiglass state in the magnetoelectric multiferroic compound Ba3NbFe3Si2O14

(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.