Dielectric relaxations in La_2NiMnO_6 with signatures of Griffiths phase

摘要

Polycrystalline La_2NiMnO_6 prepared by sol-gel auto combustion technique is found to possess dual crystallographic phases R-3c (71 vol. %) and Pbnm (29 vol. %), with respective ferromagnetic transitions at 277 K and 160 K, respectively. The impedance data are analysed via Havriliak-Negami equivalent for impedance (HNEI). Using HNEI equation, the resistance "R," relaxation time "τ," and symmetric "α" and asymmetric "β" broadening parameters, for the two crystallographic phases are obtained. The dielectric relaxations for the two crystallographic phases are found to be independent of each other. The activation energy of relaxation for the R-3c and Pbnm phases is found to be 147 meV and 88 meV, respectively. Strong correlations of dielectric relaxations with the magnetic phases are seen. Before the onset of ferromagnetic transition, i.e., above 160 K for Pbnm and 277 K for R-3c phase, relaxations are dominantly non-Debye type. Once the ferromagnetic state is achieved, relaxations attain Debye nature. Hopping type conduction mechanism is confirmed from electrical transport measurements: nearest neighbour hopping near room temperature and gradual transition to variable range hopping (VRH) at low temperature. Signature of short range magnetic ordering, reminiscent to Griffiths like phase, is seen in the temperature range 295K ＞ T ＞ 277K.