A variable temperature EPR study of the manganites (La 1/3Sm 2/3) 2/3Sr xBa 0.33− xMnO 3 ( x=0.0, 0.1, 0.2, 0.33): Small polaron hopping conductivity and Griffiths phase

Abstract

Four manganite samples of the series, (La 1/3Sm 2/3) 2/3Sr x Ba 0.33− x MnO 3, with x=0.0, 0.1, 0.2 and 0.33, were investigated by X-band (∼9.5 GHz) electron paramagnetic resonance (EPR) in the temperature range 4–300 K. The temperature dependences of EPR lines and linewidths of the samples with x=0.0, 0.1 and 0.2, containing Ba 2+ ions, exhibit similar behavior, all characterized by the transition temperatures ( T C) to ferromagnetic states in the 110–150 K range. However, the sample with x=0.33 (containing no Ba 2+ ions) is characterized by a much higher T C=205 K. This is due to significant structural changes effected by the substitution of Ba 2+ ions by Sr 2+ ions. There is an evidence of exchange narrowing of EPR lines near T min, where the linewidth exhibits the minimum. Further, a correlation between the temperature dependence of the EPR linewidth and conductivity is observed in all samples, ascribed to the influence of small-polaron hopping conductivity in the paramagnetic state. The peak-to-peak EPR linewidth was fitted to Δ B pp( T)=Δ B pp,min+ A/ Texp(− E a/ k B T), with E a=0.09 eV for x=0.0, 0.1 and 0.2 and E a=0.25 eV for x=0.33. From the published resistivity data, fitted here to σ( T)∝1/ T exp(− E σ/ k B T), the value of E σ, the activation energy, was found to be E σ=0.18 eV for samples with x=0.0, 0.1 and 0.2 and E σ=0.25 eV for the sample with x=0.33. The differences in the values of E a and E σ in the samples with x= 0.0, 0.1and 0.2 and x=0.33 has been ascribed to the differences in the flip-flop and spin-hopping rates. The presence of Griffiths phase for the samples with x=0.1 and 0.2 is indicated; it is characterized by coexistence of ferromagnetic nanostructures (ferrons) and paramagnetic phase, attributed to electronic phase separation.