A variable temperature EPR study of the manganites (La_1/3Sm_2/3)_2/3Sr_xBa_0.33−xMnO₃ (x=0.0, 0.1, 0.2, 0.33): Small polaron hopping conductivity and Griffiths phase

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• 作者：Sushil K. Misra ; Sergey I. Andronenko ; Saket Asthana ; Dhirendra Bahadur
• 关键词：Doped manganites ; Electron paramagnetic resonance (EPR) ; Small-polaron hopping ; Griffiths phase
• 刊名：Journal of Magnetism and Magnetic Materials
• 出版年：2010
• 出版时间：October 2010
• 年：2010
• 卷：322
• 期：19
• 页码：2902-2907
• 全文大小：817 K

文摘

Four manganite samples of the series, (La_1/3Sm_2/3)_2/3Sr_xBa_0.33−xMnO₃, 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 Ba²⁺ 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 Ba²⁺ ions) is characterized by a much higher T_C=205 K. This is due to significant structural changes effected by the substitution of Ba²⁺ ions by Sr²⁺ 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 ΔB_pp(T)=ΔB_pp,min+A/Texp(−E_a/k_BT), 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_BT), 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.