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|Title: ||A variable temperature EPR study of the manganites (La(1/3)Sm(2/3))(2/3)Sr(x)Ba(0.33-x)MnO(3) (x=0.0, 0.1, 0.2, 0.33): Small polaron hopping conductivity and Griffiths phase|
|Authors: ||MISRA, SK|
|Keywords: ||spin dynamics|
|Issue Date: ||2010|
|Publisher: ||ELSEVIER SCIENCE BV|
|Citation: ||JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS, 322(19), 2902-2907|
|Abstract: ||Four manganite samples of the series,(La(1/3)Sm(2/3))(2/3)Sr(x)Ba(0.33-x)MnO(3), with x=0.0, 0.1, 0.2 and 0.33, were investigated by X-band (similar to 9.5 GHz) electron paramagnetic resonance (EPR) in the temperature range 4-300 K. The temperature dependences of EPR lines and line widths 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 line width exhibits the minimum. Further, a correlation between the temperature dependence of the EPR line width 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 line width was fitted to Delta B(pp)(T)=Delta 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 Ea=0.25 eV for x=0.33. From the published resistivity data, fitted here to sigma(T)proportional to 1/T exp( -E(sigma)/k(B)T), the value of E(sigma), the activation energy, was found to be E(sigma)=0.18 eV for samples with x=0.0, 0.1 and 0.2 and E(sigma)=0.25 eV for the sample with x=0.33. The differences in the values of E(a) and Es in the samples with x=0.0, 0.1 and 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 para-magnetic phase, attributed to electronic phase separation. (C) 2010 Allrightsreserved.|
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