Abstract
Temperature dependent electron spin resonance (ESR) studies were carried out on two samples of Bi0.55Ca0.45MnO3 (grain size 21 and 15 nm) synthesized by sol-gel process and compared with the studies on corresponding bulk sample. The double integrated intensity and linewidth (ΔH) of the ESR signal verses temperature is used to get information about magnetic interactions and values of Charge-Ordering temperature, TCO and Néel temperature, TN. The TCO value decreases from 313K for bulk to 306K for 15 nm size sample. The TN values remain around 153 K for all the samples. The magnetic phase for T>TCO is dominated by ferromagnetic (FM) correlations which can be attributed to activated Mn3+–Mn4+ hopping of a small polaron. In the temperature range TCO>T>TN, the domains of FM and antiferromagnetic (AFM) spin correlations coexist. The FM phase volume decreases and the paramagnetic (PM)-PM+charge ordered (CO) and (PM+CO)−(AFM+CO) transitions become sharp with decrease in grain size. The critical exponent for PM-AFM transition for the charge ordered sample with grain size 15 nm is incidentally found to be close to the theoretically predicted value for plain PM to AFM transition. The Griffiths phase physics is found to be an appropriate theoretical model for describing the ESR data in the present system.
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