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Electron spin resonance and resistivity studies of charge-ordered Bi1−xCaxMnO3

In this paper we report the temperature dependent electron spin resonance (ESR) and electrical resistivity studies on Bi(1−x)CaxMnO3 (x = 0.4, 0.45, 0.5, 0.6). The values of charge-ordering temperature TCO and Neel temperature TN estimated from ESR data are comparable to values obtained by other techniques and reported in the literature. The double integrated (DI) intensity of the ESR signal versus temperature and ln DI versus 1000/T plots have been used to get information about magnetic interactions. It is found that 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 contribution of AFM correlations progressively increase with the increase in Ca content. The sharp change in the slope of ln DI versus 1000/T plot at ∼170–190 K indicates the onset of long-range AFM order. For T < TN, temperature independent ln DI indicates the freezing of FM microdomains/inhomogeneities in the AFM long-range ordered state. The temperature dependent resistivity data are analysed in view of the polaron model. The resistivity data analysed in view of Mott's VRH show that lnρ varies linearly with T−1/4, with a change in slope at TN. It is also found that ln ρ varies linearly with T−1/2 in the entire temperature range below TCO for all the compositions. The Griffiths phase physics may be an appropriate theoretical model for describing the ESR and resistivity data in the present system.