Effect of Al substitution on the charge ordered Pr₀.5Ca₀.5MnO₃: Structure, Magnetism and Transport

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We report the effect of Mn site substitution of Al on the structural, magnetic and transport properties of charge ordered Pr₀.5Ca₀.5MnO₃. This substitution introduces impurities in the Mn-O-Mn network, causes the Mn³+/Mn⁴+ ratio to deviate away from unity and is seen to predominantly replace Mn⁴+. The strength of charge ordering is supressed with increasing Al doping, as is seen from ac susceptibility, the magnetocaloric effect and dc resistivity. The observation of traces of charge ordering over an extended range of impurity doping of 10% besides being indicative of the robust nature of charge ordering in Pr₀.5Ca₀.5MnO₃, also underscores the relatively insignificant effect that Al substitution has on the lattice distortions and the importance of non-magnetic substitutions. For all the samples, the conductivity in the temperature range above the charge ordering transition is observed to be due to the adiabatic hopping of small polarons, whereas below the charge ordering transition, Mott's variable range hopping mechanism is seen to be valid.