Effect of aluminium for manganese substitution upon the GMR properties of the praseodymium manganites

Abstract

The substitution of Al for Mn in the GMR perovskites Pr_0.7Ca _0.2Sr_0.1MnO₃, Pr_0.7Ca_0.1Sr_0.2MnO₃ and Pr_0.5Sr _0.5MnO₃ has been studied. For the first two series of compounds that belong to the type I GMR manganites [ferromagnetic (FM)–paramagnetic (PM) transition]T_c decreases significantly as the aluminium content increases, by about 12 K per percent of Al atoms introduced into the Mn sites, whereas the maximum magnetoresistance ratio R₀/H_H is not modified dramatically, reaching 10³ at 71 K for 6% Al per Mn in a magnetic field of 7 T. This increase of R₀/R_H is, in fact, correlated to the decrease of T_C, i.e. to the increase of the semiconductive character of the samples. For the series Pr_0.5Sr_0.5Mn_{1 –x}Al_xO₃, which belongs to type II [antiferromagnetic (AFM)-FM-PM transitions], an increase of T_N from the AFM to the FM state is observed by Al doping: T_N increases from 136 K for x= 0 to 170 K for x= 0.06. In contrast, T_c decreases as x increases.