Substitution of manganese by trivalent and tetravalent elements in the CMR perovskites Pr_1-x(Ca, Sr)_xMnO₃

Abstract

The substitution of trivalent (M = In, Ga) and tetravalent elements (M = Sn, Ti) for Mn(III) and Mn(IV) respectively has been studied in the colossal magnetoresistant (CMR) perovskites Pr_0.7Ca_0.2Sr_0.1MnO₃ (type I) and Pr_0.5Sr_0.5MnO₃ (type II). For the former compound, whatever the element, the temperature transition (T_max or T_c) separating the ferromagnetic metallic (FM) state and the paramagnetic semiconductinc (PSC) state decreases dramatically when the substituted element content is increased, the saturated magnetic moment at low temperature being slightly decreased. For these type I perovskites the maximum magnetoresistance is achieved for Pr_0.7Ca_0.2Sr_0.1Mn_0.99Ga_0.01O₃, reaching a resistance ratio of 600 at 127 K against 275 at 151 K for the pristine sample. These results show that by decreasing or increasing the hole concentration via M(IV) or M(III) substitutions on the manganese site the Curie temperature T_c is always decreased. In contrast to the type I perovskites, two different effects are evidenced for the Pr_0.5Sr_0.5Mn_1-xM_xO₃ substituted type II phases depending on the M valence. On one hand for M = Ga, In when x increases the antiferromagnetic semiconducting (AFSC) state and the PSC state are favoured at the expense of ferromagnetism. On the other hand for M = Sn, Ti the low temperature AFSC state tends to disappear and for x > 0.04 only a FM to PSC transition still exits similarly to that observed in type I perovskites. Correspondingly the magnetization versus temperature curves evolve from the bell shape curve typical of the charge ordering state to that of a ferromagnetic compound. The effect of valence and d⁰, d¹⁰ electronic configurations in both type I an II CMR perovskites is discussed.