Charge order and its destruction effects on magnetocaloric properties of manganites

Abstract

Colossal magnetoresistance manganites also exhibit a large magnetic entropy change (Δ S m) or magnetocaloric effect (MCE) around their magnetic phase transitions. Although many MCE studies in manganites have been reported around the Curie temperature ( T C), there is a lack of studies around their orbital and charge ordered temperature ( T CO). The MCE exhibits quite different behavior at T C from T CO. Here, we made an effort to understand the effects of charge ordered phase and its destruction on MCE properties by studying the temperature as well as magnetic field dependence of magnetocaloric properties in (i) the charge and orbital ordered La 0.5Ca 0.5MnO 3 and (ii) the charge ordered destructed La 0.5Ca 0.5Mn 0.96Ni 0.02O 3. The studies reveal that even 2% Ni substitution in La 0.5Ca 0.5Mn 1 − x Ni x O 3 destroys charge ordered state and induces long range ferromagnetic phase. The parent compound shows both negative (Δ S m = −1.8 J/kgK) as well as positive magnetocaloric (Δ S m = + 2.9 J/kgK) effects under 3 T field at ferromagnetic and charge order transitions, respectively. The Ni-substituted compound shows only negative magnetocaloric effect (Δ S m = −2.9 J/kgK). The parent and 2% Ni-substituted samples show relative cooling power (RCP) of −68 J/kg at T CO and + 145 J/kg at T C under 3 T field, respectively. These Δ S m and RCP values at T CO are much larger than other 50% Ca-doped manganites. The results were interpreted in terms of charge ordering and its destruction with Ni substitution.