The nature of charge ordering in rare earth manganates and its strong dependence on the size of the A-site cations

Abstract

Charge ordering in rare earth manganates of the type Ln_0.5A_0.5MnO₃(Ln=rare earth, A=alkaline earth) is highly sensitive to the average radius of the A-site cations, <r_A>. In the small <r_A> regime (e.g., Y_0.5Ca_0.5MnO₃), charge ordering occurs in the paramagnetic state, the transformation to an antiferro-magnetic state occurring at still lower temperatures. At moderate <r_A> values (e.g., Nd_0.5Sr_0.5MnO₃), a ferromagnetic metallic state transforms to a charge-ordered antiferromagnetic state with cooling. These two distinct types of charge ordering and associated properties are explained in terms of the variation of the exchange couplings J_FM and J_AFM with <r_A> and the invariance of the single-ion Jahn-Teller energy with <r_A>. A qualitative temperature-<r_A> phase diagram, consistent with the experimental observations, has been constructed to describe the properties of the manganates in the different <r_A> regimes.