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 typeLn0.5A0.5MnO3(Ln=rare earth,A=alkaline earth) is highly sensitive to the average radius of theA-site cations, left angle bracketrAright-pointing angle bracket. In the small left angle bracketrAright-pointing angle bracket regime (e.g., Y0.5Ca0.5MnO3), charge ordering occurs in the paramagnetic state, the transformation to an antiferro-magnetic state occurring at still lower temperatures. At moderate left angle bracketrAright-pointing angle bracket values (e.g., Nd0.5Sr0.5MnO3), 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 couplingsJFMandJAFMwith left angle bracketrAright-pointing angle bracket and the invariance of the single-ion Jahn–Teller energy with left angle bracketrAright-pointing angle bracket. A qualitative temperature–left angle bracketrAright-pointing angle bracket phase diagram, consistent with the experimental observations, has been constructed to describe the properties of the manganates in the different left angle bracketrAright-pointing angle bracket regimes.