Charge, spin, and orbital ordering in the perovskite manganates, Ln_1−xA_xMnO₃ (Ln = rare earth, A=Ca or Sr)

Abstract

Charge ordering occurs in some mixed-valent transition metal oxides. The perovskite manganates of the formula Ln_1−xA_xMnO₃ (Ln=rare earth; A=Ca, Sr) are especially interesting because long-range ordering of the Mn³⁺ (t³_2ge¹_g ) and Mn⁴⁺ (t³_2ge⁰_g) ions in these materials is linked to antiferromagnetic spin ordering, and also to the long-range ordering of the Mn³⁺ (e_g) orbitals and the associated lattice distortions. Charge ordering occurs at a higher temperature than spin ordering in some of the manganates (T_CO>T_N), whereas in some others T_CO=T_N. Orbital ordering occurs without charge ordering in the A-type antiferromagnetic manganates, but in the manganates where charge ordering occurs, antiferromagnetism of CE-type is found along with orbital ordering. The subtle relations between charge, spin, and orbital ordering are discussed in the article, with special attention to the effects of cation size, chemical substitution, dimensionality, pressure, and magnetic and electric fields. Unusual features such as phase separation and electron-hole asymmetry are also examined.