Charge, spin, and orbital ordering in the perovskite manganates, Ln(1-x)A(x)MnO(3) (Ln = rare earth, A = Ca or Sr)

Charge ordering occurs in some mixed-valent transition metal oxides. The pe rovskite manganates of the formula Ln(1-x)A(x)MnO(3) (Ln = rare earth; A = Ca, Sr) are especially interesting because long-range ordering of the Mn3(t(2g)(3), e(g)(1)) and Mn4+ (t(2g)(3), e(g)(0)) ions in these materials is linked to antiferromagnetic spin ordering, and also to the long-range orde ring of the Mn3+ (e(g)) orbitals and the associated lattice distortions. Ch arge 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 ord ering occurs without charge ordering in the A-type antiferromagnetic mangan ates, but in the manganates where charge ordering occurs, antiferromagnetis m of CE-type is found along with orbital ordering. The subtle relations bet ween charge, spin, and orbital ordering are discussed in the article, with special attention to the effects of cation size, chemical substitution, dim ensionality, pressure, and magnetic and electric fields. Unusual features s uch as phase separation and electron-hole asymmetry are also examined.