Phase separation in manganites induced by orbital–ordering strains

Abstract

Doped manganite perovskites AMnO₃ exhibit a rich variety of electronic properties, resulting from the interplay of charge (Mn³⁺/Mn⁴⁺), spin (Mn magnetic moment) and orbital (Mn³⁺ Jahn–Teller distortion) degrees of freedom. Magnetisation measurements and ESR spectra have been used to study a series of eight AMnO₃ perovskites, in which the A cation sites are occupied by a distribution of 70% trivalent lanthanide and 30% divalent Ca, Sr or Ba ions. These all have a mean A cation radius of 1.20 Å but different values of the cation size variance σ². A change from orbital disorder to order (cooperative Jahn–Teller distortions) was previously found in the insulating regime at σ² ≈ 0.005 Ų. This work has shown that co-existence of the orbitally ordered and disordered phases is found in σ² = 0.0016–0.0040 Ų samples, with a difference of 40 K between their Curie temperatures. This is ascribed to competition between orbital ordering and microstructural lattice strains. At larger σ² > 0.005 Ų, the orbital ordering strains are dominant and only this phase is observed. This intermediate temperature phase segregation is one of many strain-driven separation phenomena in manganites.