Phase separation in hole-doped perovskite manganese oxides (manganites)

Manganites crystallize into the ABO₃ perovskite structure. Hole-doping is achieved by substituting divalent cations at the A-site which converts some of the Mn³⁺ ions (occupying the B-site) to the Mn⁴⁺ oxidation state. Hence the general formula of a hole-doped manganite is RE_{1 - x}AE_x MnO₃( RE is a trivalent rare-earth ion such as La, Nd, Pr and AE is a divalent alkaline-earth ion such as Ca, Sr). Depending on the doping x , in manganites there are two dominant phases at low temperatures: a double-exchange ferromagnetic metal (FMM) and charge-ordered antiferromagnetic insulator (COI). These two phases are separated by a first order transition. At high temperatures there is a paramagnetic insulator phase (PMI) for all dopings. Experiments also suggest that the free energy of the FMM and COI phases are very similar and this results in multiphase coexistence and the unique sensitivity of these materials to external perturbations (such as magnetic fields, electric fields, light, hydrostatic pressure and biaxial strain).

Recent theoretical studies have incorporated these experimental results. These studies have shown that a mixed phase model, in which the volume ratio of the FMM, COI and PMI phases changes with temperature, magnetic field and electric field, gives the most satisfactory explanation for the observed behavior in manganites. It is still not clear how a material which is chemically and physically homogeneous (i.e. there is no large scale charge segregation) exhibits micrometer scale phase separation under certain conditions. In order to understand this phenomenon another essential piece of the puzzle is that the COI phase has an orthorhombic distortion due to the Jahn-Teller effect. Hence when a manganite goes through the first-order transition from an FMM to a COI phase (or vice versa) there is a built-in strain in the material due to the di.erence in structures of the two phases. The accommodation of this strain generates long range interactions in these materials. When this strain is coupled with the inherent disorder in these materials the homogeneous first-order transition between the two competing (COI and FMM) phases is preempted by a micrometer scale phase separated state. Disorder can be introduced in these materials due to chemical co-doping which results in a mismatch in the ionic radii of the constituent elements and non-uniform strain in thin films. The multiphase coexistence is initiated by the presence of disorder which leads to a built-in strain in the manganites. The accommodation of this strain leads to long range interactions and the observed large scale phase separation, analogous to a martensitic transition. This phase separated state can now be manipulated extremely sensitively with external parameters such as strain, electric field, light etc. which gives rise to the notion of “electronic soft matter” in manganites similar to phases observed in materials such as liquid crystals.