Phase separation and resistivity jumps in Co compounds and other materials with low-spin-high-spin transitions

States of charge carriers in transition metal compounds with low-frequency zero-spin-nonzero-spin transitions (e.g. some Co compounds) are investigated. Such transitions indicate the appearance of magnetic excitons on the transition metal ions. Charge carriers interact strongly with the excitons via exchange forces. As a result, magnetoexcitonic self-trapping of carriers (holes) is possible in the ions when a complex consisting of a hole and one or several magnetic excitons arises. According to the parameters of the system, the magnetic ordering in the complex may be either collinear ferromagnetic or antiferromagnetic, or indeed canted antiferromagnetic. In the ground state of degenerate semiconductors, spontaneous separation into magnetoexcitonic and nonmagnetic phases is possible, with all of the holes concentrated in the former phase. The same magnetic structures are allowed in the ground state as in the hole-exciton complexes. On increase of the hole density, percolation of holes and magnetic excitons takes place, which leads to a transition from the insulating phase-separated state to the highly conducting phase-separated state.