Emission Mössbauer study of CMR manganite La_0.8Ca_0.2MnO₃ . II. Step by step snapshots of the metal-insulator transition

We follow the step by step progression of events while approaching Curie temperature T_C from below using ⁵⁷Co substituent as microprobe in an emission Mössbauer study combined with resistivity measurements. In the temperature range 0.33 < T/T_C ≤ 1, the material consists of a mixture of ferromagnetic and paramagnetic regions within the same matrix. An increase of the amount of paramagnetic fraction is accompanied by a decrease in electron delocalization in the ferromagnetic regions. At T ≥ T_C , the electrons are localized to neighboring Mn⁴⁺/Mn³⁺ pairs only, in about 46% of the paramagnetic species. The strength with which Mn atoms are bound to the neighbors also decreases progressively and rather steeply in the range 0.65 ≤ T/T_C = 1. Zero field resistivity, ρ₀, follows linearly with the amount of paramagnetic phase in the range 0.65 ≤ T/T_C ≤ 1 and still show metal-like behavior up to T/T_C ≈ 1.03.