Anomalous magnetism and charge transport in dielectric La1-xCaxMnO3 (x≤xMIT∼0.22).

A study of the charge transport, magnetization, and high frequency (ω/2π=60 GHz) electron spin resonance (ESR) has been carried out on La_1-xCa_xMnO₃ single crystals corresponding to the dielectric side of the concentration driven metal-insulator transition (x≤x_MIT∼0.22). For x=0.22 the onset of ferromagnetism below the Curie temperature T_C≈183 K was found to induce crossover from holelike (T>T_C) to electronlike (T<T_C) conductivity. The activation behavior of resistivity and Seebeck coefficients with the respective activation energies E_a^ρ∼123 meV and E_a^S∼32 meV was detected in the paramagnetic state of x=0.22 compound for T>T_C. A large enhancement of the Hall effect, which is accompanied by the increase in Hall mobility to the values of μ_H∼-20 cm²/V s, was observed at intermediate temperatures 150 K<T<T_C. Besides, a single ESR line with g factor g≈2 observed for x=0.2 at T>T_C was shown to evolve into an intricate two-mode ESR spectrum with irregular fine structure characterizing the ferromagnetic insulating (FI) state formed below T_C. The anomalies of charge transport and magnetic properties found in FI state seem to be associated with the peculiarities of strongly disordered state with phase separation (ferromagnetic metallic domains embedded in the insulating matrix of La_1-xCa_xMnO₃). [ABSTRACT FROM AUTHOR]