Electron transport, magnetic state, and type of holes in (La_0.8Ca_0.2)_{1 − y} Mn_{1 − z} O_{3 + δ}

Abstract

The electrical resistivity ρ and magnetization of La_0.8Ca_0.2MnO_3.06, (La_0.8Ca_0.2)_0.975MnO_3.01, and La_0.8Ca_0.2Mn_0.975O3.06 polycrystalline samples have been measured in magnetic fields 0 ≤ H ≤ 9 T at temperatures in the range 2 ≤ T ≤ 300 K. The results obtained for the first and second samples are typical of manganites with the colossal magnetoresistance effect: the electrical resistivity increases with a decrease in temperature to T ≈ T _C and then sharply decreases. The composition of the third sample differs from the composition of the first sample by the content of Mn vacancies, which is ∼2.5% higher in the former case, and this difference leads to a very strong difference in the properties of the samples. First, the ferromagnetism in the third sample is almost completely suppressed and, at low temperatures (T ∼ 50–70 K), there occurs a transition to the state close to the cluster glass state. Second, the electrical resistivity of the third sample at low temperatures and H = 0 is four orders of magnitude higher than that of the first sample, even though the third sample contains a considerably higher concentration of holes. It has been shown that, for all three samples over the entire temperature range under investigation, the local activation energy E depends on the magnetic field. Of particular interest is the fact that the local activation energy E significantly decreases with increasing field in the paramagnetic region, which is not typical of conventional semiconductors. These specific features of the properties of the samples have been explained by assuming the hypothesis according to which holes in all three samples can be in the form of Mn⁴⁺, as well as in the form of O⁻, and can transform into each other. It has been concluded that a decrease in the temperature T and an increase in the magnetic field H favor the formation of Mn⁴⁺ holes and result in a decrease in the values of ρ and E with the formation of the ferromagnetic order, whereas an increase in the content of vacancies in the Mn-sublattice favors the formation of O^- holes and leads to an increase in the electrical resistivity ρ and the suppression of ferromagnetism in the sample.