Spin dependent variable range hopping in manganites La1-xCaxMnO3 (x > 0.5)

Manganites La1-xCaxMnO3 belong to the family of manganese oxides with a general chemical formula R1- xAxMnO3, where R represents a rare earth element or Bi and A an alkali or alkaline earth metal or Pb. Replacement of trivalent R and divalent A introduces a mixed valence state Mn3+/Mn4+ and depending on the concentration x causes a presence of various phases in the phase diagrams of R1-xAxMnO3. A huge interest for this family of compounds started in 1994. when Jin et al. found a huge decrease of resistivity in external magnetic field close to Curie temperature known as colossal magnetoresistance (CMR). CMR is present in the part of the phase diagram with ferromagnetic metallic ground state which for the investigated system La1-xCaxMnO3 appears for concentrations 0.2 < x < 0.5. Here we focused on the poorly explored part of the phase diagram x > 0.5 [3] with a paramagnetic insulating phase at high, charge order phase at intermediate and antiferromagnetic insulating phase at low temperatures. We performed magnetotransport and magnetization measurements in the temperature interval 4.2-300 K and magnetic fields up to 5 T on thin film manganites for different concentrations 0.5 < x < 0.8. Our results show traces of CMR and ferromagnetism and a strong coupling between magnetic and transport properties which can be interpreted in terms of spin dependent variable range hopping mechanism and a coexistence of ferromagnetic and antiferromagnetic phase.