Phase Coexistence in Manganites

Abstract

The doped perovskite manganite La1-xCaxMnO3 (0<x<1) has been extensively studied due to the interactions between the electronic, magnetic and crystal lattices, and the wide range of phases that can coexist. The region of greatest interest in the bulk material is around x~0.5, where there is often mesoscopic phase coexistence between a ferromagnetic metal (FM) and an antiferromagnetic insulator (AF). The first part of the dissertation describes a systematic study on a series of La1-xCaxMnO3 films deposited onto SrTiO3 (001) by pulsed laser deposition with compositions in the range 0.40<x<0.45. From electrical transport and magnetisation measurements, the limit of metallic behaviour was found to be x=0.41 whereas ferromagnetism was seen up to x=0.45. Although the transition temperatures of 150-200 K were comparable with the bulk material, the saturation moment at 20 K was about 40% of the fully spin-aligned value, which suggests the possibility of a phase separated mixture of FM and AF regions. The deviation from the bulk behaviour is thought to be due to substrate-induced strain altering the crystal symmetry and making the cubic ferromagnetic state less favourable. In the remainder of this work, the nature of phase separation in 60 nm La0.59Ca0.41MnO3 and La0.60Ca0.40MnO3 films is investigated. The effect of an external magnetic field is studied. A high-field magnetoresistance (∆ρ/ρB=0) of 41% in fields of 400 mT was observed for a La0.60Ca0.40MnO3 film, which, while not as large as the values previously reported in the literature, is still significant. The magnetic history of the films was found to be very significant, with the zero-field resistivity depending on the highest field applied. The isothermal time dependence of the resistivity was found to be exponential, with a time constant in the range 100-1000 s. Possible mechanisms for the MR effect and the dependence on magnetic history are discussed.EPSR