Mean-field theory of charge ordering and phase transitions in the colossal-magnetoresistive manganites

doi: 10.1088/0953-8984/11/43/320We study phase transitions in the colossal-magnetoresistive manganites by using a mean-field theory both at zero and non-zero temperatures. Our Hamiltonian includes double-exchange, superexchange, and Hubbard terms with on-site and nearest-neighbour Coulomb interaction, with the parameters estimated from earlier density-functional calculations. The phase diagrams show magnetic and charge-ordered (or charge-disordered) phases as a result of the competition between the double-exchange, superexchange, and Hubbard terms, the relative effects of which are sensitively dependent on parameters such as doping, bandwidth, and temperature. In accord with the experimental observations, several important features are reproduced from our model, namely, (i) a phase transition from an insulating, charge-ordered antiferromagnetic to a metallic, charge-disordered ferromagnetic state near dopant concentration x = 1/2, (ii) the reduction of the transition temperature TAFrightarrow F by the application of a magnetic field, (iii) melting of the charge order by a magnetic field, and (iv) phase coexistence for certain values of temperature and doping. An important feature, not reproduced in our model, is the antiferromagnetism in the electron-doped systems, e.g., La1-xCaxMnO3 over the entire range of 0.5le xle1, and we suggest that a multi-band model which includes the unoccupied t2g orbitals might be an important ingredient for describing this feature.This work was supported in part by the Office of Naval Research under contract No ONR N00014-95-1-0439. One of us (RP) thanks the Department of Physics, University of Missouri- Columbia for hospitality, during the time that this work was done, and UGC (India)