Phase Separation and Phase Diagram in Lightly Doped Manganites: Temperature and Magnetic Field Effects

The effects of magnetic field and temperature on the phase separation and phase diagram of lightly doped manganites are studied. Based on the double exchange model with on-site Coulomb interaction, we show that in the case of a homogeneous charge distribution, the canting angle of localised core spins and the critical doping concentration of the system from canted phase to ferromagnetic (FM) phase become large because the effective FM coupling between localised core spins is weakened when the temperature increases. The boundary of the canted phase and FM phase shifts to a high doping concentration regime at high temperatures. In comparison with with the zero-temperature result, the phase separation can take place more easily in lightly doped manganites at finite temperatures. The application of a magnetic field decreases the energy of the FM cluster in the system, favours the separation of the hole-rich FM phase from the antiferromagnetic (AFM) background, and shifts the cant-FM border to the low doping regime. The effect of the Jahn-Teller electron-phonon coupling on the phase diagram and phase separation is also discussed.

MAGNETIC STUDIES OF SUPERCONDUCTING GdBa2Cu3O6+δ AT LOW TEMPERATURE AND HIGH FIELD

The magnetization of antiferromagnetic superconducting GdBa2Cu3O6+δ has been measured for ~1.5<T≤4.2 K for field up to ~20 T. We found that all Gd3+ spins are nearly parallel at very high field, and that this saturated spin subsystem coexists with superconductivity. Below the Néel temperature, 2.22 K, we observed the transition from the “canted” phase to the paramagnetic phase by the application of a high magnetic field.