The Monte Carlo Ferromagnetic Ising model was used to study the electrical properties of manganese oxides due to the charge ordering phase occurring at doping, x = 0.5. The half-doped manganites have an insulator antiferromagnetic ground state. We ca
lculated the internal energy, specific heat, resistivity and the magneto-resistance, MR, with parallel and anti-parallel applied magnetic fields. Our simulation reveals that the resistivity decreases exponentially and the electric current increases with increasing temperature according the free charge increase, to transport from an insulator to conductor phase. The magnetoresistance has negative small values with parallel magnetic field but has positive high values with unti-parallel magnetic field. The obtained semiconductor-metal transition behavior candidates the half-doped manganites to be very good semiconductors diode junctions.
Landau theory is used to investigate the behaviour of a metallic magnet driven towards a quantum critical point by the application of pressure. The observed dependence of the transition temperature with pressure is used to show that the coupling of t
he magnetic order to the lattice diverges as the quantum critical point is approached. This means that a first order transition will occur in magnets (both ferromagnets and antiferromagnets) because of the coupling to the lattice. The Landau equations are solved numerically without further approximations. There are other mechanisms that can cause a first order transition so the significance of this work is that it will enable us to determine the extent to which any particular first order transition is driven by coupling to the lattice or if other causes are responsible.
The optical properties of a small magnetic cluster are studied in a magnetic version of Frank-Condon principle. This simple model is considered to show new basic physics and could be adopted to treat real problems. The energies and wavefunctions of t
he cluster are calculated for different spin configurations to evaluate the energies and the strengths of the allowed transitions from the relaxed excited states. The optical de-excitation energies for the likely scenarios are obtained in terms of the exchange parameters of the model.
The Jahn-Teller distortive transition of lmo is described by a modified 3-state Potts model. The interactions between the three possible orbits depends both on the orbits and their relative orientation on the lattice. Values of the two exchange param
eters which are chosen to give the correct low temperature phase and the correct value for the transition temperature are shown to be consistent with microscopy theory. The model predicts a first order transitions and also a value for the entropy above the transition in good agreement with experiment. The theory with the same parameters also predicts the temperature dependence of the order parameter of orbital ordering agreeing well with published experimental results. Finally, the type of the transition is shown to be close to one of the most disordered phases of the generalised Potts model. The short range order found experimentally above the transition is investigated by this model.
