| It has been known for some time that, by applying a magnetic field, phase changes can be induced in materials. This thesis studies La 1-xSrxMnO3 (x=0.5) and La 1-xBaxMnO3 (x=0.5, 0.52) using x-ray diffraction. Initial investigations were directed toward inducing a phase change by applying a magnetic field to the samples. The La0.45 Sr0.55MnO3 changed from an orthorhombic structural phase (Fmmm) to a tetragonal phase (I4/mcm) with the applied field. Similarly, the La0.5Ba0.5MnO3 changed from a low temperature tetragonal structural phase (P4/mmm) to a high temperature tetragonal phase (P4/mmm) with the applied magnetic field. Additionally, studies were done so that these magnetically induced transitions could be compared to transitions induced by increasing the temperature. In both cases (magnetic and temperature) there was significant phase coexistence over a range of fields and temperatures. In La0.48Ba0.52MnO3, three phases were observed to coexist below the phase transition, a low temperature monoclinic (P 2/m), low temperature tetragonal (P4/mmm), and a high temperature tetragonal (P4/mmm). Comparing these structural phases with previous neutron results appears to indicate that the tetragonal phase is associated with a complex charge-ordered/orbital-ordered phase, and the monoclinic phase is associated with a more stable orbital ordered phase. The high temperature tetragonal phases in the three studied samples are all ferromagnetic. Combined with neutron work, this study revealed the existence of a multicritical point induced by the deliberate chemical disorder on the La Layers. A strong competition exists between the three phases near this multicritical point. |
