Correlation between electrical and magnetic properties in alkali and alkaline earth metal doped lanthanum manganites

The recent colossal magnetoresistance (CMR) found in cation doped lanthanum manganites (La,A)MnO₃, where A is the cation, has evoked great interest either in possible applications or fundamental studies. The CMR manganite undergoes a paramagnetic-ferromagnetic (PM-FM) transition along with an insulator-metal (I-M) transition when it is cooled below its Curie temperature. Such a concurrence of PM-FM and I-M transitions in the CMR manganites was first qualitatively explained in the “double exchange” (DE) theory, proposed by Zener in 1951.; In the search of the correlation between electrical and magnetic transport properties in the CMR manganites, a molecular field approximation was then used by Searle and Wang to calculate the magnetization and resistivity and compared to the experimental data obtained from La_1-xPb_xMnO ₃ single crystals. Although the data fit the molecular field calculation well, it remains an open question whether such a molecular field approximation can be applied to other CMR lanthanum manganites. It is our intention to do a thorough and systematic study of the cation doped lanthanum manganites and to compare the resistivity and magnetization obtained from these materials to the molecular field calculation. The materials we choose are the alkali and alkaline earth metal doped lanthanum manganites, La _1-xA_xMnO₃, where x = 1/3 for A = Ca, Sr, and Ba; x = 1/6 for A = Na, K, Rb, and Cs. The hole doping level is the same for all the materials we study in this way. It also avoids any complexity which arises from multi-cation doping such as La_1-x-yY_yCa_xMnO ₃. Due to the difficulty of growing alkali metal doped lanthanum manganites, we chose to fabricate thin films instead. The thin films are fabricated by the e-beam/thermal co-evaporation method that has been proved to be suitable in these materials.; The magnetization as a function of temperature M(T) in all the CMR manganite thin films matches well the molecular field calculation, but the resistivity versus temperature ρ(T) cannot be explained by the theory. A simple explanation is given in chapter 5.