Magnetism and Electron Transport in Magnetoresistive Lanthanum Calcium Manganite

Abstract

It is the goal of this thesis to understand the physical properties associated with the large negative magnetoresistance found in lanthanum calcium manganite. Such large magnetoresistances have been reported that this material is being considered for use as a magnetic field sensor. However, there are many variables such as temperature, magnetic field, chemical composition and processing that greatly influence the magnitude of the magnetoresistance. After introducing the problem in Chapter 1, Chapters 2 and 3 describe the materials synthesis and physical property measurements used in this work. In Chapter 4, the intrinsic magnetic and electron transport properties of lanthanum calcium manganite are distinguished from those that depend largely on the chemical synthesis and processing. Chemical substitution of lanthanum by gadolinium, discussed in Chapter 5, not only induces ferrimagnetism, but also dramatically alters the electron transport because of slight structural changes. The physical mechanisms and empirical relationships found among the resistivity, magnetoresistance and magnetism in Chapters 3 and 4 are studied in greater depth in Chapters 6 and 7 and compared with theoretical predictions. This analysis provides a useful method for predicting the magnetoresistance as a function of temperature, magnetic field and transition temperature. The related perovskite, strontium ruthenate, proves to be a model compound for the study of metallic ferromagnets. The results of this work is presented in two appendices, and compared with the manganite results throughout the text.