MAGNETISM ELECTRON TRANSPORT 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.
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