Critical behavior and transport properties of lanthanum manganite
Lin, Peter Chuan-Yi
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https://hdl.handle.net/2142/31317
Description
Title
Critical behavior and transport properties of lanthanum manganite
Author(s)
Lin, Peter Chuan-Yi
Issue Date
2001
Doctoral Committee Chair(s)
Salamon, Myron B.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
lanthanum manganite
Language
en
Abstract
The resistivity, thermopower, magnetization, and ac heat capacity ofLa0.7Ca0.3Mn03,
La0.7(Ca,Pb)0.3Mn03, and Lao.7Sro.3Mn03 single crystals have been measured over a
broad temperature range in fields of up to 70 kOe. Below Tc, all three samples exhibit
a strong T 2 temperature dependence, characteristic of electron-magnon scattering,
which abruptly disappears around 20 K. Above Tc, the transport properties ofLCMO
are consistent with thermally activated hopping of polarons. The charge carriers
appear to consist of two different types which add according to the Nordheim-Gorter
rule. The mixing fraction of the two populations around the transition temperature
can be extracted with an effective medium approach. Magnetization data further
support this two fluid picture. Although the inverse susceptibility follows a Curie-
Weiss law, it has an anomalously small slope. This is indicative of the presence of a
Griffiths phase where ferromagnetic clusters emerge in a paramagnetic background
well before the entire system orders. The manganites' magnetic heat capacity peak
has been observed to shift upward in temperature with applied magnetic field unlike
that of ordinary ferromagnets. In the double exchange picture proposed by Zener,
magnetism is closely linked to the nature of the charge carriers. We postulate that the
exchange energy changes with the mixing fraction of the two carrier populations. By
modifying the Oguchi model with this assumption we can reproduce the anomalous
behavior of the heat capacity.
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