MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Intrinsic Properties of La 0.67 Ca 0.33 MnO 3
electron ferromagnets Sc 3 In and ZrZn 2 . The term in La 0.67 AE 0.33 MnO 3 (Table
4-2) is about 1000 times larger than that in Fe, Co and Ni, which have
comparable M 0 's. Furthermore, the value of R 2 is expected to decrease in a
magnetic field (proportional to H -1/3 ) due to the suppression of spin
fluctuations; however, the field dependence of R 2 in the manganites is too
small for us to detect. Thus the theory of spin fluctuations does not
completely explain the observed T 2 dependence of the resistivity.
General electron-electron scattering mechanisms within the Fermi liquid
model give a T 2 dependence of the resistivity which is not necessarily field
dependent. A value of R 2 as large as that seen in this work has been found in
the nonmagnetic semimetal TiS 2 [120].
4. 2. 1. 3 Relationship to magnetism
A quite interesting correlation [114, 115, 121, 122] has been found between
the magnetization and the resistivity, namely ρ = R e exp(-M(T, H)/M r ) where
R e and M r are fitting parameters. This is clearly preferable to a polynomial fit
in the critical region near T C . If, at low temperatures, M = M 0 (1-M 2 T 2 ) where
M 2 is largely independent of field, then this expression predicts the resistivity
will vary as R 2 T 2 + R 4 T 4 . Thus a three parameter fit to ρ =
R e exp(-M(T, H)/M r ) is essentially equivalent to R 0 + R 2 T 2 + R 4 T 4 and
therefore fits our data very well. However, this equivalency breaks down
when one considers the field dependence: Since both R e and M r are
independent of field, the field dependence of ρ comes from the field
dependence of M 2 which would affect R 2 and R 4 equally. In summary, the
exponential fit combined with a T 2 dependence of the magnetization uses
four free parameters: R 0 , R e , M r and M 2 (7T) which does not fit the low
temperature data as well as the four free parameters R 0 , R 2 , R 4.5 (0T), R 4.5 (7T)
used in this work. This is discussed further in chapter 7.
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