MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

88 Chapter 4
dependence of R 2 is so small (less than 10% variation) it has not been
measured.
The range of validity can be extended to about 200K for La0.67Ca0.33MnO3 and 350K for La0.67Sr0.33MnO3 with the addition of a T n term where 4 < n < 5.
The data was analyzed with a fit to R 0 + R 2 T 2 + R 4.5 (H)T 4.5 since a T 4.5
temperature dependence has been predicted for electron-magnon scattering in
the double exchange theory [112]. Evidence for a T 4.5 term (as opposed to T 4 or
T 5 ) cannot be claimed from this data, only that it allows the determination of
the R 0 and R 2 terms to higher accuracy in a much larger temperature range.
The maximum temperature for which the data were analyzed was
determined by finding the maximum temperature where the free parameters
remained stable and the fit was visibly accurate.
The results of this analysis are given in Table 2. As expected, R 4.5 (H)
decreases as the magnetic field increases. In a field of 70 kOe, R 4.5 (H) decreases
by 25-50% (Table 2).
Recently other authors have analyzed the resistivity data for
RE 0.67 AE 0.33 MnO 3 compounds using models different than the one presented
above. The resistivity of polycrystalline La 0.67 Ca 0.33 MnO 3 has been fit to R 0 +
R c T c [29] while a polynomial fit such as the one used in this work will also fit
their data. The R 0 + R 2 T 2 + R 4.5 T 4.5 fit, which has the same number of free
parameters, is preferable since there seems to be some universality in R 2 . The
low temperature zero field resistivity of La 0.67 Ba 0.33 MnO 3 films [109] has been
fit to R 0 + R 1 T + R 2 T 2 , where R 1 < 0. R 1 may not have physical meaning even
though a small correction is necessary to explain the almost constant
resistivity at the lowest temperatures. A dominant T 2 term has been also
recently found in the resistivity of single crystals [113].