MAGNETISM ELECTRON TRANSPORT MAGNETORESISTIVE LANTHANUM CALCIUM MANGANITE

Intrinsic Properties of La 0.67 Ca 0.33 MnO 3
carriers. Above T MI the polarons conduct via thermally activated hopping,
which has a negative dρ/dT. Although this qualitatively explains the data,
the quantitative predication of the temperature dependence does not fit the
data very well.
4. 2. 7 Colossal Magnetoresistance
In general, manganite films reported in the literature which exhibit CMR
as shown in Figure 4-12 [21, 23, 26, 127, 128]: 1) do not have square hysteresis
loops at low temperatures, leading some to believe superparamagnetic
behavior exists in those materials; 2) have ill-defined T C 's which are much
less than those reported for the bulk materials with the same nominal
composition; 3) have T MI well below T C , with T MR slightly less than T MI ; and
4) have high resistivities in the metallic state, at low temperatures.
Figure 4-12 Colossal magnetoresistive La 0.67 Ca 0.33 MnO 3 film
from [21, 127].
The annealed films in this study (which do not exhibit CMR) are much
like bulk polycrystalline or single crystal material: They are clearly itinerant
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